Enhancement of the orographic effect in summer light rains over eastern China since the early 2010s

Abstract

<p indent=0mm>In this study, the long-term changes of the precipitation orographic effect were investigated by comparing daily precipitation records for 9 mountain stations with those for 27 nearby plain stations over eastern China during summer from 1979−2019. Given a parameter of interest, e.g., the number of rainy days or precipitation amount, its orographic effect is estimated as the standardized difference between mountain and plain stations. Climatological statistics over the entire data period show evident orographic effects for all analyzed precipitation categories, including the light, moderate, heavy, and torrential rain. And their orographic effects tend to be stronger with the increase of rainfall intensity. Their long-term changes, however, experienced different characteristics. The orographic effect for moderate, heavy, and torrential rain shows no evident change in trends. In contrast, the orographic effect for light rain frequency showed a trend reversing in the early 2010s. Its linear trend is −2.24%/10a (<italic>P</italic><0.1) during 1979−2010 but becomes +34.02%/10a (<italic>P</italic><0.05) during 2011−2019. Such change results mainly from the long-term changes in mountain rainfall. Furthermore, the co-variations in meteorological factors were explored. It was found that the year-to-year variations in the orographic effect of light rain frequency was significantly related to the near surface wind speed (<italic>r</italic>=0.51, <italic>P</italic><0.01). Meanwhile, the orographic effect shows no evident relation with relative humidity, air temperature, and the total column water vapor though these factors are closely related to the mountain light rain frequency. Note that large-scale weather systems would lead to concurrent precipitation at mountain and nearby plain stations; in this case, the possible influence of wind would be hard to identify. If count only the mountain station’s rainy days which co-occur with no-rain nearby plain stations, the influence of large-scale weather system would hopefully be excluded. The mountain-only light rain frequency shows a more robust positive correlation with surface wind speed (<italic>r</italic>=0.60, <italic>P</italic><0.01). Station observation shows that the trend of near surface wind speed has also changed in the early 2010s, with a weakening trend of <sc>−0.10 m/(s </sc>10a) (<italic>P</italic><0.01) during 1979−2010 and a strengthening trend of <sc>+0.19 m/(s</sc> 10a) (<italic>P</italic><0.05) during 2011−2019. This implies that the near surface wind speed likely plays a key role in the enhancement of the orographic effect since the early 2010s over eastern China. In addition, the orographic effects in four gridded precipitation datasets with different resolution, including the CN05.1, APHRO, WFDE5 and ERA5-Land, were compared. Generally, the presence of the orographic effect is clear for light, moderate, heavy, and torrential rains in these datasets. And the orographic effect for ERA5-Land datasets, which is of the highest resolution, was the most outstanding one among four datasets. Note that the orographic effect of light rain frequency in ERA5-Land did not show any trend reversing during the entire data period. On the other hand, there is a consistent and significant relationship between near surface wind speed and the frequency of mountain grid light rain which co-occur with no-rain nearby plain grids (<italic>r</italic>=0.53, <italic>P</italic><0.01). However, the near surface wind in ERA5-Land shows no evident trend from 1979 to 2019. This bias is very likely responsible for the lack of long-term trend in orographic effect. It is a challenge to improve the assimilation and analysis skills of the boundary layer wind.