Warm Frontal Zone Research Articles

Searching for the Elusive Cold-Type Occluded Front

Abstract According to the Norwegian cyclone model, whether a warm-type or cold-type occluded front forms depends upon which cold air mass is colder: the prewarm-frontal air mass or the postcold-frontal air mass. For example, a cold-type occlusion is said to occur when the occluded front slopes rearward with height because the prewarm-frontal air mass is warmer than the postcold-frontal air mass. This temperature difference and the resulting occluded-frontal structure in the Norwegian cyclone model is part of what is called the temperature rule. Paradoxically, no clear example of a rearward-sloping, cold-type occluded front has been found in the literature, even though the required temperature difference has been documented in several cases. This article presents the first documented, rearward-sloping, cold-type occluded front. This occluded front forms in a cyclone over the North Atlantic Ocean on 3–5 January 2003 and is documented in model output from the European Centre for Medium-Range Weather Forecasts. Cross sections through the evolving cyclone show the occluded front forms as the less statically stable warm-frontal zone ascends over the more stable cold-frontal zone. Such a stability difference between the cold- and warm-frontal zones is consistent with a previously published hypothesis that the less stable air is lifted by the more stable air to form occluded fronts, in disagreement with the temperature rule. Because warm-frontal zones and the cold air underneath tend to be more stable than cold-frontal zones and the postcold-frontal air, warm-type occluded fronts are much more common than cold-type occluded fronts, explaining why well-defined, rearward-sloping, cold-type occluded fronts are not common in the meteorological literature.

Multiple satellite observations of cloud cover in extratropical cyclones

AbstractUsing cloud observations from NASA Moderate Resolution Imaging Spectroradiometer, Multiangle Imaging Spectroradiometer, and CloudSat‐CALIPSO, composites of cloud fraction in southern and northern hemisphere extratropical cyclones are obtained for cold and warm seasons between 2006 and 2010, to assess differences between these three data sets, and between summer and winter cyclones. In both hemispheres and seasons, over the open ocean, the cyclone‐centered cloud fraction composites agree within 5% across the three data sets, but behind the cold fronts, or over sea ice and land, the differences are much larger. To supplement the data set comparison and learn more about the cyclones, we also examine the differences in cloud fraction between cold and warm season for each data set. The difference in cloud fraction between cold and warm season southern hemisphere cyclones is small for all three data sets, but of the same order of magnitude as the differences between the data sets. The cold‐warm season contrast in northern hemisphere cyclone cloud fractions is similar for all three data sets: in the warm sector, the cold season cloud fractions are lower close to the low, but larger on the equator edge than their warm season counterparts. This seasonal contrast in cloud fraction within the cyclones warm sector seems to be related to the seasonal differences in moisture flux within the cyclones. Our analysis suggests that the three different data sets can all be used confidently when studying the warm sector and warm frontal zone of extratropical cyclones but caution should be exerted when studying clouds in the cold sector.

Occluded Fronts and the Occlusion Process: A Fresh Look at Conventional Wisdom

Traditionally, the formation of an occluded front during the occlusion process in extratropical cyclones has been viewed as the catch-up of a faster-moving cold front to a slower-moving warm front separating the warm-sector air from the low center, as first described in the Norwegian cyclone model over 90 yr ago. In this article, the conventional wisdom, or the commonly held beliefs originating from the Norwegian cyclone model, about occluded fronts and the occlusion process are critically examined. The following four tenets of this conventional wisdom are addressed. First, the occlusion process is better described not by catch-up, but by the wrapping up and lengthening of the warm-air tongue as a result of deformation and rotation around the low center. Second, the merger of the cold front and warm front does not result in the frontal zone with the warmer air ascending over the other frontal zone. Instead, the occluded frontal zone tilts over the more statically stable frontal zone. Because a warmfrontal zone tends to be more stable than a cold-frontal zone, this process usually produces a warm-type occlusion, confirming that cold-type occlusions are less common than warm-type occlusions. Third, occlusion does not mean that the cyclone has stopped deepening, because many cyclones continue to deepen 10–30 mb for 12–36 h after the formation of the occluded front. Fourth, clouds and precipitation associated with occluded fronts differ from their widespread stratiform depiction in textbooks. Embedded precipitation bands may be parallel to the front, and little relationship may exist between the fronts and the cloud mass. These four tenets help to explain anomalies in the Norwegian cyclone model, such as how occluded fronts that spiral around the low center do not require catch-up to form, how Shapiro–Keyser cyclones undergo occlusion, why some cyclones do not form occluded fronts, how some cyclones deepen after occlusion, why few cold-type occlusions have been observed, and why occluded cyclones are often associated with heavy precipitation. This reexamination of conventional wisdom leads to a new paradigm for occluded fronts and occluded cyclones. A supplement to this article is available online: DOI: 10.1175/2010BAMS3057.2

