Spatial and temporal scales of precipitating tropical cloud systems.

摘要

Precipitation, radiative forcing, and aerosol scavenging in tropical cloud systems over the wintertime Indian Ocean are examined in satellite observations and global atmospheric simulations. Measurements of surface rain rate and top-of-atmosphere radiative fluxes from the TRMM satellite, as well as brightness temperature measurements from the METEOSAT-3 satellite, are used to identify the boundaries of cloud systems, track their evolution, and determine the spatial and temporal scales of cloud thermodynamic forcing. The resulting quantitative, statistical description of monsoonal cloud systems is compared with simulated cloud systems in the NCAR CCM3 model.; Monsoonal clouds span a spectrum of spatial scales from smaller than 25 km2 to greater than 107 km2. Atmospheric heating owing to precipitation and the cloud greenhouse effect, as well as surface cooling owing to cloud albedo, increases with the spatial scale of cloud systems. As a result, thermodynamic forcing of the monsoonal environment is dominated by the contribution from giant semi-permanent decks of overcast cloud that persist for days to weeks. Embedded within such cloud decks are numerous rain cells reaching up to 1 million square-kilometers because deep convection organizes into clusters of narrow overturning cells attached to a broad stratiform region of precipitation. A relatively few such mesoscale convective systems are greater than 105 km2, yet are responsible for up to 70% of monsoonal precipitation. In contrast, simulated cloud systems in the model gently precipitate throughout their duration and everywhere within their boundaries. The model lacks a process that acts to organize convection into mesoscale episodic structures.; Precipitation is the principal means by which particulate pollution is removed from the atmosphere. The effect of model biases in the distribution of precipitation is tested by integrating satellite precipitation measurements into the MATCH chemical transport model. Mesoscale convective systems in the equatorial Indian Ocean are a substantial barrier to the transport of aerosols from South Asia to the Southern Hemisphere. Using observations of the spatial coverage of precipitation in the model reduces the amount of South Asian aerosol transported to the remote Northern Hemisphere by more than a factor of 2 compared to a simulation using model derived precipitation.