Abstract. Summer weather conditions along the west coast of Africa near 34â S, 18âE are in- vestigated using doppler acoustic sounder profiles. Case studies ...
DOPPLER WINDS
SOUNDER
AND
OBSERVATIONS
SEA BREEZES
WEST COAST
MARK
JURY
ALONG
NEAR
and GARTH
OF TRADE THE
AFRICAN
34”S, 19”E
SPENCER-SMITH
Oceanography Dept., Univ. Cape Town, Rondebosch, 7700, South Africa
(Received in final form 24 March, 1988) Abstract. Summer weather conditions along the west coast of Africa near 34” S, 18”E are investigated using doppler acoustic sounder profiles. Case studies were selected from a two-year record to form composite analyses over the diurnal cycle. The SE trade wind exhibited a low level jet at the level of the temperature inversion due to a sharp reversal in the thermal wind vector aloft. Mean wind speeds reached 14 m s-i just before midnight as the surface and upper inversions strengthened. Seabreezes were categorised by the supporting gradient wind and found to have mean depths of 400 m, speeds of over 6 m s-r at the 200 m level, and advance/retreat times of 09 hr and 16-20 hr. During seabreezes and weak on-shore gradient flow conditions, the thermal internal boundary layer (TIBL) was monitored with sounder transects in the first 12 km of the coastal zone. The growth height was observed to be 1: 20 in the first 5 km and 1: 50 farther inland. The sounder climatology, together with surface network and aerial survey results, illustrate the four-dimensional characteristics of trade winds and seabreezes near Cape Town.
1. Introduction To describe the atmospheric boundary layer, space and time variations in the vertical profile of the wind and its associated turbulence are pre-requisite. Traditionally, the meteorological characteristics of the atmospheric boundary layer (O-1000 m) have been approximated using data from synoptic surface networks and radiosonde profiles. Such technology does not provide sufficient space, time and height resolution to investigate mesoscale phenomena. In many boundary-layer applications such as air pollution dispersion, data from tall (100 m) towers are assumed to represent the entire boundary layer up to 10 times that height. Such extrapolations are rarely justified. Since the early 1980s alternative technology in the form of tri-axial doppler acoustic radars (sounders) has been available to observe the horizontal and vertical wind and turbulence structures within the atmospheric boundary layer. Here we present the results of a summer-time data collection programme whose objective was to describe the characteristics of the atmospheric boundary layer along the African west coast near Cape Town (Figure 1). We focus on the two most prevalent summer circulation systems: the along-shore trade wind and the cross-shore seabreeze. To study mesoscale circulations such as these, the role of the larger scale forcing is recognised at the outset. Synoptic pressure gradients along the southwestern tip of the African continent Boundary-Layer Meteorology 44 (1988) 373-405. @ 1988 by Kluwer Academic Publishers.
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32-
study area *
Fig. la-b. (a) Topographic map of the west coast of Africa (29-35” S, 15-20” E, showing the plateau (elevations above 500 m shaded). (b) The study area including the data network and local mountains. Light and dark shading indicates elevations above 150 and 300 m. The surface network is shown by the squares, the sounder site by a dot, and the sounder transect by the dashed line.
near Cape Town are typically steep during the summer months (November to February) owing to a contrast between the marine high pressure cell and a thermal low over the western plateau (Taljaard, 1953; Newton 1972). The semi-permanent South Atlantic anticyclone is centered near 30” S 0” E with a pressure of 1020 Pa (Hastenrath, 1985) while the 1008 hPa low over the interior extends southwards along the 20” E longitude to 30” S. The sub-tropical ridge associated with the South Atlantic high extends to the south of the continent in the summer mean (Figure 6 for example) and subsidence results in a temperature inversion at 1200 m (Preston-Whyte et al., 1977), creating dewpoint depressions of over 10 “C (Newton, 1972). Below the inversion, SE trade winds blow with a probability of over 50% at a mean speed of 6 m S-I at the 10 m level (Jury and Guastella, 1987). The summer climatology along this southwestern fringe of Africa is, however, far from constant in either time or space. Meridional fluctuations in the subtropical ridge, forced by eastward moving Rossby waves in the circumpolar westerlies, modulat- the coastal pressure gradient (Jury, 1980). Typically, a transient high pressure cell migrates eastward following each Rossby wave trough. The South Atlantic high becomes, at first, stronger and then weaker as the transient high crosses from the Atlantic to the Indian Ocean at a
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speed of about 15 m SK’. Concurrently, a shallow mesoscale low is guided along the west coast, following on the back of the transient high as a coastally trapped atmospheric wave (Gill, 1977). Both the pressure gradient and elevated inversion strengthen and then weaken due to the passage of coastal lows. The trough/ridge/coastal-low cycle is repeated at intervals of about 6 days (PrestonWhyte and Tyson, 1973). Here we filter the synoptic weather variations through the use of composite analysis of selected case studies. We consider the mean summer condition when the sub-tropical ridge produces SE trade wind flow. We also examine seabreezes which occur during the pressure “~01” following the coastal low. Both circulations are investigated using sounder vertical profiles coupled with horizontal network analyses to provide a four-dimensional perspective. The flow patterns are analysed over the 24 hr cycle to assessthe role of the vertical and horizontal temperature gradients which form between the continental and marine air masses. In Section 3, a summer climatology is sketched. In Sections 4 and 5, the temporal nature and vertical structure of the trade wind and seabreeze circulations are described by sounder time-height composites. Spatial structure within the inversion-capped flows arises from the geometry of the southwest tip of Africa shown in Figure la. Much of the coast to the north of 34” S is oriented 340-160” and the interior plateau rises gradually to more than 500 m in a horizontal distance of less than 100 km, ultimately reaching over 1000 m. The synoptic scale surface pressure gradient is often coastally aligned (see Figure 6). Locally, mountain peaks reach 1OOOm at Cape Town and in a long N-S chain some 30 km to the east of the study area (see Figure la). The inversion-capped trade winds and seabreezes inevitably become obstructed or channeled by these mountain ridges (Jury 1984). Figure lb reveals the proximity of the sounder site to a short arc of mountains in the southwest of the study area. To the north of the sounder site, the terrain becomes considerably smoother, rising to about 200 m some 15 km inland and then falling off again into the Berg River Valley running N-S along 19” E. A smaller river valley cuts diagonally across the coastal orientation just north of the sounder site. 2. Data In November 1984 a Remtech Doppler Acoustic Radar (sounder) was located approximately 10 km to the north of Cape Town, near 33”50’ S, I8”30’ E (shown by the dot in Figure lb). The sounder was sited at an elevation of 30 m, 4 km from the west coast, bordered by a low-lying marsh to the west and a rounded hill 15 km to the east. The sounder operated at a frequency of 1600 Hz in tri-axial configuration: one antenna was aligned vertically and the other two were orthogonal at 18” from vertical. The tilted cones were aligned N-S and E-W to capture data on the horizontal wind vector. The vertical cone provided in-
