Preliminary Investigation Of Extreme Weather And Climate Events In China

Extreme weather and climate events are usually referred to the events characterized by the occurrence with low frequency in the sense of statistics (IPCC,2007). In recent years, there is an increasing concern on weather and climate extremes, since they may cause serious disasters to human society and nature. This work is aimed to provide a preliminary investigation of the variations of extreme weather and climate events (including temperature extremes, precipitation extremes and tropical cyclones) in recent years over the area of China. The conclusion is summarized as follows:1. Variations of temperature extremes have been investigated over the area of China for48years (1960-2007). The48years are firstly divided into two periods with the first period from1960to1989and the remaining for the second. During the second period, four percentile-based indices derived from daily minimum and maximum surface air temperature (total numbers of meteorological observing stations is532) are computed and compared to the corresponding values of the first period. According to the results, within the entire country, the occurrence frequencies of annual cold nights and warm nights (derived from daily minimum temperature) have respectively changed by-5.4%and5.3%, while relatively smaller variations of-2.7%and3.4%for annual cold days and warm days (derived from daily maximum temperature) have been found in recent two decades. Among various regions, most notable decreases of cold extreme frequencies are found in Northwest China, North China and Northeast China, whereas warm extremes increase profoundly in South China, Southwest China, North China and Northwest China. For seasonal changes, the frequency decreases of cold extremes mainly occurr in Winter. However, warm night frequency displays remarkable increasing in Winter and Summer, warm days vary significantly mainly in Autumn. The monthly temperature extremes except warm days all have the largest variations in February.2. Variations of precipitation extremes have been investigated under the rapid warming over China during the two previously mentioned periods. Two percentile-based indices derived from daily precipitation (total numbers of meteorological observing stations is532) are computed and compared. Annual extreme precipitation amount and extreme precipitation frequency have separately increased by10.9mm and0.12times, with significant increases in the mid-lower Yangtze River valley and Northwest China but relatively slight variations in Northeast China and North China. Considerable increases are also found in South China and Southwest China, yet they are not statistically significant. Seasonally, precipitation extremes increase significantly in the rainy season from April to September and exhibits similar spatial patterns to those of annual extremes. However, in the dry seasons from October to next March, significant increases of precipitation extremes are detected in Northeast China and North China instead. Monthly extreme precipitation amount displays most remarkable increases in June and July. Dissimilarly, monthly extreme precipitation frequency displays remarkable increases in January, February and March.3. In order to better understand the relationship between environmental factors and tropical cyclone (TC) activity in the western North Pacific (WNP). The sea surface temperature (SST),1000hPa temperature, specific humidity index (between925-500hPa) and vertical wind shear (between200-850hPa) are analyzed at first over the WNP region during a time period from1970to2001. However, the indices above either merely represent the oceanic warming (such as SST), or only reflect the atmospheric conditions. Based on these analyses, two new indices are proposed in this study. One is the difference between1000hPa temperature and SST (ΔT) which represents Neumann boundary condition. The other reflects static stability of air between level850hPa and1000hPa (Δθse). It is found that ΔT is of significant impact on TC frequency, while both ΔT and Δθse play important role in accumulated cyclone energy.4. Interannual variability of landfalling TCs in China during1960-2010has been investigated. By using the method of partial least squares regression (PLS-regression), canonical ENSO and ENSO Modoki are identified as the factors that contribute to the interannual variability of landfalling TCs. El Nino Modoki years are considered to associate with a greater-than-average frequency of landfalling TCs in China, but reversed results are obtained in canonical El Nino years. Significant difference in genesis locations of landfalling TCs in China occurs dominantly in the northern tropical WNP for the two types of climate change. The patterns of low-level circulation anomalies and outgoing longwave radiation (OLR) anomalies associated with landfalling TC genesis within El Nino and El Nino Modoki years have been examined as well. During canonical El Nino years, a broad zonal band of positive OLR anomalies dominates the tropical WNP, while the circulation anomalies exhibit a meridionally symmetrical dipole pattern with an anticyclonic anomaly in the subtropics and a cyclonic anomaly near the tropic. However, In El Nino Modoki years, a vast region of negative OLR anomalies, roughly to the south of25°N with a strong large-scale cyclonic anomaly over the tropical WNP, providing a more favorable condition for landfalling TC genesis compared to its counterpart during canonical El Nino years. As more landfalling TCs formed in the northern tropical WNP in El Nino Modoki years, more TCs make landfall on the northern coast of China than in canonical El Nino years as well. The number of landfalling TCs is slightly above the normal value. In addition, enhanced convection is found in the South China Sea and the west of the tropical WNP, which results in landfalling TCs forming more westward than in canonical La Nina years. During La Nina Modoki years, owing to an unfavorable condition for TC genesis persisting in a broad zonal band from5°N to 25°N, the landfalling TC frequencies are below normal. Since in La Nina Modoki years the western North Pacific subtropical high is located in the westernmost region, TCs mainly make landfall on the south coast of China.5. In order to investigate the possible mechanism for the frequent occurrence of extreme weather and climate events, variations of typical scale of atmospheric motion at low to middle latitudes (20～60°N) over the northern hemisphere during the period1948-2010are analyzed. It is found that the terrestrial typical scale of atmospheric motion shows a significant decrease, whereas in marine areas this decrease trend is not that significant. It is worthy to mentioning that this downward trends of typical scale of atmospheric motion are significant both at land and sea since the1990s. We choose the percentage of rainstorm (the ratio of rainstorm amount to total precipitation amount) in the rainy season from April to September over eastern China during the period1960-2007as an example to show the relationship between typical scale of atmospheric motion and the outbreak of extreme weather and climate events. The conclusions are that the percentage of rainstorm shows a significant increasing trend, whereas the typical scale of atmospheric motion shows an opposite change in long-term trends. The percentage of rainstorm and typical scale of atmospheric motion show the same anti-phase relationship. Moreover, since the mid-1980s, this negative correlation between them becomes particularly significant.