The mesoscale convection life
cycle: building block or prototype for
large-scale tropical waves?
BRIAN MAPES, STEFAN TULICH, JIALIN LIN, PAQUITA ZUIDEMA
A cumulonimbus cloud may ascend and spawn its anvil cloud,
precipitation, and downdrafts
within an hour or so. This paper inquires why a similar progression of
events (life cycle) is
observed for tropical weather fluctuations with time scales of hours,
days, and even weeks.
Regressions using point data illustrate the characteristic unit of rain
production: the mesoscale
convective system (MCS), covering 10s of km and lasting several hours,
convective rain cells. Meanwhile, averages over larger spatial areas
indicate a self-similar
progression from shallow to deep convection to stratiform anvils on
many time scales.
Synthetic data exercises indicate that simple superpositions of
fixed-structure MCS life cycles
(the Building Block hypothesis) cannot explain why longer-period life
cycles are similar. Rather,
it appears that an MCS may be a small analogue or prototype of
larger-scale waves. Multiscale
structure is hypothesized to occur via a Stretched Building Block
conceptual model, in which the
widths (durations) of zones of shallow, deep, and stratiform anvil
clouds in MCSs are modulated
by larger-scale waves.
Temperature (T) and humidity (q) data are examined and fed into an
entraining plume model,
in an attempt to elucidate their relative roles in these large-scale
convection zone variations. T
profile variations, with wavelengths shorter than troposphere depth,
appear important for high-
frequency (2-5 day period) convectively coupled waves, as density
directly links convection (via
buoyancy) and large-scale wave dynamics (via restoring force). Still,
the associated q anomalies
are several times greater than adiabatic, suggesting a strong
amplification by shallow convective
feedbacks. For lower frequency (intraseasonal) variability, q anomalies
are considerably larger
compared to T, and may be dominant.
Published in October 2006.
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Revised March 2006