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dependent information on the vertical wind vector. All data were recorded with a height resolution (thickness slice) of 50 m to the 1000 m level. Using Remtech software, mean U, V, W wind components and associated turbulence indices (standard deviations) were calculated and stored every 15 min. The data were then transferred to a larger computer for the production of the results given here. Sounder data quality was evaluated at the lowest level by comparison with data from research-grade wind sensors mounted on a tall tower at Koeberg (Figure 1b). Further international comparisons (NOAA, 1984) have lent credibility to sounder technology. The performance of the sounder during its operation in Cape Town was 100% up to the 300 m level; thereafter its efficiency dropped off exponentially to about 30% at the 1000 m level. To eliminate interpretation of data availabilities below 50%, the sounder time-height analyses are limited to the first 800m. Nocturnal soundings gave a 10% better return at the upper levels owing to lower background noise (from nearby traffic and industry). For the season used to characterise the summer climatology: mid-November 1984 to end-February 1985, the availability of sounder data exceeded 98%. Case studies forming the composite analyses were selected to avoid periods of poor sounder data availability. To obtain cross-sections of the seabreeze, the sounder was mobilised from 2 1 March to 10 April 1986 and transported along a line 15 km inland near Koeberg (shown as the dashed line in Figure lb). Profiles to the 500 m level were obtained at 4 km intervals between the hours of 12 and 14 hr. Simultaneously, air temperature profiles were made to the 100m level using a thermistor attached to a kite. Surface data were available from the SA Weather Bureau synoptic network to establish the local pressure gradient. Mesoscale data at the 1Om level were obtained from an automatic weather station network operated by the Koeberg Nuclear Power Station as part of the Emergency Response facility (Mulholland and Jury, 1987). The six stations provided horizontal wind vectors, wind standard deviations and air temperatures at hourly intervals for matching with the sounder data set. A synoptic view of the mesoscale circulation features was obtained by flying a research aircraft in a grid pattern across the coast at the 150 m level between lo-15 hr. During the second and third summer (1980 and 1981) of an oceanic upwelling project (Shannon, 1985), useful data were collected on the spatial structure of SE trade winds and seabreezes over the study area around Cape Town. The aircraft measured wind vectors, air temperatures, dewpoint temperatures and sea surface temperatures as described in Jury (1987). Four cases are presented from the aerial survey data set. 3. Summer Boundary-layer
Climatology
Before describing mesoscale circulation features such as topographically guided trade winds or seabreezes, it is meaningful to analyse the mean summer clima-
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tology of the coastal atmospheric boundary layer using doppler sounder information. We do this in the form of wind roses for the period mid-November 1984 to end-February 1985 and diurnal analyses of the vertical wind and its standard deviation for the month of January 1985. Sounder results are supplemented with wind roses from the 10 m meteorological network for the month of February 1985. The sounder wind roses were stratified by day (06-18 hr) and night (18-06 hr) and by the levels 100 and 400 m. Information on the diurnal cycle of the vertical wind component and its variance was contoured as percentages for the levels 50 and 600 m. Sounder wind roses for the 1984-1985 summer show that highest probabilities and speeds originated from the SSE sector (Figure 2a, b), indicating the deflection of trade winds by the upstream valley. Particularly in the lower 100 m layer at night time, summer winds showed a 50% preference for the south quadrant. By the 400 m level, equal amounts of S and SSE winds were obtained, more SSE at night and more S during the day. The anti-clockwise rotation of the most prevalent wind quadrant with height was attributed to warm air advection and frictional turning in the boundary layer. The day-time wind rose at the 100 m level highlights the presence of thermal forcing by a 25% occurrence of
m/s 0 4
b
8 12
Day
Night
Fig. 2. Wind roses for the period November, 1984 to February, 1985, stratified by day (06-18 hr, left) and night (18-06 hr, right) for the 100 (top) and 400 m (bottom) sounder levels. The speed and percentage scales are shown in the I.Cddle.
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AND
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seabreeze flow from the W and WSW sectors. At night, flow from these maritime sectors fell off to about 8%. When day and night wind roses were compared, a nocturnal increase in speed and directional shear was evident owing to increased stratification. The possibility of a nocturnal jet within the mean summer climatology is suggested. The low probability of SSW and SW winds was a consequence of the obstructing effect of Table Mountain (elevation 1086 m) 15 km from the sounder site (Figure lb). Other important aspects of the summer climatology are revealed by the diurnally averaged vertical wind and its standard deviation in Figure 3a, b, c, d. Subsidence predominates during summer, especially at night at both the 50 and 600 m levels. Maximum downward vertical motions (-W) occurred at about 05 hr at the lower 50 m level and before midnight at the upper 600 m level. Positive upward motions were most common, as could be expected, during the
r
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o\
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t2 h
18
10 w’“1 Jb 1 I 24 01 06
r2o--l.
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( 24
Fig. 3. Diurnally averaged percengages of the vertical wind (a, b) and vertical wind deviations (c. d) for January, 1985 at the sounder levels 50 (top) and 600m (bottom). Contours are shown at intervals of 10% against a vertical scale in cm SK’. The horiLonta1 scale is local time.
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mid-day period coincident with a warming of the land by solar insolation (>lOOO W m-‘, Guastella, 1988). Sharp upward peaks in the 50 m level 20% isoline at 09-10 hr and 17-18 hr (Figure 3a) marked the boundary-layer transition from stable to unstable and can be related to the advance and retreat of the seabreeze. The vertical wind components rose over the zero value only for a brief period around 14 hr. At 19 hr, a rapid collapse in the convective boundary layer was observed coincident with sunset. The standard deviation of the vertical wind (sigma W, Figure 3c, d) highlights a contrast between the upper and lower levels. Sigma W values at the 50 m level remained above 40 cm s-l, while at the 600 m level, values ranged between 20 and 35 cm SK’. A diurnal wave in vertical turbulence was evident throughout the boundary layer. Highest values were recorded between 12 and 18 hr near the surface, representing a lag after peak insolation. Sigma W values reached a minimum before sunrise. Certain features of the summer climatology may be reviewed using wind roses from the 10 m network for the month of February 1985 (Figure 4). One station
Fig. 4. Wind roses (as in Figure 2) at the 10m level for the month February, 1985, superimposed onto a topographic map of the study area. The scale shown at left is similar to the sounder roses collected where indicated by the dot.
