to have been similar to that of pre-frontal squall lines. ..... level jet stream, Zurich, Schweiz Met Zentre, ... associated intermediate scale disturbances in the lower ...
314
Journal of the Meteorological Societyof Japan
Mesoscale
Structure
of Rain
Systems
in the
Vol. 52, No. 3
British
Isles*
By Keith A. Browning MeteorologicalOfficeResearch Unit, Royal Radar Establishment,Malvern,England. (Manuscriptreceived10 Junuary 1974) Abstract This paper describes the three-dimensionalstructure of precipitation and some associated mesoscaledynamicalfeatures observedin frontal depressions. The materialis derivedmainlyfrom a recent seriesof case studiesin the British Isles undertakenjointly by the MeteorologicalOffice and the Royal Radar Establishment. Thesestudieshavegivenfreshinsight into the structureand mechanismof such features as rainbands, line convection,low level jets, and orographicrain.
1.
Introduction
An understanding of the structure and mechanism of mesoscale rain systems is necessary because of the economic importance of short-term forecasts of precipitation - particularly in the field of water conservation and flood allevation - and also because of the need to parameterise the effect of such phenomena in models of the larger scale circulation of the atmosphere. Unfortunately, our present understanding of mesoscale systems is meagre. This is partly because of the immense difficulties of observing mesoscale systems; routinely available observations fail to resolve them adequately and logistically complex research programs need to be established. An obvious example is the forthcoming GARP Atlantic Tropical Experiment. However, much also remains to be learned about the mesoscale structure of mid-latitude rain systems and, although the spot-light of International activity is not focussed in this area, I propose to restrict myself in this review to a consideration of the mid-latitude systems, especially those regions of more or less widespread rain that are associated with frontal depressions. I shall not attempt a comprehensive review of the literature but, rather, give a personal view of some aspects of the subject biased toward the observational studies made by my group in England * Text
of a Scientific
Session
of the World
lecture
presented
Meteorological
Commission for Atmospheric November 1973.
at the Sixth Organization
Sciences,
Versailles,
during the last five years. Using a case study approach, I have attempted to build up physical pictures of the mesoscale interaction of air motion and precipitation growth in different parts of large (synoptic) scale systems, framing each picture within the context of the large-scale situation. 2.
Scales
Let us begin in a general way by considering, as Matsumoto (1968) has done, a composite system with motions on the large, meso, and convective scales having characteristic horizontal dimensions of order 103, 102 and 10km, respectively. Denoting the vertical velocity of these motions by W1, Wm and Wc, we have: W1=0(1cms-1) Wm=0(10cms-1) Wc=0(1ms-1) corresponding to characteristic divergence fields of order 10-5, 10-4 and 10-3s-1, respectively. Fig 1 shows schematically mesoscale motions superimposed on a large-scale disturbance, it being assumed that the areas of mesoscale ascent may be composed of aggregates of convective updrafts. According to Fig 1, the magnitude of the updraft averaged over the area of ascent, and hence also the precipitation intensity averaged over the largescale disturbance, is increased substantially compared with that due to the large-scale disturbance alone. There is an approximately two-fold gain in precipitation efficiency. This is achieved at the
June 1974
Keith A. Browning
315
are clusters of convective cells; typical dimensions are 30km. The clusters sometimes occur in bands which may be very long. The bands are situated on the order of 100km apart and they occur on the warm side of, and are aligned parallel to, either warm or cold baroclinic zones. Sometimes a large-scale system is dominated by bands of one orientation; often both orientations occur separately within different parts of the same Fig. 1. Schematic representation of the vertical depression (Browning and Harrold 1969; Kreitzberg velocity field for a composite system and Brown 1970). Areas of relatively heavy of large-scale and mesoscale motions. precipitation associated with clusters tend to per(After Matsumoto 1968). sist as recognisable entities for some time in expense of a drying out of air in the descending regions of large-scale slantwise ascent even after branches of the mesoscale circulations, in agree- much of the convection within them has died away ment with the observational evidence that very and so the looser term Mesoscale Precipitation dry air often occurs in close proximity to organised Area is sometimes adopted. Austin and Houze areas of precipitation. Multi-level numerical- (1972), studying precipitation patterns in Northeast dynamical models, such as that of Bushby and USA, distinguish between small mesoscale areas, Timpson (1967), underestimate rainfall intensity with area less than 103km2, and large mesoscale and this is consistent with the view that a large areas, with area greater than 103km2, within which part of the rainfall is initiated by important the small mesoscale areas are embedded. Although mesoscale circulations that are not represented the large mesoscale areas appear to correspond in directly in the models. area to Harrold's cluster bands, Austin and Mesoscale features of the precipitation pattern Houze found they were often irregularly distributed occur over a wide range of size