The Vertical Profile of Entrainment Rate Simulated by a Cloud-Resolving Model and Application to a Cumulus Parameterization

Abstract

To investigate the vertical profiles of fractional entrainment rate as to cumulus convection, numerical simulations of a tropical cyclone rainband are conducted, using a high-resolution three-dimensional cloud-resolving model (CRM), with the 200-m horizontal resolution. On the basis of the results of CRM simulations, venically variable entrainment rate is applied to the Arakawa-Schubert (AS) cumulus parameterization.
Fractional entrainment rate, derived from the calculation based on the vertical gradient of cloud mass flux, clearly shows larger near cloud base and top. Between the heights of cloud base and top, entrainment rate is smaller, and even negative in many cases, suggesting laterally detrained air from a cumulus into the environment. From the analyses where entrainment rate is divided into three terms, it is found that the contributions of updraft is relatively large near the cloud base and top, compared to that in between. The cloud amount contribution depends on whether cloudy areas expand or shrink accompanying cloud growth or decay, respectively.
On the basis of the result of the CRM simulations, vertically variable entrainment rate is applied to the AS scheme. For investigating the effect of the modifications of the scheme, simulations of typhoon Saomai (2000) are conducted. The simulations show significant improvements: underestimates of moisture in the mid- to upper troposphere are reduced. The result is predominantly attributed to cloud mass flux, greatly influenced by lateral detrainment. The peak height of the mass flux corresponds to that in the moisture tendency.