Basic Convective and Mesoscale Research

We learn and teach about the processes which produce local weather phenomena with direct impact on everyday human activity, such as thunderstorms and mesoscale convective systems, as well as the gentler processes which produce cumulative effects on local and larger scale environments, such as cumuliform and layered cloud arrays. Although a general understanding of most of these processes exists, it is insufficient to allow accurate prediction of either the intense elements or the statistical effects of the more benign systems. The funding of the work proposed here will be used to conduct studies and participate in field programs that would lead to improvements in both the basic understanding and its application in the following areas.

1) Dynamic and Synoptic Meteorology

Storm and mesoscale weather systems cause hourly changes in local weather, but are embedded in and partially controlled by the larger scale flow fields. Detailed understanding of the dynamics of mesoscale weather and its relations to large scale driving forces, as well as small-scale dissipation, is needed to predict the evolution of local weather. Since most intense storms are fueled by latent heat release, it is especially important to study the interactions between moist processes and vertical motion on various scales. Several investigations to address these issues are described below.

(a) Storm-scale data assimilation

Further development and improvement of the two- and three-dimensional simple adjoint methods for single-Doppler retrievals are being pursued. These methods are expected to be able to retrieve not only three-dimensional winds, but also temperature and pressure perturbations. These methods will be used, together with the Advanced Regional Prediction System (ARPS) model and its adjoint, for storm-scale data assimilation. The goal is to improve numerical weather analyses and predictions on the storm scale. We are also generalizing the adjoint theory for discontinuous systems and applying the generalized adjoint to diabatic data assimilation. Exploration of the combined uses of data from various sources for storm-scale data assimilation is being done, and these include (but are not limited to) Doppler radars, the Oklahoma Mesonet, wind profilers, and even satellites (GPS-LEO). Operational soundings, and profiler and aircraft data, will also be used to provide background fields through the Oklahoma Local Analysis and Prediction System (OLAPS) three-dimensional data analysis package.

(b) Studies of balanced and unbalanced mesoscale dynamics

Since the balanced component of airflow is often long-lived and can be better observed than the unbalanced transient component, understanding the balanced and unbalanced dynamics for various mesoscale systems will improve our understanding of mesoscale predictability and/or observability. It will also provide a theoretical basis for improving the schemes for four-dimensional mesoscale data assimilation and initialization. To this end, three research areas are being examined: 1) the unbalanced generation of the density-current feature from a balanced synoptic front; 2) the effect of surface friction on the unbalanced generation of the density-current feature; 3) balanced and unbalanced dynamics for mesoscale convective systems.

(c) Interactions between land surface characteristics and mesoscale convective precipitation systems

This work is relevant to several questions and issues raised in the science plan of the GEWEX-GCIP Project. For example, a recent ECMWF model study noted that the 1993 Midwest flooding ". . . is probably due to feedback processes [involving deep, moist convection and moisture convergence in the ABL, which] require further study [along with development of] strategies for determining soil moisture fields to initialize models". CIMMS scientists and collaborators will employ the CSU-RAMS mesoscale model to investigate the feedbacks between vegetation, soil moisture, and the life cycles of deep convection and Mesoscale Convective Systems (MCSs) over the U.S. Southern Great Plains with focus on the GEWEX-GCIP/GIST domain. Particular emphasis will be given to: 1) the coupled mesoscale simulation of soil moisture and boundary layer evolution over the GIST domain and the life cycles of mesoconvective systems influencing the GIST domain; 2) the role of thunderstorm precipitation in influencing subsequent boundary layer evolution and storm initiation via localized increases of soil moisture from antecedent modeled precipitation events; and 3) the role of transpiration gradients across the GIST domain in producing mesoscale soil moisture gradients on a seasonal time-scale. The quantification of mesoscale heat and water budgets and the interactions of land surface, boundary layer, and convective phenomena provide data to serve as a basis for evaluating ground, boundary layer, and subgrid convective parameterizations in General Circulation Models (GCMs).

(d) Effects of supercell storm characteristics on lightning

Previous studies have documented large differences in the evolution of lightning rates in various types of supercell storms, but the reasons for these differences are poorly understood. Investigators have used the variability of non-electrical aspects of supercell storms, which is comparable to the variability of lightning, to develop hypotheses to explain specific lightning observations, but tests of the hypotheses have been few and very limited. To test these hypotheses, storm measurements are necessary, but by themselves are inadequate. Numerical cloud models that incorporate electrification mechanisms can help by providing a complete, physically consistent framework of winds, thermodynamics, hydrometeors, charge, and electric fields with which to examine how various electrification processes are affected by marked differences in storm characteristics. CIMMS' scientists propose a research program that incorporates both observational data analysis and numerical storm simulations to improve understanding of the electrification of supercell storms.

