thunderstorms and associated weathers, like squall, hailstorm etc. is well known. ... of thunderstorms/squalls and hailstorms in stations covered by Doppler ...
No. ESSO/IMD/Nowcasting/Synoptic Met-TS/03(2015)/19
GOVERNMENT OF INDIA MINISTRY OF EARTH SCIENCES INDIA METEOROLOGICAL DEPARTMENT NEW DELHI
OPERATIONAL MANUAL for Thunderstorm NOWCAST
Compiled By NOWCAST UNIT, OFFICE OF DIRECTOR GENERAL OF METEOROLOGY, NEW DELHI
No. ESSO/IMD/Nowcasting/Synoptic Met-TS/03(2015)/19
GOVERNMENT OF INDIA MINISTRY OF EARTH SCIENCES INDIA METEOROLOGICAL DEPARTMENT NEW DELHI
OPERATIONAL MANUAL FOR THUNDERSTORM NOWCAST
NOWCAST UNIT, OFFICE OF DIRECTOR GENERAL OF METEOROLOGY, NEW DELHI i
DECEMBER, 2015 INDIA METEOROLOGICAL DEPARTMENT (IMD) Number: ESSO/IMD/Nowcasting/Synoptic Met-TS/03(2015)/19 Title: Operational Manual for Thunderstorm Nowcast Author(s): Kamaljit Ray, Bikram Sen, Pradeep Sharma, Anwar Husain Warsi, Manik Chandra, S.C. Bhan, Soma Sen Roy. Type of Document: Scientific Manual Number of Pages and Figures: 106, 73 Number of References: 26 Reviewing and Approving Authority: Director General of Meteorology, IMD Security Classification: Unclassified Distribution: Unrestricted Date of Publication: December, 2015
ii
ACKNOWLEDGEMENTS Nowcast Operational Manual is the result of valuable inputs provided by various scientists, officers working in different fields like Nowcasting, DWR and NWP etc. in India Meteorological Department. I express my deep sense of gratitude to them. The manual has been prepared by Nowcasting Unit in the office of DDGM (Services), New Delhi. I express my sincere thanks to officers and staff of the Nowcasting unit for provided full technical support.
Kamaljit Ray Scientist, ‘E’ & Head, Nowcast Unit
iii
PREFACE The importance and reliability of DWR based information for nowcast of thunderstorms and associated weathers, like squall, hailstorm etc. is well known. Nowcasting of thunderstorms/squalls and hailstorms in stations covered by Doppler Weather Radars was implemented in December 2012 for 125 cities, which have now increased to 156 cities with the expansion of Doppler Weather Radar networks and in future this number is expected to increase further. It has also benefited from major developments in observational meteorology and computer-based interactive data processing and display systems in IMD. In present day scenario, with inclusion of SMS Alert services to farmers and various disaster managers by IMD for severe weather events like thunderstorms squalls and hailstorm, the accurate nowcasting of these weather events is very crucial and can prevent a lot of damage to life and property. Therefore, it is very important for a nowcaster to understand nowcasting and have a basic knowledge of Doppler Weather Radar products and how to use them for nowcasting purpose. So, there was a need to have an operational manual on Nowcast for capacity building & to have uniformity and better co-ordination between various IMD nowcasting offices across the country. The present document is an outcome of dedicated efforts of the nowcast team of IMD. I congratulate them for coming out with this useful and handy document. The spirit of coordination and support extended by authors who provided valuable inputs for the successful completion of Nowcast Operational Manual is greatly appreciated and duly acknowledged. I hope that the document would prove a useful reference material for improving nowcast skills of various nowcasters engaged in nowcasting of severe weather events across different MCs/RMCs.