The influence of solar wind on extratropical cyclones – Part 2: A link mediated by auroral atmospheric gravity waves?

Abstract. Cases of mesoscale cloud bands in extratropical cyclones are observed a few hours after atmospheric gravity waves (AGWs) are launched from the auroral ionosphere. It is suggested that the solar-wind-generated auroral AGWs contribute to processes that release instabilities and initiate slantwise convection thus leading to cloud bands and growth of extratropical cyclones. Also, if the AGWs are ducted to low latitudes, they could influence the development of tropical cyclones. The gravity-wave-induced vertical lift may modulate the slantwise convection by releasing the moist symmetric instability at near-threshold conditions in the warm frontal zone of extratropical cyclones. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis and severe weather. The circumstantial and statistical evidence of the solar wind influence on extratropical cyclones is further supported by a statistical analysis of high-level clouds (<440 mb) extracted from the International Satellite Cloud Climatology Project (ISCCP) D1 dataset. A statistically significant response of the high-level cloud area index (HCAI) to fast solar wind from coronal holes is found in mid-to-high latitudes during autumn-winter and in low latitudes during spring-summer. In the extratropics, this response of the HCAI to solar wind forcing is consistent with the effect on tropospheric vorticity found by Wilcox et al. (1974) and verified by Prikryl et al. (2009). In the tropics, the observed HCAI response, namely a decrease in HCAI at the arrival of solar wind stream followed by an increase a few days later, is similar to that in the northern and southern mid-to-high latitudes. The amplitude of the response nearly doubles for stream interfaces associated with the interplanetary magnetic field BZ component shifting southward. When the IMF BZ after the stream interface shifts northward, the autumn-winter effect weakens or shifts to lower (mid) latitudes and no statistically significant response is found at low latitudes in spring-summer. The observed effect persists through years of low and high volcanic aerosol loading. The similarity of the response in mid-to-high and low latitudes, the lack of dependence upon aerosol loading, and the enhanced amplitude of the effect when IMF BZ component shifts southward, favor the proposed AGW link over the atmospheric electric circuit (AEC) mechanism (Tinsley et al., 1994). The latter requires the presence of stratospheric aerosols for a significant effect and should produce negative and positive cloud anomalies in mid-to-high and low latitudes, respectively. However, if the requirement of aerosols for the AEC mechanism can be relaxed, the AGW and AEC mechanisms should work in synergy at least in mid-to-high latitudes.

Mesoscale Dynamics of the Trowal and Warm-Frontal Regions of Two Continental Winter Cyclones

