Three-Dimensional Circulation Structure of Summer Heavy Rainfall in Central North China

Abstract

Based on daily rainfall observations and Japanese 25-year Reanalysis Project data during ;1981-2010, a three-dimensional circulation structure that formed before heavy summer rainfall in central north China (CNC) is revealed in this study. Composite analyses of circulation in advance of 225 heavy rain days show that the circulation structure is characterized by a remarkable upper-tropospheric warm anomaly (UTWA), which covers most of northern China with a center at ;300 hPa. Under hydrostatic and geostrophic equilibriums, the UTWA contributes to the generation of an anticyclonic (cyclonic) anomaly above (below). The anticyclonic anomaly strengthens (weakens) westerly winds to the north (south) of the warm center and pushes the highlevel westerly jet to the north. The cyclonic anomaly deepens the trough upstream of CNC and intensifies lower southwesterly winds to the mideast of the warm center. As a result, the northerly stretched high-level jet produces upper divergence in its right-front side and the intensified southwesterly winds induce lower moisture convergence in its left-front side, causing heavy rainfall in CNC. Correlation analyses further confirm the close connections between UTWA and circulation in the upper and lower troposphere. The correlation coefficients between UTWA and the upper geopotential height, upper westerly jet, and lower southerly flow reach 0.95, 0.70, and 0.39, implying that the two critical factors leading to intense rainfall in CNC, the high-level jet and the low-level southerly flow, are closely connected with the UTWA. Consequently, in the future analyses and forecasts of heavy rainfall over northern China, more attention should be paid to the temperature in the upper troposphere.

Two major circulation structures leading to heavy summer rainfall over central North China

“…Similar to the study of Sun et al . [], the daily records are converted to rainfall amounts from 0800 Beijing time (BJT) of the present day to 0800 BJT of the next day, and the large‐scale circulations are described by 6‐hourly Japanese 25 year reanalysis data [ Onogi et al ., ] with the same period. The reanalysis data set is 1.25° × 1.25° horizontally and contains 23 vertical levels.…”

“…To investigate the precursor circulation signals associated with heavy rainfall events, all circulations are depicted by the reanalysis data at 0800 BJT on heavy rainfall days, which are just in advance of the daily heavy rainfall events (0800–0800 BJT) in this study. Considering that the complex topography over NC may interfere with the effects of circulations on rainfall, and the observations in the Beijing‐Tianjin‐Bohai region are more capable of reflecting the heavy summer rainfall over NC [ Sun et al ., ], we selected 41 stations with altitudes less than or equal to 100 m around Beijing and Tianjin (38.5°–40.5°N, 116°–118°E, red dashed rectangle in Figure b) to represent CNC. To weaken the influences of preceding rainfall on circulations, the heavy rainfall days are defined as the dates when at least three stations in CNC appear in the 90th percentile of daily heavy rainfall, and no stations in CNC appear in any rainfall during the 6 h before the day started (0800 BJT of the day).…”

“…As the upper tropospheric temperature has important influences on heavy rainfall in NC [ Sun et al ., ], and the lower tropospheric circulation controls moisture transport, the circulation structure is preliminary revealed through Singular Value Decomposition (SVD) analysis of 300 hPa temperature and 700–850 hPa geopotential height on the 91 heavy rainfall days, and further analyzed by composite analyses of circulations with consistent major characteristics as the leading modes of SVD.…”

“…The first mode is in high accordance with the composite result of Sun et al . []: prominent 300 hPa warm anomalies cover most areas of NC before the heavy rainfall; westerly (easterly) anomalies lie in the upper north (upper south) of the warm center at 200 hPa; southerly anomalies appear in the lower south of the warm center at 700–850 hPa; and NC is accompanied by extensive positive rainfall anomalies (Figure a). This high consistency and the large explained covariance (44.8%) substantiate the study of Sun et al .…”

“…Besides, combining the upper tropospheric temperature with circulation characteristics, Sun et al . [] discovered that the chief circulation structure leading to heavy rainfall in CNC is dominated by an upper tropospheric warm anomaly (UTWA). As a result, whether there are other circulation structures in association with upper tropospheric temperature anomalies for heavy rainfall in CNC and what effects of different circulation structures would have on the rainfall events become the following issues to be addressed.…”