(L.S. Rathore) Director General of Meteorology India Meteorological Department
iv
CONTENTS
Acknowledgements
iii
Preface
iv
Chapter 1
Thunderstorm Nowcasting and Verification-2014
1
Chapter 2
Functional Aspects of Doppler Weather Radar
21
Chapter 3
Radar Elements and Displays
32
Chapter 4
Classification of Weather Radar Echoes
42
Chapter 5
Use of WDSS-II in Operational Nowcasting
79
Chapter 6
SOP for All India Nowcast Services
89
References
93
v
vi
1. THUNDERSTORM NOWCASTING & VERIFICATION-2014 Kamaljit Ray, Bikram Sen, Pradeep Sharma, A. H. Warsi
India Meteorological Department, New Delhi
I. Introduction: Thunderstorm is a severe weather phenomenon, which develops mainly due to intense convection and is accompanied by heavy rainfall, thunder, lightning, hail and often with the passage of a squall line. It is the towering cumulus or the cumulonimbus clouds of convective origin with high vertical extent that is capable of producing lightning and thunder. Usually, these thunderstorms have the spatial extent of a few kilometres and life span less than an hour. In India, these thunderstorms reach severity when continental air meets warm moist air from ocean in the lower troposphere. The eastern and north eastern part of the country i.e. Bihar, Gangetic West Bengal, Jharkhand, Orissa, Assam and other states of NE India gets most affected by severe thunderstorms during pre-monsoon months (March-May), in particular, during April & May. These thunderstorms are locally named as “Kalbaishakhi” which means calamities in the month of Baishakh. Strong heating of landmass during midday initiates convection over Jharkhand Plateau which moves southeast and gets intensified by mixing with warm moist air-mass from the Bay of Bengal. These storms are also known as “Nor-westers” as they move generally from northwest to southeast direction. Fig.1.1 shows the annual climatology of thunderstorms and Fig.1.2 & 1.3 depict respectively, the climatology of thunderstorms during April and May over India based on 30 years data (19611990) (Tyagi, et. al., 2007). In India during the year 2013, a three hourly nowcast system of thunderstorm, squall and hail storm was developed by IMD for 120 cities in India. These nowcasts are primarily made by forecasters at various MCs and RMCs of IMD. With the installation of DWR at Bhopal in 2014, Srinagar in 2015 and addition of few more stations, the number of cities covered under Nowcast has increased to 156 (Fig. 1.4).
1
Fig.1.1: Annual Climatology of Thunderstorms over India (1961-1990)
Fig.1.2: Climatology of Thunderstorms over India in April
2
Fig.1.3: Climatology of Thunderstorms over India in May
Fig. 1.4: Stations issuing Thunderstorm Nowcast
3
II. Nowcasting Tools: The forecasters use the following technology for thunderstorm nowcast: a. Synoptic Evaluation: The first step in nowcasting of thunderstorm is to analyse the prevailing and forecasted synoptic situation and assess if the conditions are favourable for thunderstorm occurrence. The climatology of thunderstorm of the station selected for nowcasting are known (monthly frequency, peak time of occurrence). Depending upon the season, the broad synoptic patterns for thunderstorm occurrence should also be known. Analysis of surface synoptic charts and streamlines indicates the presence/absence of synoptic features which will lead to instability or moisture incursion in a certain area. For example the position of induced low pressure at surface, during the passage of western disturbance and westerly trough at 200 hPa are important for thunderstorm formation over Northwest, east and Northeast India. Pressure tendencies, Wind directions, Upper air circulations also are known from synoptic chart analysis.
b. NWP Guideline: The second step would be to examine the graphical NWP generated products for the area of interest. NWP models do not forecast thunderstorms directly; however, these can predict the atmospheric conditions in advance. Various models indicate the movement of certain large scale disturbances, which may affect a certain area on a particular day. Low level convergence and upper level divergence are ideal conditions for severe thunderstorm development. Strong vertical wind shear & moisture incursion at lower levels are also important for severe thunderstorm development. Various NWP derived products like LCL, potential vorticity, 200hpa divergence, 850hpa vorticity are useful for assessing the stability condition of atmosphere at a particular place/region.
c. Thermodynamic Features: Third step would be to examine the thermodynamic features at or near the places of forecast. Many thermodynamic indices are used for thunderstorm forecasting. These need to be tested and validated for the location of interest for critical values. Based on the Radiosonde ascent, the indices mentioned below can be calculated to exactly underline the area of occurrence of convective weather. INSAT-3d sounder products give continuous
4
tracking of some of these parameters including temperature & moisture vertical profiles. These indices are: a)
CAPE (Convective Available Potential Energy)- It is the measure of the amount of energy available for convection. It represents the work done on the parcel by the environment to lift it up from LFC (Level of free convection) to equilibrium level. Higher values of CAPE indicate greater potential for severe weather.
