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[1] Short term precipitation forecasts based on Lagrangian advection of radar echoes are robust and have more skill than numerical weather prediction models over time scales of several hours. This is because the models do not generally capture well the initial precipitation distribution. We will refer to the advection-based methods as radar nowcasts. Over longer time scales, we expect the models to perform better than nowcast methods as they resolve dynamically the large scale flow. We verify this conceptual picture of the relative accuracy of radar nowcasts and model forecasts using conventional skill scores. We identify the cross-over point in time where model forecasts start to have more skill than nowcast methods. This occurs at about 6 hours after the forecast is initiated. Citation: Lin, C., S. Vasić, A. Kilambi, B. Turner, and I. Zawadzki (2005), Precipitation forecast skill of numerical weather prediction models and radar nowcasts, Geophys. Res. Lett., 32, L14801,

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To satisfy the needs of the meteorological and aeroecological communities wanting a simple but effective way of flagging each other’s unwanted echo for a variety of different operational radar systems, we evaluated the ability of an estimate of depolarization ratio (DR) based on differential reflectivity (ZDR) and copolar correlation coefficient (ρHV) measurements to separate both types of echoes. The method was tested with data collected by S- and C-band radars used in the United States and Canada. The DR-based method that does not require training achieved 96% separation between weather and biological echoes. Since the misclassifications are typically caused by isolated pixels in the melting layer or at the edge of echo patterns, the addition of a despeckling algorithm considerably reduces further these false alarms, resulting in an increase in correct identification approaching 99% on test cases.

CONCEPTLet us consider the simplest radar equation involving the index of refraction n: the time t taken by electromagnetic waves to reach a target at range r and return to the radar is t = 2r nk,,, with c,, being the speed of light in vacuum. For fixed targets, r is constant and only n varies; hence if t could be measured precisely for such targets, the average value of the refractive index over the path between the radar and these targets could be determined.Unfortunately, most radars cannot measure t and site surveys are not accurate enough to determine r with the part-per-million accuracy required to obtain useful information about n. However, if the range to the target is fixed, but only known to a fair accuracy, say better than 1 %, it would be enough to allow us to relate changes in t to changes in n; the absolute calibration would then have to be done by other means. With this scheme, we are only required to determine changes in t that can be obtained by measuring the phase of the target. 1 a) n = n, Fig. I . Illustration of the effect of changes in index of refraction on the phase of ground targets. The phases offive arbitrary targets ( T I . . . T,) at an initial condition (a) and their change as refractivity increases up to a given range and decreases beyond (b) are presented. The dials illustrate either the current phase of the targets (top), or the difference between the current phase of the targets and the reference phase when n = no (bottom).