A Comparison of Cloud and Rainfall Information from Instantaneous Visible and Infrared Scanner and Precipitation Radar Observations over a Frontal Zone in East Asia during June 1998

Abstract

The comparison between cloud information and rainfall is studied using infrared and radar data from the Tropical Rainfall Measuring Mission. Cloud information from the visible and infrared scanner was compared with rain information from precipitation radar (PR) for rain cases assembled during June 1998 over a frontal zone in east Asia. The authors selected the following four parameters: 1) radiance ratio of 0.6 and 1.6 ?m [channel 1/channel 2 (Ch1/Ch2)], 2) brightness temperature difference between 11 and 12 ?m (BTD45), 3) brightness temperature difference between 3.8 and 11 ?m (BTD34), and 4) brightness temperature (TBB) in channel 4 (Ch4) as the cloud information. The flags of ?rain certain,? stratiform rain, brightband existence, and convective rain observed by PR, and integrated rain rate from the rain bottom to rain top were used as the rainfall information. From the comparison between rain?no-rain information by PR and the four cloud parameters, it is found that values of the radiance ratio of Ch1/Ch2 larger than 25, BTD45 smaller than 1.5 K, and BTD34 smaller than 8 K are effective in delineating rain area. The probability of detection (POD), false alarm ratio (FAR), and skill score (SS) are computed and compared for the following rain and no-rain algorithms: 1) single cloud threshold of 235 K in Ch4 TBB as in the Geostationary Operational Environmental Satellite Precipitation Index, 2) single threshold of 260 K in Ch4 TBB, 3) Ch1/Ch2 larger than 25 and Ch4 TBB colder than 260 K (C12), 4) BTD45 smaller than 1.5 K and Ch4 TBB colder than 260 K (C45), and 5) BTD34 smaller than 8 K and Ch4 TBB colder than 260 K. The C12 method shows the highest SS, and the C45 method shows the highest POD. The BTD34 scores better in FAR than the BTD45 and is better than BTD45 in delineating the thicker part of cirrus clouds. The use of the second channel shows better scores than does use of the single infrared threshold algorithm. The cloud characteristics for convective rain and stratiform rain are also studied using Ch1/Ch2, BTD45, and BTD34. The percentage of occurrence of stratiform rain shows a local maximum for clouds of small BTD45/BTD34 with Ch4 TBB of 220?250 K. The higher percentage of convective rain corresponds well to the optically thicker (smaller BTD45) clouds colder than 210 K. However, there is no significant difference in Ch1/Ch2 between convective and stratiform rain, because significant convective cases are not included in the data that were processed.