Radiative Impacts on the Growth of Drops within Simulated Marine Stratocumulus. Part I: Maximum Solar Heating

Abstract

The effects of solar heating at a variety of solar zenith angles (⌰ o ) on the vapor depositional growth of cloud drops, and hence the potential for collection enhancement, is investigated. A large eddy simulation (LES) model is used to predict the evolution of marine stratocumulus clouds subject to changes in ⌰ o . During the course of each simulation, LES output is stored for 600 parcel trajectories and is used to drive an offline microphysical model that includes the influence of radiation on drop growth.Smaller ⌰ o , such as when the sun is overhead, provide strong solar heating, which tends to confine circulations to the cloud layer and leads to long in-cloud residence times for cloud drops. At larger ⌰ o , when solar heating is weak, circulations are stronger and penetrate through the depth of the boundary layer, which causes much shorter in-cloud residence times for cloud drops. Simulations show that this leads to a more rapid collection process in strongly, as compared to weakly solar-heated clouds provided that the liquid water contents of each cloud are similar. When drop vapor growth includes radiative effects, three main results emerge: 1) Solar heating at smaller ⌰ o (0°to 45°) dominates over longwave cooling effects causing a suppression of collection for lower drop concentrations (100 to 200 cm Ϫ3 ). 2) At larger drop concentrations (տ300 cm Ϫ3 ) longwave cooling dominates over solar heating and collection is enhanced. 3) At large ⌰ o (60°to 90°), solar heating is ineffective at modifying the drop size spectrum thus allowing longwave cooling to significantly enhance collection at all drop concentrations above approximately 100 cm Ϫ3.

Radiative–Dynamical Feedbacks in Low Liquid Water Path Stratiform Clouds

“…We initialize the model with random thermal perturbations to an input sounding that produces thick, overcast stratiform clouds (Hartman and Harrington 2005). A low LWP stratiform cloud was created from that sounding by lowering the vapor mixing ratio by 2.75 g kg 21 above the boundary layer inversion.…”

Evaporation and condensation of water mist/cloud droplets with thermal radiation

“…9 and 10 demonstrate the effect of LW radiant emission in enhancing cloud droplet growth, the opposite effect of SW (solar) radiant absorption could be shown by including it [24][25][26] in the radiation term Q r D, with the possibility that SW radiation could change the sign from negative to positive. This would require estimating absorption of SW radiation, which is more complicated just by the stronger spectral variation of water's absorption coefficient in the SW range compared to the LW range and the smaller absorption efficiency for SW radiation compared with the near unity emission efficiency for LW radiation (different absorption and emission efficiencies would need to be evaluated, as noted previously).…”

Long-resident droplets at the stratocumulus top

“…Long-resident droplets also grow on average considerably larger than convective droplets due to condensation induced by longwave radiative cooling, which can speed up rain formation (Roach, 1976;Hartman and Harrington, 2005). In this paper, we isolate the effect of radiation, and investigate how condensation due to radiative cooling alone broadens the cloud-top DSD.…”

“…The parcels vary their position following the flow dynamics, so that the TEM can be considered as a Lagrangian model. Harrington (2000) and Hartman and Harrington (2005) used a TEM driven by a LES to show that stratocumulus drizzle is sped up by radiative cooling at parcels that stay close to the top. Similarly, Magaritz et al (2009) used a TEM driven by two-dimensional synthetic turbulence and found that drizzle is initiated in lucky parcels that stay for a long time close to the cloud top.…”