What Controls Stratocumulus Radiative Properties? Lagrangian Observations of Cloud Evolution
Robert Pincus, Marcia B. Baker, and Christopher S. Bretherton
Journal of the Atmospheric Sciences, September 1997
Volume 54, pages 2215-2236. (© 1997 American Meteorological Society)

Abstract

Marine stratocumulus clouds have a large impact on the Earth's radiation budget. Their optical properties vary on two distinct time scales, one associated with the diurnal cycle of solar insolation, and another with the downstream transition to trade cumulus. Hypotheses regarding the control of cloud radiative properties fall broadly into two groups: those focussed on the effects of precipitation, and those concerned with the environment in which the clouds evolve. Reconciling model results and observations in an effort to develop parameterizations of cloud optical properties is difficult because marine boundary layer clouds are not in equilibrium with their local environment.

We describe a new technique for the observation of boundary layer cloud evolution in a moving or Lagrangian frame of reference. Blending satellite imagery and gridded environmental information, the method provides a time series of the environmental conditions to which the boundary layer is subject and the properties of clouds as they respond to external forcings. We combine the technique with in situ observations of precipitation off the coast of California, and compare the downstream evolution of cloud fraction in five cases which were observed to be precipitating with three cases which were not. In this small data set cloud fraction remains almost uniformly high, and there is no relationship between the presence of precipitation and the evolution of cloud fraction on one- and two-day time scales.

Analysis of a large number of examples shows that clouds in this region have a typical pattern of diurnal evolution such that clouds which are optically thicker than about 10 in the morning are unlikely to breakup over the course of the day, but will instead show a large diurnal cycle in optical depth. Morning cloud optical thickness and the resultant susceptibility to breakup have a much larger impact on diurnally averaged cloud radiative forcing than do diurnal variations in cloud properties. Cloud response is significantly correlated with lower tropospheric temperature stratification at all times, though the best correlation exists when cloud response lags stability by at least 16 hours. Sea surface temperature is also correlated with cloud properties during the period in which cloud response is measured and the 12 hours prior. We suggest that sea surface temperature plays two competing roles in determining boundary layer cloudiness, with rapid changes in SST promoting cloudiness on short time scales but tending to lead to a more rapid transition to the trade cumulus regime.

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