Intensified reduction in summertime light rainfall over mountains compared with plains in Eastern China Jing Yang Dao-Yi Gong State Key Laboratory of Earth Surface Processes and Resource Ecology (ESPRE), Beijing Normal University, Beijing, China

Introduction Orographic precipitation is a major water source on some continental regions in the subtropics. Trend of orographic rainfall has been paid more attention recently:  A temporal decrease in many mountain ranges of the western United States [e.g., Griffith et al., 2005], in central Israel [e.g., Givati and Rosenfeld, 2004] and at Mountain Huashan in central China [Rosenfeld et al., 2007].  Alpert et al. [2008] found that trends over central Israel are increasing which are somewhat inconsistent with other studies.

Introduction Orographic rainfall ratio: Total orographic rainfall:

Introduction Orographic rainfall ratio: If R is decreased, what could cause it? 1.P(mountain) decreases 2.P(plain) increases 3.P (plain) + a and P(plain) + a : spatially homogeneous increase in rainfall due to a large-scale process could result in a decrease in the orographic rainfall ratio. A decline in the orographic rainfall ratio does not necessarily correspond to the reduced precipitation in mountain areas.

Introduction Total orographic rainfall: Involving different rainfall-forming processes [Smith, 2003] : 1. large-scale synoptic precipitation (mostly causing medium-to- large rainfall), 2. uplifting-caused stably stratiform precipitation (mostly causing light rainfall), 3. strong small-scale topographically-induced strong convection (mostly causing heavy rainfall). The factors that underlie observed trends in total orographic rainfall are complex and difficult to resolve.

What this study do To examine the trend of orographic precipitation ratio over mountains in Eastern China To calculate the trends of the total rainfall and various grades of rainfall respectively over the selected mountains and their surrounding plain regions To evaluate the differences in light rainfall trends between the mountain and plain regions, and to discuss why

Datasets Daily station data (precipitation, surface wind) Covering the period from 1960 to 2007.

Strategy  Daily rainfalls are divided into four grades: Light: mm/day; Medium: mm/day; Large: 10-30mm/day Heavy: greater than 30 mm/day  Jun-Jul-Aug (JJA): wet season for most stations in Eastern China and has more warm-cloud precipitation

Strategy Retrieve trend: linear regression Statistical significance test: t test and Mann–Kendall trend test Assess the statistical significance of differences in trends between two groups: A t test

Strategy Rainfall amount: the daily mean precipitation rate during JJA Rainfall frequency: total number of rainfall days during JJA

How to select the mountain/plain stations in Eastern China 1.The station location east of 105 o E; 2.Historical rainfall record at the station available for the period 1960– Difference in altitude between the mountain and surrounding plain region of at least 1000m 4.The nearby urban stations were selected based on their proximity to the mountains. The small distance between each mountain and the adjacent plain region ensured a similar large-scale meteorological background for the two areas.

Nearby Plain stations

Major results

most ratios have decreasing trends (17 out of 21 ratios) with some exceptions in Group C and Group E

 A trend of typical “north drought and south wet” which is closely related to the variation in East Asian summer monsoon [e.g., Gong and Ho, 2002; Wang et al., 2008].  Among those groups with negative trend slopes, the mountain stations have larger decreasing trends than do most of their adjacent plain regions.  A trend of typical “north drought and south wet” which is closely related to the variation in East Asian summer monsoon [e.g., Gong and Ho, 2002; Wang et al., 2008].  Among those groups with negative trend slopes, the mountain stations have larger decreasing trends than do most of their adjacent plain regions.

The evident decreasing trend exists over most stations in both frequency (22 out of 28 stations) and amount (20 out of 28 stations). Decreasing trend is larger over mountains than over plain regions in each group. The evident decreasing trend exists over most stations in both frequency (22 out of 28 stations) and amount (20 out of 28 stations). Decreasing trend is larger over mountains than over plain regions in each group.

 Both the amount and frequency of light rainfall show a significant decreasing trend during the past 50 years over both mountain and plain regions.  The amount and frequency of light rainfall at mountain stations show stronger decreasing trends than at plain stations  Both the amount and frequency of light rainfall show a significant decreasing trend during the past 50 years over both mountain and plain regions.  The amount and frequency of light rainfall at mountain stations show stronger decreasing trends than at plain stations

Discussion: Q1: why is light rainfall over China suppressed?  The increase of aerosol. (e.g, Gong et al., 2007; Qian et al., 2009; Choi et al. 2008; Jin et al 2008)

the increased cloud droplet number concentration and the reduced droplet size in a polluted case the declined raindrop concentration and the delayed raindrop formation.

Discussion:  Q2 : why are the decreasing trends of rainfall over mountain larger than those over plain particularly in light rains?  what is the major difference of precipitation forming between over mountain and plain regions.

The orographic rains (especially light rains) are often caused by forced lifting of moisture air as it passes over mountains [Dore et al., 2006]; The rains over plain regions are caused by local convection [Cotton and Yuter, 2009) compared with plain rainfall forming, the orographic rainfall forming depends on wind speed [Hill et al., 1981].

 the maximum wind speed at 10 meter is decreased over most stations (24 out 27 stations) during the past half century;  the prevalence of windy days with surface daily mean wind speed more than 5m/s are decreased in most stations (25 out of 28 stations)  the daily averaged surface wind speed is reduced over more than half stations (17 out of 28 stations)

A general decline of summer wind speed over China has been also reported in some previous studies [e.g., Zou et al., 2005; Xu et al., 2006]. Reduced wind speed Decrease the lifting of moisture air into the mountain Suppress the development of stratiform precipitation Reducing water content over mountain

Discussion: Q3: what are the reasons for the wind speed reduction ?  Urbanization [e.g., Jiang et al., 2009].  However, the decreasing trends of wind speed are evident over both plain and mountain stations. Therefore, we believe that the urbanization should not be a major effect to cause the wind speed reduction over mountain regions.

Discussion:  Aerosol particles [e.g., Ackerman 1977; Jacobson and Kaufman 2006]:  Scattering aerosol particles (like sulfate) increase the air’s stability by reducing solar radiation to the ground.  Soot and soil dust absorb solar absorption enhance the air’s stability by heating the air aloft.  An increase in the air’s stability reduces vertical turbulence and the vertical flux of horizontal momentum [e.g., Archer and Jacobson, 2003] so that reduces the wind speed.  The cooling surface of the ground due to some scattering aerosols can decrease the land-sea thermal contrast in the summer, so that reduce the summer monsoon intensity over China (Xu et al. 2000)

Conclusion Decreasing trends in the frequency and amount of light rainfall over mountains are greater than those over nearby plain regions. Wind speed has weakened in recent decades at both mountain and nearby urban stations, indicating reduced orographic lifting and consequently suppressed mountain rainfall (especially light rainfall).

Implication whether/how the decreasing wind speed in East China is related to the air pollution needs further observation and simulation studies.

Yang J., Gong DY 2010: Intensified reduction in summertime light rainfall over mountains compared with plains in Eastern China. Climatic Change 100:807–815 DOI /s

Separating Nighttime rainfall and daytime rainfall