A fixed-layer estimation of upper tropospheric specific humidity from the GOES water vapor channel: Parameterization and validation of the altered brightness temperature product

A derived-product image (DPI) of specific humidity is determined at the pix el-level using the GOES 6.7 mum water vapor channel and ancillary model tem perature fields. By treating the brightness temperature., T-6.7, as a compo sition of three effectively independent influences (specific humidity, temp erature and zenith angle), the ancillary temperature fields allow an estima tion of the sole effect of specific humidity on the variance of T-6.7. prim arily in the midlatitudes. This variance is approximately linear with the l ogarithm of a vertically weighted average of specific humidity. As an inter mediate step, we have created a term called the altered brightness temperat ure product. T-6.7(alt), that represents the variance of T-6.7 due to speci fic humidity plus an approximate mean of T-6.7 in the midlatitudes. The dom ain of this derived product extends to zenith angles often greater than 70 degrees within the GOES field of view. However, strong vertical gradients i n specific humidity in the midtroposphere to upper troposphere were found t o violate the linearity of the regression. In the validation, the slope of the first-order regression is robust but the constant is inconclusive, rend ering the product better for relative rather than absolute associations of specific humidity. Comparison to potential vorticity (PV) shows that the de rived product corresponds best to PV at the 300 hPa pressure level at value s of PV associated with the tropopause. The combined use of the DPI and mod el-derived PV contours yields an effective toot for analyzing stratosphere- troposphere exchange in the midlatitudes.