b)
CIN (Convective Inhibition Energy) - Its gives measure of how unlikely thunderstorm development is. It gives the amount of energy that will prevent an air parcel from rising from the surface to the level of free convection. Therefore, for convection to occur high values of CAPE & low value of CIN are required.
c)
Lifted Index (LI) =T500-TP500 where, T500 -Environmental temperature ( 0C) & TP500 - 500mb temperature, which a parcel will achieve if it is, lifted dry adiabatically from the surface to its lifted condensation level(LCL) & then moist adiabatically to 500mb. Threshold LI < 0 - possible thunderstorms LI < -4 - possible severe thunderstorms
d) Total Total Index, TT= Td850 + T850 - 2(T500 )
or (Td850 - T500 ) + (T850 - T500 )
Threshold TT > 44 - Slight chance of Thunderstorm TT > 50 - Moderate chance of severe thunderstorms TT > 55 - Strong chance of severe thunderstorms e) K Index, K = T850 - T500 + T850 - (T700 - Td700) Threshold K > 35: 80-90% probability of thunderstorm K > 40: 100% probability of thunderstorm d. Final Step: Once the current & forecasted synoptic condition have been assessed as favourable for thunderstorm occurrence & the NWP products also ensure the same, the thermodynamic parameters/Indices are examined. On concluding that the overall inputs indicate a situation and environment which is favourable for thunderstorm occurrence over the location of interest, the forecaster has to target the most probable time of occurrence and that is where the nowcast comes into play. Utilising the latest satellite imagery and Doppler Radar data, the 5
nowcast is issued. DWR tracks the convective echo for its intensity and direction of movement. Therefore regular monitoring of the following products is done for thunderstorm Nowcast:
DWR products such as Max Z (250 km), PPI_Z (500 km), VVP_2, PPI_V (250 km) and also animation sequence of above images at every half an hour interval.
Satellite pictures at every one hourly interval have to be observed for convective system development.
All India weather forecast issued by NWFC and Storm Bulletin issued by Nowcast Unit under STORM Project to be reviewed each day for any thunderstorm advisory.
Data from Automatic Weather Station (AWS), Current weather data from Automatic Weather Observing System (AWOS) and RAPID for satellite guidance (link available in website).
NWP model guidance.
All RWFCs and SWFCs having expert system WDSSII installed in the Doppler Weather Radar should issue Nowcast as per the guidance of the expert system. The officers will be trained by NWP Division, New Delhi for utilising this product.
III. Nowcast Verification: The verification of Thunderstorm Nowcasts issued every three hourly (daily) was done based on the past weather reported every three hourly by IMD observatories located in various cities. Due to non-availability of observatory at all locations, it was difficult to verify the nowcast for all stations. Therefore, for a total of 85 stations (Fig. 1.5), the nowcast issued for thunderstorms, every three hourly was verified based on the actual data collected in the nearby IMD observatories. The occurrence/ non-occurrence of the thunderstorm event was verified using various statistical parameters like; Forecast Accuracy (ACC), Probability of Detection (POD), False-Alarm Ratio (FAR), Critical Success Index (CSI) and Equitable Threat Score (ETS). Unlike POD and FAR, CSI does not use the correct non-events value and is sensitive to the climatology of the event, tending to give poorer score for rare events. ETS is designed to help offset this tendency. It removes the hits recorded by chance from the scores. This paper describes the nowcasting system of India Meteorological Department and its verification results.
