Abstract
Summertime continental shallow cumulus clouds (ShCu) are detected using Geostationary Operational Environmental Satellite (GOES)-16 reflectance data, with cross-validation by observations from ground-based stereo cameras at the Department of Energy Atmospheric Radiation Measurement Southern Great Plains site. A ShCu cloudy pixel is identified when the GOES reflectance exceeds the clear-sky surface reflectance by a reflectance detection threshold of ShCu, ΔR. We firstly construct diurnally varying clear-sky surface reflectance maps and then estimate the ∆R. A GOES simulator is designed, projecting the clouds reconstructed by stereo cameras towards the surface along the satellite’s slanted viewing direction. The dynamic ShCu detection threshold ΔR is determined by making the GOES cloud fraction (CF) equal to the CF from the GOES simulator. Although there are temporal variabilities in ΔR, cloud fractions and cloud size distributions can be well reproduced using a constant ΔR value of 0.045. The method presented in this study enables daytime ShCu detection, which is usually falsely reported as clear sky in the GOES-16 cloud mask data product. Using this method, a new ShCu dataset can be generated to bridge the observational gap in detecting ShCu, which may transition into deep precipitating clouds, and to facilitate further studies on ShCu development over heterogenous land surface.
Highlights
The assumption for this algorithm is that land surfaces are stable and slow-varying over a restricted time period and that the signal measured by the Geostationary Operational Environmental Satellite (GOES) instrument during this period can be considered only a function of the scene-viewing geometry and cloudiness [54]
We firstly show the results using the estimated dynamic ∆R based on each of the satellite images. The estimation of these dynamic ∆Rs depends on the availability of observations from ground-based stereo cameras as a constraint
If there are no ground-based observations, can we use a constant threshold ∆R to detect shallow cumulus clouds (ShCu)? In the following, we pursue an optimal constant reflectance threshold. Based on this optimal constant reflectance threshold, we further provide the cloud size distribution and the diurnal cycles of cloud fraction and cloud size
Summary
Continental shallow cumulus clouds (ShCu) are tightly coupled with the land surface and have strong impacts on the surface energy partition and the turbulent transport of momentum, heat, and moisture [1,2,3,4,5,6]. They are important for their seeding role in transitioning into hot-tower deep convective clouds with heavy precipitation [7]. Systematic model biases in the surface energy balance and precipitation are often attributed to the parameterized processes associated with ShCu, e.g., the shallow-todeep convection transition and the interactions among land surface, atmospheric boundary layer, clouds, and precipitation [15,16]
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