Cumulus convection is a key process in controlling the water vapor content of the atmosphere, which is in turn the largest feedback mechanism for climate change in global climate models. Yet scant attention has been paid to designing convective representations that attempt to handle water vapor with fidelity, and even less to evaluating their performance. Here the authors attempt to address this deficiency by designing a representation of cumulus convection with close attention paid to convective water fluxes and by subjecting the scheme to rigorous tests using sounding array data. The authors maintain that such tests, in which a single-column model is forced by large-scale processes measured by or inferred from the sounding data, must be carried out over a period at least as long as the radiative-subsidence timescale—about 30 days—governing the water vapor adjustment time. The authors also argue that the observed forcing must be preconditioned to guarantee integral enthalpy conservation, else er...

Development and Evaluation of a Convection Scheme for Use in Climate Models

During the past two decades, convective scale models have advanced sufficiently to study the dynamic and microphysical processes associated with mesoscale convective systems. The basic features of these models are that they are non-hydrostatic and include a good representation of microphysical pro­ cesses. The Goddard Cumulus Ensemble (GCE) model has been extensively applied to study cloud-environment interactions, cloud interaction and merg­ ers, air-sea interaction, cloud draft structure and trace gas transport. The GCE model has improved significantly during the past decade. For example, ice-microphysical processes, and solar and infrared radiative transfer pro­ cesses have been included. These model improvements allow the GCE model to study cloud-radiation interaction, cloud-radiation-climate relations and to develop rain retrieval algorithms for Tropical Rainfall Measuring Mission (TRMM). In Part I, a full description of the GCE model is presented, as well as several sensitivity tests associated with its assumptions. In Part II (Simpson and Tao, 1993), we will review GCE model applications to cloud precipitating processes and to the Tropical Rainfall Measuring Mission (TRMM), a joint U.S.-Japan satellite project to measure rain and latent heat release over the global tropics.

Goddard Cumulus Ensemble Model. Part I: Model Description

A simple parameterization has been developed to simulate global lightning distributions. Convective cloud top height is used as the variable in the parameterization, with different formulations for continental and marine thunderstorms. The parameterization has been validated using two lightning data sets: one global and one regional. In both cases the simulated lightning distributions and frequencies are in very good agreement with the observed lightning data. This parameterization could be used for global studies of lightning climatology; the earth's electric circuit; in general circulation models for modeling global lightning activity, atmospheric NO(x) concentrations, and perhaps forest fire distributions for both the present and future climate; and, possibly, even as a short-term forecasting aid.

A simple lightning parameterization for calculating global lightning distributions

Even the most casual observer notices the changes in the wind. Though it has a certain persistence in time, the details of its swirls and eddies seem infinitely variable. Its strength changes day to day as weather systems evolve, and day to night as the sun rises and sets. It is modulated strongly by terrain features and by urban architecture. How can we hope to describe such a complicated phenomenon?

Structure of the PBL

Evidence for the tripole structure of electrified clouds is summarized. The observed structure is quantitatively consistent with the results of laboratory simulations and suggests that collision between ice crystals and rimed graupel particles is the dominant mechanism for charge separation in thunderstorms. The physical basis for this mechanism is poorly understood.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JD094iD11p13151%4010.1002/%28ISSN%292169-8996.ELECCONF2

The tripole structure of thunderstorms

This is Part I of a two-paper describing the structure of the vertical profile of radar reflectivity in convective cells which are of part of mesoscale convective systems in GATE. Earlier work has established that characteristic mean vertical velocities in such convective clouds in GATE were rather weak, <3–5 m s−1. The microphysical implications of the weak updrafts have been proposed to include the rarity of large particles above the freezing level. As a working hypothesis for these papers it is proposed that cells with weak updrafts have characteristic vertical reflectivity profiles exhibiting modest reflectivities at low levels, and decreasing rapidly with height above the freezing level. Vertical radar reflectivity profiles are compiled from 296 convective cells having at least 40 dBZ surface reflectivity, and it is found that the profiles are consistent with the above reasoning. The, mean profile is of modest strength—45 dBZ echo at the surface and a 20 dBZ echo top of 8.2 km, and the refle...

