Abstract
Abstract. An artificial neural network cloud classification scheme is combined with A-train observations to characterize the physical properties and radiative effects of marine low clouds based on their morphology and type of mesoscale cellular convection (MCC) on a global scale. The cloud morphological categories are (i) organized closed MCC, (ii) organized open MCC and (iii) cellular but disorganized MCC. Global distributions of the frequency of occurrence of MCC types show clear regional signatures. Organized closed and open MCCs are most frequently found in subtropical regions and in midlatitude storm tracks of both hemispheres. Cellular but disorganized MCC are the predominant type of marine low clouds in regions with warmer sea surface temperature such as in the tropics and trade wind zones. All MCC types exhibit a pronounced seasonal cycle. The physical properties of MCCs such as cloud fraction, radar reflectivity, drizzle rates and cloud top heights as well as the radiative effects of MCCs are found highly variable and a function of the type of MCC. On a global scale, the cloud fraction is largest for closed MCC with mean cloud fractions of about 90%, whereas cloud fractions of open and cellular but disorganized MCC are only about 51% and 40%, respectively. Probability density functions (PDFs) of cloud fractions are heavily skewed and exhibit modest regional variability. PDFs of column maximum radar reflectivities and inferred cloud base drizzle rates indicate fundamental differences in the cloud and precipitation characteristics of different MCC types. Similarly, the radiative effects of MCCs differ substantially from each other in terms of shortwave reflectance and transmissivity. These differences highlight the importance of low-cloud morphologies and their associated cloudiness on the shortwave cloud forcing.
Highlights
Marine stratocumulus (Sc) clouds are an important component of the climate system as they cover vast areas of the Earth’s ocean surface and affect the radiation balance of the Earth
We use an artificial neural network cloud classification scheme to identify and discriminate marine low clouds based on their morphology
The cloud morphology involves the type of mesoscale cellular convection (MCC) and the level of organization within the cloud field
Summary
Marine stratocumulus (Sc) clouds are an important component of the climate system as they cover vast areas of the Earth’s ocean surface and affect the radiation balance of the Earth. Closed MCC are the dominant type of low-cloud morphology in the Arctic Ocean east of Greenland (i.e., the Greenland Sea) This result should be taken with caution due to the inherent uncertainty in MODIS cloud retrievals (i.e., cloud optical depth and effective radius) at high latitudes mentioned in Sect. In order to derive reliable statistics of the effects of different MCC types on the microphysical properties and radiative effects of low clouds, A-train observation are mapped onto the MCC type classification based on MODIS cloud scene data as discussed in Sect. Besides the differences in the low-cloud fraction and precipitation characteristics, the open and closed MCC regions differ considerably in terms of the instantaneous reflected shortwave radiation and TOA cloud radiative forcing (CRF). All statistics are based on 1 year of combined MODIS and CloudSat/CALIPSO observations from 2008
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