Abstract The dynamic and thermodynamic structure and associated frontal circulations within the trowal and warm-frontal regions of two extratropical winter cyclones are examined using numerical simulations. In each cyclone, the warm, moist airstream originating in the warm sector was found to bifurcate upon reaching the warm front. One branch of the flow turned anticyclonically eastward, corresponding to the warm conveyor belt, while the second branch turned cyclonically westward becoming the trowal airstream. The dynamic forcing of vertical motion within the two airstreams was investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), both from an analysis of the Sawyer–Eliassen (SE) equation in two dimensions and from complete model solutions. Shearing deformation, associated with the jet stream and the low-level cyclonic flow, dominated confluent deformation near the trowal in both cases. The shearing deformation was accompanied by cold advection associated with an intrusion of cold, dry air aloft. The configuration of isentropes and the wind field led to frontogenesis on the equatorward side of the trowal and frontolysis farther south on the poleward side of the jet stream. The SE solution showed a circulation centered on the frontogenesis–frontolysis couplet, with air rising in the trowal and sinking within the dry air mass on the trowal’s equatorward side. The rising branch of the circulation was responsible for the wide swath of snowfall coincident with the trowal. From the vicinity of the bifurcation axis eastward along the warm-frontal zone, confluent deformation dominated within the troposphere. Frontogenesis in this region produced a direct circulation whose rising branch accounted for the production of precipitation over the warm-frontal zone. Diabatic processes associated with latent heating and cooling produced frontogenesis–frontolysis couplets and significantly modified the transverse frontal circulations. The ascending motion was amplified by a factor of 2 or greater compared with the ascending motion solely due to horizontal deformation. The width of the ascending branch was also narrowed compared with that solely from deformation. Vertical tilting, a result of the secondary circulation generated by horizontal deformation, produced frontogenesis–frontolysis couplets that acted to oppose and reduce the magnitude of the secondary circulation. A conceptual model of the effect of these processes on the production and organization of snowfall in the two cyclones is presented.

The Role of Shearwise and Transverse Quasigeostrophic Vertical Motions in the Midlatitude Cyclone Life Cycle

Abstract The total quasigeostrophic (QG) vertical motion field is partitioned into transverse and shearwise couplets oriented parallel to, and along, the geostrophic vertical shear, respectively. The physical role played by each of these components of vertical motion in the midlatitude cyclone life cycle is then illustrated by examination of the life cycles of two recently observed cyclones. The analysis suggests that the origin and subsequent intensification of the lower-tropospheric cyclone responds predominantly to column stretching associated with the updraft portion of the shearwise QG vertical motion, which displays a single, dominant, middle-tropospheric couplet at all stages of the cyclone life cycle. The transverse QG omega, associated with the cyclones’ frontal zones, appears only after those frontal zones have been established. The absence of transverse ascent maxima and associated column stretching in the vicinity of the surface cyclone center suggests that the transverse ω plays little role in the initial development stage of the storms examined here. Near the end of the mature stage of the life cycle, however, in what appears to be a characteristic distribution, a transverse ascent maximum along the western edge of the warm frontal zone becomes superimposed with the shearwise ascent maximum that fuels continued cyclogenesis. It is suggested that use of the shearwise/transverse diagnostic approach may provide new and/or supporting insight regarding a number of synoptic processes including the development of upper-level jet/front systems and the nature of the physical distinction between type A and type B cyclogenesis events.

Measurements of dissipation rate in frontal zones

This paper demonstrates that it is possible to use measurements of the Doppler spectrum width from a high-resolution radar to obtain quantitative estimates of dissipation rate during the passage of precipitating frontal zones. Using the high-resolution Chilbolton radar, this method has been applied, for the first time, to infer the rate at which kinetic energy is dissipated by turbulence within mid- and lower-tropospheric frontal zones. Analysis of detailed measurements for one cold-frontal region showed multiple shallow sheets of high dissipation rate, with spatially averaged values around 1 x 10−3 m2s−3. Comparable values were also observed in a warm-frontal zone. The magnitude of the observed dissipation rate supports the hypothesis that mixing played a major role in determining the mean properties of the fronts, in that the turbulence acted to destroy the frontal shear layer on a time-scale of several hours. These issues have particular relevance to the representation of fronts within future very high-resolution numerical weather-prediction models.

