Role of land surface in controlling daytime cloud amount: Two case studies in the GCIP‐SW area

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Abstract
Two summertime cases are studied, in which fields of low cumulus clouds develop out of an initially clear sky under quiescent synoptic scale conditions. Both sites fall within the southwest study area of the GEWEX (Global Energy and Water cycle Experiment) Continental‐Scale International Project (GCIP). The cases are modeled using a one‐dimensional soil‐vegetation‐boundary layer model known as the parameterization for land‐atmosphere‐cloud exchange (PLACE). PLACE accurately predicts cloud onset and amount in both cases, one from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) in Kansas and one from Oklahoma. Sensitivity tests with PLACE show further that in the Oklahoma case, morning cloud amount is highly sensitive to land surface characteristics, whereas in the FIFE case, much less sensitivity is found, primarily due to differences in the predawn atmospheric thermodynamic structure through which the boundary layer must grow. Analysis of observational data, including cloud amount inferred from GOES visible data and vegetation index from the advanced very high resolution radiometer (AVHRR) data confirm that the PLACE model predictions of sensitivities and of the sensitivity differences between cases are correct. In the Oklahoma case, atmospheric conditions and soil moisture are relatively uniform across the study area, but a significant gradient of satellite‐derived vegetation index is observed. Clouds form quickly due to a very shallow nocturnal inversion, with strong lapse conditions above. They form first over the hotter, more sparsely vegetated areas. Over areas covered with deciduous forest, clouds were observed to form 1 to 2 hours later due to the suppression of vertical mixing caused by increased latent heat release. In the FIFE case, because of a deep preexisting stable layer in the atmosphere the primary cause of spatial variations in morning cloud amount appears to be the amount of moisture in low levels of the atmosphere. In the afternoon, once the stable layer has been eroded, the FIFE study area begins to demonstrate more sensitivity of cloud amount to surface characteristics.