scales. Measure- and so the relation between the two classifications ments of the spatial spectrum of the amount of is not entirely clear. Mesoscale precipitation areas radar echo in Japan indicate a broad spectrum with exist over both land and sea but, as we show a peak at a wavelength of 150km, (Matsumoto, later, in some circumstances fresh mesoscale 1973). A classification of mesoscale precipitation precipitation areas are generated orographically areas has been devised by Harrold (1972) for in addition to the normal orographic enhancement baroclinic disturbances in the British Isles and this of the rainfall intensity from existing precipitation is shown in Fig 2. It should be emphasized that areas. this classification is based upon a rather small sample of case studies and there is wide variability 3. The conveyor belt and the generation of potential instability from case to case. The so-called clusters in Fig 2 The conventional Norwegian model of a frontal depression (Bjerknes, 1919) does not account for the mesoscale aspects of the rainfall distribution. As a first step toward formulating a more realistic model, Harrold (1972) has analysed the largescale flow in the warm air in a way that provides a useful framework in which to consider the mesoscale structure. The main features of Harrold's model are shown in Fig 3. This shows the flow relative to the moving system in a warm sector depression at a mature stage. Production of baroclinic precipitation occurs mainly in a tongue of moist, warm air (stippled in Fig 3). This Fig. 2. Schematic representation of the mesotongue of moist warm air is derived from a region scale structure of the precipitation of small-scale convection in the planetary boundary associated with a partly occluded depression (From Harrold 1972). layer in the warm sector and sometimes from as
316
Journal of the Meteorological Society of Japan
Vol . 52, No. 3
large-scale slantwise convection. The configuration of the flow, and the location of regions of ascent within it, largely determine the distribution of precipitation within mid-latitude depressions. The fronts are a secondary feature. In studies of the general circulation this flow has been compared to a conveyor belt transporting large quantities of heat, moisture and westerly momentum poleward and upward, and I shall use the term 'conveyor belt' to give it a clear-cut identity (Harrold 1972; Browning 1971). The top of the conveyor belt is normally defined by the base of mid-tropospheric air warmed and dried by earlier descent (as shown by the hatched arrow in Fig 3). Convection within the conveyor belt as it travels over a warm sea results in its having a higher wet-bulb potential temperature (*w) than the base of the overlying drier air (Fig 3b). As these flows ascend in the largescale baroclinic field, the potential instability is Fig. 3. Model showing the generation of realised as convection at the top of the conveyor potential instability in a mid-latitude belt. This convection occurs in clusters of mesodepression. Horizontal and vertical scale dimensions but it is not known what sections are shown in (a) and (b), determines the size of these clusters. Continued respectively, the section in (b) being large-scale ascent leads to eventual removal of the along the line AB in (a). The stippled potential instability and so, as shown by Boucher arrow in each diagram represents the flow relative to the system in the (1959), there is usually a transition from abundant small-scale convection aloft to more nearly straticonveyor belt. The hatched arrow form precipitation associated with uniform represents the flow of a layer of air with lower wet-bulb potential temslantwise ascent as the conveyor belt flow advances perature in the middle troposphere. above the elevated warm frontal zone. Isopleths in (b) are of wet-bulb potenConveyor belt flows can be identified in most tial temperature (*w) labelled in deg. mid-latitude baroclinic disturbances, including in C; the absolute values are arbitrary. so-called non-frontal depressions such as the polar The principal regions of potential lows that form in weakly baroclinic regions within polar airstreams traveling over warm seas (Harrold the surface cold front and in the and Browning 1969). The precise configuration warm sector where the hatched arrow of the conveyor belt depends on the large-scale overruns the stippled arrow. Convecfield of motion. Early in the development of a tive clouds are sketched in (b) in the system the conveyor belt turns only slightly to the region where potential instability is right (ie anti-cyclonically) as it ascends; later, as realised by large-scale ascent. the large-scale circulation develops, the conveyor (Redrawn from Browning, Hardman, belt turns more sharply to the right and it may Harrold and Pardoe 1973). ascend over a broader front. The conveyor belt far away as sub-tropical anticyclones. This air often starts ascending within the warm sector of accelerates ahead of the cold front as a well- a depression, in which case the rear edge of the defined flow typically a few hundred kilometers rain (relative to an observer on the ground) is wide and two kilometers deep before ascending not closely related to the position of the surface above the main warm frontal zone (Fig 3a). warm front. As the conveyor belt ascends above Green, Ludlam and McIlveen (1966) and Palmen the warm front zone, precipitation grown within and Newton (1969) describe this flow as being it falls into cold air. Away from the surface the most significant ascending flow in systems of warm front this cold air is descending and
instability (**w/*z
