| To examine the performance of the high-resolution Climate Prediction Center Morphing Technique (CMORPH) precipitation estimates, a comparison of six-year(2004-2009) mean summer(JJA, June, July and August) precipitation amount, frequency, and intensity between the CMORPH data and rain gauge measurements is conducted. The gauge reports used here are collected from450stations located in eastern China. Then, the hourly precipitation data from CMORPH in summer during2004-2009is used to characterize the spatial patterns and diurnal cycles of summer precipitation in several urban zones, including the three city clusters in eastern China and several others around the world. By comparing the precipitation in urban areas and surrounding areas, the possible impacts of urbanization on precipitation are investigated.To understand and evaluate better the impacts of urbanization on the summer precipitation, high-resolution sensitivity experiments were conducted in summer (JJA) over a5-year period(2003-2007) in Yangtze River Delta(YRD) by using the Weather Research and Forecasting (WRF) model coupled with a single-layer Urban Canopy model(UCM). Evaluation using TRMM precipitation data shows that this configuration of WRF reproduced the summer precipitation reasonably well in this region. Then, two parallel tests were performed by prescribing two different urban land-use scenarios in the YRD region:in the control case, land cover determined by the MODIS satellite observations in2001was employed to represent current urbanization conditions; while in the other test, all urban cover in the YRD region was replaced by cropland surface, representing conditions without urbanization. By comparing the two tests, the urban impacts on diurnal variations of precipitation in the urban areas are explored.The major findings are summarized as follows: 1. The CMORPH data is comparable to rain gauge data in revealing the spatial patterns of6-year average JJA precipitation. And the CMORPH product is capable of capturing the diurnal variations of precipitation with reasonable quality.The CMORPH summer precipitation shows large-scale patterns that resemble rain gauge observations, such as shapes and locations of three rainbands located in northern-northeastern China, between the Yangtze and Yellow Rivers and in south China. However, relative to the rain gauge observations, the CMORPH data set overestimates the precipitation hours over most of eastern China but underestimates the precipitation amount and intensity.The diurnal variations of precipitation in summer derived from CMORPH are similar to that from rain gauge observations. For example, a late-afternoon maximum of rainfall and rain hours over most regions in the southern, northern and northeastern China, and a double peak pattern of rainfall and rain hours in the middle and lower reaches of the Yangtze River are seen in both CMORPH and rain gauge products. As shown by the rain gauge records, a late-afternoon peak of rain intensity dominates in northeastern China, and the diurnal phase of rain intensity changes eastward along the Yangtze River Valley. However, there are some differences between CMORPH and rain gauge observations in diurnal phase. For instance, the early morning peak observed in the areas between the Yangtze and Yellow Rivers does not appear in CMORPH.2. Analysis of6-year(2004-2009) average summer rainfall rates derived from CMORPH indicates that precipitation in and around urban areas is much stronger and more when compared to surrounding areas in city clusters located in eastern China. Similar precipitation intensity centers are found in other city clusters around the world.The spatial distribution of summer precipitation averaged over the period of6years suggests the existence of elevated regions of rainfall rates in and around urban areas in YRD. And the strongest precipitation centers appear in the three most developed urban areas stretching from the northwest to the southeast in YRD, denoted as Shanghai, SuXichang and NingZhenyang. These precipitation centers show a maximum intensity of2.4mm/hr, which is8%higher than that of surrounding non-urban areas with similar or greater topographic relief. Therefore, urban impacts on these precipitation centers are suggested. Similarly, in the northeastern suburbs of Beijing city or urban areas in the Pearl River Delta, where it is closer to their nearby mountainous regions, the stronger precipitation are observed. The average percentage increases in mean rainfall rates in these centers over their surrounding areas with similar or greater topographic relief are15%and11%, respectively. And the combined effects of urbanization and topographic relief might contribute to such centers.In all of the three urban zones mentioned above, the spatial patterns of summer precipitation frequency essentially correlate to their topography. It can be clearly seen that rain hours over the mountains are much more than that over the plains, while there is no difference in rain hours between urban areas and surrounding areas. So. the urbanization in city clusters may have few impacts on precipitation hours. However, precipitation amount are the combination of precipitation frequency and intensity. Though the maximum rainfall is observed over mountainous areas where there exist most frequent precipitation, several weak rainfall centers are also observed in the urban areas where rain intensity centers appear.The rainfall rate centers are also evident in and around cities in London urban zone, Paris urban zone and the coastal urban zone located in northeastern America, about23%,8%and10%greater than the surrounding areas respectively.The diurnal phases of the spatial mean precipitation intensity in the hypothesized urban-impacted regions are consistent with those in surrounding areas. But in most of the day, the rainfall rates are greater in the hypothesized urban-impacted region. For example, in the coastal urban zone located in northeastern America, the diurnal peak in the hypothesized urban-impacted region is30%larger than in the other areas.3. The simulation results from the high-resolution sensitivity experiments in summer over a5-year period(2003-2007) in YRD using WRF/Noah/UCM suggest that urbanization in this region could lead to a reduction of light rain events around noon-time and an increase of convective precipitation in the late-afternoon. Due to urbanization, water vapor in the lower atmosphere is decreased in the morning, and as a result, the light rain events decreased around noon-time. However, in the afternoon, the combined effects of heat islands and strengthened convergence dominate, which enhances the convection in urban areas. Thus, the convective precipitation in this region is increased in the late-afternoon.urbanization can have significant effects on thermal distribution, dynamic process and moisture transportation of the atmosphere. The imperviousness of the urban surface leads to less evaporation, and decreased green vegetation coverage in the urban areas results in less transpiration. The combination of them may lead to the decrease of moisture, which is known as "urban dry island". The average warm-season water vapor mixing ratio at2m may decreases by1.4g/kg at most. Meanwhile, the mean near-surface temperature in much of the Yangtze River Delta rises due to urbanization, the temperature at2m increases by more than1.6K in certain parts. This strengthens the vertical movement in this region. Moreover, the surface convergence is enhanced due to the increased surface roughness in the urban areas. As a consequent, the convection in urban areas is augmented.During0800-1100local standard time(LST) in the morning, the "urban dry island" effect dominates in the urban area, and this leads to a reduction of low cloud amount. The cloud water condenses and then rain water appears. So the decrease of rain water lags that of the cloud water. Consequently, the rain water mixing ratio declines, later, at noon(1100-1400LST). So does the number of light rain events. The number of summer light rain events decreases on average by as much as3in some areas at noon.After14:00, the impacts of urbanization on water vapor in the lower troposphere weakens. Whereas, during1400-1700LST, the maximum updraft induced by urban heat islands occurs in cities. Meanwhile, the convergence is enhanced due to the increased surface roughness in the urban areas. So the convection in the urban areas reaches its daily peak. Consequently, more water vapor can be conveyed from lower troposphere to higher level and the convective cloud increases. Then more rain water is formed, and the mean summer rainfall of higher-order precipitation over urbanized areas is enhanced on average by15%during1700-2000LST. |
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