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was within 3 km of the sounder site and so it is the height, measurement technology and period (month versus season) which differ. The main point of agreement between the sounder and surface data sets was the prevalence of SSE trade winds. The influence of the local topography was noted in the acceleration of winds around the landward side of Table Mountain. At both adjacent stations, SSE winds of over 8 m-i were frequent. Farther to the north, the lower relief allowed a downstream spreading of the prevailing summer winds. Wind speeds diminished in the northern part of the study area and directions were more variable. Farther inland, southwesterly flow was evident owing to a frictional (rightward) turning of the wind. At the southern inland site, the flow became aligned with a river valley. 4. Trade Winds and Nocturnal Jets 4.1. SPATIALSTRUCTURE
Spatial structure was observed by the selection of aerial survey cases from a 600 hour data set (Jury, 1986). Two examples for 29 January and 3 February, 1981 which best represent trade wind flow are illustrated in Figure 5a, b. Wind streamlines for these two cases clearly highlight the topographic channelling of a wind axis from False Bay in the south towards the sounder site along the west coast. Along the wind axis, extending through the sounder site, cool marine air penetrated downstream from False Bay (Figure lb). A splitting of the trade wind flow was evident on either side of Table Mountain, In the mountain lee, the atmosphere was calm and stable during the daytime as shown by the isotherms for 29 January and the isotachs for 3 February, 1981. These patterns clearly identify the mountain wake in both the wind and temperature fields. The wake acted to enhance subsidence so that warm dry air was characteristic over a 40 km* area downstream. The warming in the lee of Table Mountain (Figure 5a) represents a lowering of the marine boundary layer and inversion, similar to that found by Beardsley et al. (1987) in the lee of Point Arena, California. An important feature occurring at the southeastern edge of the study area, was the obstruction of SE trade wind flow by a major ridge of mountains. Downstream over the study area, this orographically affected air mass was drier than is usually the case for maritime trade wind flow (surface dewpoint depressions were 4 “C at night, Table 2). The drying of the marine boundary layer and the increased subsidence aloft were further enhanced by the fact that trade winds typically flow down the sea surface temperature gradient from the warm Agulhas to the cool Benguela Current. In the process, the air mass is advected down the air-sea surface heat flux gradient (Holflich, 1984). This may partially explain the mean downward vertical motions noted in the summer climatology (Figure 3a, b). The aerial survey data presented for the trade wind cases represent daytime conditions. To study the night-time situation, we use surface network data for the
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a
b Fig. 5. Research aircraft data representing trade wind flow conditions at the 150m level for 29 January (a) and 3 February, 1981 (b). Isotherms (in a, at intervals of 2°C) and isotachs (in b, at intervals of 2.5 m SK’) are superimposed onto the wind direction streamlines. The horizontally channelled jet is highlighted by shading the area over 12.5 m s-i in b.
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24 hr 17 November, 1985 case study. Wind vectors, vorticity and divergence fields are shown (with the seabreeze surface analysis), in Figure 13a. It is clear that the previously calm mountain wake area was “activated” by strong trade winds, most notably at the island station to the north of Table Mountain. The divergence field was positive and reached a maximum of +3 * 10e4s-’ over the sounder site, owing to a diffluence in the wind streamlines. Such a surface divergence would require subsidence in the atmospheric boundary layer through continuity of mass. The vorticity field revealed a distinct cyclonic band of values less than -2 1 10-” s-’ stretching to the northwest of the sounder site. The nocturnal acceleration of trade wind flow in the lee of Table Mountain was first reported by Jury et al. (1985) and is investigated in the following section. The nocturnal wind speed maximum is a mean summer condition for all surface stations along the leeward slopes of the coastal mountains. DeSouza et al. (197 1) have found a similar low level jet in the trade wind flow over Barbados. 4.2. TEMPORAL
VARIABILITY
To study the diurnal nature of the SE trade wind, sounder profiles were screened using the 10 m wind speed at the island station in the lee of Table Mountain. The selection criterion was that the 10m wind speed at the island station must have exceeded 10 m ss’ between the hours 21 and 03 hr. Such a criteria would therefore identify a nocturnal jet type phenomena within the SE trade wind flow. In addition, daily maximum temperatures at the island station must have exceeded 22 “C, indicating the entrainment of the warm wake into the boundary layer at this marine location adjacent to the cool 13 “C upwelling waters. A number of possible days were selected on the basis of simultaneous data from the sounder site (outside the mountain wake). Further screening of the cases was done using the synoptic surface pressure analysis of the SA Weather Bureau. The criteria set was that the individual daily map should correspond closely with the February mean, shown in Figure 6, i.e., a high pressure ridge to the south of the African continent should connect the South Atlantic and Indian Ocean anticyclones and a thermal trough should be present over the western interior. The synoptic screening eliminated data for those days following the passage of a transient coastal low through the study area. The final set of trade wind cases comprised three sequences during January, February and November 1985 as shown in the following table. Using doppler sounder profiles, other researchers have reported higher night time drainage winds in the lee of mountains (Neff and King, 1987), but less has been said about the nocturnal acceleration of the trade wind profile. The question to be answered here was whether the nocturnal jet was confined to the mountain lee or whether the entire atmospheric boundary layer accelerated after sunset. To determine the upper boundary layer forcing, radiosonde data from the local airport near Cape Town were consulted. Composite averages of temperature, dewpoint, wind direction and speed for the nine case study days were
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Fig. 6. Mean surface pressure analysis (+lOOO hPa) for the month February 1985 from the SA Weather Bureau synoptic data set. Isobars over the African plateau are interpolated from surface adjusted values.
calculated for the standard levels 1000, 925 and 850 hPa, corresponding to the surface, 700 and 1500m levels (Table II). In addition, an individual case on 14 February, 1985 01 hr is given to isolate certain aspects of the vertical structure. Composite radiosonde data clearly show that atmospheric boundary-layer temperatures were warm. During the day, surface temperature reached 26”C, thereby establishing an unstable surface convective layer. At night, surface cooling enabled a mean inversion of 2 “C to form between 1000 and 925 hPa. A steep vertical moisture gradient was evidenced by 850 hPa dewpoint depressions approaching 20°C. Such warm advection conditions were the result of the anti-clockwise rotation of the wind vector with height. The continental air mass
TABLE Doppler 1. 2. 3. 4. 5. 6. 7. 8. 9.
