P6.9
Convective initiation without an instantaneous warm bubble
Lewis Grasso, CIRA/Colorado State Univ., Fort Collins, CO
INTRODUCTION
Convective initiation is an important forecasting issue. Knowing where and when thunderstorms will develop provides forecasters with valuable information that can be passed along to the public. Although much has been learned about thunderstorm behavior--for example, updraft splitting and the deviation of updraft movement from the mean tropospheric wind, little is known about the initiation process.
Instantaneous warm bubbles are a popular and convenient mechanism that are used to cause the development of numerically simulated thunderstorms. Warm bubbles have been used successfully in the following studies: The simulation of three-dimensional convective storm dynamics, Simulations of right and left moving storms produced through storm splitting, On the evolution of thunderstorm rotation, The influence of the shear-induced pressure gradient on thunderstorm motion , and The dependence of numerically simulated convective systems on vertical wind shear and buoyancy.
Due to the unrealistic nature of a warm bubble, study of simulated convective initiation may be forbidden. In some cases, effort has been place on using a minimum bubble to trigger simulated updrafts. Recently, Bluestein and Weisman argued, ``Using the minimum perturbation is important to ensure that the long-term solutions are minimally affected by the nature of the initialization.'' Can an initial state be chosen to minimally affect a long-term solution? Is thunderstorm evolution in the Earths atmosphere minimally affected by the initiation process? Because of the uncertainties accompanied by instantaneous warm bubbles, a different method is used to explore convective initiation in an idealized setting.
Motivation for using a different method to explore thunderstorm triggering is found in a recent dryline paper. The authors found that thermal forcing was part of the convective triggering process along drylines. In particular, thermal forcing was located and remained in the boundary layer. This idea is extended to an idealized environment; that is, create thermal forcing by adding a nudging term to the prognostic equation of a temperature variable--a temperature variable could be temperature,potential temperature, or ice-liquid water potential temperature. In contrast to an instantaneous warm bubble, an investigator can control the thermal perturbation, the volume and location of the perturbation, the nudging time scale, and the time interval over which nudging is applied. In particular, the location of a thermal perturbation can be fixed in the boundary layer for a prescribed time interval.
PRELIMINARY RESULTS
The numerical model used for this simulation was RAMS. Results indicated that storm motion varied with the nudging time scale, that is, mid level updrafts moved faster and more to the right as the nudging time scale decreased.
Poster Session 6, Numerical Modeling of Severe Local Storms
Wednesday, 14 August 2002, 3:00 PM-4:30 PM
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