6
Verification Results-2014: The difficulties in prognosticating the development of thunderstorms are well known. A successful forecast of severe thunderstorms depends as much upon the forecaster as on the timely availability of various observations. The skill and experience of the forecaster, continuous monitoring, his familiarity with the regional weather and meticulous attention to details contribute largely in timely forecasts and warning of thunderstorms. To categorise the Nowcast into excellent, good and bad category for POD, CSI & ETS and vice-versa for FAR, the scores were divided into three categories i.e. greater than 0.8, 0.4 to 0.8 and less than 0.4 respectively. Fig. 1.6 shows the percentage of stations that fall in the above three scales of FAR. FAR was excellent (0.8) for more than 60 % of the stations in July and August, 2014 and it was low i.e. categorised as bad for around 40% of stations in January, February and March, 2014 (Fig. 1.7). Fig. 1.8 gives month-wise Ratio Scores. It shows that the Ratio scores were excellent for more than 90% of the stations in all the months except February, 2014 and it was due to high number of nowcasts for “No thunderstorm” that was observed. ETS was excellent (>0.8) in July and August, 2014 for around 60 % of the stations and bad ( 0.8 for maximum number of stations (30-40 %) in January, June, July, August and September, 2014 and bad (0.8) in Rajasthan, Tamil Nadu, Punjab, Bihar & East Uttar Pradesh and bad (0.8) for Tamil Nadu, Rajasthan & Punjab and bad (55 dBZ precipitation at its centre, will be moving rapidly. Conventional radar cannot see the actual tornado, but apparently can see the parent tornado cyclone which spawns the smaller tornado. Thus, at the vertex of the well-defined hook, there is usually greater than a 75% chance that a funnel or tornado cloud could be spotted. Please note that the tornado hook occurs as much as 10 miles away from the heavy rain and hail. Tornadoes most often occur on the edge of a severe thunderstorm, not deep within its core. Useful as hook echoes are, only about 20% of tornadoes actually produce a hook-like signature on radar. Modern Doppler Weather Radars can map the actual tornado cyclone, and even sometimes the tornado vortex winds, greatly improving warning reliability.
57
Fig. 4.17: A model of Hook echo showing features of TVS and some tornado pictures
A tornado vortex signature or tornadic vortex signature, abbreviated TVS, is a Doppler weather radar detected rotation algorithm that indicates the likely presence of a strong mesocyclone that is in some stage of tornado genesis (Fig. 4.17). It may give meteorologists the ability to pinpoint and track the location of tornadic rotation within a larger storm.
TVS-Product of DWR (Fig. 4.18) indicates about the place where wind
directions are changing, known as shear within a small area and there is rotation. There is also a strong possibility that a tornado will form in that area. The forecaster could issue a tornado warning based on radar signature. It is often visible on the Doppler radar storm relative velocity product as side by side inbound and outbound velocities, a signature known as a velocity couplet or "gate-to-gate" shear.
Tornado signature
In bound velocities
Out bound velocities
Fig. 4.18: Tornado signature in PPI-V radar image
A TVS can be measured by gate to gate wind shear, which is the change of wind speed and direction across the two gates of inbound and outbound velocities. Gates are the individual pixels on the radar display. For example, if the inbound velocity is −48 knots
58
(−88.9 km/h) knots and the outbound is 39 knots (72 km/h), then there is
87
knots(48+39=87) (161 km/h) of gate to gate shear. In many cases, the TVS is a strong mesocyclone aloft, not an actual tornado, although the presence of an actual tornado on the ground can occasionally be inferred based on a strong couplet in concerned with a debris cloud signature, or through confirmation from storm spotters.
d. Bounded Weak Echo Region (BWER): This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually. The BWER has been noted on radar imagery of severe thunderstorms since 1973 and has a lightning detection system equivalent known as a lightning hole. The bounded weak echo region, also known as a BWER or a vault, is a radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward and is surrounded by higher reflectivity aloft. The BWER is related to the strong updraft in a severe convective storm that carries newly formed atmospheric particulates, called hydrometeors, to high levels before they can grow to radar-detectable sizes. The BWER is a nearly vertical channel of weak radar echo, surrounded on the sides and top by significantly stronger echoes. BWERs are typically found at midlevel of convective storms, 3 kilometres to 10 kilometres above the ground, and are a few kilometres in horizontal diameter. Identifying the location of the updraft region is important because it is linked to locations where severe weather normally occurs. The presence of a BWER has been part of a method to diagnose thunderstorm strength as part of the Lemon technique since 1977. The updraft strength within the BWER supports the growth of large hailstones just above the vault, which can be displaced slightly into the direction of motion of the parent super cell storm. The bounded weak echo region (BWER) is a region of low radar reflectivity bounded above by an area of higher radar reflectivity which shows evidence of a strong updraft within mesocyclones. Radar analysts have recognized this phenomenon since at least 1973, using different elevation scans. A BWER associated with a mesocyclone, can be confirmed by the interpretation of Doppler weather radar’s precipitation velocity products. A cross-section of the three-dimensional reflectivity of a thunderstorm shows the vault better. The development of a pronounced BWER can lead to tropical cyclone-like radar signatures over land when located with a low angle plan position indicator (PPI). When using the lightning detection system, lightning holes (uncovered in 2004) correspond to where a BWER would be seen on radar {Fig. 4.19 (a), (b) and (c)}. 59
.
