Stratiform Rain Experiment

STRATIFORM RAIN EXPERIMENT

Purpose

The purpose of this experiment is to verify and improve multi-parameter-radar techniques to measure precipitation and to determine hydrometeor type, size and number density. A model has been developed that helps with the interpretation of precipitation observations made with the polarization diversity radar. Preliminary tests indicate that it is possible to determine the relative contributions of rain and hail to the total precipitation rate and to obtain an estimate of the distribution of hailstones with size. The model makes assumptions about the size, shape, and orientation of rain and hail. It is not perfect, however, and must be verified or improved. In the process, the interpretation of radar observations of precipitation will be verified. These tasks can be accomplished by obtaining information about precipitation with the cloud physics research aircraft. Observations, using the appropriate PMS probes rotated so that the shape of hydrometeors can be observed, will be compared with radar observations made at the time. The aircraft observations will be used to substantiate or modify the radar model.

If it is possible to determine through radar observations the relative contributions of hail and rain to the total precipitation rate, the distribution with size of hailstones, and whether or not melting snow or graupel is being observed, then the radar can be applied to several problems.

Since the presence of hail, melting snow, or graupel can introduce errors in the measurement of rainfall by weather radar, the application of polarization techniques can improve these rainfall measurements. In this case hydrologists and agriculturists requiring rainfall measurements over large areas will benefit.

Experiment Procedures

The experiment requires the cloud physics aircraft to fly through precipitation at the base of convective and stratiform systems, at all levels in stratiform system, (especially the region at and below the melting level), and in the first echo region of convective storms. The S-band polarization diversity radar will be in operation before, during, and after all flights.

In the case of stratiform rain, it is anticipated that the research aircraft will be dedicated to this mission and will fly extensive flight tracks at various heights (to be determined in conjunction with a cloud physicist). The flight tracks will follow radials to and from the radar site and will lie between 10 and 50 km range from the radar. It is anticipated that both the S-band and C-band radars will be operating since cloud seeding should not be required. The aircraft will be required to obtain images of the snow above the melting layer, the hydrometeors within the melting region, and the hydrometeors well below the melting region.

During convective showers or storms, it is anticipated that other experiments will have higher priority. Therefore, observations will be obtained whenever possible. For example, on the way to a storm and on returning from a storm, the aircraft may make a few passes below cloud base in the rain areas. During regular cloud penetrations, the Aircraft Mission Scientist should note whenever precipitation size particles are encountered so that these events can be used for the study. Since cloud seeding operations will probably be in progress, only the S-band radar will be operated to prevent interference with the real-time displays. It is hoped that rain and graupel can be observed.

In addition to PMS probe data, the standard thermodynamic information will be required, together with accurate position information.