The Diurnal Cycle of West Pacific Deep Convection and Its Relation to the Spatial and Temporal Variation of Tropical MCSs

Abstract

Infrared (IR) and visible satellite data from the Japanese Geostationary Meteorological Satellite (GMS-4) with 5-km spatial and 1-h temporal resolution were used to examine the diurnal cycle of deep convection over a sector of the tropical west Pacific warm pool bounded by 0°?20°N, 140°E?180°. Data were analyzed for 45 days of summer from 22 June 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 temperature (TBB) threshold techniques. Based on previous studies, a ?65°C cloud-top TBB threshold was chosen to isolate pixels containing active, deep convection. Spectral analysis of time series constructed from hourly cold cloud (??65°C) pixel counts revealed a powerful diurnal cycle of deep convection significant at the 95% confidence level during summer and winter. Composited hourly statistics of fractional areal cloud cover documented a 0500?0600 local standard time (LST) maximum with a 1500?1900 LST minimum of convection for both seasons. Objective analysis techniques were developed to analyze the phase and amplitude of the diurnal cycle of deep convection and its relation to the satellite-observed daily spatial and temporal variation of tropical mesoscale convective systems (MCSs). Results showed that the diurnal cycle of convective rainfall with an early morning maximum was disproportionately dominated by the largest ?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 algorithm called ?threshold initiation? showed that all scales of organized, intensifying deep convection existed at all times of day and night. In addition, the early morning peak was largely composed of building convection. Conditional recurrence probabilities of deep convection associated with MCSs were computed at 24- and 48-h intervals. Results for summer and December 1994 revealed that when early morning convection associated with a large MCS occurred at any location, the same region contained convection the next morning nearly half the time. Convection was less likely at the 48-h point. These results are not consistent with diurnal theories based on sea surface heating, afternoon initiation of convection, and nocturnal evolution of mesoscale 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 clusters 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 diurnal convective variability. However, all findings are consistent with the work of Gray and colleagues that emphasizes the role of day?night variations in net tropospheric cooling in clear and longwave cooling in cloudy versus clear regions as an explanation of the observed daily variation of tropical convective rainfall and its significant relationship to organized mesoscale convection.