Geoscience Reference
In-Depth Information
5. Radar propagation condition forecasting
This section deals with anomalous propagation forecasting using mesoscale numerical
weather prediction models. It is illustrated with several examples, discussing capabilities
and limitations found in this application.
5.1 VRG forecasts
Anticipating the occurrence of AP may be an advantage for monitoring purposes of radar
quality control or to obtain a deeper understanding of processes related to anomalous
propagation. Numerical Weather Prediction (NWP) systems provide the capability to obtain
forecasts of propagation conditions from temperature and humidity forecast profiles in a
similar way as they are obtained from radiosonde observations. Despite NWP systems allow
to study anomalous propagation events with more spatial detail than that given by the
synoptic radiosonde network, they have a number of accuracy limitations that may hamper
the operational production of AP forecasts. For example Bech et al. (2007a) compared 4
months of vertical refractivity gradient forecasts over Barcelona retrieved from numerical
model output of the MASS system (Codina et al. 1997a, 1997b; Koch et al. 1985) with actual
radiosonde observations and found a systematic bias of the model towards subrefraction
(Fig. 14).
In order to reduce the bias, a simple heuristic approach was suggested combining linearly
model output and previous radiosonde observations. As illustrated in the Taylor diagram
(Taylor, 2001) shown in Fig. 15, the modified forecasts, labelled here as H2b, H4b, H6b and
H8b, produced better results in terms of RMS and correlation compared to the original
forecasts (MASS).
Fig. 14. Time series of Vertical Refractivity Gradient (VRG) over Barcelona from NWP-
derived forecasts (dashed line) and radiosonde-based diagnostics (solid line).
