![]() It is variations of this “base value” that we wish to isolate with our methodology. For the radar product analyzed the averaged total reflectivity is around 20 dbZ under normal conditions and is evident in the plot as the most frequent value. Around -33h and - 9h there was precipitation, as indicated by the Doppler corrected reflectivity approaching the total reflectivity. Around -42 h there is reduced clutter and images suggest sub-refraction. At times of -44 h, -25 h and - 4h there appeared to be super-refraction, giving increased ground clutter. It was selected because it shows a wide variety of variations, but no single deviation is remarkable. The period shown covers two days, with times given relative to the end time. The Exeter radar is surrounded by farmland and is not far from one of the Great Lakes, which can apparently result in strong temperature and humidity gradients in the boundary layer. 4 illustrates some of the complications due to refraction issues for the Exeter radar (CWSO). Time series of averaged total reflectivity and Doppler corrected reflectivity (yellow) around the Exeter Radar for 48 hours preceding 0400 UTC. If we don’t get the expected Gaussian distribution of dBTclt that is an indication that the orography of the analyzed radar site is not suitable for our monitoring methodology, or the technical condition of the radar measurement were unstable. However, because there are many factors during a year (season) that can change the averaged clutter reflectivity value, for example refraction or attenuation in precipitation, in reality we will have a Gaussian distribution such as the one shown in Fig. In the ideal situation for unchangeable clutters, the obtained dBTclt will have the same value after each volume scan. As it was mentioned in Section 2.1, all clutter bins with reflectivity smaller than 15 dBT were set to zero. After the clutter bins were detected (Section 2.1) it is straightforward to obtain a mean uncorrected clutter reflectivity dBTclt after each volume scan as an arithmetic average of reflectivity from all clutter bins. It is a trade off between usage time and sensitivity and finding the right balance without losing the accuracy of the methodology requires a fine-tuning for each radar site. A narrower range is a more sensitive monitoring tool and vice versa. The range width is actually related to the sensitivity of the methodology for radar performance monitoring. 2 which dBTmin and dBTmax values correspond to it. We can increase the Ncrt2 value until we reach a satisfied usage time and see from the Fig. If we are unsatisfied with the obtained usage time we can do the reverse of the described procedure. The ratio of the number of points in the bin range (Ncrt1, Ncrt2) and total number of points in Fig.2 gives the expected time (usage time) when we could have ‘clutter clear weather’. All points above this critical bin number value (blue colored) are unsuitable for our ground clutter analyses since the weather reflectivity compared with the clutter echo cannot be ignored. 2 scatterplot, we can determine which number of bins corresponds to the acceptable range and that is our desired Ncrt2 value. Applying established dBTmin and dBTmax values in the Fig. For arbitrary coverage of data, lets say 95%, we can find a range, dBTmin and dBTmax values, that define a chosen subset of data. The upper critical number of bins (Ncrt2) per volume scan we can estimate analyzing the clutter reflectivity frequency distributions of dBTclt, which we are expecting to have Gaussian shape, Fig. Points below this line indicate a radar failure. The lower critical number of radar bins is a total number of ground clutter bins (Ncrt1). The horizontal green lines defines a critical range of bins when is ‘clutter clear weather’. Each point in the graph was obtained from a single volume scan. In figure 2, we presented a scatterplot of the total number of bins (Nall) versus the mean uncorrected reflectivity from clutter bins (dBTclt) only.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |