Identification and Forecasting of the Potential for the Occurrence of Extended-Range Vhf-Uhf Transhorizon Propagation from Synoptic-Scale Data.

An objective method is presented for identification of tropospheric areas with a potential for supporting extended -range VHF-UHF transhorizon propagation (ERTP), and for forecasting the potential for changes in this structure over short periods of time. Rawinsonde data of unique accuracy and sampling interval obtained from the third Atmospheric Variability Experiment (AVE III) sponsored by the National Aeronautics and Space Administration were used in an evaluation of the contribution of sub-synoptic-scale processes to variations in the refractive index structure as a function of time, space, air mass, and geography. AVE III data also were used in an evaluation of the contribution of the kinematic processes of vertical motion, temperature advection, moisture advection, differential temperature advection, differential moisture advection, and horizontal velocity divergence to temporal and spatial variations of the time rate-of -change of the vertical gradient of refractivity (VGR). Because of the small contribution from sub-synoptic-scale processes (less than 10 per cent except in frontal zones or convective regions) rawinsonde data may be used to determine the VGR over a region. Synoptic-scale differential moisture advection is most often the major contributor to the time rate-of -change of the VGR. An evaluation of the vertical structure of the gradient of refractivity (VSGR) along paths which support ERTP was conducted using conventional rawinsonde data collected during periods where ERTP had been observed. It was determined that the potential for ERTP is indicated by the presence of an elevated layer which is continuous along the propagation path and through which the VGR is less than 1.4 times the standard gradient for the layer. The effectiveness of forecasting the VGR through a given layer on the basis of synoptic-scale kinematics at observation time was evaluated using AVE III data. This evaluation revealed that this forecast method shows skill in forecasting the VGR over periods of at least 3 h and over longer periods in some instances. An objective method for the identification of tropospheric areas with a potential for supporting ERTP, and for forecasting the potential for changes in the required VGR structure over short periods of time was developed on the basis of the analyses described above. Evaluation of this method using an independent data set collected during a period of persistent ERTP revealed that the method accurately identified the potential for the occurrence of ERTP in the region where it had been observed, and forecasted the conditions to persist.