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Description:
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In this study , a month -long simulation of gravity waves over North America and
North Atlantic Ocean is performed using the mesoscale model MM5 for January 2003 ,
verified with Advanced Microwave Sounding Unit -A (AMSU -A ) radiance observations
in the upper troposphere and lower stratosphere . According to the monthly mean
statistics , four regions of strong gravity wave activities are found both in the simulation
and the AMSU -A observations : northwestern Atlantic , Appalachian Mountains , Rocky
Mountains and Greenland , respectively . Those over the northwestern Atlantic Ocean are
strongly associated with the midlatitude baroclinic jet -front systems , while the other
three regions are apparently collocated with high topography .
Imbalance diagnosis and numerical sensitivity experiments of a strong gravity
wave event during January 18 -22 show that the gravity waves are strongly linked to the
unbalanced flow in the baroclinic jet -front system . The gravity waves are usually
radiated from the upper tropospheric jet exit region with maximum nonlinear balance
equation residual ( Δ NBE ; key indicator of flow imbalance ) , distinctly different from other surface sources . Flow imbalance related strongly to tropopause folding and
frontogenesis of the large -scale background flow . Similar wave characteristics are
simulated in experiments with different microphysics and grid resolutions . The Δ NBE is
again shown to be a good predictor for jet -front related gravity waves , suggesting its
potential application to gravity wave parameterizations for global and climate models . |