Sensitivity of tropical convection to sea surface temperature in the absence of large-scale flow

The response of convection to changing sea surface temperature (SST) in the absence of large-scale Bow is examined, using a three-dimensional cloud re solving model. The model includes a five-category bulk microphysical scheme representing snow, ice, graupel, rain, and cloud amounts in addition to an interactive radiation scheme for the shortwave and infrared. Long integrat ions are made to achieve a radiative-convective equilibrium state for SSTs of 298, 300, and 302 K, for which cloud and convection statistics are analy zed. The main conclusion of the paper is that, despite significant temperature s ensitivities in many of the conversion terms between bulk water categories, convection is very insensitive to changing SST in the absence of large-sca le how. This is a result of the moist adiabatic temperature profile that th e tropical atmosphere is constrained to take. A parcel of air rising throug h a deep convective cloud experiences approximately the same range of tempe ratures but at higher altitudes as SST increases. Thus the vertical profile s of cloud fraction and other cloud-related statistics are simply shifted i n height, but not changed in overall magnitude. The small changes in cloud properties that do occur lead to a small reducti on in cloud fraction as SST increases. This appears to be due to an increas e in graupel amounts with respect to snow, giving smaller cloud fractions s ince graupel has a higher fall velocity. The radiative effects of the chang es in atmospheric properties are examined and it is found that the model at mosphere exhibits no supergreenhouse effect since atmospheric relative humi dity is not altered significantly by the SST changes. The water vapor feedb ack effect is largely canceled by the change in temperature. Clouds have a negligibly small, but highly nonlinear, feedback in the model climate, in t he absence of large-scale flow.