Observed Diurnal Cycle of Summer Precipitation over South Asia and ...

ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2015, VOL. 8, NO. 4, 201207

Observed Diurnal Cycle of Summer Precipitation over South Asia and East Asia Based on CMORPH and TRMM Satellite Data ZHANG Xin-Xin1,2, BI Xun-Qiang2, and KONG Xiang-Hui2,3 1

College of Atmospheric Sciences, Plateau Atmosphere and Environment Key Laboratory of Sichuan Province, Chengdu University of Information Technology, Chengdu 610225, China 2 Climate Change Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 3 University of Chinese Academy of Sciences, Beijing 100049, China Received 11 January 2015; revised 9 February 2015; accepted 4 March 2015; published 16 July 2015

Abstract The characteristics of the summer precipitation diurnal cycle over South Asia and East Asia during 2001–13 are investigated based on the high spatiotemporal resolution estimates of the CPC (Climate Prediction Center) Morphing (CMORPH) technique. The results show that summer precipitation over South Asia and East Asia possesses a remarkable diurnal cycle, with obvious regional differences. Over the coastal areas, plateau, and high mountains, summer precipitation peaks in the late afternoon; while over low altitude areas, such as valleys, basins, and inshore seas, it peaks during midnight to early morning. In addition to these general features consistent with previous studies, the high resolution CMORPH technique can depict finer regional details, such as the less coherent phase pattern over a few regions. Besides, through comparative analysis of the diurnal cycle strength and precipitation fields, the authors find that for humid areas the summer precipitation diurnal cycle is especially significant over Southeast China, the Sichuan Basin, Hainan Province, Taiwan Province, the Philippines, and Indonesia. And it is relatively weak over the south of Northeast China, central East China, Yunnan Province, the central Indian Peninsula, and most oceanic areas. Comparisons between two satellite datasets—those of the CMORPH and Tropical Rainfall Measuring Mission (TRMM) 3B42 products—are also presented. For summer precipitation and the main diurnal cycle features, the results from both products agree over most regions, except a few areas, e.g., the Tibetan Plateau.  Keywords: diurnal cycle, precipitation, CMORPH, TRMM 3B42, South Asia, East Asia Citation: Zhang, X.-X., X.-Q. Bi, and X.-H. Kong, 2015: Observed diurnal cycle of summer precipitation over South Asia and East Asia based on CMORPH and TRMM satellite data, Atmos. Oceanic Sci. Lett., 8, 201–207, doi:10.3878/AOSL20150010.

1

Introduction

Precipitation is a major component of the water cycle. Basically a result of the regular variation of solar radiation, the diurnal cycle is a basic characteristic of precipitation, and is important to hydrology (e.g., runoff, evapotranspiration, etc). In addition, the diurnal cycle can *

Corresponding author: BI Xun-Qiang, [email protected]

also alter the surface solar radiation and longwave terrestrial radiation, by changing the pattern of land surface temperature and affecting atmospheric wet convection and the formation of clouds (Byon et al., 2005). Therefore, investigating the diurnal cycle features of precipitation is helpful not only for understanding the physical processes involved in precipitation, but also for evaluating the performance of weather and climate models in simulating and forecasting precipitation. Previous studies of the precipitation diurnal cycle have mostly been based on observational data (e.g., Wallace, 1975; Dai et al., 1999; Dai, 2001; Yu et al., 2007a, b). In recent years, with the rapid development of satellite products, the diurnal cycle of precipitation has emerged as a hot topic. Many studies have examined the diurnal variation of regional and even global precipitation. Generally speaking, summer precipitation tends to peak in the afternoon over continental areas and in the early morning over oceanic areas (e.g., Dai, 2001; Bowman et al., 2005; Yang and Smith, 2006). One influential finding is that the diurnal cycle of precipitation comes mostly from its frequency rather than its intensity over most low and middle latitudes, based on four kinds of satellite datasets (Dai et al., 2007). As is well known, the Asian monsoon is one of the most important subsystems of the global climate system, and can significantly affect the local weather and climate. Yu et al. (2014) systematically summarized the recent progress in studies of the diurnal variation of precipitation over contiguous China. Mao and Wu (2012) investigated the diurnal cycle of precipitation over the Asian monsoon region, showing that there are regional features. Yuan et al. (2010) addressed that the intraseasonal movement in the early morning precipitation peak is associated with a shift of the East Asian monsoon rain band. Li et al. (2010) analyzed the diurnal cycle of precipitation over the South China Sea (SCS), and found that its features are related to the activities of the SCS summer monsoon and ENSO events. Yu et al. (2007b) examined the characteristics of diurnal variations in summer precipitation over China, and found that there are two comparable diurnal peaks over the region between the Yangtze River and the Yellow River: one appearing in the early morning and the other in the late afternoon. Many studies have focused on the Tibetan Plateau and its adjacent areas, uncovering remarkable differences between the central plateau and the Si-

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chuan Basin. And over different ground types (e.g., mountain, valley, lake, etc.), the characteristics of the diurnal cycle of precipitation are also different (Bai et al., 2008; Liu et al., 2009; Singh and Nakamura, 2009; Liu and Ma, 2013). Although studies of the precipitation diurnal cycle have made great progress, most are based on observational data from a limited number of meteorological stations, or satellite data with a shorter duration and 3 h frequency. The present paper analyzes the diurnal cycle of summer precipitation over South Asia and East Asia based on high spatiotemporal resolution CPC (Climate Prediction Center) Morphing (CMORPH) data over the recent 13 year period from 2001 to 2013. The remainder of the paper is organized as follows: In section 2, we briefly describe the data and methods used in the study. In section 3, we discuss the results, including a comparison between CMORPH and Tropical Rainfall Measuring Mission (TRMM) 3B42 data, and a detailed analysis based on CMORPH data. Conclusions are given in section 4.