A Radar Study of Convective Cells in Mesoscale Systems in GATE. Part I: Vertical Profile Statistics and Comparison with Hurricanes

The evolution of the 26 July 1985 Erie, Colorado tornado is described using data from NCAR's CP-2 Doppler radar. This tornado develops within 20 km of the radar site under weakly forced synoptic conditions and weak tropospheric flow, and is not accompanied by a mesocyclone. The initial circulation forms near the surface at the intersection of two mesoscale boundaries and develops vertically, intensifying into an Fl tornado when it becomes collocated with the intense updrafts of a rapidly developing cumulonimbus. This tornado appears to be the land equivalent of a waterspout, and comparisons between the two vortices are made. It is speculated that Florida and portions of the western High Plains may be prone to nonmesocyclone tornado development, and that vortex-intensification processes associated with nonmesoscyclone tornadoes may be important in mesocyclone tornadogenesis. Suggestions on how to better forecast these tornadoes are also presented.

A case study of nonmesocyclone Tornado development in northeast Colorado: similarities to waterspout formation

On the afternoon of 3 June 1981 a severe thunderstorm spawned two tornadoes which moved across a portion of metropolitan Denver. The tornadoes were classified as strong F2 intensity, and caused damage totaling over $1 5 million. The synoptic-scale setting for this event was similar to that associated with many other occurrences of severe convection in eastern Colorado, with post-frontal moist southeasterly upslope flow at low levels and southwesterly flow aloft in advance of an approaching trough. We chose to study this event in part because of its occurrence within the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork. Emphasis is placed on mesoscale evolution culminating in the formation of the tornadic storm. A zone of surface convergence and cyclonic vorticity developed during the early daylight hours over and north of Denver between southeasterly flow over the plains and a region of lighter, generally northerly flow just east of the foothills. The tornadic s...

A Subsynoptic Analysis of the Denver Tornadoes of 3 June 1981

This is Part II of a two-part paper describing the vertical profile of radar reflectivity in GATE convective cells. Time-height radar life histories for 42 cells over three GATE days are examined, using data from the Quadra radar with 5-minute resolution. Mean profiles and plots of cell characteristics are generated, and confirm that the mean profiles in Part I are representative of the active portion of the cell lifetime. There are marked differences between the cell life histories of isolated cells and the longer-lived cells associated with mesoscale systems. In contrast to cells sampled in organized systems, the isolated cells are often of very limited vertical extent and must be dominated by the warm rain process. When forcing features exist such as gust fronts and intersecting lines of convection, they appear to dominate the generation of new convection, and isolated strong echoes are not observed. Composite life histories for typical GATE cells are constructed. The typical radar echo forms ...

A Radar Study of Convective Cells in Mesoscale Systems in GATE. Part II: Life Cycles of Convective Cells

The tilt with height of the leading edge of seven mesoscale convective lines in GATE is determined by two independent methods. When averaged over time and along the line axis, the tilt is found to he surprisingly shallow, between 20 and 35 degrees from the horizontal. This is distinct from the slopes of the individual towers, which can be much steeper. The line leading-edge slope corresponds to the ratio of the vertical to horizontal velocity, relative to the line motion, of “representative” embedded convective cores.

Edward J. Szoke

Papers

A Radar Study of Convective Cells in Mesoscale Systems in GATE. Part I: Vertical Profile Statistics and Comparison with Hurricanes

A case study of nonmesocyclone Tornado development in northeast Colorado: similarities to waterspout formation

A Subsynoptic Analysis of the Denver Tornadoes of 3 June 1981

A Radar Study of Convective Cells in Mesoscale Systems in GATE. Part II: Life Cycles of Convective Cells

The Tilt of the Leading Edge of Mesoscale Tropical Convective Lines