24 25 26 27 11 12 13 17 18
Jan., Jan., Jan., Jan., Feb., Feb., Feb., Nov., Nov.,
1985 1985 1985 1985 1985 1985 1985 1985 1985
I
acoustic sounder data for Trade Wind cases 12 hr-25 12 hr-26 12 hr-27 12 hr-28 12 hr-12 12 hr-13 12 hr-14 12 hr-18 12 hr-19
Jan., Jan., Jan., Jan., Feb., Feb., Feb., Nov., Nov.,
1985 1985 1985 1985 1985 1985 1985 1985 1985
11 hr (14 shown in Figure 7) 11 hr 11 hr 11 hr 11 hr 11 hr 11 hr 11 hr (shown in Figure 13a) 11 hr
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II
Trade wind radiosonde Composite cases 13 hr. daytime ascent
01 hr. nighttime
ascent
data Individual case 14 February 1985 01 hr
P 1000 hPa
T 7-d Dir W
925 hPa
T
850hPa
7-d Dir Spd T Td Dir Spd
17.O”C 15.5 192” 5.0 m ss’ 22.6 9.9 180 1.4 19.8 1.3 330 3.2
1000 hPa
925hPa
T 7-d Dir Spd
T Td Dir W
850hPa
T Td Dir W
19.2 15.3 181 7.4 21.3 10.4 179 4.9 19.7 2.9 014 0.3
1008 998 977 900 893 877 873 859 824 800 790
T
18.2 16.8 20.1 17.0 16.5 18.0 17.7 16.6 13.4 14.1 13.4
Td 16.6 15.3 16.8 12.8 12.3 6.5 3.8 9.4 7.0 -13.8 -10.3
Dir
Spd
220 155 145
6 8 2
015
1
245
3
was drawn off the African plateau and became entrained into the top of the coastal boundary layer. Topographic channelling of trade wind flow appeared to extend to the 925 hPa level where southerly wind directions persisted. The potential for channelling can be assessed using the Froude number F = U/N(h), where U is the maximum wind speed in a layer, h is the mountain elevation (set = 1000 m) and N is the buoyancy frequency = [g/8(d0/dt)]“2 where g is gravity, 8 is the potential temperature and dz is the thickness of the layer. Applying the ideas of Walter and Overland (1982) to our case, a buoyancy frequency of lop2 s-i (day), 2 . lop2 s-l (night) and a Froude number of about 0.7 are obtained from the mean profile, (dz = 1000-925 hPa, and U = 8 m s-l day, 14 m s-l night, Figure 8). Such F values indicate that a supercritical hydraulic jump of the trade wind jet over Table Mountain is not possible. However, evidence has already been presented (Figure 13a) that trade winds preferentially accelerate in the mountain’s lee at night. The forcing of the nocturnal downslope jet may be obscured by averaging the profile data. Closer inspection of profile information for the case of 01 hr 14 February, 1985 (Table II) reveals the formation of a lower inversion at 977 hPa which corresponded with the sounder wind maximum in the 200-300 m layer. An upper inversion was present at g77 hPa, some 200 m above the summit of Table Mountain or 1200 m ASL. From the top of the surface inversion to the base of the upper inversion, an unstable layer was found. The buoyancy frequencies for the surface inversion, upper inversion and unstable layer were 2.5, 1.9 and 1.2 . lop2 s-‘, respectively. Froude numbers were calculated for the three levels using radiosonde winds of 6, 8, and 2ms-’ (Table II), yielding results of 0.23, 0.10 and 0.63, for the surface and upper inversion and unstable layer, respectively. Similar results were
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obtained for other night-time profile cases. The mean heights of the surface and upper inversions were found to be 971 hPa and 876 hPa. The upper inversion strictly limits the trade wind flow, while the lower inversion primes the low level jet preferentially at night when convective heating subsides. A supercritical Froude number of 1.6 can be obtained in the unstable layer using a buoyancy frequency of lo-* and a wind speed of 16 m se2 (Figure 7a). The jet can then rise through the unstable layer and become compressed between the summit and upper inversion. We conclude that the nocturnal decay of the surface convective layer sets up a laminar flow which attains geostrophic magnitude at the top of the 200-300 m surface inversion. The upper inversion
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26 Jan 85
27
28
Fig. 7. Time-height sounder chronology for a period of trade wind conditions extendingg from 25-28 January, 1985. Wind direction streamlines (north = up) and isotachs (at 2 m s-t intervals, dashed) are analysed in (a). In (b) the vertical wind (dashed, unlabelled) and vertical wind deviations (solid, labelled) are analysed at intervals of 25 and 15 ems-‘, respectively. W values below -75 cm s-r (indicating subsidence) are shaded.
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then acts to accelerate the horizontal jet over Table Mountain, limiting the wake in the process. Winant et al. (1987) have examined a similar wake phenomena in the lee of Point Arena, California. They make use of two-layer hydraulic theory to explain that the wake is initiated by a hydraulic jump. A modified Froude number F = U/(g’h)1’2 is used, where U is defined as before, g’ is the density difference across the inversion and h is the height of the marine layer. Both the area of acceleration and the angle of the hydraulic jump relative to the coast can be determined. Applying these ideas here it is apparent that the daytime F is limited by a thickening of the marine layer (h). The nighttime F is supercritical as U is increased by Coriolis alignment of the gradient and thermal winds and h is reduced from 1200 to 300 m by the formation of the surface inversion. Further analysis of sounder trade wind profiles provide additional evidence on the dynamics of the downslope jet. Time-height analyses were constructed from the sounder data for each of the three sequences by plotting wind speed isotachs and direction streamlines and vertical wind speed and sigma W contours. These provide a chronological description of trade winds in the SO-800m boundary layer for the period 25-28 January, 1985 (Figure 7a, b). It is clear that the SE trade wind pulses over the 24 hr cycle in response to horizontal temperature gradients and the gradual Coriolis (anti-clockwise) turning of the underlying seabreeze vector through the afternoon hours. After sunset, atmospheric stability limited the effect of surface friction and the SE trade wind increased in speed and depth. The jet stretching northward from False Bay appeared to be concentrated at night-time, a condition somewhat out of phase to that found along the California coast by Beardsley et al. (1987), where the inshore movement of an along-shore jet was found to be a day-time phenomena. Throughout this particular sounder trade wind sequence, a gradual reduction in the level of the nocturnal jet was evident, implying that the synoptic weather forcing was not stationary. Overall though, the SE wind profile varied sufficiently uniformly over the 24-hr cycle to make a diurnal composite analysis meaningful. The vertical wind and turbulence structures for the January 1985 time-height analysis again exhibit a sharp 24-hr cycle (Figure 7b). At night the coastal plain cooled and subsidence (- W values) reached -1.0 m s--r within the layer 300600 m. The stability of the nocturnal boundary layer was illustrated by the low sigma W values during the night and early morning. The vertical wind became negative quite suddenly after sunset, while the sigma W lagged in time, owing to the thermal inertia of the lower atmosphere. It is noted that the sigma theta (standard deviation of the horizontal wind) showed no coherent 24-hr pattern, perhaps because of its inverse relationship with wind speed. Composite profiles of the trade wind speed, direction, sigma theta, W and sigma W were constructed by averaging all nine cases for each hour over the 24-hr cycle. Profiles of the 5 parameters at 6-hourly intervals are shown in Figure 8 centered on midnight. Most surprisingly, the general shape of the trade wind
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Compositeprofilesduring trade wind conditions for the times 12, 18, 24 and 06 hr of the
parameters(top to bottom): wind speed, wind direction, sigma theta, W and sigma W. Profilescales are shown on the 12 and 24 hr soundings. Shading is employed for speeds over 10 m SC’ to highlight the jet.
speed profile did not exhibit a diurnal change except at the lowest level. Wind speed reached a peak at 300m near sunset. The low-level wind maximum gradually descended to 200 m by sunrise. Profiles became most flattened during mid-afternoon convection, and were most jet-like after midnight. This type of wind maximum at the top of the marine layer has been extensively studied along west coast upwelling regions (Bunker, 1965; Johnson and O’Brien, 1973; Elliot and O’Brien, 1977; Enfield, 1981; Lester, 1985; and Zemba and Friehe, 1987). The direction profiles were consistently sloped from S to E with height. Sigma theta showed high values near the surface and aloft in contrast to low values in the jet layer. The W vertical wind component became quite negative after sunset while during the day convection again flattened the W profile. Sigma W
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Fig. 9. Composite analysis (from the same data set as in Figure 8) of speed, direction, sigma theta, W and sigma W (top to bottom) using contours to identify significant features in the diurnal cycle of trade wind flow.
