Brian E. Mapes
Cooperative Institute for Research in the Environmental
Sciences
University of Colorado, Boulder, Colorado
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 continuous vertical spectrum of tropospheric heating in an unbounded, realistically stratified atmosphere is approximated with two spectral bands that capture the essence of radar-observed MCS heating profiles. The equations governing the wind and height amplitudes of each spectral band are shallow-water equations, combined with a time-dependent spatial smoothing which mimics the finite spectral widths of the bands. The finite band widths represent the tropospheric effects of vertical wave propagation.
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 is not an "up-scale" evolution: the spectrum of meso-size convective heating is red, not meso-scale. Small scales preferentially propagate vertically, 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 wind and temperature fields 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 MCS-like heating events. Rawinsonde
array-scale
vertical motion adjusts rapidly to convection, with an
adjustment time
(LS ~ 1--2 h) small compared to the observed lifetime of MCSs
(6--12h).
These results indicate that the observed quasi-equilibrium
between
large
scale vertical motion and convection may be due to the rapid
response
of the
former to the latter, not vice versa as has been commonly
supposed.
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