The natural variability of precipitating clouds over the western Pacific warm pool

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Abstract

The natural variability of precipitating cloud systems over the western Pacific ‘warm pool’ is investigated by analysing aircraft C-band radar data collected on 24 aircraft missions during the Coupled Ocean-Atmosphere Response Experiment of the Tropical Ocean and Global Atmosphere programme (TOGA COARE) in relation to the infrared (IR) temperature patterns measured contemporaneously by geosynchronous satellite. the data are analysed at fine (24 km × 24 km) and coarse (240 km diameter) horizontal resolutions, which correspond to typical resolutions of mesoscale and general-ciruclation models, repectively. The analysis is statistical and imposes no a priori conceptual model or subjectively decided-upon structure categories. The mean IR temperature of cloud tops, and the sizes of rain areas mapped by radar (and objectively subdivided into convective and stratiform subareas), are tallied and relatted to each other to obtain statistics respresentative of the four-month period of the TOGA COARE.

At fine resolution the precipitation region underlying very cold tops (<208 K) always contained stratiform precipitation and had less than a 50% probability of containing some amount of convective precipitation. The frequency of occurrence of precipitation, the area covered by all types of precipitation, and the area covered by stratiform precipitation all increased with decreasing mean IR temperature. In contrast, the area occupied by conective precipitation was independent of mean IR temperature.

Over a period of sustained convective cell activity, the cells never instantaneously occupied more 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 stratiform precipitation, and the stratifrom region grew as each cell finished its active convective phase and was added to the stratiform area. The sustainability of convective cells over time thus determined the overall size of precipitation area.

Rain-area size did not correlate lineraly with mean IR temperature nor any other statistical measure of the IR temperature pattern at coarse resolution. However, two-dimensional joint-probability distributions of mean IR temperature (a measure of cloud structure) and radar echo parameters (which measure precipitation structure) on the coarse scale yield insight into the natural variability of precipitation processes over the warm pool: most notabaly, large precipitation areas (>50% of a coarse area) tended to have low mean cloud-top temperatures (<235K), and high mean cloud-top temperatures (>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 temperatures renging from 210–295 K.

Once precipitation regions reached a large enough size (near 20% of a coarse area), the cloud systems were eicient at spreading ice outside of the boundaries of the precipitation regions. Larger rain areas (>20% of a coarse area) were predominantly stratiform (>75% by area and >50% by rainfall amount). Precipitating cold clouds (containing ice) with stratiform area fractions <50% were not observed.

During active phases of the intraseasonal oscillation (ISO), the probaility that there were no satellite-observable clouds within a coarse-resolution area of the aircraft sample was practically zero (maximumn mean IR temperature never >286 K). While during suppressed phases the near or total absence of clouds was common. Large rain area with very clod cloud tops were common under active conditions but not under suppressed conditions. The large rain areas with very cold cloud tops (>20% of the coarse area and <235 K mean IR temperature) occurred primarily during the active phase and were 75–100% stratiform by area and >70% by rain fraction. In the suppressed phases, stratiform rain fractions varied widely since most of the rainfall was associated with small precipitation areas.

Although the joint-probability distributions differ substantially between active and suppressed phases of the ISO, the overall precipitation-area size is not strongly related to wind speed, wind direction, or thermodynamic variables, as seen in the storm inflow soundings obtained by the TOGA COARE aircraft.

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