WMOD00511 - Laserfiche WebLink

Home Browse Search

~>~....,' '....... so. ftI ~~...._.....,~u..'l-j,o>".~...._~_..,. .~.. , PRECIPITATION GAUGE SITES 687 ~1[Z]6. 10 . . . o o 10 20 "'"[2] "'I2J o 10 20 o 10 <XR 20 I j . ! I I t i ~ i ( f Figure 5. Plots of weighted means of vertical angle Cl of obstacles against the arithmetic mean of eight sectors, ClS' From the left: the mean of 16 sectors versus the mean of eight sectors; the weight mean, ClSw' by the doubled values of westerly sectors (NW, W, SWI versus the mean of eight sectors, ClS; and the mean of three westerly sectors Cllw .versus the mean of eight sectors cxs possible in cases where there is a series of pictures of the gauge site taken from various sectors by different people in consecutive years, complemented by drawings and written reports on site peculiarities, noting possible effects of the surrounding objects on GSE. Even in such cases errors can still occur when the :x values of adjoining sectors vary considerably. A further possible source of error stems from pictures taken that seem more to reflect the investigator's concept how the GSE for a particular sector should appear, rather than the reality. In Figure 5, the weighted means of ex are plotted against the eight-sector arithmetic mean. It shows that the difference between the averages of:x computed for 16 and for 8 sectors is small. The latter seems to slightly overestimate the 0: average. Almost the same can be said of the doubled values for westerly sectors. They do not change the average considerably. The difference rarely exceeds a value of :t I o. However, the difference between the eight-sector 0: and the arithmetic average ex of the three westerly sectors is absolutely clear. It seems that the more-or-Iess protected sites are relatively better protected, particularly from westerly precipitation-bearing winds. In contrast, for more-or-less open sites the westerly sectors are even more 'exposed' than other sectors. DISCUSSION The relative accuracy of GSE class estimates from the station history by trained personnel, of less than 0,5 ofa GSE class out of four, shows the practicability of the suggested method. Brown and Peck (1962) pointed out that two meteorologists who had the opportunity to discuss and jointly classify several gauge sites usually did not vary by more than one category out of seven. This conforms with the results obtained in the present study. Interestingly the error in GSE estimate of :to'5 of a class results in an error for wind-induced losses of between :t 0,5 and :t 1.0 per cent in the summer season, from April to September, and between :t 1.0 and :t 2.0 per cent in the winter season, from October to March. These figures represent the maximum error in the lowlands, up to 600 m a.s.l. and for wind speeds up to 4 m s - t. In contrast, for wind speeds up to 6 m s - 1 and at altitudes up to 2000 m a.s.l., they can be three times greater. The accuracy of GSE estimates can be increased if the classification is made by personnel who frequently visit the sites. Of course, the best method of classification is the direct measurement of 0: or by assessing photographs taken by a camera fitted with a fish-eye lens from the gauge orifice. Here, the arithmetic average based on eight wind directions of the wind rose can be taken as a reference. An average based on 16 directions produces only a small improvement. The establishment of four GSE classes, as shown in Table I, will be sufficient for most practical purposes. An additional three interim classes can be used in windy regions, in the mountainous areas, and for research purposes. The possibility of using weighted averages of ex in the direction of precipitation-bearing winds must also be considered. . APPLICATIONS The results of this study show that it is possible to classify the GSE fairly accurately from the station history records. Providing such records have not been lost or destroyed, there are prospects for dealing with