THE NATURAL VARIABILITY OF PRECIPITATING CLOUDS OVER THE WESTERN PACIFIC WARM POOL

The natural variability of precipitating cloud systems over the wester n Pacific 'warm pool' is investigated by analysing aircraft C-band rad ar data collected on 24 aircraft missions during the Coupled Ocean-Atm osphere Response Experiment of the Tropical Ocean and Global Atmospher e programme (TOGA COARE) in relation to the infrared (IR) temperature patterns measured contemporaneously by geosynchronous satellite. The d ata are analysed at fine (24 km x 24 km) and coarse (240 km diameter) horizontal resolutions, which correspond to typical resolutions of mes oscale and general-circulation models, respectively. The analysis is s tatistical and imposes no a priori conceptual model or subjectively de cided-upon structure categories. The mean IR temperature of cloud tops , and the sizes of rain areas mapped by radar (and objectively subdivi ded into convective and stratiform subareas), are tallied and related to each other to obtain statistics representative of the four-month pe riod of the TOGA COARE. At fine resolution the precipitation region un derlying very cold cloud tops (<208 K) always contained stratiform pre cipitation and had less than a 50% probability of containing some amou nt of convective precipitation. The frequency of occurrence of precipi tation, the area covered by all types of precipitation, and the area c overed by stratiform precipitation all increased with decreasing mean IR temperature. In contrast, the area occupied by convective precipita tion was independent of mean IR temperature. Over a period of sustaine d convective cell activity, the cells never instantaneously occupied m ore than a small fraction of the coarse area, and never more than 30% of a fine-grid element. However, as cells weakened they evolved into s tratiform precipitation, and the stratiform region grew as each cell f inished its active convective phase and was added to the stratiform ar ea. The sustainability of convective cells over time thus determined t he overall size of a precipitation area. Rain-area size did not correl ate linearly with mean IR temperature nor any other statistical measur e of the IR temperature pattern at coarse resolution. However, two-dim ensional joint-probability distributions of mean IR temperature (a mea sure of cloud structure) and radar echo parameters (which measure prec ipitation structure) on the coarse scale yield insight into the natura l variability of precipitation processes over the warm pool: most nota bly, large precipitation areas (>50% of a coarse area) tended to have low mean cloud-top temperatures (<235 K), and high mean cloud-top temp eratures (>235 K) were associated with small precipitation areas (<50% of a coarse area). However, the converse of these two results was not true. Low mean cloud-top temperatures (<235 K) were associated with a wide range of precipitation areas from near 0 to 90% of a coarse area , and small precipitation areas (<20% of a coarse area) had mean IR te mperatures ranging from 210-295 K. Once precipitation regions reached a large enough size (near 20% of a coarse area), the cloud systems wer e efficient at spreading ice outside of the boundaries of the precipit ation regions. Larger rain areas (>20% of a coarse area) were predomin antly stratiform (>75% by area and >50% by rainfall amount). Precipita ting cold clouds (containing ice) with stratiform area fractions <50% were not observed. During active phases of the intraseasonal oscillati on (ISO), the probability that there were no satellite-observable clou ds within a coarse-resolution area of the aircraft sample was practica lly zero (maximum mean IR temperature never >286 K), while during supp ressed phases the near or total absence of clouds was common. Large ra in areas with very cold cloud tops were common under active conditions but not under suppressed conditions. The large rain areas with very c old cloud tops (>20% of the coarse area and <235 K mean IR temperature ) occurred primarily during the active phase and were 75-100% stratifo rm by area and >70% by rain fraction. In the suppressed phases, strati form rain fractions varied widely since most of the rainfall was assoc iated with small precipitation areas. Although the joint-probability d istributions differ substantially between active and suppressed phases of the ISO, the overall precipitation-area size is not strongly relat ed to wind speed, wind direction, or thermodynamic variables, as seen in the storm inflow soundings obtained by the TOGA COARE aircraft.