This is a brief description of the value of trying to forecast a GLASH image using 48-hour forecast fields and reverse domain filling Lagrangian trajectories to produce a 48-hour average upper tropospheric specific humidity field. The example shown below illustrates the location of a dry trough that has tropopause folding occurring along the downstream edge as evidenced in the ozonesonde profile from Northern Michigan depicting a layer of high stratospheric ozone just below the tropopause. Using a 48-hour forecast and sampling 350hPa specific humidity along 48-hour reverse domain filling Lagrangian trajectories, it is possible to roughly estimate the average upper-tropospheric specific humidity field, and to view this as a forecast GLASH image.

	Provides a qualitative estimate of the expected water vapor gradients.
	Allows for flight planning to sample tropopause folds.

Evidence

Observation 1

There was clearly strong folding evident in the upper troposphere with a deep layer of dry air and high associated with the advancing upper level trough.

Observation 2

The 48 hour forecast field of 350hPa specific humidity predicts a streamer of dry air, with a strong anchor shape, located west of the Appalachians, and extending SW to NE from eastern Tennesee to Western New York, with the center of the upper level low over Southern Ontario just north of Lake Niagara Falls. The real GLASH image from this time of 12UTC July 28, 2004 shows that the forecast was very accurate and could effectively be used to predict the location of future stratosoperhic/tropospheric exchange on the basis of the ozone profile that showed the folded layer of high ozone, with additional layers of moist/low ozone air mixed into it in the center of the upper level vortex.