Radiative Cooling of Stratocumulus

Theory predicts that the infrared (i.r.) cooling rate at stratocumulus (Sc) cloud top could be as much as -40 C/hr depending on the cloud-top LWC and the above cloud top environment. The actual decrease in temperature in Sc due to i.r. flux divergence at cloud top can obviously not reach -40 C, because much smaller temperature decreases at cloud top will cause negative buoyancy and descend of the cooled air so that large cooling values are not reached at cloud top. Thus the total effect of i.r. cooling as well as the effect of evaporative cooling on Sc depends on how the cooled air is distributed within the Sc.

How such cooling expresses itself in Sc is still not fully understood; even though, the cooling can cause negative buoyancy and affect the evolution of the Sc. A recent POST (Physics of Stratocumulus Top; Sc field study) data analysis (Gerber et al., 2013, JGR) with high data-rate aircraft measurements of temperature and microphysics in Sc provided some new insight on this cooling behavior. Some Sc showed entrained parcels with reduced LWC that appeared to contain air cooled primarily by radiative cooling. They also showed that entrained parcels with decreased temperature did not always have reduced LWC.

Here we describe results of a further look at POST data where we estimate the average cooling rate near cloud top based on the measured integral temperature decrease per aircraft flight distance near cloud top times the mean vertical velocity in the entrained parcels. This permits calculation of the mean temperature cooling rate (C0/hr) near cloud top, and permits calculation of i.r. flux divergence if the cooling is dominated by this process. The implications of entrained parcels with and without reduced LWC are described. Further a comparison is made between this calculation and the i.r. flux divergence measured directly on the POST aircraft with broadband i.r. radiometers.