Effect of Cloud Overlap Assumptions in Climate Models on Modeled Earth-Atmosphere Radiative Fields

A new subgrid-scale Stochastic Cloud Generator(SCG),which can produce different overlap assumptions,has been implemented into the NCAR/CAM3 climate model.Four types of overlapping schemes are generated from the SCG,including Maximum Overlap(MO),Random Overlap(RO),Maximum-Random Overlap(MRO),and a recently developed General Overlap(GenO).Using GenO as a reference,the authors studied the sensitivity of radiative fields to cloud overlap changes.Results showed that the total cloud fractions generated by MRO,MO,and RO differed from that of the GenO by-0.012,-0.034 and 0.026,respectively,with MRO being closest to the GenO.The Downwelling Shortwave Radiation(DSR) at the surface was affected by the cloud overlap assumptions significantly.The largest regional differences of DSR are more than 16 W/m2 in the tropical convective area,which is about 8%-12% of the DSR for the GenO there and is statistically significant at the 95% confidence level.The second largest differences of 4-8 W/m2 occurred at middle or higher latitudes where the low cloud covers are often large.However,the differences of outgoing longwave radiation(OLR) among the four schemes are quite smaller,with the largest value over 3-4 W/m2 also located in the tropical convective area.The cloud overlap assumptions can also change the profiles of radiative heating rate and thus change the thermodynamic structure of the atmosphere.It was found that the change in longwave heating rate(with the largest value of about 0.1-0.26 K/d) is about an order higher than that in shortwave heating rate(with the largest value of about 0.01-0.025 K/d),thus becomes the major factor influencing the thermodynamic structure.Further exploration of cloud radiative forcing(CRF) at both the surface and the top of atmosphere(TOA) and column CRF suggests that the cloud overlap assumptions affect energy absorption of the whole atmospheric column at some latitudes,through which the energy distribution at different latitudes can be systematically changed,and finally affect the evolvement of the modeled climate system.