Estimating the Gross Moist Stability of the Tropical Atmosphere*

Abstract

Recent theoretical studies have indicated that large-scale circulation in deep convective regions evolves subject to an overall static stability-termed the gross moist stability-that takes into account both dry static stability and moist convective effects. The gross moist stability has been explicitly defined for a continuously stratified atmosphere under convective quasi-equilibrium constraints. A subsidiary quantity-the gross moisture stratification-measures the overall effectiveness in producing precipitation subject to these quasi-equilibrium constraints. These definitions are relevant in regions that experience deep convection sufficiently often; criteria based on climatological precipitation and maximum level of convection are used to define a domain of applicability. In this paper, 10-yr monthly mean rawinsonde data, and European Centre for Medium-Range Weather Forecasts (ECMWF) and National Meteorological Center (NMC) analyses are used to estimate the magnitude and horizontal distribution of these two quantities in the Tropics within the domain of applicability.The gross moist stability is found to be positive but much smaller than typical dry static stability values. Its magnitude varies modestly from 200 to 800 J kg Ϫ1 and exhibits relatively little dependence on sea surface temperature (SST). These values correspond, for instance, to a phase speed change from 8 to 16 m s Ϫ1 for the Madden-Julian oscillation. The gross moisture stratification is larger and exhibits strong dependence on SST, varying from 1500 to 3500 J kg Ϫ1 between cold and warm SST regions. A high degree of cancellation between effects of increasing low-level moisture and maximum level of convection, respectively, tends to keep the gross moist stability values relatively constant. Differences among the ECMWF and NMC analysis products and the rawinsonde data affect the estimate, but there is qualitative agreement. It is encouraging that reasonably robust estimates of a small, positive gross moist stability (as the difference between larger dry static stability and gross moisture stratification quantities) can be obtained. This helps justify use of small, constant moist phase speeds in some simple models of tropical circulation, although it also points out inconsistencies in how such models neglect variations in the height of convection.

Analysis of MJO Wind-Flux Feedbacks in the Indian Ocean Using RAMA Buoy Observations

“…Previous studies estimated that deep convection discharges column MSE by vertical advection at a rate of approximately 20 % of precipitation in warm pool regions and approximately 14 % over the Maritime Continent (Yu et al 1998). Recent MSE analysis by Sobel et al (2014) for the DYNAMO MJO events found a similar rate of MSE discharge by vertical advection during peak MJO activitybetween 10 % and 20 % of precipitation (their Figs.…”

“…A highly idealized theoretical analysis of Yu et al (1998) suggested that the export of column MSE by vertical motions associated with deep convection occurs at a rate of approximately 20 % of precipitation over the west Pacific warm pool and southeast Indian Ocean and at a rate of approximately 14 % of precipitation near the Maritime Continent. In more recent findings, Sobel et al (2014) showed vertical advection discharges column MSE at a rate of 10 % -20 % of precipitation at the peak of the CINDY/ DYNAMO (i.e., Cooperative Indian Ocean Experiments on Intraseasonal Variability in the Year 2011; Dynamics of the MJO) field campaign (Yokoi et al 2014;Moum et al 2013).…”

“…In more recent findings, Sobel et al (2014) showed vertical advection discharges column MSE at a rate of 10 % -20 % of precipitation at the peak of the CINDY/ DYNAMO (i.e., Cooperative Indian Ocean Experiments on Intraseasonal Variability in the Year 2011; Dynamics of the MJO) field campaign (Yokoi et al 2014;Moum et al 2013). We note that in both Yu et al (1998) and Sobel et al (2014) precipitation was converted to energy units to compute the ratio of MSE discharge from vertical motions associated with convection. If the ratio of surface fluxes to precipitation is a substantial fraction of these values during active MJO events, then the conclusion could be made that surface fluxes are important for maintaining MJO convection as surface fluxes would counteract a large portion of the MSE discharge associated with vertical motions in deep convection areas.…”

Seasonal and Interannual Variations of the Energy Flux Equator and ITCZ. Part II: Zonally Varying Shifts of the ITCZ

“…Therefore, the divergent component of the AET can often be related to the ITCZ. Since MSE typically increases with altitude, AET generally diverges where mean vertical motions in the tropics are upward (Yu et al 1998)-potentially with the exception of a narrow region in the mid-Pacific (Back and Bretherton 2006). We therefore define the zonally varying EFE as the latitude where the meridional component of the divergent columnintegrated MSE flux both vanishes and has a positive meridional gradient.…”

Modulation of the Madden-Julian Oscillation by ENSO: Inferences from Observations and GCM Simulations

“…The warm and cold ENSO composites for the moist stability in the model atmosphere, based on a definition analogous to that of Neelin and Held (1987), are also computed. Weaker moist stability over the central Pacific (not shown) is found during El Niñ o years, which might be conducive to the slower MJO propagation during warm events (see, e.g., Wang and Li 1994;Yu et al 1998). Another possible mechanism for the change of the MJO phase speed is the advection by the time mean flow, as the mean upper-level zonal wind is decreased during warm ENSO events in the same central Pacific region in the November-March period (not shown).…”