The effect of temperature and humidity fluctuations on the liquid water path of non-precipitating closed cell stratocumulus clouds.

The spatial variability of the liquid water path (LWP) is analyzed from a large-eddy simulation of the diurnal cycle of stratocumulus clouds as observed during FIRE I. In stratocumulus clouds the temperature and the total specific humidity cannot fluctuate independently but are tightly connected to fluctuations in the liquid water potential temperature. If the latter are relatively small, a strong positive correlation between the temperature and the total specific humidity can be expected. The effect of temperature fluctuations on the magnitude of liquid water fluctuations must be considered to properly compute the LWP distribution in stratocumulus clouds.

The simulated stratocumulus cloud fields are used to compute the albedo inhomogeneity factor $chi$ according to the effective thickness approach. During the day the mean LWP decreases due to shortwave radiative warming. Also, the probability density function for the LWP becomes positively skewed due to cumuli that have their base well below the mean stratocumulus cloud base height. For this situation the inhomogeneity factor $chi$ obtains a minimum value of about 0.85. For a solid (unbroken) cloud with an assumed Gaussian distribution for the optical depth we find that the minimum albedo bias factor will be about 0.8. This minimum value for the inhomogeneity factor, and the LES results are larger than reported from FIRE I observations by Cahalan et al. (1994).