Using GOES layer average specific humdity (GLASH) and Lagrangian reverse domain filling trajectories to forecast Stratospheric/Tropospheric Exchange (STE)

A derived product image of upper tropospheric specific humidity from the Geostationary Operational Environmental Satellites (GOES) was recently developed at UVA. It is based on a linearization of the relationship between brightness temperature from the 6.7 um water vapor channel on the GOES Imager and layer average relative humidity for the upper troposphere. Using the vertical weighting function for the channel along with temperature fields from a meteorological model, images can be “corrected” for temperature and zenith angle biases (Moody et al., 1999). The result is a product that represents GOES layer average specific humidity (GLASH), a field that depicts the sharp boundaries between polar and subtropical air masses. Given the wavelength of the water vapor channel, the GLASH signal is influenced by moisture variations from 250 to 500hPa, with the peak contribution from about 350 hPa. As a result, the imagery shows a maximum gradient in moisture along the polar tropopause break, where dry air on the poleward side of the boundary has a greater contribution from the stratosphere and air on the equatorward side of the gradient represents an entirely (or largely) tropospheric contribution. To the extent that layer average specific humidity is a quasi-conservative property, GLASH imagery provides an effective tracer of atmospheric motion in the upper troposphere. Animations of this field depict features at a range of scales from synoptic scale ridges and troughs, to finely scaled streamers and rolled vortices, representations of advective processes that lead to irreversible mixing. Previous work has shown that tropopause folding activity, an important component of STE, is correlated with strong gradients in remotely sensed specific humidity (Wimmers et al, 2002). In this paper, we will report on the use of reverse domain filling (RDF) trajectories and Lagrangian Liapunov exponents to develop a mixing forecast for the upper-troposphere. Liapunov exponents provide a measure of the stretching rate, the deformation of the flow field by velocity shear. Previous work has shown that Lagrangian simulations capture filamentary tracer structures associated with chaotic advection like those observed in the real stratosphere (Pierce and Fairlie, 1993). This work will report on the application of these methods to predict and diagnose mixing of stratospheric and tropospheric air in the troposphere. The RDF trajectories based on wind fields from operational National Center for Environmental Prediction models will define regions where mixing associated with the filamentation and fragmentation of dry air intrusions are expected. The net Lagrangian vertical displacement will be used to assess the depth of STE. These diagnostic fields will be compared directly with GLASH imagery, allowing us to assess the predictive capability of the satellite imagery, and GLASH gradients for locating regions of STE. We will present data from the INTEX/NA field study, an integrated atmospheric chemistry field experiment over eastern North America planned for the summer of 2004. Additionally, data will be presented for a case during the Tropospheric Ozone about the Spring Equinox (TOPSE) field campaign which took place in the spring of 2000, at a time when we have ozone cross sections as supporting evidence of STE.

Moody, J. L., A. J. Wimmers, and J. C. Davenport, Remotely sensed specific humidity: Development of a derived product from the GOES Imager Channel 3, Geophys. Res. Lett., 26(1), 59-62, 1999.

Wimmers, A.J., J.L. Moody, E.V. Browell, J. W. Hair, W.B. Grant, C.F. Butler, M.A. Fenn, C.C. Schmidt, J. Li and B.A. Ridley, Signatures of tropopause folding in satellite imagery, J. Geophys. Res., 108(D4), 8360, doi:10.1029/2001JD001358, 2003.

Pierce, R. Bradley, T. Duncan A.Fairlie, Chaotic Advection in the Stratosphere: Implications for the Dispersal of Chemically Perturbed Air from the Polar Vortex, J. Geophys. Res., 98, 18589-18595, 1993.