The Area-Time-Integral Technique to Estimate Convective Rain Volumes over Areas Applied to Satellite Data—A Preliminary Investigation

Abstract

Early work attempting to apply GOES rapid scan satellite data to a recently developed simple radar technique used to estimate convective rain volumes over areas in a semiarid environment (the northern Great Plains) is described. Called the Area-Time-Integral (ATI) technique, it provides a means of estimating total rain volumes over fixed and floating target areas. The basis of the method is the existence of a strong correlation between the radar echo area coverage integrated over the lifetime of the storm and the radar estimated rain volume. The technique does not require the consideration of the structure of the radar intensities to generate rain volumes. but only the area covered by radar echoes. This fact might reduce the source of errors generated by the structure differences between the radar and the satellite signatures above given thresholds. Satellite and radar data from the 1981 Cooperative Convective Precipitation Experiment (CCOPE) and the North Dakota Cloud Modification Project (NDCMP) are used. Consecutive time steps with both radar reflectivities and satellite (VIS and IR) rapid wan data were considered during the evolution of six convective clusters: three on 12 June, and three on 2 July 1981. Radar echoes with reflectivity values ≥ 25 dBZ were used to define the area of rainfall and the respective digital unit thresholds within the satellite data delineating the rainy part of the cloud area. Correlation of the ATI versus IR digital count values was obtained for every time step and for the storm lifetime, respectively. A comparison of the stepwise evolution of radar parameters such as echo areas maximum echo heights, maximum reflectivities and satellite parameters such as threshold count values and coldest cloud top temperature is presented graphically and reflects the multicell characteristics of the convective clusters. Also, a comparison of radar and satellite parameters for the cluster lifetime is made. Satellite parameters pertaining to the cluster lifetime were derived both dependently and independently of radar data. The main purpose of this investigation is to compute convective rain volume of a convective cluster by application of the ATI technique based only on satellite data. As such, the key element is to determine the ATI from satellite data without consideration of radar data. This is possible if trends of satellite products generated independently are similar to those of satellite products based upon radar observations as done here. A parallel with the two-step techniques generally used to estimate rain volume from satellite data is made. To delineate the rainy part of a cloud area, a regression analysis is used. The regression relate a satellite-independent product to a satellite-dependent product. For a given storm. the satellite-independent product is first computed; then the regression equation gives the ATI, Finally, the rain volume is obtained by using the ATI versus rain volume relationship. By applying the ATI/rain volume relationship to satellite data, the errors generated by the complicated multiple area-volume transform relations am reduced, as similar errors were reduced when the technique was applied to radar data. In addition, a regression analysis gives more accurate estimates than a single threshold when used to delineate an area covered by rain events from an area covered by clouds. The advantages of the ATI technique are based on the fact that the technique operates on a storm lifetime integrated basis, while the previous techniques operate on a time-step basis. The new technique generates only total rain volume estimates (not rain rates). This indeed is a limitation. The analyses of six convective clusters suggest that the extension of the ATI technique using satellite data holds promise.