Trends in pre‐summer frontal and diurnal rainfall activities during 1982–2012 over Taiwan and Southeast China: characteristics and possible causes

Abstract

Frontal convection (FC) and diurnal convection (DC) are the two most frequently observed weather systems affecting the pre-summer (May and June, MJ) rainfall formation over Taiwan and Southeast China. Focusing on the time period of 1982-2012 MJ months, this study found that the occurrence frequency of FC has declined, but the occurrence frequency of DC has significantly increased over Taiwan and Southeast China. Diagnoses of the atmospheric thermodynamic conditions over the East Asian-Western North Pacific (EAWNP) region indicate that the area favourable for the FC formation (i.e. the area with a locally enhanced meridional temperature gradient) has shifted northward from 20 o -30 o N to north of 30 o N during the recent three decades; this shift has led to a decline in FC numbers over Taiwan and Southeast China. Analyses also indicate that the recent increase in DC activities over Taiwan and Southeast China after 1982 occurred in association with an observed strengthening of the daytime land-sea thermal contrast coupled with an intensification of the afternoon sea-breeze over the EAWNP region and a locally enhanced moisture flux convergence over Taiwan and Southeast China. Possible causes for the observed changes in the atmospheric thermodynamic conditions over the EAWNP region are also discussed.KEY WORDS Mei-Yu season rainfall; frontal activity; diurnal activity; long-term variations.

Impact of atmospheric changes on the low‐frequency variations of convective afternoon rainfall activity over Taiwan

“…The thick line in Figure a represents the low‐pass‐filtered frequency through 11 year running average. The linear trend of the frequency during 1961–2012 (the time period focused in this study; blue line) and during 1982–2012 (the time period examined by Huang and Chen []; red line) are added in Figure a for discussions. (b) The temporal variation of CAR frequency (thin line), its related 10–20 year band‐pass‐filtered values (green line) and 4–8 year band‐pass‐filtered values (yellow line).…”

“…By examining the trend in summer CAR in the Taipei basin (located in northern Taiwan), Chen et al [] noted an intensification of the summer CAR over the past 50 years and attributed this change to the strengthening urban heat island effect. Recently, Huang and Chen [] examining the trend in CAR activity over Taiwan during 1982–2012 Mei‐Yu season have suggested that the CAR has become more frequent and more intense since 1982. Because the formation of CAR in Taiwan is geographically dependent [ Huang and Wang , ], differences are likely to exist between the low‐frequency variations of CAR activity over different subregions in Taiwan.…”

“…Because the formation of CAR in Taiwan is geographically dependent [ Huang and Wang , ], differences are likely to exist between the low‐frequency variations of CAR activity over different subregions in Taiwan. However, the method used in Huang and Chen [] for identifying CAR did not consider the striking regional differences over Taiwan.…”

Future changes in propagating and non-propagating diurnal rainfall over East Asia

“…3 we show that Fig. 1 The first mode of EOF analysis on the long-term, 3-hourly TRMM 3B42 precipitation, following Huang and Chen (2015): a eigen-vector (E1) and b eigen-coefficient (PC1). In a, the domains of the Yangtze River regime (red dotted box) and southeast China regime (orange outline) are marked to help the discussions in the manuscript.…”

“…The spatial-temporal variations of diurnal rainfall were depicted by the Empirical Orthogonal Function (EOF) analysis applied on the long-term, 3-hourly TRMM precipitation, following Huang and Chen (2015). The first EOF (Fig.…”