Abstract
Abstract. A statistical cirrus cloud scheme that accounts for mesoscale temperature perturbations is implemented in a coupled aerosol and atmospheric circulation model to better represent both subgrid-scale supersaturation and cloud formation. This new scheme treats the effects of aerosol on cloud formation and ice freezing in an improved manner, and both homogeneous freezing and heterogeneous freezing are included. The scheme is able to better simulate the observed probability distribution of relative humidity compared to the scheme that was implemented in an older version of the model. Heterogeneous ice nuclei (IN) are shown to decrease the frequency of occurrence of supersaturation, and improve the comparison with observations at 192 hPa. Homogeneous freezing alone can not reproduce observed ice crystal number concentrations at low temperatures (<205 K), but the addition of heterogeneous IN improves the comparison somewhat. Increases in heterogeneous IN affect both high level cirrus clouds and low level liquid clouds. Increases in cirrus clouds lead to a more cloudy and moist lower troposphere with less precipitation, effects which we associate with the decreased convective activity. The change in the net cloud forcing is not very sensitive to the change in ice crystal concentrations, but the change in the net radiative flux at the top of the atmosphere is still large because of changes in water vapor. Changes in the magnitude of the assumed mesoscale temperature perturbations by 25% alter the ice crystal number concentrations and the net radiative fluxes by an amount that is comparable to that from a factor of 10 change in the heterogeneous IN number concentrations. Further improvements on the representation of mesoscale temperature perturbations, heterogeneous IN and the competition between homogeneous freezing and heterogeneous freezing are needed.
Highlights
Cirrus clouds cover about 30% of the Earth’s area (Wang et al, 1996; Rossow and Schiffer, 1999; Wylie and Menzel, 1999) and are important in maintaining the global radiation balance (Ramanathan and Collins, 1991)
Homogeneous freezing occurs through the freezing of liquid solutions such as sulfate droplets (Koop et al, 2000), and this usually occurs at low temperature and high relative humidity over ice (RHi)
The representation of subgrid-scale fluctuations of temperature, humidity, and cooling rates that are believed to control the cirrus microphysical formation process are highly simplified in these studies, since the effects of subgrid-scale fluctuations are only represented in the calculation of ice crystal number concentrations and cloud fraction only depends on the grid-mean relative humidity
Summary
Cirrus clouds cover about 30% of the Earth’s area (Wang et al, 1996; Rossow and Schiffer, 1999; Wylie and Menzel, 1999) and are important in maintaining the global radiation balance (Ramanathan and Collins, 1991). Global models have been used recently to study the effect of homogeneous and heterogeneous freezing on cirrus cloud properties (Lohmann and Karcher, 2002; Hendricks et al, 2005; Lohmann et al, 2008; Liu et al, 2009) In these studies, the individual GCMs were updated to allow supersaturation with respect to ice, but cloud fraction was still diagnosed based on grid-mean relative humidity and the same humidity was assumed both inside and outside of cirrus clouds. The representation of subgrid-scale fluctuations of temperature, humidity, and cooling rates that are believed to control the cirrus microphysical formation process are highly simplified in these studies, since the effects of subgrid-scale fluctuations are only represented in the calculation of ice crystal number concentrations and cloud fraction only depends on the grid-mean relative humidity.
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