Radial velocity patterns observed in a RHI scan through a Bounded Weak Echo Region (BWER) in a hail-producing thunderstorm.
A cursor cross has been placed in the echo hole associated with the BWER
(a)
(b)
BWER
The outer yellow-shaded translucent region depicts reflectivity levels between 39 and 48 dBZ. This reflectivity surface shows the BWER overhang on the southeast side of the storm. The interior red-shaded region contains reflectivity values at and above 61 dBZ.
(c) Fig. 4.19 (a), (b) & (c): Bounded weak echo regions
B. Classification based on Seasons: The classification of weather radar echoes based on seasons is being discussed here. In general, if we consider the span of a year, over the country following significant weather patterns are prevailing with respect to various seasons in following Table 4.2
60
Table 4.2 Season wise classification of radar echoes Winter season
Pre-monsoon season
Southwest Monsoon
Post-monsoon season
(Dec. to Feb.)
(March to June)
(June to Sep.)
(Oct. to Nov.)
Echoes associated
Andhis (NW-India)
Steady rainfall
Cyclonic storms in the
with cold fronts
Bay of Bengal and Arabian Sea
Squall line
Nor-westers (E-India)
Showers
Hail
Cyclonic storms in the Bay of Bengal and Arabian Sea
1. Winter Season (December to February): In winter the weather is generally associated with Western Disturbances over northern parts of India. Radar echoes from thunderstorms have generally clearly defined edges and well developed vertical structures. The tops of the echoes may or may not show the anvil shape on RHI images. Thunderstorm cell have a tendency to conglomerate in to a band or lines.
a. Cold front Thunderstorm: Cold front thunderstorms are associated with the passage of western disturbances or their induced during winter season in the northern parts of the country. These disturbances move in an easterly or north-easterly direction. The radar echoes associated with the cold front of the western disturbances are mostly in the form of convective type of cellular echoes aligned in a line. The heights of tops of clouds are about 10 to 14 km. After the passage of these thunderstorms, sometimes conditions become favourable (winds calm or sufficiently weak) for super-refraction. Radar images shown below (Fig. 4.20 a to d) represent different feature of the echoes associated with cold front. As soon as cold front comes within detectable radar range, echoes from the upper portion of cumulonimbus clouds appear on images as rarely continuous and generally narrow bands due to the finite beam width and consequent poor discrimination by the radar. These features can be seen in the radar images.
As the cold front comes nearer, the band of echoes begins to appear as composed of a
larger number of cells, often with very little separation, which break up and form again and change constantly in shape and size as they pass across the range of radar. Although the actual cloud structure along the cold front may be almost solid with narrow bands, the band
61
of echoes often appear discontinuous on images as the portion of cloud associated with the front contains drops of too small a size and poor concentration to give detectable echo. When the front is very near the radar station, the radar echoes again lose their cellular structure which can be easily seen by reducing dBZ scale to detect weaker echoes caused by drops of smaller sizes. The individual cells on RHI images show typical cellular structure representing strong vertical movement associated with cumulus or cumulonimbus cloud at the leading edge of the front. This trend continuous until the front passes over the radar station when the echoes from the more distant storm cells become weak because of rain attenuation and the precipitation appears to be almost evenly distributed around the radar stations over a distance of many miles. The apparent length of the front shows an increase as the front approaches the station due to decrease in rain attenuation except when the disturbance is over the station itself and causes considerable absorption of radar energy due to heavy rain thus masking the echoes from the outer portions of the front. The width of the cold front echoes varies greatly with the distance as well as activity. The width increases with increased activity and as the front approaches the station.
