The source of discrepancies in aerosol–cloud–precipitation interactions between GCM and A-Train retrievals

Takuro Michibata,Toshihiko Takemura,Yousuke Sato,Kentaroh Suzuki

doi:10.5194/acp-16-15413-2016

Abstract

Abstract. Aerosol–cloud interactions are one of the most uncertain processes in climate models due to their nonlinear complexity. A key complexity arises from the possibility that clouds can respond to perturbed aerosols in two opposite ways, as characterized by the traditional “cloud lifetime” hypothesis and more recent “buffered system” hypothesis. Their importance in climate simulations remains poorly understood. Here we investigate the response of the liquid water path (LWP) to aerosol perturbations for warm clouds from the perspective of general circulation model (GCM) and A-Train remote sensing, through process-oriented model evaluations. A systematic difference is found in the LWP response between the model results and observations. The model results indicate a near-global uniform increase of LWP with increasing aerosol loading, while the sign of the response of the LWP from the A-Train varies from region to region. The satellite-observed response of the LWP is closely related to meteorological and/or macrophysical factors, in addition to the microphysics. The model does not reproduce this variability of cloud susceptibility (i.e., sensitivity of LWP to perturbed aerosols) because the parameterization of the autoconversion process assumes only suppression of rain formation in response to increased cloud droplet number, and does not consider macrophysical aspects that serve as a mechanism for the negative responses of the LWP via enhancements of evaporation and precipitation. Model biases are also found in the precipitation microphysics, which suggests that the model generates rainwater readily even when little cloud water is present. This essentially causes projections of unrealistically frequent and light rain, with high cloud susceptibilities to aerosol perturbations.

Highlights

Aerosol particles play an important indirect role in the climate system by modifying cloud micro- and macrophysical properties, which is referred to as aerosol–cloud interactions (Twomey, 1977; Albrecht, 1989)
We apply a threshold of R > 0.14 mm day−1, which is equivalent to a radar reflectivity of −15 dBZe (Terai et al, 2015), for precipitation in the model to enable a fair comparison with satellite observations
We explored the source of discrepancy in the aerosol– cloud–precipitation interaction for warm clouds between an aerosol–climate model and A-Train satellite retrieval

Summary

Introduction

Aerosol particles play an important indirect role in the climate system by modifying cloud micro- and macrophysical properties, which is referred to as aerosol–cloud interactions (Twomey, 1977; Albrecht, 1989). An increase in aerosols supplies more numerous cloud condensation nuclei, resulting in numerous and smaller cloud droplets leading to brighter clouds, which is known as the “albedo effect” (Twomey, 1977). Smaller cloud droplets suppress the onset of precipitation in warm clouds due to the less efficient collision– coalescence process, resulting in a longer cloud lifetime, which is known as the “lifetime effect” (Albrecht, 1989). One of the most important factors that quantify the magnitude of aerosol–cloud interactions is the response of the cloud liquid water path (LWP) to aerosol perturbations. This factor characterizes aerosol impacts on the global hydro-

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Results

Conclusion