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Old February 20th 17, 04:08 AM posted to rec.aviation.soaring
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Default Dr Jack forecast high too low

On Saturday, February 18, 2017 at 9:05:38 PM UTC-5, wrote:
It seems that for many months Dr Jack's forecast temperature is off by -4° F consistently,


Well one good work around is to use the Hcrit for the height, then add to it any additional height shown in the Thermal Height Variability, parameter. Just by coincidence(?) it shows what would happen if 4 degrees hotter.


Thermal Height Variability
This parameter measures the atmospheric stability above the BL and thereby indicates the variability of the BL top (TI=0) height which can arise from (1) actual variations in surface temperature over the region encompassing a model grid cell due to surface changes, etc., (2) variations in actual surface elevation which are omitted by the smoothed topography over a model grid cell (since surface elevation changes are effectively changes in surface temperature), or (3) error in the model's surface temperature prediction. The value given is the expected height change which would be produced by a surface change of 4 degF, but is usually best evaluated in a relative sense. (Numerically this parameter is calculated as the difference between the TI=+4 and TI=0 heights - strictly speaking this only give the effect of an increase in surface temperature, since the effect of a surface temperature decrease cannot be easily estimated.) Weak stability above the BL top gives large variability values which are often good for soaring, since thermal heights due to small sub-grid-scale variations can then be much higher than the predicted average BL height. However, high variability values can also be accompanied by soaring conditions being much poorer than those predicted if actual surface temperatures are much cooler than those predicted by the model. In short, this parameter represents the uncertainty of the predicted BL height. [see the BL Variability diagram].

http://www.drjack.info/BLIP/INFO/DOC/blvariability.jpg