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values were clearly highest at the surface and decreased aloft, being closely related to surface convection and roughness. The most significant finding was that the slope and shape of the trade wind speed profiles did not change appreciably over the 24-hr cycle. Another view of the same data set is taken using contour analysis (Figure 9) to illustrate the 24-hr trade wind cycle. The wind speed pattern exhibited a 14 ms ~-’ “bulls eye” centered on the 200 m level between sunset and midnight. The sunset maximum was attributed to Coriolis turning of the underlying seabreeze (best illustrated in Figure IO), adding to the pre-existing gradient trade wind. This effect is possible through resonance between the inertial period (21 hr) and the diurnal insolation cycle (Bain, 1988). Increasing stratification (i.e., radiosonde 1000 hPa temperatures dropped by 8 “C between 13 hr and 0 1 hr while 925 hPa temperatures remained relatively constant. Table II) maintained the high wind speeds during the night by diminishing the effect of surface friction. The composite wind direction analysis suggests an additional influence. Wind directions varied from 100-190” over the 24-hr cycle. Unexpectedly, most of the variance was contained at the top of the boundary layer, where a pre-dawn burst of easterly winds off the plateau dominated: winds were from 100” between 600-800 m from 04-06 hr. Very little seabreeze influence could be identified near the surface. The otf-plateau flow was thought to be a drainage at the top of the nocturnal inversion, which may compress the lower level jet, Smith el al. (1986) have reported a similar phenomenon occurring over northeastern Australia. Turbulence and vertical winds (in the lower panels of Figure 9) show the expected diurnal patterns. Low sigma thetas were recorded in the 200-500 m jet layer from 18-07 hr. Sigma thetas increased rapidly above 600 m during the day and above 800 m at night, defining the top of the coastal boundary layer during trade wind conditions. The vertical wind component showed that downward motions commenced immediately after sunset with the collapse of the convective boundary layer. After sunrise, neutral W values occurred. The subsidence associated with the surface inversion and low level jet was clearly at a maximum in the 200-300 m layer, well above the monitoring capability of tall (100 m) towers. The sigma W again lagged the rest of the parameters across the 24-hr cycle. The greatest depth of convection was recorded in the afternoon period, whereafter the sigma W contours descended to a minimum near sunrise. The stability of the lower atmosphere near Cape Town was most notable above 400 m in the morning hours. The primary result of the nocturnal stratification was an increase in strength and depth of the SE trade wind flow. 5. Seabreezes 5.1. ANALYSIS
SC-HEME
The study of seabreezes has consequences for air pollution transport, because unlike the stronger trade winds, seabreezes and their associated weather con-
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ditions produce low rates of dispersion in the coastal boundary layer. Seabreeze influences also play havoc with mesoscale forecasts, such as would be necessary for a Nuclear Emergency (Thuiller, 1987). The seabreeze can cause a rapid transition in near-shore wind directions and stabilities as it advances inland in the morning and retreats in the afternoon (Von Gogh, 1981; Jury and Guastella, 1987). If pollutants become embedded in the seabreeze, as would happen for most coastal emissions, they may become concentrated on the front edge and vertically recirculated (Lyons and Cole, 1973). It is therefore useful to characterise the depth, duration and spatial extent of the seabreeze and the turbulence associated with it. As shown in the first section (Figure 2a), the mean frequency of seabreeze occurrence is about 25% on summer days. Studies of mid-latitude seabreezes have illustrated the possible variety of this phenomenon. Maximum velocities in the seabreeze have been found to range from as low as 2.2 m s-l (Hsu, 1970) to 7.2 m s-* (Johnson and O’Brien, 1973). Average onset times range from 07 hr to as late as 14 hr (Sturman and Tyson, 1981) owing to offshore gradient winds. Average times of seabreeze-retreat range from 17 hr (Gill, 1968) to 24 hr. The depth of seabreezeshas been found to vary from 120m (Craig et al., 1945) to over 1 km (Johnson and O’Brien, 1973). In some cases, the base of the upper return flow ranged from 670 m (Frizzola and Fisher, 1963) to 1500 m (Eddy, 1966), while other researchers could not find the return flow at all, particularly if there was an upper-level onshore gradient wind. Midlatitude seabreezes tend to penetrate 20-40 km inland depending on the terrain slope (Atkinson, 1981). More recently, doppler sounders have been used to characterise seabreezes (Aggarwal et al., 1980; Bacci et al., 1984; and Raghu Kumar et al., 1986). Along the coast of Africa, the first review of seabreeze circulations was that of Jackson (1954). He considered that seabreezes were most prevalent at the lower latitudes and were substantially masked by stronger gradient winds poleward of the 33” S latitude. Preston-Whyte (1969) found that seabreezes over the east coast of Africa near 30” S commenced about 09 hr and penetrated over 60 km inland with a depth of lOOOm, being aided by gradient onshore winds. In terms of our analysis here, a relevant study of seabreezes over the coast north of Cape Town was that of Keen (1979). Using simultaneous pilot balloon releases, a detailed view of the seabreeze was compiled. Inflow was found in the 100-1000 m layer at speeds of 5-7 m s-‘. The seabreeze was found to penetrate only 15 km inland and then return seawards with updrafts of over 1 m s-l and speeds of 2-3 m s-l in the 400-2000 m layer. We compare our results obtained here using a different observational technology with those of Keen (1979). Redding et ~1. (1982) investigated the relationship between synoptic forcing and mesoscale circulations near Koeberg and found that seabreezes occurred about 23% of the time when the horizontal surface pressure gradient was less than lop2 hPa km-‘, but less than 6% of the time when the pressure gradient exceeded that value. They observed seabreezes to be about 250m in depth, considerably shallower than that reported by Keen (1979). Recently, further work has been done on
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Seabreeze cases used in composite analysis SE seabreeze
Classic seabreeze
NW seabreeze
19 Nov. 1984 30 Nov. 1984 16 Jan.
15 Feb. 198s 10 Mar. 198.5 II Mar. 1985
16 Jan. 1985 14 Nov. 1985 19 Jan. 1986
weather conditions and phenomena indirectly associated with seabreezes. Heydenrych (1987) and Comrie (1987) have used a similar observational mix of surface network, sounder and aircraft technology to study coastal lows and thermal internal boundary layers, respectively. The analysis scheme to select seabreeze days from the sounder data set again used information from other sources. Initially, a daily averaged wind speed of less than 6ms-’ at the Koeberg 80 m tower was used as a selection criterion. Then, hourly wind vectors for the individual days were inspected for an olf-shore/onshore wind shift after sunrise. Next, the sounder data set was consulted to ensure that potential cases fell within periods of optimum sounder performance. A final criterion was added; 10 m air temperatures at the river valley site, 15 km inland must exceed 23 “C so as to avoid strong on-shore gradient winds and cloudy weather associated with frontal passages. This sorting left 20 days. It soon became apparent that different types of seabreezes could be identified on the basis of the direction of the upper boundary-layer flow and the position of the transient pressure co1 following the coastal low, in relation to the time of day. The three types were named SE, classic and NW. To ensure that each seabreeze category had a uniform number of cases, the final number of cases was further reduced as outlined in Table III. Each of the categories represented a distinct type, the SE being a weakening and veering of the gradient trade wind, the classic being the traditional land/sea breeze and the NW being characterised by NE winds in the morning and generally NW winds in the upper boundary layer. Composite analysis was then employed to form a single seabreeze type for each of the three categories. Time-height analyses of the wind vectors of the three seabreeze types are shown in Figure 10, centered on mid-day. 5.2.
