PNNL

Abstract

Uncertainties associated with the response of shallow clouds to global warming remain a great challenge for climate projection. Due to the small size of these clouds, parameterizations are required to represent them in both current- and next-generation climate models. We present a quantitative evaluation of several important assumptions used in both mass-flux and Assumed-PDF Higher-Order Closure (AP-HOC) parameterizations of shallow convection. We use large-eddy simulations (LESs) of four different shallow convection regimes (continental and marine shallow Cu, marine Cu-under-St and marine Sc) as benchmarks and apply a qt (total moisture)-w (vertical velocity) quadrant analysis technique to identify the “coherent structures” of moist and dry up/down-drafts. The statistics of these coherent structures and the environment are then used to evaluate commonly used assumptions in mass-flux parameterizations related to (i) the convective downdrafts and (ii) the subplume variability of convective updrafts. For quantitative evaluation of the PDF closure used in AP-HOCs, we perform offline calculations with the PDF closure used in the Cloud Layers Unified By Bi-normals (CLUBB) and the Simplified Higher-Order Closure (SHOC) schemes by supplying the closure with statistical quantities directly calculated from the LESs. Then, the impact of parameterization assumptions embedded in the PDF closure in the same two categories is analyzed and understood through comparisons of the output from the closure with the statistics of the moist and dry up/down-drafts from the LESs. Our main findings include, - In all four shallow convection regimes, a spectrum of downdrafts exists in the upper cloud layer, some more moist and directly associated with overshooting cumulus, others drier and possibly associated with smaller-scale entrainment. - Significant moist downdrafts near the top of cumulus layers represent a third mode of variability that the double-Gaussian PDF framework used by most AP-HOCs cannot accommodate. - When we replace the cloud core-environment decomposition with a moist updraft-environment one to better capture overshooting updrafts, the bulk-plume assumption leads to significant underestimation of cloud water and cloud water flux but allowing Gaussian subplume variability reduces the underestimation significantly. - In the CLUBB PDF closure, simplifying assumptions about subplume variability lead to error compensation in the estimation of cloud water flux and to significant biases in important terms in the moisture flux budget equation.