The Large-Scale Part of Tropical Mesoscale Convective System Circulations

Abstract

The linear response of the tropical troposphere to a heat source resembling a moderately large mesoscale convective system (MCS) is modeled. The spectral representation of vertical structure, with and without rigid lids atop the atmosphere, is illustrated graphically and discussed physically. The spectrum characterizing a mean radar-observed MCS heating profile in an unbounded, realistically stratified atmosphere can be well approximated with just two spectral bands. The equations governing the wind and height amplitudes of each spectral band are linear shallow-water equations, combined with a time-dependent spatial smoothing that accounts for the finite spectral widths of the bands. This spectral band smoothing mimics the tropospheric smoothing effects of the preferential upward propagation of high horizontal wavenumber components.During the period from 0-72 hours after the MCS, the flow evolves from initially circular expanding wavelike rings, toward the planetary-wave patterns studied by Matsuno, Gill, and others. This size expansion is not truly an "up-scale" evolution: the red horizontal spectrum of scales excited by a meso-sized heat source merely undergoes phase evolution with time. Small scales do preferentially propagate upward, however. An interesting feature strongly excited by an equatorial MCS is a zonally elongated inertio-gravity motion along the equator, which cools the troposphere 48 hours after a brief heating event. This oscillation has a large zonally-symmetric component.The linear solutions developed here are superposed to obtain estimates of the wind and temperature changes forced by satellite-observed ensembles of MCSs. Low-level winds comparable to observed Australian monsoon winds spin up from rest in just 2-3 days, while some of the monsoon convective heating escapes into the equatorial waveguide as downwelling Kelvin and mixed Rossby-gravity waves.Synthetic data constructed from model fields illustrate how observing systems experience the effects of MCS heating events. Deep vertical motion on tropical rawinsonde-array scales responds rapidly to convective heating, with a response time (1-2h) small compared to the observed lifetime of MCSs (6-12h). These results indicate that rawinsonde array observations of a "quasi-equilibrium" between largescale vertical motion and convection may be due to the rapid response of the former to the latter, not vice versa as has sometimes been supposed.

On the Dynamics of Easterly Waves, Monsoon Depressions, and Tropical Depression Type Disturbances

“…The purpose of this note is to suggest that some aspects of these synoptic-scale tropical disturbances may be understood by considering the linear response of a hypothetical dry, initially resting tropical atmosphere to an imposed pulse of heating, such as might represent (in an idealized way) a large mesoscale convective system. Studies in which the response of an otherwise adiabatic atmosphere to an imposed heating is examined are extremely numerous.1 A number of these are similar to the present work either in the form of the heating or the emphasis on some aspects of the Rossby wave part of the response (Silva Dias et al 1983;Salby and Garcia, 1987;Lim and Chang 1987;Bergman and Salby 1994;Holland 1995;Horinouchi and Yoden 1996;Mapes 1998). The present work reveals no fundamentally new physics.…”

Equilibrium Vs. Activation Control of Large-Scale Variations of Tropical Deep Convection

“…5a, Fig. 5b shows a similar plot from Mapes (1997), constructed using synthetic data from a simple linear dry primitive-equation model. In the model, a heating process scaled to resemble a typical mesoscale convective system is specified to fluctuate within a synthetic tropical radiosonde array similar in dimensions to the Marshall Islands array.…”

“…The form of the warming caused by convective heating is a non-steady expanding pattern of downwelling wavefronts in the stratified environment (Bretherton and Smolarkiewicz 1989, Nicholls et al 1992, Bretherton 1993, Mapes 1997). Unfortunately, the terms 'compensating subsidence' and 'cloud-induced subsidence,' which might appear to describe this response, were taken long ago to describe a mathematical term that arose as a definitional artifact in literature on the interaction of convection with large scales (Ooyama 1971, Yanai et al 1973.…”

“…Tests of the quasi-equilibrium hypothesis, for a) Marshall Islands rawinsonde array data (AS74 Fig. 13) and b) imposed-heating model output (Mapes 1997 Fig. 15).…”

“…Although the heating is positive-only, some negative temperature perturbations at low levels are apparent near r=450 km. When ensembles of such MCS-like heating events come and go, on a rotating earth, both the 1-signed and 2-signed temperature signals can appear, with either phase, at various times and places (Mapes 1997).…”