TEMPORAL
VARIABILII‘Y
In the early morning hours, the SE type exhibited 10 m s-’ trade wind flow. About 3 hr after sunrise (10 hr), surface winds veered from SE to SW. Up to the time of maximum heating (14 hr), the depth of the sea-breeze increased to 500 m. Using a seabreeze criteria that the wind should be: 1. within the range 210-290” (40” either side of 250” perpendicular) 2. speeds should be greater than 2.5 m s-‘,
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OBSERVATIONS
TABLE IV Characteristics of composite seabreezes
Onset
Retreat
Maximum depth Direction Depth
Strongest winds Height Speed
0930 hr 0930 hr 0830 hr
1630 hr 2030 hr 2030 hr
500 400 400
8.4 6.2 200
Category SE Classic NW
233 at 1330 hr 275 at 1530 hr at5.3
150 at 1430 hr 150 at 1330 hr 200 at 1230 hr
the seabreeze was determined to be overcome by the SE trade wind after 17 hr. Towards evening, the trade winds increased to 12 m ss’ at the 200 m level, while above 500 m SW winds persisted. The most significant feature of the SE type seabreeze is that lower-level wind speeds were weakest during the mid-day period. In the classic case, horizontal winds were relatively light in the early morning and a shallow offshore landbreeze flow developed. In the upper boundary layer, northerly winds prevailed. Between 09 and 10 hr, a sudden shift in winds occurred. Seabreeze speeds exceeded 6 m s-’ and the depth grew to 400 m by 1530 hr. The seabreeze then decayed gradually around sunset, falling outside the abovementioned criteria by 21 hr. Similarly in the NW case, surface winds started off light, then a sudden shift to westerly on-shore flow near 09 hr was experienced. Upper gradient wind support was more evident in this case, making a depth determination more difficult. We summarise the mean characteristics for each category in Table IV. From these results it is clear that each composite seabreeze type has distinctive features. From the mean directions and speed distributions over the 24-hr cycle, the SE case is most unusual and may be considered to be a temporary cross-shore deflection of the SE trade wind. For the most part, the temporal patterns of the secondary turbulence variables and vertical winds were sufficiently similar to allow the three types to be grouped together to form a single seabreeze composite. These results are shown in Figure 11. The diurnally-averaged sigma theta time-height pattern (top) was dominated by high values preceding the leading edge of the seabreeze air mass as it advanced across the sounder site, first affecting the surface at 08 hr but only reaching the 600 m level by 11 hr. On either side of this line of horizontal wind variability, sigma theta values were of order 10”. Such a line of variability, encompassing the upper levels through the mid-day period, was maintained in the composite analysis of the vertical wind and its deviation (middle and bottom). Before sunrise, subsidence and low vertical turbulence prevailed. After the seabreeze had moved across the sounder site, vertical uplift and convection gradually increased from the surface upwards in time, representing the growth of
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a
b
a- 6w
C
01
06
12h
18
24
Fig. 11. Composite time-height analysis for all seabreeze cases; of sigma theta (top), W (middle) and sigma W (bottom). Lines of maximum values are shown in bold in the top and middle panels. The convective layer is shaded in the bottom panel.
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the thermal internal boundary layer. Maximum + W values were recorded at the lowest 50 m level at 12 hr but only at 17 hr at the 600 m level. Sigma W values reached a maximum in the afternoon period, to the trade wind examples.
5.3.
SPATIALSTRUCTURE
To incorporate a wider perspective on seabreezes near Cape Town, we consider the synoptic surface air pressure maps (Figure 12) and radiosonde profiles for the time of maximum heating (13-14 hr) on the case study days discussed above. The significant pressure features for seabreeze days include a transient high pressure cell receding to the south east of the African continent, a weak coastal low moving over the study area and a cold front approaching from the west. The mean positions for each of the categories and pressure features are given in Table V. Using radiosonde data from the local airport, 15 km to the SE of the sounder site, the upper boundary layer was dominated by a thermal wind, yielding N to NW flow of over 5 m s-l at the 925 (700 m) and 850 hPa (1500 m) levels for all seabreeze cases. The seabreeze mean wind direction at the 850 hPa level was 345” at 8 m s-l. Such a strong on-shore gradient flow would very likely damp out any upper-layer return circulation associated with the seabreeze. Nearer the surface, NW flow also prevailed in the classic and NW categories. In the SE seabreeze type, SSW flow was observed at the 1000 hPa level. A few seabreeze days exhibited off-plateau northeasterly flow above 925 hPa, indicating warm advection. The temperature profile on seabreeze days was characterised by weak instability, owing to the location of the radiosonde site within the inland convective boundary layer. 1000 hPa temperatures ranged from a warm 30.2” down to 22.7 “C, while at the 850 hPa level, temperatures were 8 “C cooler, in the range 13.0 to 2O.O”C, with little distinction as to seabreeze type. The mean temperatures at the 1000 and 850 hPa levels were 25” and 17 “C, respectively, giving a lapse rate of -1 “C 200 m-‘. A 2-5 “C temperature inversion at heights ranging from 834 to 958 hPa was present on all seabreeze days. Calculating the buoyancy frequency across the inversion for a number of cases, we found N values of 2.5 * lo-‘s-l. Using a mean 925 hPa speed of 5 m s-l, the Froude number was 0.2. Clearly the potential for channelling of seabreezes by the local topography is significant. In the SE seabreeze cases, upper level flow was reversed in comparison to that beneath. At this wind shear transition, the F number must drop to near 0, suggesting pronounced topographic channelling. With little directional shear, higher gradient winds (8 m s-l at 925 hPa), and N values of 1.16 . lo-* s-l, the classic and NW seabreezes exhibited F numbers below 0.7, indicating that the flow should also be channelled and perhaps dammed on the windward side of Table Mountain. TO capture the horizontal dimension in our analysis, aerial survey data collected on 23 February, 1980 and 26 January, 1981, were selected to represent the
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Fig. 12. Synoptic surface pressure analysis for three days corresponding to the three categories of the seabreeze: SE (top), classic (middle) and NW (bottom). The pressure co1 between the coastal low and the approaching westerly trough is evident. Map borders are approximately 15”-45”S, 5” W-5.5” E. The dot shows the study area.
