Diagnosing DYNAMO convection with weak temperature gradient simulations

Abstract

Determining relationships between convective and environmental diagnostics can improve our understanding of mechanisms controlling tropical convection, and consequently, result in better representations of convection in coarsely resolved models. We identify important diagnostic relationships in observations taken during the Dynamics of the Madden-Julian Oscillation (DYNAMO) campaign and perform weak temperature gradient (WTG) simulations of DYNAMO convection to determine if the observed relationships are reproduced in our model. We find that the WTG approximation models local changes in the diagnostics used in the study-precipitation rate, atmospheric stability, moisture, and gross moist stability (GMS)-and reproduces diagnostic relationships suggested in previous studies; an increase in precipitation rate is correlated with increased atmospheric moisture content, which, in turn, is correlated with greater atmospheric stability. Large-scale atmospheric stability-changes of which might be related to balanced dynamics, we speculate-seems to be a candidate for a convective controlling mechanism. Observed and modeled interactions of local convection with the large-scale environment-quantified by the GMSare in agreement with the theory of Inoue and Back (2015b); the GMS increases from small, positive or negative, values during developing convection and further increases for decaying convection past a critical GMS found at peak precipitation rates, atmospheric stability, and moisture content. Understanding the link between the critical GMS and the diagnostics-still a standing problem-could further our understanding of interactions between local convection and the large-scale environment.

On the cumulus diurnal cycle over the tropical warm pool

“…Using the same WTG scheme, Sentić et al . [] demonstrate that the evolution of precipitation, lapse rate, and humidity during DYNAMO are well‐represented by the WTG framework. Comparison between measured vertical motion ( w dynamo ) and WTG‐balanced vertical motion from DYNAMO during SP/MJO2 (i.e., w balanced ) further supports that WTG is a reasonable approximation for this period (Figure ) ( , where Q 1 is the apparent heat source for SP/MJO2, and the overline indicates a horizontal domain average) (the profile from the control simulation, w ctl , is discussed in more detail later).…”

The role of radiation in organizing convection in weak temperature gradient simulations

“…The time scale over which the domain mean potential temperature is relaxed to the reference profile ( t θ in equation ) represents the time scale over which gravity waves counteract buoyancy anomalies induced by convective heating. Typical values in WTG investigations are on the order of an hour [ Sessions et al ., ; Wang and Sobel , ; Daleu et al ., ; Anber et al ., ; Wang et al ., ; Sentić et al ., ], though some studies have used strict enforcement ( ) [ Sobel and Bretherton , ]. We choose a relaxation time scale of approximately 11 min ( s −1 ).…”

“…WTG simulations have been used successfully to investigate various properties of convection and convective organization. For example, it has been implemented in a simple model of the Hadley circulation [ Polvani and Sobel , ], the Madden‐Julian Oscillation [ Wang et al ., ; Sentić et al ., ], and cyclogenesis [ Raymond and Sessions , ].…”

Modeling the MJO in a cloud‐resolving model with parameterized large‐scale dynamics: Vertical structure, radiation, and horizontal advection of dry air

“…For the same MJO events, Sentic et al . [] also performed numerical simulations using the SWTG method and found that their simulations were capable of reproducing important relations between convection and large‐scale environment—including atmospheric stability, moisture, and gross moist stability.…”