Structure and Evolution of Winter Cyclones in the Central United States and Their Effects on the Distribution of Precipitation. Part VI: A Mesoscale Modeling Study of the Initiation of Convective Rainbands

Abstract A cyclonic storm that moved over the central United States on 8–9 March 1992 developed two convective rainbands, namely, a pre–dry trough rainband and a cold front aloft (CFA) rainband. This study extends the results of previous investigations of these two rainbands by examining their initiation with the use of a nested-grid mesoscale model simulation with spatial resolution down to 8.3 km. The model simulation reproduced the synoptic-scale setting in which the rainbands developed, as well as the mesoscale processes that initiated the rainbands. The pre–dry trough rainband was produced by the gradual ascent of a convectively unstable airstream above a gently sloping warm-frontal zone east of the dryline. After sufficient lifting, the instability was released through upright convection. The gradual ascent is well estimated by quasigeostrophic diagnosis, but the location and timing of the rainband are very sensitive to the convective stability characteristics within the airstream. The CFA rainband ...

Piecewise Frontogenesis from a Potential Vorticity Perspective: Methodology and a Case Study

Output from a numerical simulation by the University of Wisconsin Nonhydrostatic Modeling System is used to investigate the evolution of the surface frontal structure of an intense winter cyclone that occurred off the east coast of North America. Using the model data as input to the piecewise potential vorticity (PV) inversion method developed by Davis and Emanuel, the total surface frontogenesis is partitioned into contributions from discrete PV anomalies. These contributions are calculated using the nondivergent balanced winds associated with PV anomalies in the upper, interior, and surface layers as well as the balanced irrotational winds in what is termed piecewise frontogenesis. The cyclone of interest developed into a nearly stationary, well-occluded storm characterized by a cutoff PV anomaly at the tropopause and near-surface frontogenesis concentrated in the warm frontal zone. During the open wave stage of the cyclone life cycle, this frontogenesis was produced largely by the balanced flow, particularly that portion of it associated with the upper-level PV anomaly. Throughout the remainder of the cyclone life cycle, the upper-level contribution to the frontogenesis retained a distribution similar to that of the total frontogenesis. After the cyclone occluded, however, the frontogenesis forced by the unbalanced portion of the flow became increasingly significant despite the persistent smallness of the unbalanced winds. In fact, a substantial portion of the lower-tropospheric frontogenesis observed in the occluded quadrant, including that along a developing bent-back front, was eventually associated with the unbalanced flow. These results suggest a potentially significant role is played by unbalanced motions in the development and maintenance of lowertropospheric frontal structure in midlatitude cyclones.

Release of potential shearing instability in warm frontal zones

Potential shearing instability (PSHI) exists in the atmosphere when the Richardson number in a stably stratified shear layer would be less than 1/4 if the layer were lifted to saturation. Sometimes the air entering a warm frontal zone near the position of the surface warm front is initially unsaturated, its Richardson number is greater than 1/4 but PSHI exists over a deep layer. This paper presents a study of a warm frontal zone using observations from the high-resolution Doppler radar at Chilbolton, supported by radiosondes and output from the mesoscale version of the operational United Kingdom Meteorological Office Unified Model. Very large-amplitude Kelvin-Helmholtz billows were observed to form where the PSHI was realised following the onset of saturation. the billows were probably intense enough to be a hazard for air traffic and an important source of momentum transfer. Rough estimates from the radar of vertical momentum fluxes due to the billows are shown to be broadly consistent with measurements by the same radar of the change in velocity profile across the frontal zone from start to end of the billow train. the present version of the Unified Model does not parametrize vertical mixing due to shearing instability in the free atmosphere; this may be an important omission.

Release of potential shearing instability in warm frontal zones

AbstractPotential shearing instability (PSHI) exists in the atmosphere when the Richardson number in a stably stratified shear layer would be less than 1/4 if the layer were lifted to saturation. Sometimes the air entering a warm frontal zone near the position of the surface warm front is initially unsaturated, its Richardson number is greater than 1/4 but PSHI exists over a deep layer. This paper presents a study of a warm frontal zone using observations from the high‐resolution Doppler radar at Chilbolton, supported by radiosondes and output from the mesoscale version of the operational United Kingdom Meteorological Office Unified Model. Very large‐amplitude Kelvin‐Helmholtz billows were observed to form where the PSHI was realised following the onset of saturation. the billows were probably intense enough to be a hazard for air traffic and an important source of momentum transfer. Rough estimates from the radar of vertical momentum fluxes due to the billows are shown to be broadly consistent with measurements by the same radar of the change in velocity profile across the frontal zone from start to end of the billow train. the present version of the Unified Model does not parametrize vertical mixing due to shearing instability in the free atmosphere; this may be an important omission.