(a)
(b)
(b)
(d)
Fig. 4.20: (a) Cold front echoes far from radar (b) Cold front echoes nearer to radar (c) Cold front echoes very near to radar
(d) Cold front passes over the radar station
62
b. Squall line: Sometimes the frontal system of thunderstorms is preceded by another prefrontal squall line type thunderstorm, of comparatively weak activity, parallel to the main front. This prefrontal activity may be observed almost 80 to 100 km ahead of the main front (Fig. 4.21 a & b) and merging of both systems yields to long squall line pattern. Squall line echoes are characterised by cellular bands oriented parallel and close to the surface position of the convergence zone. The movement of the bands generally approximates to the movement of the squall line but sometimes the speed of the band is greater. In these cases, the bands apparently form in the rear of the convergence zone, move through and dissipate on the leading edge of the zone.
(a)
(b)
Fig. 4.21 (a) & (b): Merging of frontal squall line with prefrontal squall line
c. Hailstorms: Hail is invariably associated with violent convection and the radar echoes which have been positively identified with hail have shown the sharp-edge, high intensity echoes (more than 50 dBZ above freezing level) characteristic of convective type {(Fig. 4.22 (a) & (b)}. In fact in most of the cases, the extra brightness of the echo is a good criterion for the presence of hail. An unusually bright convective echo should always be suspected to be with hail formation. In many cases, hail has been found to be associated with protuberances or hooks from the edge of very bright convective type echoes. But it should be clearly born in mind that the shape of the echo alone can do no more than indicate the possibility of the presence of hail and much more information can be gathered from consideration of intensity of the echo, especially in case of hail of damaging size. Local experience will show that for a particular radar set, there is a threshold value of echo intensity at a specified range above which there would be a strong probability that hail of damaging size may be present. 63
(a)
(b)
Fig. 4.22: (a) High dBZ echoes & (b) Hail warning areas
2. Summer or Pre-monsoon Season (March to May): a. “Andhis” of Northwest India: “Andhis” are characterised by their great violence, huge blinding columns of dust, squally weather, lighting, rainfall and sometimes hail also. These are associated with an upper level surface of discontinuity when almost sudden development takes place as the heads of growing towering cumuli force their way through the surface of discontinuity due to the presence of adequate insolation. The radar echoes first appear like the air-mass thunderstorm echoes. Suddenly some of them are found to grow in intensity and on RHI display the tops can be seen rising fast. Within matter of minutes, they align themselves in a squall line pattern and move, giving rise to “Andhis” {Fig. 4.23 (a), (b) & (c)}. The rest of the randomly distributed air mass type echoes dissipate after some time when convection due to insolation gets weakened. Sometime due to extreme dryness of the atmosphere, rain streaks do not reach the ground.
(a) Max-Z (250km range)
(b): PPI-V (250km range)
(c) Dust Storm warning (50km range)
Fig. 4.23 (a), (b) & (c): Echoes related to “Andhis” of North-West India
64
b. Nor ‘westers of East India: Nor ‘westers of eastern India are the real aviation hazards. They are locally known as “Kal-Baisakhi” in East India which literally means the “Doom of Baisakh” (April) season. These storms are associated with a high level surface of discontinuity with warm and dry air above cold and moist air. They are characterised by huge, black, ominous, rolling clouds associated with specially severe up and down drafts, very severe turbulence, lightning and excessive rainfall. They are known for their suddenness as shown in Fig. 4.24. The echo characteristics are similar to those of “Andhis”. The only difference is that Andhis raise huge columns of dust, there is almost no dust associated with nor’westers. The resultant echoes are very bright in their case. It is also thought that the heights of tops of nor’wester echoes are perhaps the highest observed anywhere any time.
