Future Changes in Midlatitudes Convective Available Potential Energy (CAPE) and their Relationship with Moist Static Energy

Abstract:

Convective available potential energy (CAPE), the integral buoyancy of an air parcel given undiluted ascent, is a metric strongly associated with severe convective weather events. CAPE is assumed to increase in future, warmer conditions, with the strong effect of increased surface specific humidity partially offset by a reduced environmental lapse rate. Theory developed for strongly convecting regimes suggests that CAPE should increase following the Clausius-Clapeyron (C-C) relationship, by ~6%/K. We use high-resolution model output (4 km dynamically downscaled WRF over North America) to investigate whether this relationship holds in the summertime mid-latitudes. We show that mean CAPE increases are substantially larger than C-C scaling, i.e. that lapse rate changes are smaller than assumed. Entrainment models based on zero-buoyancy assumptions, which relate CAPE to the mid-tropospheric deficits in specific humidity, also cannot capture the midlatitudes CAPE distribution. Instead, CAPE appears tightly correlated with the ‘MSE deficit’, the difference between specific humidity at surface and at 650 hPa. In all model simulations and in radiosonde observations, atmospheric profiles develop appreciable CAPE only when surface MSE exceeds that in mid-troposphere; beyond this point, mean CAPE increases as ~13% of the rise in surface MSE. This robust relationship suggests that future changes in mid-latitudes CAPE can be reproduced with a relatively simple transformation of the present-day distribution. We show that the high tail of the future model CAPE distribution (> 1000 J/kg) is well-captured by adjusting present-day profiles using three parameters derived from model output: mean temperature increases at surface and upper troposphere (4.24 K and 4.96 K, respectively) and a diurnally varying surface RH adjustment (about -2%). We also discuss differences between midlatitudes and tropics, and the role of general circulation in setting the observed CAPE-MSE relationship.