Lateral mixing as a source of subtropical tropospheric water vapor

R. T. Pierrehumbert
Department of Geophysical Sciences
University of Chicago

It is hypothesized that the subtropical water vapor distribution results from the interplay of three simple factors: subsidence, which brings down dry air from aloft; lateral mixing, which brings in moistened air from the convective region at various rates; and drying by processing of air through the cold extratropics. A simplified Lagrangian model is formulated, and used to study how this process works during the CEPEX period (March, 1993). A key result is that the Northern subtropics should be viewed as a general background of dry air with mass mixing ratio of 10-4 or less on the 330K surface, interrupted by a few coherent moist plumes with mixing ratios on the order of 10-3. It is suggested that the Lagrangian model would also be useful in interpreting satellite water vapor data, and in diagnosing water vapor transport errors in general circulation models.

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