The Structure and Evolution of a Continental Winter Cyclone. Part I: Frontal Structure and the Occlusion Process

The frontal structure and occlusion process in a cyclone of moderate intensity that affected the central United States in January 1995 is examined. The deep warm-frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that, although characterized by cold-air advection near the surface, had many of the characteristics of a warm front aloft. In fact, this feature had a structure similar to the so-called bent-back fronts previously documented only in association with explosively deepening maritime cyclones. The development of a warm-occluded structure was investigated with the aid of a numerical simulation of the event performed using the University of Wisconsin–Nonhydrostatic Modeling System. The development of the warm-occluded structure was asynchronous in the vertical; occurring first at midtropospheric levels and later near the surface, in contrast to the classical occlusion process. Near the surface, the warm-occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical cold-frontal zone. At midtropospheric levels, the warm occluded structure formed as a result of the cold-frontal zone approaching, and subsequently ascending, the warm-frontal zone in accord with a component of the classical occlusion mechanism. The observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper-level potential vorticity (PV) anomaly that controls the cyclogenesis. It is suggested that the occlusion process begins aloft, where the associated vortex strength is greatest, and gradually penetrates downward toward the surface during the cyclone life cycle. Additionally, a characteristic“treble clef” shape to the upper-level PV anomaly is shown to be a sufficient condition for asserting the presence of a warm-occluded structure in the underlying troposphere.

Radar observations of wind-shear splitting within evolving atmospheric Kelvin-Helmholtz billows

The high-resolution radar at Chilbolton was used to measure the velocity structure of large-amplitude Kelvin-Helmholtz billows within a precipitating warm-frontal zone. A deep layer of frontal shear was observed to split into thin layers of more concentrated shear displaying the characteristic braided structure previously reported within clear-air radar reflectivity patterns and attributed to a splitting of the thermal stratification. the billows were situated within a region of strong wind veer with height and they were orientated at right angles to the mean flow such that air parcels within them were tending to follow helical trajectories up the inclined frontal zone. Radar scans normal to the axes of the billows at different distances from their origin, close to the position of the surface warm front, provided an indication of the evolution of the split-shear structure.

Extreme warm frontal icing on 25 February 1994 causes an aircraft accident near Uttoxeter

AbstractAn active, double warm‐frontal zone, aligned WNW‐ESE over the British Isles, produced copious precipitation over a wide area from southern Scotlant to the English Midlands during the evening of 25 February 1994. A turboprop cargo aircraft carrying Royal Mail parcels from Edinburgh to the intended destination of Coverntry suffered severe icing during its descent from 18000 ft over Manchester Airport, flying SSE wards in frontal nimbostratus. Then, some freezing rain was encountered below about 3000 ft over snow‐covered ground during the final stages of the flight. At one time, three of the four engines had ‘flamed‐out’, and subsequent attempts to re‐start both of the failed starboard (windward) side engines were unsuccessful. While the official report criticises the crew for following incorrect drills, and for not using their weather radar; eventually the asymmetric build‐up of ice on the windward‐side airframe and dead engines made the aircraft uncontrollable.

