K. A. Browning

Airflow and Precipitation Trajectories Within Severe Local Storms Which Travel to the Right of the Winds

Abstract A kinematic airflow model is proposed for severe local storms which travel to the right of the winds in the middle troposphere. The model is an extension of that proposed by Browning and Ludlam (1962), and represents an open system in which an updraft and a precipitation-maintained downdraft are fed persistently by air approaching the storm from its right flank. A qualitative consideration of precipitation trajectories enables the principal features of the radar structure of many severe local storms to be explained.

The Fronts and Atlantic Storm-Track Experiment (FASTEX): Scientific Objectives and Experimental Design

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as ...

Conceptual Models of Precipitation Systems

Abstract Imagery from radars and satellites is one of the main ingredients of nowcasting. When used to provide very detailed forecasts of precipitation for a few hours ahead, the imagery needs to be interpreted carefully in terms of synoptic and mesoscale phenomena and their mechanisms. This paper gives an overview of some conceptual models that are useful for this purpose. The models represent a variety of systems associated with midlatitude cyclones and also mesoscale convective systems in the tropics and midlatitudes. Specific phenomena discussed are warm conveyor belts, including those with rearward- and forward-sloping ascent in ana and kata cold frontal situations, respectively; cold conveyor belts ahead of warm fronts; narrow rainbands associated with line convection at the boundary of a pre-cold-frontal low-level jet; wide mesoscale rainbands associated with midtropospheric convection; squall lines in the tropics and midlatitudes; nonsquall mesoscale convective systems in the tropics and midlatitu...

The dry intrusion perspective of extra‐tropical cyclone development

The dry intrusion is a coherent region of air descending from near tropopause-level. It often has a clear signature in satellite imagery, especially in the water vapour channel, where it is seen as a ‘dark zone’. Parts of dry intrusions are characterised by high potential vorticity and, upon approaching a low-level baroclinic zone, rapid cyclogenesis may be expected to ensue. The leading edges of dry intrusions are defined by cold θw-fronts (moisture fronts). In some places the dry intrusion undercuts rearward-ascending warm air to give an ana-cold front. In other places it overruns the warm air to produce an upper cold θw-front in advance of the surface cold front. Here the dry intrusion is associated with the generation of potential instability and its eventual release as showers or thunderstorms. Identification of dry intrusions provides the forecaster with additional nowcasting evidence that is especially helpful when issuing severe weather warnings. The identification of water vapour dark zones associated with dry intrusions can also form the basis of methods for validating NWP models. Through their relationship to high potential vorticity, they can provide guidance for bogussing NWP models in situations of potentially severe weather. This article provides an introduction to the structure and behaviour of dry intrusions and their relationship to other aspects of extra-tropical cyclones.

Organization of Clouds and Precipitation in Extratropical Cyclones

A number of models accounting for the distribution of cloud and precipitation in extratropical cyclones were proposed during the 19th and early 20th centuries. A history of these has been recounted by Bergeron (1959, abridged version in Bergeron 1981) and also by Ludlam (1966) in his inaugural lecture as professor of meteorology. These models culminated in the classical Norwegian polar-front cyclone model of the Bergen school Bjerknes and Solberg 1922) in which the patterns of cloud and precipitation were related to vertical air motions resulting from the relative movement of different air masses along inclined frontal surfaces. This model is still widely used today. During the past quarter-century, however, the availability of imagery from satellites and radars has revolutionized the capability to observe cloud and precipitation. The imagery has drawn attention to many synoptic-scale and mesoscale features not explained by the classical model, as discussed by Reed in Sec(tion 3.3.3. It is now clear that the Norwegian model, despite its popularity, is a broad-brush model that fails to explain many important variations in structure among cyclones.