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TABLE
V
Mean position (“S, “E) and strength of synoptic weather features SE
Classic
NW seabreeze
Coastal low position strength
28, 16 1013 hPa
34,25 1010
34,20 1009
Cold front at 40 “S strength
9” E 1015
10 1004
8 1005
High pressure position strength
37,30 1021
41,50 1024
40, 39 1025
classic and NW seabreezes in Figure 14a, b, respectively. In the classic case, a cyclonic rotation of the flow trajectory was evident. Trade wind flow curved around the upstream mountain ridge in the southeast of the study area and became entrained into a southerly seabreeze over the False Bay coast, penetrating 10 km inland. In the lee of Table Mountain, weaker westerly flow was drawn coastward towards the sounder site and along the west coast, accelerating in the process from 2 to 6 m s-l. The cool west-coast seabreeze and warmer False Bay seabreeze converged along a front extending eastwards from Table Mountain across the valley. Both these seabreezes and their convergence front ultimately became entrained into the Berg Valley thermal low (Jury, 1980), an area
3 Fig. 13. 10m network analyses of individual days representing: (a) a trade wind nocturnal jet condition, 24 hr 17 November, 1985 illustrating wind vectors and divergence (dashed) and vorticity fields (shaded, less than -2) with units 10-4s-‘; (b) a seabreeze-TIBL condition for the sounder transect 13 hr 27 March, 1986 illustrating wind vector, isotherm and sigma theta distributions (light shading 15” of variance). The sounder site is shown by the dot; the wind vector scale is given at upper right in (a), m s-‘.
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of lower pressure and higher temperature, hence lower density, located near 33”3O’S, 19” E. (At the 1000 hPa level, the Berg Valley thermal low is typically 2 hPa lower and 8 “C warmer than the air over the western False Bay coast.) In the NW seabreeze example of 26 January, 1981, only a slight wind trajectory curvature was present. A splitting and deceleration of flow approaching Table Mountain was observed. The isotach pattern indicates the acceleration of the
a
b Fig. 14. Research aircraft data representing the mesoscale structure of seabreezes occurring on (a) 23 February, 1980 and (b) 26 January, 1981, representing the categories classic and NW, respectively. The sounder site is shown by the large dot; smaller dots in (b) represent the aircraft wind data distribution. Note that the mesoscale structure for the SE type is well represented by aerial survey data in Figure Sa.
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west-coast seabreeze from 4 to 8 m s-l over the sounder site. A similar although less intense increase in speed occurred farther north along the west coast. No separate seabreeze circulation was evident over the False Bay coast during the NW seabreeze condition represented here. 5.4. SEABREEZECROSS-SECTIONS
From 21 March to 10 April, 1986 the sounder was mobilized and operated along a transect extending inland from the coast some 12 km to the southeast of Koeberg as shown in Figure lb. Profile data were collected at approximately 4 km intervals between 12 and 14 hr, when the seabreeze was at full strength. Some problems experienced with ambient noise levels limited the analysis of the sounder transect data to the first 500 m. Simultaneous temperature profiles were made to the 100 m level and data from Koeberg and the inland valley station (at either end of the transect) were consulted. From the available data set, 27 March, 1986 most closely corresponded with the previous seabreeze examples and was therefore selected for interpretation. The sounder wind speed, direction, sigma theta and sigma W patterns are shown in Figure 15a. The temperature cross-section and changes with time of the direction, speed and temperature at the two surface stations are shown in Figure 15b. In Figure 13b the spatial structure of the 27 March, 1986 seabreeze is illustrated using vector, isotherm and sigma theta distributions. All parameters are discussed in relation to inland distance. Starting at the coast, the marine air mass of 27 March, 1986 was characterised by very low wind speeds in the layer lOO-200m. The wind vector rotated in a clockwise sense with height, changing from southwesterly near the surface to northwesterly aloft, most abruptly across the wind minimum layer at 100m. This is likely due to the orientation of the horizontal temperature gradients, causing cross-isotherm flow at the surface and a N to NW thermal wind through the upper boundary layer. The horizontal and vertical turbulence indices were very low right through the profile. The vertical temperature structure at the coast was characterised by a cool tongue near the 30 m level which extended inland a short distance into the cross-section. Looking at cross-section data collected between 2 and 8 km inland, a sharp increase in the speed (6 m s-l) and depth (400 m) of the seabreeze was noted. The vertical shear in wind directions gradually relaxed along an upward slope (using the 270” isoline: 200 m at 2 km to 400 m at 8 km inland). It should be mentioned that some of the structure, particularly for direction, may be the result of aliasing: the coast profile was obtained before 12 hr, while the most inland profile was obtained after 14 hr. During this time, the seabreeze underwent Coriolis rotation (-20”, as in Figure lo), while the inland convective boundary layer deepened (by 100 m, as in Figure 11~). Sigma theta values were very low just above the cool tongue around the 200 m level. Perhaps the eastward acceleration of the seabreeze (+aU/&) damped out the horizontal eddies. Vertical turbulence probably best exemplifies the growth of the convective (thermal internal) boun-
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dary layer in the bottom panel of Figure 15a. In agreement with simple prediction schemes (Hanna, 1987) a sudden upward slope in sigma W values was observed between 2 and 5 km inland. The 30 cm s-r isoline started at the surface at 2 km, reached 200 m by 5 km inland and then leveled off to 300 m at 12 km. The pattern of sigma W closely corresponded to the temperature pattern (compare with top panel of Figure 15b, note different height scale). Temperatures sloped upwards rather more gradually. The thermal structure was, of course, dominated by the land-sea temperature difference. Growth of the TIBL was suppressed by the on-shore advection of marine air, cooled over the 11 “C upwelling waters at the western end of the transect. At the eastern end, land temperatures were observed to exceed 30 “C. Near the 8 km mark, surface sigma W values were over 60 cm s-’ and air temperatures exceeded 26 “C. Similarly, the sigma theta values rose abruptly from 10 to 30” near the surface between 7 and 12 km inland. At the eastern end of the cross-section (12 km), the seabreeze decelerated, particularly above 200 m, as winds swung more to the SW. The turbulence indices exhibited maximum values within the inland convective region. The temperature pattern contained an upward step above the 60 m level at about 10 km inland, a suggestion of the wedge shape of the marine layer as it was advected across the coast by the seabreeze. In general, the sounder and radiosonde profiles for this and other cross-sections indicate that upper gradient winds from the NW opposed any return flow aloft. Because the patterns contained in the 27 March sounder transect were replicated on a number of other occasions, we conclude that features such as the inland slope of the surface convective layer and the preferential acceleration of the seabreeze across the tightest portion of the temperature gradient (aTlax) are characteristic of a majority of seabreezes along the west coast of Africa near 34” s. Considering the temporal variability of the 27 March, 1986 seabreeze, wind directions at the inland station, 15 km to the ESE of Koeberg, showed a 2 hr lag in seabreeze onset. The rate of advance could then be estimated at 2 m s-r. The onset time at the coast was about 3.5 hr after sunrise or 1030 hr. This compares with the 09 hr onset time for the mid-summer composites (which coincided with an inherently earlier sunrise time). Wind speeds at the coastal station remained low throughout 27 March, while the inland station experienced a sharp acceleration to 8 m s-’ during the afternoon. This indicates that the seabreeze was channelled up the river valley towards the interior thermal low. The 10 m air temperatures at the inland station exhibited a range of over 12 “C, compared with a coastal range of less than 4 “C. The maximum E-W differential of 8 “C occurred at 14 hr, just preceding the wind maximum at the inland site. These meteorological patterns were judged to be typical when considering time series for similar sounder transect cases. The only exception was the wind speed at the coastal site, which usually experienced an early afternoon increase of over 2 m s-l, leading the inland site by 2-3 hr.
