Understanding Severe Weather Processes
Planning a Convection Initiation Study (TIMEX)
The weather research community, through the U. S. Weather Research Program (USWRP), has established the objective to improve the specificity, accuracy, and reliability of weather forecasts for disruptive, high impact weather, particularly for the initiation and evolution of severe storms and heavy convective precipitation. A key to the mesoscale quantitative precipitation forecast (QPF) problem is our lack of understanding of the definitive boundary layer (BL) and internal processes that are responsible for the wide range of observed modes and evolution of storms and convective systems. One of the fundamental issues in convective precipitation forecasting is the timing and location of the initiation of storms, or, in many cases, the failure of initiation. The term "convection initiation" (CI) is defined herein as the process(es) leading to the formation of a cumulonimbus (AMS Glossary 2000), a deep, moist convective cloud with an anvil, an active precipitation growth process, and intra-cloud or cloud-to-ground lightning.
Clearly, any convective precipitation forecast will fail completely if CI is forecast and fails to occur, and vice versa. Further, quantitative errors are strongly dependent on errors in the location and timing of CI. For example, research on seabreeze fronts, terrain-induced convergence bands, and drylines has suggested the strong dependence of CI upon:
- the trajectories that parcels follow while rising through the BL from the ground toward the level of free convection;
- the amount of mixing that occurs along these plumes of moisture;
- the low-level temperature and humidity stratification.
In turn, the foregoing depends on the shear profile and variations in the vertical motion field associated with boundaries. None of these processes are adequately understood, and appear to be misrepresented in mesoscale models in the presence of weak larger-scale forcing.
Beginning in 1997, Conrad Ziegler (NSSL), Erik Rasmussen, and Jeanne Schneider began envisioning a field project, dubbed the Thunderstorm Initiation Mobile Experiment (TIMEX), to study the CI process. Following the example of VORTEX, testable hypotheses were refined and debated among a group of interested researchers. TIMEX meetings in 1997-1998 and related discussion prompted evolution in plans and approaches with consideration of the observations required to test the hypotheses. Some hypotheses could be tested with existing systems, while other hypotheses required the development of new or improved observing systems. The NCAR/NOAA lower-tropospheric water vapor workshop, held in June 1998, and the third TIMEX meeting in September 1998, contributed to groundwork for a major new field experiment, the International H2O Project (IHOP).