A classification of FASTEX cyclones using a height-attributable quasi-geostrophic vertical-motion diagnostic

A systematic and objective procedure is developed for applying the simple cyclone classification scheme of Petterssen and Smebye. This method uses a height-attributable solution of the quasi-geostrophic ! equation to identify and quantify the relative importance of upper- and lower-tropospheric forcing and also the time trend in the horizontal spacing of the forcing at these two levels. By applying this classification method to a sample of cyclogenesis events, during their maximum intensification stage, from the Fronts and Atlantic Storm-Track EXperiment field experiment, the Type A and Type B scheme of Petterssen and Smebye is reproduced and extended to include a Type C. Type C consists of upper-level dominated cyclones that form at high latitudes and in their initial stages resemble comma-cloud-type polar lows. Detailed examples of each of the three types are presented. Some cyclones can undergo more than one period of development and it is found that each development period can be classified as a different cyclogenesis category, A or B: these cyclones are classified as hybrid Type A=B. There is some evidence that cyclone forecast accuracy depends on cyclone type, thereby suggesting the potential for this method to be used to assign confidence levels for forecasts of cyclogenesis produced by numerical weather-prediction models. Type B cyclones appear more difficult to predict, because their development depends, initially, on the interaction between significant features in the upper and lower troposphere. Copyright

On the Forecasting of Frontal Rain Using a Weather Radar Network

Abstract This paper is concerned with the quantitative forecasting of hourly rainfall for the period 0–6 h ahead using linear extrapolation techniques. It deals with results obtained as part of the Meteorological Office Short Period Forecasting Pilot Project. The primary data used in this study are composite maps of rainfall echo distribution generated automatically and in real time using digital data received from a network of four weather radars covering parts of England and Wales. Forecasts have been derived during a total of 29 frontal rainfall events between November 1979 and June 1980. The forecasts wore derived both subjectively in real time and objectively using a computerized echo centroid tracking technique. The objective procedure, which was used to derive forecasts on a grid of 32 × 32, 20 km squares, is a practical way of quickly producing detailed forecasts for a large, number of target areas but its accuracy suffers from a number of factors. The subjective procedure, which was applied to a ...

Some Inferences About the Updraft Within a Severe Local Storm

Abstract Various features of the radar echo from a severe local storm are interpreted with the purpose of defining the nature of its updraft. The analysis illustrates the methodology of using conventional radar data to study airflow and also confirms a number of important features of an airflow model previously proposed by the author (1964). For example, it is confirmed that an updraft was present within, and was largely responsible for, the radar vault. The updraft within the vault was inclined upward with a component toward the storms left flank below the level of the tropopause and in the reverse sense at higher levels. A novel feature of this analysis is that shafts of precipitation descending from neighboring storms are shown to have been drawn toward, and upward within, the vault. A study of the shape of these shafts of precipitation has permitted inferences to be made concerning the nature of the inflow toward the updraft.

The Evolution of Tornadic Storms

Abstract A life cycle classification for severe local storms is proposed, which adds a fourth stage, the quasi-steady SR Mature Stage, to the well-known Byers-Braham classification. A combined analysis of the surface weather and radar echoes from three neighboring severe local storms reveals that each storm went through a similar, well-defined metamorphosis at the onset of its SR Mature Stage. The distinctive character of this stage is attributed to the rotational properties of the updraft, which in the storms studied developed when the updrafts had become vigorous enough to produce giant hail.

Mesoscale Aspects of Extratropical Cyclones: An Observational Perspective

The understanding of the mesoscale structure of frontal cyclones has, as shown in this review, benefited greatly from special mesoscale observations such as dropwindsondes and other measurements from aircraft. These observations are usually interpreted within the context of numerical weather prediction (NWP) model analyses based on the operational synoptic-scale observations. A few studies have also used high-resolution mesoscale models to assimilate the observations. Another tool for mesoscale analysis—and a major source of inspiration for this review—is imagery. This includes cloud and water-vapor imagery from satellites, especially geostationary satellites, and also precipitation imagery from ground-based radars, especially in networks. By providing the overall pattern of the mesoscale phenomena and an indication of the relationship to the larger-scale forcing, the imagery is invaluable for giving qualitative understanding of the processes at work. Often it provides the only clues as to what is happening on the mesoscale.