Assessment of Slantwise Convection in ERICA Cyclones

Abstract Atmospheric stability properties for cumulus and slantwise convection in oceanic midlatitude cyclones are analyzed using dropsonde observations from the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). Vertical cross sections perpendicular to the low-level wind shear are selected in the frontal regions for four ERICA storms. To assess the stability properties for conditional symmetric instability (CSI), a sounding analysis is carried out along surfaces of constant absolute angular momentum M. The buoyancy of the parcel along the slanted M surface is determined, both with and without the water loading effect. Our analysis suggests that a systematic bias toward overestimation of slantwise instability occurs when the loading effect is neglected. The major finding of our analysis is that the lower-tropospheric air on the warm side of the warm-frontal zone is stable or neutral with respect to vertical cumulus convection but unstable for slantwise convection. Convective instabilit...

The structure and evolution of a wintertime occluded front

A study, making use of serial radiosonde soundings, weather radar measurements and aircraft reports, has been made of the structure of a deeply occluded wintertime front. The sounding interval during the frontal passage was exceptionally short. The synoptic-scale front-relative flow could be described using the concepts of cyclone warm and cold conveyor belt flows (WCB and CCB) only in its initial stages. During the final stages of the occlusion process, the WCB became diffuse, the CCB weakened and penetrated below the occluded front, and a third relatively dry, cool, and diffluent airflow of mid-tropospheric origin was established above the occluded front. The subsynoptic structure of the occlusion was characterized by a warm frontal zone and, lying above it, a series of “warm tongues” and “cold surges” with a wavelength of about 100 km. The related circulations were also responsible for the formation of the observed banded structures in the snowfall, and agreed with the observed distributions of wind, humidity and temperature. As a whole, the warm tongues and the cold surges together with their related circulations transferred warm moist air upwards and northwards as required in the occlusion process. The observed wavelength, as well as the thermal and kinematic features of the circulations agreed well with the theory of conditional symmetric instability (CSI), though in this case, only small areas with pure CSI were found. The most significant area with CSI was found behind the occluded surface front at the top of the moist layer, while the other areas of CSI, found at higher elevations both above and on the occluded frontal surface were clearly smaller. This was considered to be related to the time history and the life-cycle of the roll circulations probably produced by the CSI. DOI: 10.1034/j.1600-0870.1990.00012.x

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. XIII: Structure of a Warm Front

Abstract Mesoscale measurement from radars, aircraft and rawinsondes, and synoptic and satellite data are used to provide a detailed description of a warm front as it approached the Washington Coast. In many respects, the warm front was consistent with the classical model: temperature rises were concentrated within a forward-sloping frontal zone, winds veered with height and lapse rates were more stable within the frontal zone, clouds and precipitation were produced by upglide over the warm-frontal surface and, as the warm front approached, clouds lowered and precipitation generally increased. However, in several important respects the warm front differed from the classical picture. Air flowed through the warm front and the warm-frontal zone. Also, the warm-frontal zone had a “staircase” profile, with some segments nearly horizontal and other segments with steep slopes. Finally, precipitation was by no means uniformly distributed; instead, it occurred in both irregular and banded-shaped mesoscale features.

Doppler Radar Study of a Warm Frontal Region

The paper presents a case study of the structure of a warm frontal region as deduced from Doppler radar observations. The precipitation occurring ahead of the surface warm front was banded. The dominant precipitation bands were oriented transverse to the mid-level winds, and they were spaced 110 km apart. It is suggested that these bands were formed by highly organized vertical circulations in a 2.5 km thick layer just above the warm frontal zone. The precipitation bands extended from this layer down to the surface. Near the surface additional circulations were produced by pressure perturbations resulting from cooling associated with melting snow. Some diagnostic calculations of ageostrophic winds, frontogenesis and vorticity production are presented. The frontogenesis calculation gives approximately a 2-4 h doubling time of the horizontal potential temperature gradient associated with the warm front, at mid-levels. The highly organized band-associated circulations suggest the importance of their inclusion in diagnostic calculations.

