Vertical grid refinement for stratocumulus clouds in the radiation scheme of the global climate model ECHAM6.3-HAM2.3-P3

Paolo Pelucchi,Ulrike Lohmann,David Neubauer

doi:10.5194/gmd-14-5413-2021

Abstract

Abstract. In this study, we implement a vertical grid refinement scheme in the radiation routine of the global aerosol–climate model ECHAM-HAM, aiming to improve the representation of stratocumulus clouds and address the underestimation of their cloud cover. The scheme is based on a reconstruction of the temperature inversion as a physical constraint for the cloud top. On the refined grid, the boundary layer and the free troposphere are separated and the cloud's layer is made thinner. The cloud cover is recalculated either by conserving the cloud volume (SC-VOLUME) or by using the Sundqvist cloud cover routine on the new grid representation (SC-SUND). In global climate simulations, we find that the SC-VOLUME approach is inadequate, as there is a mismatch, in most cases, between the layer of the inversion and the layer of the stratocumulus cloud, which prevents its application and is itself likely caused by an overly low vertical resolution. Additionally, we find that the occurrence frequency of stratocumulus clouds is underestimated in ECHAM-HAM, limiting a priori the potential benefits of a scheme like SC-VOLUME targeting only cloud amount when present. With the SC-SUND approach, the possibility for new clouds to be formed on the refined grid results in a large increase in mean total cloud cover in stratocumulus regions. In both cases, however, the changes exerted in the radiation routine are too weak to produce a significant improvement in the simulated stratocumulus cloud cover. We investigate and discuss the reasons behind this. The grid refinement scheme could be used more effectively for this purpose if implemented directly in the model's cloud microphysics and cloud cover routines, but other possible ways forward are also discussed.

Highlights

Stratocumulus (Sc) clouds belong to the low-level stratiform clouds
The validation in the singlecolumn model (SCM) was carried out using a forcing derived from observations made during the East Pacific Investigation of Climate (EPIC) campaign (Bretherton et al, 2004), from a segment between 16 and 22 October 2001 in the southeastern Pacific, where the vertical structure of the boundary layer capped by a persistent stratocumulus cloud was observed using radiosondes and remote sensing (Bretherton, 2005)
In comparison to the inversion reconstruction performed by Siegenthaler-Le Drian (2010), who tested it in the SCM with the EPIC data, with the modifications added in our method, the inversion is found in cases where the method previously failed – for example, at the start of the day 1 or during day 6

Summary

Introduction

Stratocumulus (Sc) clouds belong to the low-level stratiform clouds. They occur in many regions and cover large areas of the Earth’s surface, but they appear most frequently over the oceans. Stratocumulus clouds are of considerable importance to the Earth’s radiative budget, as they exert a very strong net negative cloud radiative effect. This is due to the combination of a weak long-wave effect due to their low-lying position and an especially strong reflection of short-wave solar radiation accentuated by their location over dark oceans. Model grid boxes at the typical level of stratocumulus clouds are gener-

Methods

Results

Conclusion