Sensitivity of aerosol and cloud effects on radiation to cloud types: comparison between deep convective clouds and warm stratiform clouds over one-day period

S S Lee,L J Donner,V T J Phillips

doi:10.5194/acp-9-2555-2009

Abstract

Abstract. Cloud and aerosol effects on radiation in two contrasting cloud types, a deep mesoscale convective system (MCS) and warm stratocumulus clouds, are simulated and compared. At the top of the atmosphere, 45–81% of shortwave cloud forcing (SCF) is offset by longwave cloud forcing (LCF) in the MCS, whereas warm stratiform clouds show the offset of less than ~20%. 28% of increased negative SCF is offset by increased LCF with increasing aerosols in the MCS at the top of the atmosphere. However, the stratiform clouds show the offset of just around 2–5%. Ice clouds as well as liquid clouds play an important role in the larger offset in the MCS. Lower cloud-top height and cloud depth, characterizing cloud types, lead to the smaller offset of SCF by LCF and the offset of increased negative SCF by increased LCF at high aerosol in stratocumulus clouds than in the MCS. Supplementary simulations show that this dependence of modulation of LCF on cloud depth and cloud-top height is also simulated among different types of convective clouds.

Highlights

Among the many atmospheric processes that play a role in the Earth’s radiation budget, clouds are among the most important and difficult to understand
Larger counterbalance in deep convective clouds than in shallow stratiform clouds at the top of the atmosphere is observed by Ramanathan et al (1989). They found that shortwave cloud forcing (SCF) was substantially counterbalanced by the reduction of outgoing LW in deep convective regions mainly associated with Asian and Indian Monsoon, storm tracks, and ITCZ
In SHALLOW, less than 20% of SCF is offset by longwave cloud forcing (LCF), whereas, in deep MCS (DEEP), the offset is 45% at high aerosol and as much as 81% at low aerosol at the top of the atmosphere

Summary

Introduction

Among the many atmospheric processes that play a role in the Earth’s radiation budget, clouds are among the most important and difficult to understand. Clouds affect the energy balance in the atmosphere by regulating the flow of radiation at the top of the atmosphere. This regulation process is complicated by cloud microphysics involving numerous processes among different types of hydrometeors such as droplets, ice crystals, rain, snow, and hail. Increasing aerosols with industrialization are known to change cloud microphysics. Increasing aerosols decrease droplet size and increase cloud albedo (first aerosol indirect effect) and possibly suppress precipitation and alter cloud lifetime (second aerosol indirect effect). Uncertainties of radiative forcing associated with aerosol indirect effects are comparable to radiative forcing by an anthropogenic increase in green house gases (Ramaswamy et al, 2001)

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