The derivation of a quasi-geostrophic (QG) system from the rotating shallow
water equations on a midlatitude beta-plane coupled with moisture is presented.
Condensation is prescribed to occur whenever the moisture at a point exceeds a
prescribed saturation value. It is seen that a slow condensation time scale is
required to obtain a consistent set of equations at leading order. Further,
since the advecting wind fields are geostrophic, changes in moisture (and
hence, precipitation) occur only via non-divergent mechanisms. Following
observations, a saturation profile with gradients in the zonal and meridional
directions is prescribed. A purely meridional gradient has the effect of
slowing down the dry Rossby waves, through a reduction in the "equivalent
gradient" of the background potential vorticity. A large scale unstable moist
mode results on the inclusion of a zonal gradient by itself, or in conjunction
with a meridional moisture gradient. For gradients that are are representative
of the atmosphere, the most unstable moist mode propagates zonally in the
direction of increasing moisture, matures over an intraseasonal timescale and
has small phase speed.Comment: 9 pages, 8 figures, Quarterly Journal of the Royal Meteorological
Society, DOI:10.1002/qj.2644, 201
