Vertical-Mode and Cloud
Decomposition of Large-
Scale Convectively Coupled Gravity Waves in a
Two-Dimensional Cloud-Resolving Model
Stefan N. Tulich, David A. Randall, and Brian E. Mapes
JAS, (re)submitted Dec 2005
ABSTRACT
This paper presents an analysis of large-scale [O(1000 km)]
convectively coupled gravity
waves simulated using a two-dimensional cloud-resolving model. The
waves develop spontane-
ously under uniform radiative cooling and approximately zero-mean-flow
conditions, with wave-
number-2 of the domain appearing prominently and right-moving
components dominating over
left-moving components for random reasons. The analysis discretizes the
model output in two dif-
ferent ways. First, a vertical-mode transform projects profiles of
winds, temperature, and heating
onto the vertical modes of the model’s base-state atmosphere. Second, a
cloud-partitioning algo-
rithm sorts sufficiently cloudy grid columns into three categories:
shallow convective, deep con-
vective, and stratiform anvil.
Results show that the tilted structures of the large-scale waves can
largely be regarded as
the superposition of two dominant vertical spectral “bands”, each
consisting of a pair of vertical
modes. The “slow” modes have propagation speeds of 16 and 18 m s-1 (and
roughly a full-wave-
length vertical structure through the troposphere), while the “fast”
modes have speeds of 35 and
45 m s-1 (and roughly a half-wavelength structure). Deep convection
anomalies in the waves are
more-or-less in phase with the cold temperature anomalies of the slow
modes, and in quadrature
with those of the fast modes. Owing to the characteristic life cycle of
deep convective cloud sys-
tems, shallow convective heating peaks roughly ~ 100 km to the right of
the most intense deep
convection (implying a ~2 hr lead), while stratiform heating peaks ~
150 km to the left (implying
a ~3 hr lag). Time series of the spectral energy production indicate
that the propagation speed of
the waves (~16 m s-1) is set by the “intrinsic” speed of the slow
modes, rather than a reduced-sta-
bility of the fast modes.
A PDF
manuscript is available.