MM5 contrail forecasting in Alaska
Martin Stuefer, Xiande Meng and Gerd Wendler
Geophysical Institute University of Alaska Fairbanks, Alaska 99775
Abstract: 2005, paper in special issue of Monthly Weather Review on
High Latitude Numerical Weather Prediction; MWR, Vol. 133, No. 12, pages 3517-3526.
Fifth-generation mesoscale model (MM5) is being used for forecasting the atmospheric layers of aircraft condensation trail (contrail) formation. Contrail forecasts are based on a conventional algorithm describing the adiabatic mixing of aircraft exhaust with environmental air. Algorithm input data are MM5 forecasted temperature and humidity values at defined pressure or sigma levels, and an aircraft relevant contrail factor that is derived statistically from a contrail observation database.
For comparison purposes we introduce a mean overlap (MO), which is a parameter quantifying the overlap between forecasted contrail layers and contrail layers derived from radiosonde measurements. Mean overlap values are used as test for the altitude and thickness of forecasted contrail layers. Contrail layers from Arctic MM5 and Airforce Weather Agency (AFWA) MM5 models agree well with contrail layers derived from corresponding radiosonde measurements for certain forecast periods; a steady decrease of the MO shows a decrease of contrail forecast accuracy with the increasing forecast period. Mean overlaps around 82% indicate reasonable results for the 48 hours forecasts. Verification of MM5 with actual contrail observations shows slightly better performance of Arctic MM5. A possible dry bias might occur in humidity measurements at low temperature levels due to temperature dependence errors of the humidity sensor polymer, which might also affect forecasts of humidity of the upper troposphere or lower stratosphere. Despite this fact, our contrail verification study shows hit rates higher than 82% within forecast periods up to 36 hours using Arctic MM5.