Transition as a Constraint on Entrainment in Climate Models

5 Properties of the transition to strong deep convection, as previously observed in satellite pre6 cipitation statistics, are analyzed using parcel stability computations and a convective plume 7 velocity equation. A set of alternative entrainment assumptions yield very different char8 acteristics of the deep convection onset boundary (here measured by conditional instability 9 and plume vertical velocity) in a bulk temperature-water vapor thermodynamic plane. In 10 observations the threshold value of column water vapor above which there is a rapid increase 11 in precipitation, referred to as the critical value, increases with temperature, but not as 12 quickly as column saturation, and this can be matched only for cases with sufficiently strong 13 entrainment. This corroborates the earlier hypothesis that entraining plumes can explain 14 this feature seen in observations, and places bounds on the lower tropospheric entrainment. 15 Examination of a simple interactive entrainment scheme in which a minimum turbulent en16 trainment is enhanced by a dynamic entrainment (associated with buoyancy-induced vertical 17 acceleration) shows that the deep convection onset curve is governed by the prescribed min18 imum entrainment. Results from a 0.5 degree resolution version of the Community Climate 19 System Model, whose convective parameterization includes substantial entrainment, yield 20 a reasonable match to satellite observations in several respects. Temperature-water vapor 21 dependence is seen to agree well with the plume calculations and with offline simulations per22 formed using the convection scheme of the model. These findings suggest that the convective 23 transition characteristics, including the onset curve in the temperature-water vapor plane, 24 can provide a substantial constraint for entrainment assumptions used in climate model deep 25 convective parameterizations. 26