| Persistent extreme precipitation (PEP), with a high potential for causing severe disaster, represents agreat threat to the safety of both people and property, and results in substantial economic losses. Anin-depth understanding of the underlying causes and precursor signals of PEP events will be of greatimportance in improving the predication of such high-impact weather. The studies on PEP may thereforebetter guarantee the safety of people and property, support sustainable economic development, andenhance disaster prevention and mitigation capabilities.In this study, firstly, a rational definition of PEP is given, taking extremity, persistence, disaster-causingcapability and precipitation totals into consideration comprehensively. Based on this new definition, thefrequency, seasonality, persistence, intensity and climate change characteristics of individual stationbased PEP events are analyzed during1951to2010. Then, considering the temporal persistence andspatial coherence of the individual station based PEP, a method for identifying regional PEP events isdesigned. Thereafter, spatial distribution and climate change of regional PEP are discussed. The resultsshow that: Individual station based PEP events mainly occur in the Yangtze-Huai River Valley (YHRV),regions south of the Yangtze River and South China. These events are of longer duration and higherintensity compared with the events occurred in northern part of China (north of34°N). Regional eventsalso mainly take place in southern part of China (south of34°N). All the stations involved in regionalevents experience at least3-day extreme precipitation and they are concentrated spatially, constitutingthe core of precipitation amount in a whole intense precipitation process. Both individual station basedevents and regional events increasingly occurred in the YHRV and regions south of the Yangtze River after1990. During recent two decades, the individual station based events are more intense and endure longer,and the regional events tend to affect a larger area, strengthen in intensity, and last longer. Additionally,according to the occurrence time, region and dominant weather system, the70events in southern part ofChina are divided into three categories: Central-Eastern China type (including Yangtze-Huai River Valleyand regions south of the Yangtze River), South China type, and typhoon influencing type.Using a composite analysis for ‘Event-days’ and ‘Non-Event days’, two characteristic circulation patternsresponsible for persistent extreme precipitation events in Central-Eastern China are identified: a doubleblocking high type and a single blocking high type. The frequency and persistence of key elements ofanomalous circulation patterns are also analyzed to further illustrate the applicability of typical pattern. Main conclusions are summarized as follows: At500hPa, the double blocking high type is characterized bythe development of two blocking highs near the Ural Mountains and the Sea of Okhotsk, with a troughbetween them. This long-lived pattern continuously steers cold/dry air from mid–high latitudes toCentral–Eastern China, where it encounters anomalously warm/moist air from lower latitudes conveyedby intensified southeasterlies associated with a westward shift of the western Pacific subtropical high(WPSH). The single blocking high type is characterized by the appearance of a blocking high to the southof Lake Baikal. This blocking high is accompanied by a deep trough located to its east, which extends intothe southern Yangtze River Valley. The anomalous anticyclone associated with the westward extension ofthe WPSH is located further south and slightly east of that observed during the double blocking high type.Accordingly, the intensified southeasterlies convey anomalously moist air to the southern and easternparts of Central–Eastern China. In the upper troposphere (200hPa), the displacement of the South AsianHigh and jets favors robust divergences for both typical circulation patterns. The frequency analysis of thekey elements and complementary EOF analysis further confirms that both double high pattern and singleblocking high pattern derived from composite analysis are the leading modes during PEP and they areshared by a group of high impact evens, rather than only suitable for a specific case. The comparisonbetween the composites and additional cases that share similar circulation patterns but do not result inPEP in Central–Eastern China suggests that PEP events typically result from set combinations of persistentanomalies from lower to upper levels, rather than from isolated anomalies of specific weather systems.Finally, the precursor signals (about1-2weeks prior to the onset of PEP) of typical double blocking highpattern are investigated. Different phases of circulation anomalies from lower to upper levels arecomposited at biweekly (10-20days) and intra-seasonal (20-50days) time scale, by performing waveletanalysis and reconstruction technique as well as Butterworth filter. Such composites of low-frequencycomponents aim to explain why the leading time length differs in different key elements. The results showthat: obvious anomalous signals can be detected in mid-high latitudes at500-hPa about12days prior tothe onset of PEP. From12days to7days prior to PEP, Ural blocking high was intensifying and progressingeastward; meanwhile, positive anomalies originated from East Asia coast migrated to the southwest; theWPSH extended westward gradually. From6days to1day prior to PEP, Ural blocking high continued tostrengthen accompanied by a extension towards high latitudes; Okhotsk blocking high shifted to the westand further develops; the trough between these two blocking highs intruded into the regions south of40°N. Simultaneously, the WPSH advanced towards west more rapidly and its westernmost pointextended to the west of120°E. It seems that the dominance of the intra-seasonal oscillation in mid-highlatitudes circulation led these precursor signals to develop ahead of the onset of PEP about12days. At200-hPa, the South Asian High began to move to the east obviously5days prior to PEP, accompanied witheast-moving and intensifying westerly jets. These anomalous signals are mainly modulated by thebiweekly oscillations: positive geopotential anomalies expanded towards southwest and in the northwestquadrant of these positive anomalies, negative anomalies developed as well as extended to the southeast. The enhanced geopotential height gradient between them favored the acceleration and south-eastmoving of westerly anomalies, which finally enhanced the westerlies located from Eastern China to theSea of Japan. At850-hPa, the wind composites for the first five phases of biweekly oscillations resemblethe composites from7days to1day prior to the onset of PEP: an anomalous cyclone appeared inCentral-Eastern China then it weakened and finally dissipated, followed by a intensifying and north-westprogressive anomalous anticyclone. Near the onset of PEP, this anomalous anticyclone anchored in theimmediately south of Central-Eastern China. In addition to the composites of anomalies, normalizedanomalies composites are also performed to further suggest how usually large departure from normal ofthese anomalous precursor signals may be used as a tool to provide some clues in assessing the potentialfor a disaster-causing high impact. |
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