(e) Morphology and evolution of an intense macroburst observed by Doppler radar

The squall line is a mesoscale phenomenon that often produces severe weather in various forms (e.g., damaging winds and flash flooding). Many studies have focused on the mature stage of squall line evolution, yet under the proper conditions severe weather can occur when the line is in the dissipation stage. A unique data set has been gathered whose analysis will allow advancement of basic understanding of the relation of squall line dissipation to the generation, structure, and evolution of a very intense macroburst. The proposed project has the following specific objectives: 1) to diagnose the causative relations between the dissipation of the squall line and the associated evolution of the macroburst; 2) to provide an observational base to investigate the validity of the theory that relates squall line evolution to balances between low-level environmental wind shear and internally-generated vorticity; 3) to extend conceptual models such as the gust front model proposed by Wakimoto to the case of intense macrobursts; and 4) to provide observational verification for the Droegemeier and Wilhelmson model results that contained breaking Kelvin-Helmholtz waves on the interface between the macroburst flow and displaced airmass. The results of this research should provide insight and guidance to those responsible for forecasts of squall line evolution and warnings of extreme thunderstorm winds to the public. The completed project will include the availability of a data set for those interested in physical simulations and predictions of extreme gust flow phenomena.

2) Boundary layer meteorology and turbulence

Fluxes of heat, moisture, and momentum through the atmospheric boundary layer make significant contributions to the dynamics and thermodynamics of mesoscale processes. The effective parameterization of boundary layer processes in operational and mesoscale models has been seriously hampered by a scarcity of relevant data, and CIMMS has a continuing interest in participating in the acquisition of such observations and the development of simplified models detailing important boundary layer processes.

(a) Boundary layer flux climatology and mesoscale model initiation

With few exceptions, past boundary layer observations have focused on extremely simple environments and offer little guidance on many situations of importance for mesoscale studies. The development of new observing technologies and facilities represents an opportunity to significantly increase the number and quality of the observations of boundary layer fluxes for a wide range of mesoscale environments. These technologies include the NWS network of WSR-88D radars, the NOAA 405 MHz Profilers with RASS, and the promising work at NCAR and the University of Massachusetts to develop 915 MHz Profilers with RASS that can observe boundary layer fluxes of momentum and buoyancy. If the potential of the 915 MHz flux profilers is realized, a true climatology of boundary layer fluxes is within reach. And beyond the advances in understanding of boundary processes, such a statistical data base of fluxes and coincident WSR-88D and 405 MHz data could make it possible to key the sub-grid-scale parameterization of boundary layer fluxes to the larger-scale WSR-88D data itself. CIMMS proposes to develop such a climatology, and assess the utility of such parameterizations as part of the research on mesoscale model initialization and data insertion/correction.

(b) Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX)

Over the last two decades, significant progress has been made in the area of identification of severe thunderstorms using Doppler radar data. In particular, it is recognized that many significant tornadoes in the Great Plains are associated with thunderstorms containing mesocyclones. However, we are faced with significant gaps in our understanding of tornadogenesis and tornadic storms. For example, it is not clear why some storms with mesocyclones produce tornadoes and others do not. We also do not understand what determines which storms, out of a group of storms on any given day, will become tornadic. In order to begin resolving these issues, scientists from NSSL and other institutions mounted the VORTEX field experiment in the springs of 1994 and 1995, in a fairly large region of the southern and central Plains (roughly from 33 to 38 degrees north, and between 96 and 102 degrees west). A group of testable hypotheses were defined, and observational strategies were developed specifically to generate the necessary test data. Observational platforms included the NOAA WP-3 Orion aircraft, NSSL mobile soundings and Mobile Mesonet, the NWS WSR-88D radars and Profiler network, the Oklahoma Mesonet, and a mobile C-band radar, supported by real-time runs of cloud and mesoscale models. The field phase of VORTEX was extremely successful, producing an unprecedented collection of observations on the structure and life cycle of both tornadic and non-tornadic storms. The subsequent necessary data analysis and development of case studies have begun. One major thrust in exploiting these data will involve study of tornadogenesis and supercell thunderstorms.

(c) Cooperative Oklahoma P-3 Studies Program (COPS-98) Thunderstorm Initiation Mobile Experiment (TIMEX)

This broad-collaboration field program will be hypothesis-driven (following the VORTEX model) and focused on the mechanisms associated with the convective initiation of severe weather. NOAA De Havilland Twin Otter in situ measurements of turbulent fluxes of moisture, heat, and momentum in and above the convective boundary layer, and on either side of boundaries (dryline, outflow boundary, front, etc.), will be a critical factor in the definition of physical mechanisms responsible for the change in state from fair-weather convection to a storm-producing environment near the boundary. We are proposing that the Twin Otter will fly in tandem with the NOAA WP-3 Orion, supported by the NSSL Mobile Mesonet, the NOAA/OU dual-Doppler facility, and the NWS operational data stream.