Purpose:

To improve understanding of satellite-observed cloud top structures and their link to internal storm processes, with emphasis on microphysical and electrical properties.

Background:

The proposed study is a continuation of the Multispectral Observations of Storm Tops (MOST) project'. This project began by using the AVHRR channel 3 (3.7 microns) imagery from the NOAA polar-orbiting meteorological satellites to detect apparent microphysical changes at storm tops, seen as spots or plumes in the imagery. Later, this work included imagery from channel 2 (3.9 microns) of the GOES-8 and 9 geostationary satellites and some WSR-88D radar data on internal storm structure.

In some cases, persistent plumes are observed to form just downwind of penetrating tops in long-lived rotating storms. The coldest cloud top temperature is usually observed upwind from this area in the shape of a "U" or "V". A locally warm spot (in the thermal infrared imagery) is observed just downwind of the coldest point nearby the leading edge of the plume. From visible imagery, the plumes appear to be as much as 1 km above the anvil. In the 3.7-3.9 micron imagery, enhanced solar reflection is often observed from the plumes as compared to the anvil. The enhanced reflection has been attributed to different microphysical characteristics of the plume, however no in-situ measurements are available to substantiate the properties of these features.

In addition to plumes, shorter lived areas of higher reflectance are also observed in other regions, such as within the more uniform cloud shield behind an intense line of convection. Unlike the plumes, these features do not remain fixed with respect to a storm. In contrast, the entire areas of high refectivity are advected with the wind near cloud top level. It is possible that electrical fields near the top of the storms may play a role in changing the amount of reflection through hydrometer alignment. Until now, no comparisons have been made between lightning and cloud top feature locations.

An attempt was made to study polarimetric radar data when a small plume was observed within range of the Cimmaron radar (Nov, 1996). A strong updraft with a minimum in snow crystals was inferred from the data below the cloud top level. Unfortunately, the radar scans were not sufficiently elevated to sample the very top of storm near the altitude of the plume.

Data Collection:

a. Satellite:

Satellite imagery will be examined on a regular basis for plumes and other features of high reflectivity in the 3.7/3.9 micron channels during MEaPRS operations. The period of interest would be from the beginning of operations until dark. Satellite data will be archive or acquired from GOES-8/9 (at U. Wisconsin or NSSL) and from AVHRR (at the Czech Hydrometeorological Institute from the NOAA Satellite Active Archive). Super rapid scan data (30 s to 1 min) may be requested if possible. However, the long lead time of such requests minimizes probability of successful collection. If super rapid scan in unavailable, the interval between GOES data will be 7.5 or 15 minutes, depending on the extent of severe weather.

b. Radar:

1. When we can identify a significant feature from satellite within range of the Cimmaron radar in time to notify the radar operator, we request the following data be collected (on 1-2 days):

- Several RHI's across the feature including altitudes above cloud top (13-15 km maximum). Otherwise,

- Routine PPI's which also include scans above cloud top while the feature is within range of Cimarron.

2. In cases where satellite features cannot be identified in time to direct collection of radar data, we will consider Cimmaron and lightning data collected for analysis of all substantial storms with or without plumes.

c. Aircraft:

It is not expected that the microphysical data collected from the P-3 will be of direct use since the flight altitude will be well below anvil level. Nevertheless, the data could provide some insight into the microphysical structure and airflow below some of the cloud top features over stratiform-like regions. In addition, data from the airborne radar may be of use in mapping storm top height nearby plumes. No special data acquisition from the aircraft is requested.

d. Lightning Mapper:

Data routinely collected by the lightning mapper network will be utilized to examine the frequency of lightning near cloud top level in vicinity of plumes and other areas of enhanced reflectivity in the 3.7/3.9 micron imagery. Since the mapper covers a relatively small area, lightning frequency from the national lightning network will also be utilized with respect to the features, although location at cloud top cannot be identified.

e. Electric Fields:

If available nearby storm top features, measurements will be examined to characterize the electric fields in those regions. Analysis: The data collected during MEaPRS will be used to infer microphysical and electrical structure in vicinity of plumes. We anticipate a detailed analysis of a single case or two where radar, satellite, and lightning data overlap in time and space, and comparisons with storms without plumes or spots, if comparable data are obtained.

For more information, look at the Multispectral Observations of Storm Tops (MOST) project page.