Deep-Tropospheric Gravity Waves Created by Leeside Cold Fronts

F. M. Ralph Environmental Technology Laboratory, NOAA/ERL, Boulder, Colorado

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P. J. Neiman Environmental Technology Laboratory, NOAA/ERL, Boulder, Colorado

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T. L. Keller Research Applications Program, NCAR, Boulder, Colorado

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Abstract

The water vapor channel of the Geostationary Operational Environmental Satellite GOES-8 reveals narrow (30 km wide), elongated (500–1000 km) bands that propagate southward on the eastern side of the Rocky Mountains from Colorado to Texas. Two events in which surface and wind profiler observations show that these bands are associated with leeside cold fronts are documented in detail, and several other cases are summarized. The wind profilers observe vertical motions exceeding 1 m s−1 in narrow plumes at the leading edge of the fronts, in broader zones in the upper troposphere, and in the lower stratosphere. These cause vertical displacements of up to 1 km and are responsible for the signature in water vapor images.

The bands occur when the Rocky Mountains block either arctic leeside cold fronts coming from the north or northeast or Pacific cold fronts coming from the northwest. The blocking changes the frontal orientation and disrupts geostrophic thermal wind balance near the terrain-modified fronts. This imbalance is manifested as strong (20 m s−1) prefrontal, front-relative, cross-front flow Vr. Observations and numerical simulations are presented showing that deep-tropospheric gravity waves are produced in this region by the obstacle effect of the surface leeside cold front. Farther east, Vr is near zero, and the waves are weak or absent.

Along the western portion of the front the waves propagate with the front and resemble trapped lee waves; however, farther east the waves appear ahead of the surface front by up to 100 km. These prefrontal gravity waves occur when the wave forcing decays along the eastern portion of the front and the trapped waves that had developed there become decoupled from the front and propagate away. Numerical simulations of a well-observed event confirm that trapped waves would have developed, and profiler data confirm the trapped nature of the observed gravity wave’s vertical structure. Such waves could create convection, including prefrontal squall lines, and can be seen in real-time satellite imagery before the convection is triggered.

Corresponding author address: Dr. F. Martin Ralph, NOAA/ERL/ETL, Mail code R/E/ET7, 325 Broadway, Boulder, CO 80303.

Email: fralph@etl.noaa.gov

Abstract

The water vapor channel of the Geostationary Operational Environmental Satellite GOES-8 reveals narrow (30 km wide), elongated (500–1000 km) bands that propagate southward on the eastern side of the Rocky Mountains from Colorado to Texas. Two events in which surface and wind profiler observations show that these bands are associated with leeside cold fronts are documented in detail, and several other cases are summarized. The wind profilers observe vertical motions exceeding 1 m s−1 in narrow plumes at the leading edge of the fronts, in broader zones in the upper troposphere, and in the lower stratosphere. These cause vertical displacements of up to 1 km and are responsible for the signature in water vapor images.

The bands occur when the Rocky Mountains block either arctic leeside cold fronts coming from the north or northeast or Pacific cold fronts coming from the northwest. The blocking changes the frontal orientation and disrupts geostrophic thermal wind balance near the terrain-modified fronts. This imbalance is manifested as strong (20 m s−1) prefrontal, front-relative, cross-front flow Vr. Observations and numerical simulations are presented showing that deep-tropospheric gravity waves are produced in this region by the obstacle effect of the surface leeside cold front. Farther east, Vr is near zero, and the waves are weak or absent.

Along the western portion of the front the waves propagate with the front and resemble trapped lee waves; however, farther east the waves appear ahead of the surface front by up to 100 km. These prefrontal gravity waves occur when the wave forcing decays along the eastern portion of the front and the trapped waves that had developed there become decoupled from the front and propagate away. Numerical simulations of a well-observed event confirm that trapped waves would have developed, and profiler data confirm the trapped nature of the observed gravity wave’s vertical structure. Such waves could create convection, including prefrontal squall lines, and can be seen in real-time satellite imagery before the convection is triggered.

Corresponding author address: Dr. F. Martin Ralph, NOAA/ERL/ETL, Mail code R/E/ET7, 325 Broadway, Boulder, CO 80303.

Email: fralph@etl.noaa.gov

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