Tellus, 56A, 56-67.
Suzana J. Camargo
IRI, Lamont-Doherty Earth Observatory
of Columbia University, Palisades, NY
Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences,
Columbia University, New York, NY.
Abstract
The formation of tropical storms in a low-resolution Atmospheric
General Circulation model is studied on the Western North Pacific
region during the June-October season. The model simulates the mean
annual cycle of storm number in this basin quite well. Time-dependent
composites of the storms are formed and analyzed, with a focus on the
temporal evolution of quantities averaged in space around the storm
centers. Day zero of each composite corresponds to the time at which
the disturbance passes criteria for detection.
The composites depict the model storms as convectively-coupled,
synoptic-scale vortices whose degree of coupling to convection
increases at some point, leading to intensification. Variables
related to disturbance intensity have significant anomalies at day
-7,
indicating a finite amplitude disturbance prior to ``genesis''. Many
of these variables show similar temporal evolution, with a local
minimum two or three days before day zero, and a strong increase after
that for several days, followed by an eventual decrease. The
precipitation reaches its maximum on day 2, the net moist static
energy forcing (surface fluxes minus net tropospheric radiative
cooling, each of which has an anomaly of 20-30 $W~m^{2}$ in the
sense of warming the atmosphere)
a day later, and dynamical variables such as vorticity and
temperature still later, with broad plateaus centered around day 4
or 5. The vorticity increases at the surface at the same time as at
midlevels, unlike in observed storms. The mean composite
environmental vertical wind shear has a maximum amplitude on day -2
and then decreases. This could indicate a causal role of shear in
limiting development, but would also be consistent with a coincidental
storm motion to regions of lower shear, with development controlled by
other factors. A signal in the skewness of the lower-level relative
humidity distribution over the ensemble suggests that a dry lower
troposphere can prevent development of a model tropical disturbance.