Moisture and temperature perturbations in the course of convection development

1. Tropical convection and quasi-equilibrium hypothesis

Schematic Tropical convective clouds are often thought of as a "mediator" acting against a large-scale forcing destabilizing the atmosphere such as moisture convergence and surface heat flux. This neutrizing effect of convection has been formulated by a school of scientists (initiated by Aarakawa and Schubert, 1974) into the quasi-equilibrium hypothesis, which has been widely accepted as the basis for many cumulus parameterization schemes. Some recent studies, on the other hand, found observational evidence that urges updates to the quasi-equilibrium hypothesis as originally postulated.

Satellite data are analyzed to explore the thermodynamic evolution of tropical and subtropical atmospheres prior and subsequent to moist convection, motivated to offer an observational testbed for convective adjustment central to the quasi-equilibrium hypothesis. Tropical Rainfall Measuring Mission (TRMM) and Aqua satellite measurements are projected onto a composite temporal sequence over an hourly to daily time scale by exploiting the temporal gap between the local satellite overpasses that changes from one day to another (Figure on right).

The atmospheric forcing and response to convection are investigated separately for deep convective and congestus clouds. In the deep tropics, a quick ventilation of atmospheric boundary layer (ABL) into the free troposphere is found Schematic in association with deep convection but is preceded by a steady buildup of ABL moisture, which does not imply continuous adjustment to equilibrium. The evolution of convective available potential energy (CAPE) (black curve in left figure) is controlled not only by the ABL moisture (red curve) but also largely by a coincident ABL cooling (blue curve). CAPE exhibits a rapid drop for 12 h preceding convection and a following restoring phase lasting a day or two as the cool anomaly recovers (Left figure a). The behavior of CAPE is quite different when moist convection is brought by congestus clouds with no deep convection nearby. CAPE gently increases over a period of 1-2 days until congestus occurs and then declines as slowly back to the initial level (Left figure b). Convective adjustment is not efficiently at work even in the deep Tropics at times when convective clouds are not developed so deep as to penetrate the whole troposphere.

2. Water and heat budget analysis

The above compositing technique is further expored with substantial updates to facilitate a moisture and thermal budget analysis of the tropical atmosphere. A variety of satellite sensors are employed including radars (TRMM PR and CloudSat radar), an infrared and microwave sounder unit (Aqua AIRS/AMSU suite), and microwave radiometer (Aqua AMSR-E) and scatterometer (QuikSCAT SeaWinds) aboard different platforms. Satellite measurements of atmospheric parameters including air temperature, water vapor, cumulus cloud cover, and surface wind are composited with respect to the temporal lead or lag from TRMM detected convection to obtain statistically continuous time series just as illustrated above. Schematic AIRS observed temperature and humidity profiles, representing cloud-cleared sounding, are combined with semi-theoretical estimates of in-cloud temperature and humidity to construct the large-scale mean field. Those measurements are ingested to the moisture and thermal budget equations integrated vertically over each layer separated by cloud base. This strategy makes it possible to evaluate the free-tropospheric (FT) convergence of moisture and dry static energy and their vertical flux at cloud base, which was difficult to estimate from satellite observations alone.

The main findings include: 1) Vertical moisture transport at cloud base (dotted curve in right figure) is the dominant source of FT moistening prior to isolated cumulus development (top panel) while overwhelmed by horizontal moisture convergence (solid curve) for highly organized systems (bottom panel); 2) FT diabatic heating is largely offset on an instantaneous basis as expected; and 3) FT moistening by convective eddies amounts to a half of the total cloud-base moisture flux in the background state, while large-scale mean updrafts modulate the variability of cloud-base flux when highly organized systems develop. The known correlation between congestus clouds and FT moisture before deep convection may be accounted for by large-scale mean moisture updraft rather than congestus eddy moistening.

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Last modified: Wed May 22 12:10:21 2013