A CLIMATOLOGY OF MIDLATITUDE CONTINENTAL CLOUDS FROM THE ARM SGP CENTRAL FACILITY: CLOUD FRACTION AND RADIATIVE FORCING

Data collected at the Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility (SCF) are analyzed for determining the monthly and hourly variations of cloud fraction and radiative forcing between January 1997 and December 2002. Cloud fractions are estimated for total cloud cover and for single-layered low (0-3 km), middle (3-6 km), and high clouds (>6 km) using ARM SCF ground-based paired lidar-radar measurements. Shortwave (SW) and longwave (LW) fluxes are derived from up- and down-looking standard precision spectral pyranometers and precision infrared radiometer measurements with the uncertainties of ~10 Wm^-2. The annual averages of total, and single-layered low, middle and high cloud fractions are 0.49, 0.11, 0.03, and 0.17, respectively. Total and low cloud amounts were greatest from December through March and least during the July-August period, high clouds occur more frequently than other types of clouds with a peak in summer. Total and low clouds have less downwelling SW fluxes (151 and 138 Wm^-2) than middle and high clouds (188 and 201 Wm^-2), and more downwelling LW fluxes (349 and 356 Wm^-2) than middle and high clouds (337 and 333 Wm^-2). Low clouds have the largest LW warming effect (55 Wm^-2) and SW cooling effect (-91 Wm^-2) with the maximum values (absolue) in spring and the minimum values in summer, while high clouds have the least LW warming effect (17 Wm^-2) and SW cooling effect (-37 Wm^-2) compared other clouds. A sensitivity study has shown that the total and low cloud transmissions and radiative forcings (CRF) are more sensitivity to cloud fraction than other clouds, where their LW cloud transmission and CRFs increase, and their SW cloud transmissions and CRFs decrease, with increased cloud fraction. An uncertainty analysis demonstrates that the calculated CRFs are not significantly affected by the difference between clear-sky and cloudy conditions, and the uncertainties in data sampling and clear-sky screening.