Figure 1: Photograph looking north at newly-developing deep dryline convection at 0040 UTC on 16 May 1991 during the COPS-91 field experiment (courtesy C. Hane, NSSL). The NSSL-2, a mobile ballooning laboratory operated by the Joint Mobile Research Facility (JMRF) of the Oklahoma Weather Center, is in foreground while a developing Wheeler County, Texas storm is in background. Valuable experience gained from COPS and the later VORTEX project on obtaining mobile soundings and other observations both preceding and near developing storms will be applied during TIMEx.

TIMEx is a field study designed to investigate convective initiation on the mainland United States

Planning TIMEx:
The Thunderstorm Initiation
Mobile Experiment

by Conrad Ziegler

Anticipating thunderstorm initiation is a very difficult and challenging problem of considerable importance to both warm season quantitative precipitation and severe weather forecasting. To help improve the accuracy and specificity of storm forecasts, NSSL scientists Jeanne Schneider, Conrad Ziegler, and Erik Rasmussen are leading the planning of the "Thunderstorm Initiation Mobile Experiment" (TIMEx), a field study designed to investigate convective initiation on the mainland United States. Community discussions of the proposed field study are taking place via an interactive web page . NSSL has organized two TIMEx planning meetings, the initial meeting hosted in Norman in November 1997 and the second meeting held in Phoenix, Arizona in January 1998 during the AMS Meeting. The TIMEx planning meetings have brought together scientists from several universities, NCAR, other Oklahoma Weather Center elements, the National Weather Service, and other federal laboratories to discuss hypothesized convective initiation processes and the observational strategies required to detect those processes in actual cases. We intend for TIMEx to be one of a series of field programs designed to answer specific questions about the life cycles of storms and Mesoscale Convective Systems (MCSs). (See also the article on the "MCS Electrification and Polarimetric Radar Study - MEaPRS" in this issue.) As realized in TIMEx we begin with a focus on convective storm initiation.

One of the fundamental issues in precipitation forecasting is the timing and location of the initiation of storms, or in many cases, the failure of initiation. Clearly, any precipitation forecast will fail completely if initiation is forecast and fails to occur, and vice versa. Further, quantitative errors are strongly dependent on errors in the location and time of storm initiation. Moreover, large errors in forecast temperature may be caused by poor forecasts of storm development. Recent case studies of seabreeze lines, drylines, and other sharply defined zones of contrasting winds, temperature, and moisture near ground have documented how airflow convergence and lifting of moist air along such "boundaries" can initiate storms (Fig. 1). Atmospheric airflow disturbances such as gravity waves or strong, localized air currents known as "jet streaks" may help trigger storms along boundaries by providing additional lifting of moist, unstable air. Strong vertical wind shear and mixing of moist rising air with drier air from higher in the atmosphere may limit the tendency to achieve water saturation and cloud formation, thus suppressing the development of deep convective clouds and storms. None of these processes are adequately understood, and may be misrepresented or completely unresolved in mesoscale numerical weather prediction models.

The plan for TIMEx is to conduct a relatively small, very focused field experiment as early as the spring of 2000 to document structure and morphology of the planetary boundary layer and lower troposphere on spatial scales from 2-20 km in the horizontal dimension. Various remote and in-situ measurements from mobile platforms will be concentrated in areas with the potential for deep, moist convection. Airflow in the optically clear planetary boundary layer will be detected with existing sensitive airborne and ground-based Doppler radars (Fig. 2), while "lidars" (laser light Doppler radars) that measure water vapor content based on a differential water vapor absorption principle must be developed for mobile application. Following a year of analysis of data collected during the preliminary field phase, a second and more comprehensive TIMEx field phase will be conducted. Though the site of the preliminary field phase has not been firmly decided upon, the scientists attending the planning meetings favored the Texas-Oklahoma-Kansas region owing to the excellent visibility for ground-based mobile Doppler radar observations and the possibility of utilizing dense observing networks already in place on the U.S. Southern Plains.

In TIMEx we plan to collect data on as many types of boundaries as possible, including stationary fronts, warm fronts, outflow boundaries, drylines, and the myriad of other low-altitude features detectable as radar fine lines in WSR-88D reflectivity data but of unknown origin and character. Additionally, a variety of low-altitude shear, temperature, and humidity regimes will be sampled. Analysis of this unique set of observations will assist the NWS by providing the basis for conceptual models of the convective initiation process that in turn will help improve forecasts of storm development.

For more information contact: Conrad Ziegler at conrad.ziegler@nssl.noaa.gov