Figure 4.24: Radar echoes of Nor‘westers of East India
3. Southwest Monsoon Season (June to September):
This season is the main season in India, when detectable rainfall is available over large areas of the country with exception of a small shadow zone in the state of Madras (Chennai). We can classify monsoon echoes into two types, viz. those coastal areas and those over inland areas. In coastal areas, with the break of monsoon and only for a few days after that, there is thunderstorm activity. Thereafter it may be all steady downpours with no lightning or thunder. It is therefore obvious that on the first few days of the break monsoon in coastal areas, radar echoes are mainly convective or at the most mixture of convective and stratiform types. These showers are of stronger intensity and extend to higher heights than the stratiform
65
echoes observed in other seasons. Bright band is almost invariably seen in this season. Over land areas, monsoon activity is always augmented by local surface heating due to insolation. It is therefore seen that the echoes almost always start as convective types becoming later on, a mixture of both convective and stratiform type and finally setting to purely stratiform type with high tops and intense brightness. In this final stage, bright band is displayed.
a. Steady Rainfall: Steady precipitation from heavy altostratus clouds may be detected by the weather radar and may appear as a relatively featureless echo on the radar display. An example is shown in Fig. 4.25. It is mentioned here that this presentation is not true picture of the distribution of precipitation under such conditions. The precipitation is possibly too light to be detected by the radar beyond a certain range or is so widespread that it is lost below the radar horizon due to the curvature of the earth. The echo generally appears brightest in the centre of radar display and the intensity decrease steadily with increasing range until it gradually merges with the background noise.
Fig. 4.25: Steady Rainfall
b. Showers: Air-mass showers, during the period of relatively strong convective activity, appear as scattered echoes with sharply defined edges and well developed vertical structures. Often these echoes merge into bands and move as squall lines. Air-mass showers tend to form, grow and dissipate rather quickly. It is necessary to keep a close check on PPI display pattern to watch the formation of new cells. The growth from a detectable echo cell to a welldeveloped one may take place within ten or fifteen minutes. Fig. 4.26 shows a typical example of echoes from air-mass showers. However, when convective activity is weak or the
66
moisture supply is inadequate, the individual cells may be very small and may show poor vertical development. They sometimes arrange themselves in close groups and have a longer life-cycle than the more intense convective cells.
Fig. 4.26: Showers
4. Post–monsoon Season (October to November):
a. Tropical Cyclone: Tropical cyclones affect the east and west coast of the country during the periods of pre-monsoon (April to May) and post-monsoon (October to December) and cause extensive damage to life and property. Due to very heavy precipitation in the storm area, only S-band radars are suitable for studying the distribution of rainfall and tracking the movement of cyclone once it is in the range of respective radar. The cyclone detection radar network provides wealth of information on the precipitation pattern, dynamics and mechanism of formation and decay of the cyclones. A few typical pictures of tropical cyclones observed with the help of IMD S-band radar network at various stations are shown in Fig. 4.27.
Fig. 4.27: Some images of Tropical Cyclones
67
III. Classification of Non-Meteorological (non-precipitation) Radar Echoes: Due to Non-meteorological target, the spurious echoes may be displayed on radar display. The patterns produced by these effects are very different from real meteorological echoes and their occurrence is easily identifiable. These echoes may occur due to stationary objects on the earth’s surface (like Hills, Buildings and Trees), the transient objects (like ships, aircraft, bird and insects) and interference from other sources (such as nearby radars). The non-meteorological or non-precipitation echoes mainly occur close to the radar site, where the beam is at a low elevation and they can be identified by their persistence. The different types of non-meteorological radar echoes are as under: a. Ground Clutter: Ground clutter is a type of anomalous propagation in which the radar beam bounces off objects on or near the ground. Echoes from objects like tall buildings (especially if the radar site is near a large city or in a valley), cars on a high-traffic road, TV/Radio towers, and wind farms can be seen in almost all radar reflectivity images if the conditions are favourable. This "ground clutter" generally appears within a radius of 40-50km of the radar as a roughly circular region with a random pattern {Fig. 4.28 (a), (b) & (c)}. Ground clutter can be substantially reduced by the use of radar software.
(a)
(b)
(c)
Fig. 4.28: An example of ground clutter from a single Radar site on a cloudless day (a, b & c).
