The basic equations - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 83

layer roll circulations in a moist atmosphere, called ''horizontal convective rolls''
(HCRs). We also see them over the ocean. Observed linear patterns of seagulls
soaring have been also been interpreted as evidence of roll circulations. Rolls have
been inferred from measurements made aboard aircraft, in situ instruments
mounted on towers, by Doppler radars and lidars ( Figure 2.18 ), when the under-
lying land is being heated (rolls have also been measured when the stratification is
stable; these rolls owe their existence to mechanisms involving dynamic instabil-
ities). Insects and aerosols are accumulated in regions of convergence underneath
the rising branch of rolls, so they become detectable by radar in ''clear air''. It is
thought that it is likely that all rolls in nature may owe part of their existence to
thermal instability and part to dynamic instability. Thus, the theories of Rayleigh-
Be´ nard convection in a sheared environment may have some qualitative relevance
to convection in the real atmosphere. Such roll circulations when tilted by gradi-
ents in vertical velocity (e.g., along the edge of updrafts and downdrafts) may be
sources of vertical vorticity along boundaries and in some convective clouds
initiated along the boundaries ( Figure 2.19 ), and when they intersect boundaries
along which rolls of another nature (e.g., frontogenetical or solenoidal circula-
tions) are present they may influence where moist convection does or does not get
triggered ( Figure 2.20 ).

2.10 RESPONSE OF A BOUSSINESQ ATMOSPHERE TO
HEAT SOURCES

In convective storms, latent heat is released by water molecules when condensation
occurs and the heat is absorbed by the atmosphere. Latent heat is absorbed by
water molecules and the atmosphere is cooled as precipitation evaporates, subli-
mates, or frozen precipitation melts. When latent heat is exchanged, buoyancy
changes. In addition, when water substances change form, the loading term in
(2.23) changes as the mixing ratios of various water substances change. Precipita-
tion falling to the ground is an irreversible process: precipitation does not rise up
out of the ground to become cloud droplets again (unless someone puts a power-
ful vacuum cleaner to the ground

)! Furthermore, latent heat release is a highly
nonlinear process: it is completely off until phase changes occur, which do so
suddenly. The consequences of cooling will be discussed in the next chapter. Here,
we look briefly at how the atmosphere responds to a sudden turning-on of the
precipitation process and the accompanying sudden latent heat release.
Analysis of the effects of sudden onset of a heat source was carried out by
Chris Bretherton and Piotr Smolarkiewicz in a 1989 journal article and reproduced
in Emanuel's text. The atmosphere is assumed to be hydrostatic for both
simplicity and to isolate the effects of sudden heating; it is also assumed to be
resting, inviscid, and non-rotating. The equations of motion (cf. (2.203)-(2.205)),
the thermodynamic equation (cf. (2.161)), and the continuity equation (2.207) are

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