High-Resolution Clouds and Radiative Fluxes from Satellites: Transferability of Methods and Application to Monsoon Regions
Pinker, Rachel T.
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High-resolution information on clouds and radiative fluxes is produced for the Indian and African monsoon regions of interest to the GEWEX Project as articulated under the Coordinated Energy and Water Cycle Observations Project (CEOP). Such data are needed to provide forcing parameters for regional climate models, to evaluate them, and to facilitate their transferability to various climatic regions. Emphasis is placed on capturing the small-scale spatial variability and the diurnal cycle of cloud systems and on improving flux retrievals under the challenging conditions of high elevation and abundant aerosol loads that are characteristic of the various monsoon regions. Once developed, the data are applied to several issues investigated under CEOP and related to hydro-climate and aerosols. Documentation of the diurnal cycle of clouds and convection throughout the progression of the Indian monsoon has been limited due to lack of hourly satellite data over the region prior to 1998. This study adds to the base of knowledge by contrasting the diurnal cycle of clouds and convection in six diverse sectors of the Indian monsoon region and compositing the data for the pre-, peak-, and post-monsoon seasons to better understand the evolution of the monsoon. Comparison of satellite-observed clouds to model-predicted values points out model deficiencies in simulating clouds during the peak-monsoon season and at locations with elevated terrain. The high-resolution cloud information and cloud optical depth data derived with the radiative flux inference scheme are used to re-evaluate the "Elevated Heat Pump" (EHP) hypothesis. The hypothesis predicts early initiation and enhancement of monsoon precipitation in northern India and the Bay of Bengal due to anomalous warming caused by high aerosol loads in the Indo-Gangetic Basin. Newly derived information on convection is used to study the contrast in precipitation patterns during years with high and low aerosol loads. Evidence of the EHP effect is not found. This may be attributed to aerosol indirect effects or air-sea interactions which are not accounted for in the model simulations that were used to develop the hypothesis. Experiments are conducted with different aerosol treatments in the radiative flux inference scheme over Africa with the goal of determining whether using observed aerosol inputs can improve on fluxes retrieved with climatological aerosol values. This question is pertinent to the African Monsoon Multidisciplinary Analysis (AMMA) program, a subprogram of CEOP, which seeks to improve prediction of the West African Monsoon. The radiation component of the surface forcing database used for all AMMA land surface models overestimates clear-sky radiation under high aerosol loads due to poor representation of aerosols. The experiments show that flux retrievals improve when observed aerosol values are used, but biases are reduced even more significantly when aerosol absorbing properties are incorporated into the inference scheme as well. The improved scheme is then used to study the spatial and seasonal variations in downwelling surface shortwave flux and surface albedo over the African continent.