A diagnostic test of the closure assumption in Arakawa and Schubert's cumulus parametrization scheme

Abstract

AbstractThe parametrization of cumulus convection by Arakawa and Schubert is based on the assumption of quasi‐equilibrium between cumulus‐scale and large‐scale processes. Under these conditions, the cloud work function A(λ) should be constant. This assumption is tested in a diagnostic approach by using GATE data. Rawinsonde observations at the A/B‐scale ships were analysed for the period 30 August to 8 September 1974 and the cloud work functions are calculated for each observation time.The sensitivity of the cloud work function to temperature and humidity is studied. It is concluded that for the limited range of variations observed during GATE, the cloud work function can be regarded as constant for periods of up to 24 hours. Observed time changes of the large‐scale forcing indicate that a coupling between the time changes of the large‐scale temperatures and the moisture fields ensures that A(λ) remains constant.