The diurnal cycle of west Pacific deep convection and its relation to the spatial and temporal variation of tropical MCSs

Infrared (IR) and visible satellite data from the Japanese Geostationary Me teorological Satellite (GMS-4) with 5-km spatial and l-h temporal resolutio n were used to examine the diurnal cycle of deep convection over a sector o f the tropical west Pacific warm pool bounded by 0 degrees-20 degrees N, 14 0 degrees E-180 degrees. Data were analyzed for 45 days of summer from 22 J une to 5 August 1994 and for 65 days of winter between 28 November 1994 and 31 January 1995. Deep convective clouds were identified in IR imagery using brightness tempe rature (T-BB) threshold techniques. Based on previous studies, a -65 degree s C cloud-top T-BB threshold was chosen to isolate pixels containing active , deep convection. Spectral analysis of time series constructed from hourly cold cloud (less than or equal to-65 degrees C) pixel counts revealed a po werful diurnal cycle of deep convection significant at the 95% confidence l evel during summer and winter. Composited hourly statistics of fractional a real cloud cover documented a 0500-0600 local standard time (LST) maximum w ith a 1500-1900 LST minimum of convection for both seasons. Objective analysis techniques were developed to analyze the phase and ampli tude of the diurnal cycle of deep convection and its relation to the satell ite-observed daily spatial and temporal Variation of tropical mesoscale con vective systems (MCSs). Results showed that the diurnal cycle of convective rainfall with an early morning maximum was disproportionately dominated by the largest similar to 10% of MCSs for each time period. While the number of large MCSs increased only slightly throughout nocturnal hours, the area of cold cloud associated with these systems expanded dramatically. An algor ithm called "threshold initiation" showed that all scales of organized, int ensifying deep convection existed at all times of day and night. In additio n, the early morning peak was largely composed of building convection. Cond itional recurrence probabilities of deep convection associated with MCSs we re computed at 24- and 48-h intervals. Results for summer and December 1994 revealed that when early morning convection associated with a large MCS oc curred at any location, the same region contained convection the next morni ng nearly half the time. Convection was less likely at the 48-h point. Thes e results are not consistent with diurnal theories based on sea surface hea ting, afternoon initiation of convection, and nocturnal evolution of mesosc ale convective systems. Findings indicate that the diurnal cycle of deep convective cloud is driven by the internal variation of large clusters. MCSs embedded in cloud cluste rs that exist into or form during the night grow spatially larger and more intense. Some results support direct radiative forcing of clouds and large- scale clear-region radiative destabilization as possible contributors to di urnal convective variability. However, all findings are consistent with the work of Gray and colleagues that emphasizes the role of day-night variatio ns in net tropospheric cooling in clear and longwave cooling in cloudy vers us clear regions as an explanation of the observed daily variation of tropi cal convective rainfall and its significant relationship to organized mesos cale convection.