Experimental tropopause folding product
For an introduction, scroll down
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Introduction to the Tropopause Folding Product
The base image of the Tropopause Folding Product is a derived product based on the GOES water vapor channel, tentatively called the Specific Humidity Product. This derived product is designed to vary only with water vapor amount by removing the "biases" on water vapor channel brightness temperature from atmospheric temperature and zenith angle. The result of this operation is a value (in degrees K) that varies logarithmically with specific humidity in a fixed layer (~250-500 hPa) of the atmosphere. For more information on the derivation, see Moody et al. [1999] and Wimmers and Moody [2001].
This base image depicts a uniquely effective quasi-conservative tracer of water vapor, which emphasizes the otherwise obscured contiguous air masses of dry air traveling from the poles (purple and blue) and moist air traveling from the subtropics (yellow and green), showing individual air masses with a more consistent quantitative value. By extension, the image gradients highlight the boundaries of air masses at the mesoscale level, and provide a proxy for gradients in tropopause height. Recent investigation has found that strong gradients in the image-derived specific humidity correspond strongly with tropopause folding, which is an event in which the boundary between the stratosphere and the troposphere folds into the troposphere, frequently leading to dynamical instability (enhanced turbulence) [Shapiro, 1980] and chemical mixing between the two levels. Wimmers and Moody [2003a] determined a threshold that distinguishes image gradients strong enough to correspond to tropopause folds, and predicted folds in 13 out of 14 cases, with no false positives. However, the gradient magnitude does not go further to predict the size of the tropopause folds [Wimmers and Moody, 2003b]. Consequently, the resulting empirical model for estimating tropopause folding uses an "average" tropopause folding size, which was found to be without significant bias over latitude or gradient magnitude.
Tropopause folds are modeled as "ribbons" of uniform width (234 km), with one edge along the gradient maximum (shown in the images as the darker edge), presumed to be the opening of the tropopause fold. The other edge extends out in the direction of higher moisture, modeling a fold into and underneath the warmer air mass. Unfortunately, the model does not presently estimate the altitude of the fold; it only asserts that the fold intrudes into the free troposphere. The product is shown here in relation to hourly pilot reports of turbulence, where green points are approximately below the height of the convective boundary layer, and red points are in the free troposphere. Numbers displayed alongside are the altitude of the event, reported in hundreds of feet. The size of the point is proportional to the described intensity of turbulence.
Naturally, not all modeled tropopause folds correspond to reports of turbulence. One reason for this is that turbulence is under-sampled and underreported by this system, so it is an imperfect method of verification. However, the model presently shows significant bias as a predictor of turbulence in areas of "residual" tropopause folding, which means that it highlights areas of recent development, which are likely to show evidence of layering, but are not associated with the dynamical instability more common with ongoing development. This is clearly the case with dissipating midlatitude cyclones and streamers. Secondly, the modeled tropopause folding appears to correspond more strongly on the leading edges of dynamical troughs than on the trailing edge, because vertical sheering is more intense on the leading edge. These concerns are being actively investigated in order to further refine the model for turbulence prediction.
References:
Shapiro, M.A., Turbulent mixing within tropopause folds as a mechanism for the exchange of chemical constituents between the stratosphere and troposphere, J. Atmos. Sci., 37, 994-1004, 1980.
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., and J.L. Moody, A fixed-layer estimation of upper tropospheric specific humidity from the GOES water vapor channel: Parameterization and validation of the altered brightness temperature product, J. Geophys. Res., 106 (D15), 17115-17132, 2001.
Wimmers, A. J. and J. L. Moody, Tropopause folding at satellite-observed spatial gradients, I. Verification of an empirical relationship, submitted to J. Geophys. Res., 2003a.
Wimmers, A. J. and J. L. Moody, Tropopause folding at satellite-observed spatial gradients, II. Development of an empirical model, submitted to J. Geophys. Res., 2003b.
Other relevant publications:
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.
Cooper, O. R., C. Forster, D. Parrish, E. Dunlea, G. Hubler, F. Fehsenfeld, J. Holloway, S. Oltmans, B. Johnson, A. Wimmers and L. Horowitz, On the life cycle of a stratospheric intrusion and its large-scale mixing with polluted warm conveyor belts, J. Geophys. Res., accepted for publication, 2004.