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Fig. 15. Sounder transect height-distance analysis for 12-14 hr 27 March, 1986, showing (top to bottom in (a)) the wind speed, direction, sigma theta and sigma W. In (b) the temperature cross-section and 10m time series data from the coast and inland sites corresponding with the sounder transect are illustrated.
In the horizontal plane, the wind, temperature and turbulence pattern for 13 hr 27 March, 1987 is shown in Figure 13b. The offshore-inland curve in the velocity from WNW to WSW was clearly represented. Isotherms were very tightly packed just inland from the coast and appeared to be more so to the north than in the southern portion of the study area. Sigma theta values closely corresponded with the temperature pattern and surface roughness features. A broad zone of low horizontal eddies was present along the coast, and corresponded well with the transect pattern. At the offshore island, higher values were found, but this may be specific to the leeward site of the station. Farther inland, horizontal variance values rose above 15”.
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6. Summary
Mesoscale circulation systems along the African west coast near 34 “S were investigated using doppler acoustic sounder time-height analyses for the summers of 1984/85 and 1985/86. To provide a four-dimensional perspective of the two most prominent summer winds, the trade wind and the seabreeze, matching surface network space-time analyses and aerial survey data from earlier work were considered. Spatial structure within the SE trades and seabreezes was related to the limited depth of flow and the channelling and obstructing effects of the local mountains. During trade wind conditions, with a high pressure cell ridged to the south of the continent, a horizontal jet axis extending from False Bay northwards towards the sounder site was described by aerial survey analysis of the 150 m level wind field. A splitting of the shallow flow was observed on either side of Table Mountain. As the pressure gradients relaxed with the passage of a coastal low, seabreezes accelerated across both the south- and west-facing coasts, forming a convergence line to the east of Table Mountain. With a westerly trough approaching, a more uniform seabreeze of WNW origin was observed. Much of the space and time structure within the seabreeze depended on the phase relationship between the random synoptic perturbations (i.e., the surface pressure co1 preceeding an upper level westerly wave) and the regular 24 hr insolation cycle. Diurnal variability was emphasized through the use of time-height composites, carefully screened to filter the synoptic weather cycle. The trade winds behaved as a nocturnal jet, motivated by a diurnal turning of the stratified flow (Blackadar, 1957). During the afternoon, the underlying seabreeze flow near the surface was rotated by the Coriolis force to align gradually with the gradient SE wind. After sunset, a sudden burst of subsidence acted to stratify the dry trade wind air mass and limited the upward transfer of surface friction. The composite trade wind accelerated to a 14 m s-’ peak near midnight at the 200m level. Thereafter, the upper flow turned to easterly and vertically compressed the horizontal wind jet, keeping speeds high until dawn. The nocturnal acceleration of the SE trade wind had previously only been observed in the lee of the local mountains, but it is now clear that this phenomenon is more widespread, affecting the entire coastal boundary layer in the vicinity of Cape Town. An evaluation of the Froude number showed that the horizontally channelled jet is compressed against the side of Table Mountain at night. During the day, the jet relaxes away from the mountain and a wake is set up. The nature and diurnal cycle of seabreezes was found to be influenced by the non-stationarity of synoptic weather conditions. A shallow, coastally trapped low pressure area typically passes through the Cape Town area from NW to SE at a speed of between 4-8 m s-l (Heydenrych, 1987) and is followed by a pressure col, prior to the arrival of a weak westerly trough. The pressure co1 corresponds with clear skies, low inversion levels, high insolation conditions and sharp
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thermal gradients of order 10 “C in the first 10 km of the coastal zone. In response, a seabreeze of depth 400 m advanced across the coast at 2 m s-’ containing on-shore winds of over 6 m s-l at the 200 m level. Using cross-section data collected during the mid-day period, the seabreeze was observed to be sluggish and well-stratified at the coast. However, within the first 10 km, lowlevel on-shore winds u&ally doubled in speed and became thermally convective. The upward slope of the thermal internal boundary layer was of the order 1: 20 in the first 5 km and gradually flattened to 1: 50 farther inland, reaching an equilibrium height of about 300m above ground. Good agreement was found between the TIBL growth height equation of Lyons (1977) and the observed structure on 27 March, 1986. Implications for local air pollution transport are contained within these patterns, particularly for the seabreezes and associated TIBLs which coincide with low dispersion potentials in summer time. On the positive side, no return flow could be found and we suppose that little recirculation of pollutants could occur in a sustained seabreeze. On the other hand, the mesoscale patterns illustrated here, particularly in the vertical, suggest that dispersion prediction models currently in use for routine emissions and nuclear accidents need to be modified to parameterise the upper boundary-layer using doppler sounder data and incorporate interpolation schemes which recognise the channelling effects of the local topography and associated cross-shore gradients in wind and turbulence profiles. Acknowledgements I wish to thank Dr C. S. Keen, formerly of the Environmental and Geographical Sciences Dept. of the University of Cape Town for acting as project leader for the doppler sounder during its operation near Cape Town. The following persons assisted Dr Keen: C. Heydenrych - data collection and processing, M. Dutkiewicz - development of data interfaces and software packages and A. Comrie collection and processing of seabreeze sounder transect data. P. Lee, G. Sneddon and K. Levey assisted the first author with the composite analyses of sounder data for the Trade Wind cases. The Sea Fisheries Research Institute is acknowledged for research aircraft data, collected and processed by the first author during an earlier upwelling project. Eskom is thanked for surface network data. Finally, I thank the Oceanography Dept., Univ. of Cape Town for supplementary funding for these analyses and the Foundation for Research and Development of the CSIR, for funding the doppler sounder project. References Aggarwal, S. K., Singal, S. P., Kapoor, R. K., and Adiga, B. B.: 1980, ‘A Study of Atmospheric Structures Using Sodar in Relation to Land and Sea Breezes’, Boundary-Layer Meteorol. 18, 361-371.
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