Importance of Accurate Liquid Water Path for Estimation of Solar Radiation in Warm Boundary Layer Clouds: An Observational Study

A 1-yr observational study of overcast boundary layer stratus at the U.S. Department of Energy Atmospheric Radiation Measurement Program Southern Great Plains site illustrates that surface radiation has a higher sensitivity to cloud liquid water path variations when compared to cloud drop effective radius variations. The mean, median, and standard deviation of observed cloud liquid water path and cloud drop effective radius are 0.120, 0.101, 0.108 mm and 7.38, 7.13, 2.39 μm, respectively. Liquid water path variations can therefore cause 3 times the variation in optical depth as effective radius—a direct consequence of the comparative variability displayed by the statistics of the two parameters. Radiative transfer calculations demonstrate that, over and above the impact of higher liquid water path variability on optical depth, normalized cloud forcing is 2 times as sensitive to liquid water path variations as it is to effective radius variations. Consequently, radiative transfer calculations of surface flux using observed liquid water paths and a fixed effective radius of 7.5 μm have a 79% correlation with observed values. This higher sensitivity of solar flux to liquid water path is a result of the regimes of natural occurrence of cloud liquid water paths and cloud drop effective radii.