Multi‐level measurements of turbulence over the sea during the passage of a frontal zone

AbstractA Doppler radar technique originally proposed by Lhermitte (1968) has been used to measure the horizontal components of turbulence simultaneously at closely spaced height intervals from an altitude of 80m to 2500m. The observable range of turbulence scales was between about 200m and 5000m. Observations were made over the sea, with the radar situated on the Isles of Scilly, to minimize the effects of in‐homogeneities in terrain.The measurements reported in this paper were made during the passage of a warm frontal zone with strong surface winds; they show the change with height in the structure of turbulence simultaneously in both the planetary boundary layer and a free shear zone situated some way above it. Below 600m, in the planetary boundary layer, the two horizontal components of turbulence were rather similar in intensity, the total variance of horizontal velocity in the observed spectral range decreasing from 1·7m2s−2 at 80m to less than 0·1m2s−2 at 700m. For much of the observed spectral range the turbulence spectra were of the form kS(k) α k−n, with n decreasing from 1·2 below 300m to 0·7 at 700m. The lapse rate was slightly less than dry adiabatic in the planetary boundary layer with broken cloud above 300m, consistent with slight stability below 300m and some instability above. The decrease in the value of n is thought to have been due to the decrease of stability with height. The spectral scale λm increased with height reaching a value of 1·6km at 900m. Above the boundary layer for most of the time there was a placid layer several hundred metres deep and this was surmounted by a free shear layer in which the variance of velocity increased again to typically 0·5m2s−2. In the free shear layer the horizontal components of turbulence resolved parallel and perpendicular to the local shear vector were rather similar in intensity at scales less than 500m but were markedly unequal at scales between 500m and 2km. This is consistent with the occurrence of Kelvin‐Helmholtz billows with a wavelength of 1 to 2km.The Doppler radar measurements also enabled the vertical flux of horizontal momentum to be calculated. The quantity −u′w′ was found to decrease from about 0·2m2s−2 in the planetary boundary layer to about 0·01m2s−2 in the placid layer, before increasing again to about 0·1m2s−2 in the free shear layer. The energy dissipation rate ϵ was estimated from the turbulence spectra in those cases where the spectral slope n was not greatly different from 2/3. Values of ϵ decreased from 10cm2s−3 at 400m to 2cm2s−3 in the placid layer, before increasing to a second maximum of about 20cm2s−3 in the free shear zone. The rate of mechanical production of turbulence was calculated from the Doppler radar measurements of momentum flux and shear and it was found to vary with height in a rather similar manner to ϵ.

The structure of rainbands within a mid‐latitude depression

AbstractA case study is presented of a winter depression over the British Isles in which extensive banded structure was observed within precipitation ahead of the surface warm front. Measurements of the mesoscale airflow and precipitation structure of the rainbands were made using a variety of radar techniques together with multiple radiosonde and aircraft observations. The measurements were made over the sea to avoid the confusing effects of topography. The dominant rainbands were oriented parallel to the surface cold front and were typically 100 km wide. They moved with a velocity faster than the underlying warm front. For the most part the bands were characterized by clusters of weak small‐scale convective cells due to the release of potential instability produced where tongues of relatively dry air of low θw in the middle troposphere overran low‐level moist air undergoing slantwise ascent above the warm frontal zone. Although there was the usual large‐scale, and thermally‐direct, circulation associated with the active warm front, the air which ascended as small‐scale convection within the rainbands entered a region of weak cold frontal baroclinicity, whereupon it participated in a thermally direct circulation of its own. This led to each rainband having a rearward‐sloping anvil cloud canopy characterized by ascending air with colder drier air descending beneath. Precipitation falling from the canopy evaporated within the underlying drier air thereby probably intensifying the descending branch of the circulation. Very large ageostrophic winds were measured in association with these circulations.The important ingredient responsible for the convective nature of the rainbands appears to have been the incursion of tongues of relatively dry air of low θw in the middle troposphere above the moist warm‐sector air in a region where the resulting instability could be realized by large‐scale ascent. Although the potential instability was very weak in the present case, the origin of the rainbands appears to have been similar to that of pre‐frontal squall lines. The intensity of the convection within rainbands depends on the stability but the very existence of any precipitation in the first place depends on other dynamical factors leading to widespread ascent.