Aerosol Indirect Effects in Low LWP Clouds: Radiative-dynamical Feedbacks

Jonathan Petters Pennsylvania State University
Jerry Harrington Pennsylvania State University
Eugene Clothiaux Pennsylvania State University

Category: Aerosol-Cloud-Radiation Interactions

Working Group: Cloud Life Cycle

We compute cloud radiative heating through static plane-parallel adiabatic cloud layers, finding that integrated longwave radiative cooling and shortwave radiative heating are sensitive to LWP and droplet concentration Nd when LWP < 20 g m^-2. We examine feedbacks between radiative heating and dynamics within low-level low liquid water path (LWP) stratocumulus via large-eddy simulation (LES). We ran six hour nocturnal simulations and sixteen hour simulations of the diurnal cycle for the droplet concentrations of 50, 200, and 1000 cm^-3. Our simulations suggest that for low-level low LWP clouds, increasing droplet concentration leads to decreases in LWP and possible dissipation. Initial entrainment drying leads to more cloud breaks and lower LWPs when droplet concentration is high. Consequently, integrated radiative cooling is lessened for higher droplet concentrations in our nocturnal simulations. During the simulations, entrainment drying suppresses cloud growth when the droplet concentration is high (Nd = 1000 cm^-3) but is not strong enough to counteract cloud growth when the droplet concentration is low (Nd = 50 cm^-3). In our simulations of the diurnal cycle, integrated shortwave warming results in decreases in LWP and cloud fraction lowering. Consequently, radiative cooling decreases and circulations weaken, and LWP decreases further owing to the shortwave heating, creating a negative feedback loop. For Nd = 200 and 1000 cm^-3, integrated longwave cooling is reduced enough during the morning hours such that the cloud layer cannot be maintained against shortwave warming, and the cloud layer dissipates entirely. In contrast, for Nd = 50 cm^-3 the cloud layer is maintained throughout the day.

This poster will be displayed at ASR Science Team Meeting.

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