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thermodynamic measurements at tropopause altitudes (between 12-20km) over long durations (at least 12 hours every week) and over long distances (an east-west trajectory of about 5,000-10,000km to span the entire Pacific) under a variety of weather conditions, including highly disturbed tropical weather conditions. High-altitude aircraft measurements are the best platforms for undertaking these measurements. Objectives of the Measurements -Use seasonal and interannual variations, including El Nino variations, in the observed radiative energy fluxes and microphysical parameters to infer the nature of the coupling between SST, the strength of convection, the water vapor greenhouse effect, and the cloud-radiative forcing field. -Validate and advance the state-of-the-art treatment of cloud-radiation interactions in coupled ocean/atmosphere models. -Estimate the three-dimensional structure of the atmosphere cloud-radiative forcing field on sptial scales renging from few tens to thousand of kilometers. This estimate will be used to understand the relationship between the large-scale moisture and radiative heating fields and the subgrid-scale variation in the cloud parameters. This analysis will also be the fundametnal basis for improving the GCM parameterization. Specific Scientific Issues and Questions Greenhouse effect of the tropical troposphere: In order to understand the relative role of thermodynamics and convection in governing the H2O greenhouse effect, we need to measure the radiation trapped within the lower (0-3km), middle (3-10km ), and upper (10-18km) troposphere in convectively disturbed and undisturbed conditions. -What is the relationship between surface radiative heating (by H2O greenhouse effect) and atmopheric longwave cooling? -Does the atmospheric longwave cooling exhibit the same thermodynamic dependence as seen in the Top of the Atmosphere (TOA) greenhouse effect by Raval and Ramanathan? -Does the water vapor continuum greenhouse effect have a nonlinear dependence on surface temperature? -Does the clear-sky radiative cooling change systematically with SSTs in the descending branch of the Walker and Hadley cells? Response of cirrus clouds to surface warming: There is a controversy whether themid to upper troposphere will dry out or moisten with a warming of SSTs. The controversy is fueled by lack of water vapor data in the region of interest. We need accuracte mapping of the water vapor distribution in the equatorial Pacific region during all phases of convection. -Do cirrus clouds moisten or desiccate the upper troposphere and lower stratosphere? -Are optically thin cirrus clouds lifted radiatively into the lower stratosphere where they can evaporate and moisten the atmosphere? Response of cirrus clouds to surface warming: It is now well established that during an El Nino warming of the central and eastern Pacific , thick cirrus clouds form over the region of warming and reduce solar radiation reaching the ocean surface , thus limiting the warming. This natural experiment may hold the key to unravelling the cloud-climate feedback problem. -What is the correlation between SSTs, cirrus spatial extent, and thickness? -What is the dependence of cirrus particle distribution and reflectivities on SSTs, cirrus cloud tops, and water vapor transport into the region of convection? -What is the link between the cirrus radiative heating and the upper-level wind divergence?