Mathematical algorithm can be applied to the radar data to remove echoes where the echo intensity changes rapidly in an unrealistic fashion. It may be noted that these images should be use cautiously because ground clutter removal techniques can remove some real echoes also. 68
The image of fig. 4.28 (a) is the ground clutter pattern which develops on a typical summer evening after sunset. After sunset, the air near the ground cools faster than aloft forming a shallow stable layer (inversion) near the ground. The stable layer acts like a lid on the lower atmosphere and causes the radar beam to be reflected back toward the ground. When the radar is in more sensitive mode, it not only picks up ground returns like buildings and trees, but also detects insects, smoke, and dust. The way to identify ground clutter is, with the help of movement and intensity of the radar echoes. Ground clutter objects will not move with time and sometimes appear very intense, unrealistically intense, on base reflectivity. Also, base velocity will usually have a problem detecting the velocity of ground clutter and will usually alias or throw it out as bad data. Sometimes ground clutter can also be produced with the dust in the lower atmosphere about three hundred meters. This is shown by random isolated points of very low reflectivity within a 15-20km of the radar site (once the beam travels high enough, it no longer samples the lower atmosphere and doesn’t detect the dust anymore). The Fig. 4.29 (a) shows Radar reflectivity image without Ground Clutter and in Fig. 4.29 (b) shows the same image after Ground Clutter Suppression
(a)
(b)
Fig. 4.29: (a) Radar reflectivity image without Ground Clutter Suppression (b) The same image after Ground Clutter Suppression
Note: The yellow and green returns in the above example are not ground clutter by its strict definition; however, for simplicity, non-precipitation targets may be called ground clutter. When significant anomalous propagation (AP) is a problem on DWR radar as shown in Fig. 4.29(a), clutter suppression can be invoked by the radar operator {Fig.4.29(b)}. When the suppression is invoked, any targets that are determined to be not moving will be automatically removed from the display.
69
b. Side-Lobe Echoes: Every time the antenna rotates, some radiation escapes on each side of the beam—called “side-lobes.” If a target exists where it can be detected by the side-lobes as well as the main-lobe, the side-lobe echoes may be represented on both sides of the true echo at the same range, as shown in Fig. 4.30 (a) & (b). Side-lobes usually show only at short ranges and from strong tar-gets. They can be reduced through careful reduction of the sensitivity or proper adjustment. Radars transmit energy along a main beam having a typical beam-width of about 1°. There are also secondary power transmissions along with side lobes located a few degrees from the main beam centre. Normally the side lobe returns are too weak to be significant. An exception may occur with very highly reflective targets, such as columns of heavy rain or hail within a cumulonimbus cloud. Fig. 4.31 shows a schematic range height indicator presentation through a distant cumulonimbus, with the main radar beam drawn at a high elevation that passes above the physical echo top. The side lobe transmission, however, is still striking the hail column within the cloud, and the resulting echo is associated by the radar with the main beam. Thus an apparent ‘spike’ is produced, up to the main beam level, giving an exaggerated estimate of where the true echo top lies.
(a) Side Lobes
(b) Spurious Targets
Fig. 4.30: (a) Geometrical presentation of Side Lobe echo (b) Side lobe echoes indicated with arrows as spurious targets
70
Fig. 4.31: Range height indicator presentation through a distant cumulonimbus
Note: The main radar beam overshoots the cloud top but the side lobe transmission is reflected. The true top of the cloud is given by T, and T’ is the observed top. Sometimes side lobes weaker pulses can be reflected with the objects near the radar and give the appearance of weak reflectivity. This can contaminate the radar image due to nearby ground clutter.
c. Under highly stable atmospheric conditions: Typically on calm, clear nights the radar beam can be refracted almost directly into the ground at some distance from the radar, resulting in an area of intense-looking echoes. This "anomalous propagation” phenomenon (commonly known as AP) is much less common than ground clutter. Certain sites situated at low elevations on coastlines regularly detect "sea return", a phenomenon similar to ground clutter except that the echoes come from ocean waves.
d. Radar returns from birds, insects, and aircraft: Echoes from migrating birds regularly appear during night time hours between late February and late May, and again from August through early November. Return from insects is sometimes apparent during July and August. The apparent intensity and areal coverage of these features is partly dependent on radio propagation conditions, but they usually appear within 50km of the radar and produce reflectivity of
