Transition from Suppressed to Active Convection Modulated by a Weak Temperature Gradient–Derived Large-Scale Circulation

Abstract

Numerical simulations are performed to assess the influence of the large-scale circulation on the transition from suppressed to active convection. As a model tool, the authors used a coupled-column model. It consists of two cloud-resolving models that are fully coupled via a large-scale circulation that is derived from the requirement that the instantaneous domain-mean potential temperature profiles of the two columns remain close to each other. This is known as the weak temperature gradient approach.The simulations of the transition are initialized from coupled-column simulations over nonuniform surface forcing, and the transition is forced in the dry column by changing the local and/or remote surface forcings to uniform surface forcing across the columns. As the strength of the circulation is reduced to zero, moisture is recharged into the dry column and a transition to active convection occurs once the column is sufficiently moistened to sustain deep convection. Direct effects of changing surface forcing occur over the first few days only. Afterward, it is the evolution of the large-scale circulation that systematically modulates the transition. Its contributions are approximately equally divided between the heating and moistening effects.A transition time is defined to summarize the evolution from suppressed to active convection. It is the time when the rain rate in the dry column is halfway to the mean value obtained at equilibrium over uniform surface forcing. The transition time is around twice as long for a transition that is forced remotely compared to a transition that is forced locally. Simulations in which both local and remote surface forcings are changed produce intermediate transition times.

Using the Weak‐Temperature Gradient Approximation to Evaluate Parameterizations: An Example of the Transition From Suppressed to Active Convection

“…The cooling rate is 1.5 K d −1 up to 12 km and it then decreases linearly in height to reach the prescribed value of 0 K d −1 at 15 km. The horizontal wind speed components are relaxed toward vertically uniform values of 0 and 5 m s −1 , both with the relaxation time scale of 6 h, consistent with the setup in Daleu et al ().…”

“…The simulations discussed in this paper are performed with τ = 2 h, which is the value used in the same idealized experimental setup using a CRM (Daleu et al, ).…”

“…It is an innovation of the WTG which was recently proposed by Herman and Raymond (). There is another large‐scale parameterization approach, namely, the damped gravity wave (DGW), which is based on the coupling of convection to a linear gravity wave of a single horizontal wavenumber (e.g., Daleu et al, ; Edman & Romps, ; Kuang, ; Romps, ).…”

“…Moreover, Wang et al () demonstrated that in comparison to the WTG and DGW approaches, the spectral WTG approach better captures the vertical structure of the large‐scale vertical motion observed during the DYNAMO field campaign. Although the spectral WTG approach has become more popular, we used the WTG approach because it matches the approach used in the CRM study of Daleu et al (), to which this study is compared.…”

Intercomparison of methods of coupling between convection and large‐scale circulation: 2. Comparison over nonuniform surface conditions

“…The implementation of the WTG and DGW methods has always used a configuration that couples a simulated column to a reference state [e.g., Raymond and Zeng , ; Sobel et al ., ; Sessions et al ., ; Wang and Sobel , ; Kuang , ; Wang and Sobel , ; Wang et al ., ; Romps , ] until recently when Daleu et al . [] developed a new configuration that couples two simulated columns via a WTG‐derived large‐scale circulation [ Daleu et al ., ]. Much insight has been learned from these efforts.…”