Wet-day Frequency Research Articles

Climatological Thresholds of Daily Rainfall

This technical note summarizes the results from daily rainfall data analysis with relation to annual rainfall amounts. By investigating the behavior of correlation coefficients between the annual rainfall amounts and the within-the-year cumulative frequency distribution functions of wet days with various threshold rainfall depths, it was found that (1) the annual rainfall amount is weakly correlated with the number of either dry or wet days—thus, the zero rainfall depth is not a proper threshold for the explanation of dry or wet years; and (2) there exist certain thresholds of daily rainfall depth (the climatological thresholds) with which the cumulative frequency of wet days can be well related with the annual rainfall amount. Also with this threshold, the wet and dry years can be seen divided clearly into separate groups of the cumulative frequency distribution functions, without any overlaps, which were common when using the zero rainfall depth as a threshold. Because it is found that there exist certain thresholds to be used for explaining the annual rainfall amount changes, these might also be used effectively for the analysis of possible changes in flood or drought patterns caught by global warming.

Changes in average and extreme precipitation in two regional climate model experiments

Two regional climate model experiments for northern and central Europe are studied focussingon greenhouse gas-induced changes in heavy precipitation. The average yearly maximum onedayprecipitation Pmax shows a general increase in the whole model domain in both experiments, although the mean precipitation Pmean decreases in the southern part of the area, especially inone of the experiments. The average yearly maximum six-hour precipitation increases evenmore than the one-day Pmax, suggesting a decrease in the timescale of heavy precipitation. Thecontrast between the Pmean and Pmax changes in the southern part of the domain and the lackof such a contrast further north are affected by changes in wet-day frequency that stem, at leastin part, from changes in atmospheric circulation. However, the yearly extremes of precipitationexhibit a larger percentage increase than the average wet-day precipitation. The signal-to-noiseaspects of the model results are also studied in some detail. The 44 km grid-box-scale changesin Pmax are very heavily affected by inter-annual variability, with an estimated standard errorof about 20% for the 10-year mean changes. However, the noise in Pmax decreases sharplytoward larger horizontal scales, and large-area mean changes in Pmax can be estimated withsimilar accuracy to those in Pmean. Although a horizontal averaging of model results smoothsout the small-scale details in the true climate change signal as well, this disadvantage is, in thecase of Pmax changes, much smaller than the advantage of reduced noise.

Representing Twentieth-Century Space–Time Climate Variability. Part II: Development of 1901–96 Monthly Grids of Terrestrial Surface Climate

The authors describe the construction of a 0.58 lat‐long gridded dataset of monthly terrestrial surface climate for the period of 1901‐96. The dataset comprises a suite of seven climate elements: precipitation, mean temperature, diurnal temperature range, wet-day frequency, vapor pressure, cloud cover, and ground frost frequency. The spatial coverage extends over all land areas, including oceanic islands but excluding Antarctica. Fields of monthly climate anomalies, relative to the 1961‐90 mean, were interpolated from surface climate data. The anomaly grids were then combined with a 1961‐90 mean monthly climatology (described in Part I) to arrive at grids of monthly climate over the 96-yr period. The primary variables—precipitation, mean temperature, and diurnal temperature range—were interpolated directly from station observations. The resulting time series are compared with other coarser-resolution datasets of similar temporal extent. The remaining climatic elements, termed secondary variables,were interpolated from merged datasets comprising station observations and, in regions where there were no station data, synthetic data estimated using predictive relationships with the primary variables. These predictive relationships are described and evaluated. It is argued that this new dataset represents an advance over other products because (i) it has higher spatial resolution than other datasets of similar temporal extent, (ii) it has longer temporal coverage than other products of similar spatial resolution, (iii) it encompasses a more extensive suite of surface climate variables than available elsewhere, and (iv) the construction method ensures that strict temporal fidelity is maintained. The dataset should be of particular relevance to a number of applications in applied climatology, including large-scale biogeochemical and hydrological modeling, climate change scenario construction, evaluation of regional climate models, and comparison with satellite products. The dataset is available from the Climatic Research Unit and is currently being updated to 1998.

Representing Twentieth-Century Space–Time Climate Variability. Part I: Development of a 1961–90 Mean Monthly Terrestrial Climatology

Abstract The construction of a 0.5° lat × 0.5° long surface climatology of global land areas, excluding Antarctica, is described. The climatology represents the period 1961–90 and comprises a suite of nine variables: precipitation, wet-day frequency, mean temperature, diurnal temperature range, vapor pressure, sunshine, cloud cover, ground frost frequency, and wind speed. The climate surfaces have been constructed from a new dataset of station 1961–90 climatological normals, numbering between 19 800 (precipitation) and 3615 (wind speed). The station data were interpolated as a function of latitude, longitude, and elevation using thin-plate splines. The accuracy of the interpolations are assessed using cross validation and by comparison with other climatologies. This new climatology represents an advance over earlier published global terrestrial climatologies in that it is strictly constrained to the period 1961–90, describes an extended suite of surface climate variables, explicitly incorporates elevation...

Studies in fleece-rot of sheep: some ecological aspects.

Previous work on this subject has indicated the importance of climatic factors, particularly rainfall, in the development of fleece-rot. Over an 8-year period 19 outbreaks were recorded on the F. D. McMaster Field Station, C.S.I.R.O. Of these, 16 occurred during calendar months which had eight or more wet days and 4 or more in. of rain. Using these data as criteria, rainfall records for the period 1911-1940 were examined in order to give broad definition to zones in which fleece-rot might be expected as a rare, or frequent, occurrence. Records were taken from 18 selected towns in south-eastern Australia. In the zone in which occurrence of the condition was expected to be high, there were differences between localities as to the season of the year at which expectation was highest, in the frequency of wet days per month, and in the intensity of daily rainfall. In some localities shearing during certain months of the year would assist sheep to escape fleece-rot, as they would be in short wool when it was most likely to occur.