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Cloud Diurnal Variations in the Maritime Continent During Boreal Winter Ming-Dah Chou Department of Atmospheric Sciences National Taiwan University Presentation at the Department of Atmospheric Sciences, National Central University, January 2, 2008. Importance of Cloud Diurnal Variation • Net solar and IR radiative heat balance. Strong diurnal solar heating. • A fundamental mode of variability of the global climate system • A key test of physical parameterization in a global circulation model Objectives of the Study • To understand the diurnal variations of convection over different regions and seasons • To understand the impact of diurnal radiative heating and cooling on convection and clouds Possible mechanisms Land Low-level thermodynamical response to radiative heating cycle. Local topography A. Direct radiation convection interaction (Kraus 1963, Randall et al. 1991) [Nighttime enhanced convection; daytime depressed convection] Ocean B. Cloud versus cloud-free radiation difference in horizontal (Gray and Jacobson 1977) [Nighttime enhanced convection; daytime depressed convection] C. Thermodynamic (RH: Tao et al.(1996); APW: Sui et al.(1997); Chen and Houze 1997) [nighttime enhanced convection] D. Diurnal cycle of SST under weak wind clear sky. (Halpern and Reed 1976; Stramma et al. 1986) [Late afternoon enhanced convection during clear calm day] Japan’s Geostationary Satellite-5 (GMS-5) • Spectral Band & Resolution VIS (0.55 mm~0 .90 mm) IR1(10.5 mm~11.5 mm) IR2(11.5 mm~12.5 mm) IR3(6.5 mm~7.0 mm) • Temporal resolution: • Period of this study: ~1.25 km ~5 km ~5 km ~5 km 1 hour 3 boreal winters DJF Dec 1997-Dec 2000 The brightness temperature, Tbb, of IR2 was used to infer clouds. A smaller Tb indicates a higher cloud fractional cover and a higher cloud height. Definition of Tbb? Seasonal Mean Brightness Temperature September 1997 - February 2003 K GMS data from Central Weather Bureau Standard Deviation of Daily-Mean Brightness Temperature K Percentage Contribution (%) of Different Time Scales to the Total Variance Local Time of Tbb Maximum Local Hours (1) The maximum Tbb in most of the open oceanic regions occurs in daytime hours. (2) There are no open oceanic regions where the maximum Tbb occurs in the early hours after midnight from. Local Time of Tbb Minimum (1) Over dry regions centered at equator, the minimum Tbb occurs from midnight to early morning (0:00-6:00 LT). (2) Over humid regions centered at ~5ºN and south of 10ºS, the minimum Tbb occurs in late afternoon and early evening (14:00-18:00 LT). Synoptic scale Classification into Active and Inactive Phases • Criterion 1: Standard deviation (SD) of the whole 3-year DJF Active events: Tb-ave(Tb)< -0.5*SD Inactive events: Tb-ave(Tb)> +0.5*SD • Criterion 2: The above conditions continued for at least 4 days Synoptic scale Borneo (261.29 3yr mean 13.25 SD) New Guinea (262.78 3yr mean 12.90 SD) Inactive 59 days active 72 days Inactive 52 days active 60 days Diurnal Variations of Tbb in Boreal Winters Units: K Local Hours Diurnal Variations of Tbb in Boreal Winter Oceanic Regions Units: K Local Hours Diurnal Variations of Tbb in Boreal Winter Units: K Local Hours Local Time of Tbb Maximum Local Hours (1) The maximum Tbb in most of the open oceanic regions occurs in daytime hours. (2) There are no open oceanic regions where the maximum Tbb occurs in the early hours after midnight from. The 850-hPa Streamline and Wind Speed m s-1 Mean Diurnal Cycle of the Tbb Anomaly Along the 140E Meridian The ordinate is for local hours. Conclusions The land-sea contract of cloud variation is apparent • Over land, the diurnal cloud variation contributes significantly to the total variance • Over oceans, cloud variations with a time scale > one day contribute significantly to the total variance The large-scale circulation affects the magnitude, but not the phase, of the cloud diurnal variation Conclusions (continued) Three major thermodynamic processes govern the diurnal variation of clouds: • In convective regions, the solar heating causes a cloud maximum in the afternoon. • In convection-suppressed oceanic regions, the cloud-top IR cooling induces a cloud maximum in the early hours after midnight. • In oceanic regions adjacent to land, land-sea breezes and prevailing winds cause a phase propagating several hundred kilometers offshore.