2

Data and method

Satellite products with high spatiotemporal resolution are advantageous, especially over regions with complex terrain or a sparse distribution of meteorological stations. There have been many assessments of satellite datasets, all of which have demonstrated that these datasets can be applied in studies of precipitation. Besides, the TRMM 3B42 and CMORPH products are superior to other satellite products (Dai et al., 2007; Dinku et al., 2008). CMORPH combined precipitation estimates are derived exclusively from several low-orbit satellite microwave sensors by using National Oceanic and Atmospheric Administration’s CMORPH technique. During periods when microwave data are not available at certain locations, infrared data observed by geostationary satellites are instead used to determine the spatial distribution of precipitation information (Joyce et al., 2004). The CMORPH data we use have a spatial resolution of 0.07275° × 0.07277° (roughly 8 km in the equatorial zone) between 60°S and 60°N, and a temporal resolution of 30 min. The TRMM 3B42 datasets are a real-time multi-satellite precipitation analysis produced by the Distributed Active Archive Center, Goddard Space Flight Center, National Aeronautics and Space Administration (NASA). The data are based on an algorithm that combines the calibrated microwave rain estimates from a variety of satellites and the Infrared Radiation (IR) precipitation estimates collected by the international constellation of geosynchonous earth orbit (GEO) (Huffman et al., 2001). The spatial resolution is 0.25° × 0.25° between 50°S and 50°N, and the temporal resolution is 3 h. To evaluate the availability of CMORPH and TRMM 3B42 estimates, we compare them with the blended rainfall data from the Global Precipitation Climatology Project (GPCP). Except for the satellite-observed precipitation data, GPCP rainfall is merged with gauge records from more than 6000 observational stations. Here, the

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GPCP data have a spatial resolution of 1° × 1° and a temporal resolution of 1 d (Huffman et al., 1997). The observational stations are nonuniformly distributed, so the quality of GPCP data varies accordingly. Figure 1 presents the domain and topography selected for this study. The domain covers South Asia and East Asia (0°–50°N, 60–150°E). The terrain of this region is very complex, including oceans, mountains, basins, and the Tibetan Plateau. Besides, this area is under the influence of the Asian monsoon. Eight sub-regions are also illustrated in Fig. 1; six of them are in China and two in India. The diurnal variation of precipitation for these sub-regions is presented in subsection 3.3. Over the whole domain, during the study period from 2001 to 2013, the summer precipitation accounts for over 38% of the annual rainfall amount. Besides, Dai (2001) pointed out that the magnitude of the diurnal cycle of precipitation is strongest in summer among four seasons. So, in this study, we focus on the features of the diurnal cycle of summer precipitation. Because the spatiotemporal resolution of CMORPH products is considerably higher than that of TRMM 3B42 products, the CMORPH data are adjusted, using grid-cell average interpolation, into 0.25° × 0.25° grid points, and processed to 3 hourly data, so as to compare the differences between the two products in subsection 3.2. Then, in subsection 3.3, we analyze the diurnal summer precipitation cycle in detail based on the original CMORPH data. Two concepts, phase and strength, are defined to study the features of the summer precipitation diurnal cycle. The phase of the diurnal cycle refers to the time period during which the precipitation peaks appear. Since solar radiation is the dominant factor causing the diurnal cycles of meteorological variables, the calculated climatological precipitation rate for Coordinated Universal Time (UTC) is converted to the corresponding Local Solar Time (LST) based on the longitude where the grid points are located. Note that, hereinafter, the time used refers to LST. The

Figure 1 The domain used in this study and its topography (units: m). Eight sub-regions are also shown. Region 1 (40–43°N, 125–129°E); Region 2 (38–41°N, 114–118°E); Region 3 (33–36°N, 115–119°E); Region 4 (30–33°N, 105–109°E); Region 5 (28–31°N, 99–103°E); Region 6 (24–27°N, 114–118°E); Region 7 (22–25°N, 76–80°E); Region 8 (16–19°N, 75–79°E).

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ZHANG ET AL.: DIURNAL CYCLE OF CMORPH AND TRMM SATELLITE PRECIPITATION

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strength of the diurnal cycle means the probability of maximum precipitation occurrence. The percentages of the maximum precipitation occurrence within each LST hour are firstly calculated, and then we define the largest value (in percentage terms) among 24 LST hours as the diurnal cycle strength (hereinafter referred to as DCS).

3 3.1

Results Mean precipitation

Before analyzing the diurnal cycle, we first compare the mean summer precipitation rate. Figure 2 shows the distributions of mean precipitation rates during the summers (June to August, JJA) of 2001–13, derived from GPCP, CMORPH, and TRMM 3B42, respectively. All of the results reflect the non-uniform precipitation characteristics over the selected region well, which is under the combined influence of tropical and subtropical monsoons, exhibiting remarkable accordance in the major precipitation patterns. The precipitation decreases from the southeast to the northwest in China. Precipitation maxima exist mainly over three regions: the western coast of the Indian Peninsula, the Indochina Peninsula, and the Philippines. However, compared to GPCP, both CMORPH and TRMM 3B42 overestimate the rainfall amount over most regions, especially over the Tibetan Plateau and along the southeast coastal area of China. Note that the magnitude and extent of the maxima of CMORPH rainfall are smaller than those of TRMM rainfall. Besides, there is a minimum center of CMORPH estimates over Myanmar, which is located on the leeward slope of Patkai Mountain. CMORPH can identify this foehn effect, due to its high spatial resolution. Moist air brought by the southwest monsoon mostly falls as rainfall on the windward slope, causing insufficient water vapor supply on the lee side. Overall, the CMORPH precipitation pattern is more similar to the GPCP precipitation pattern than TRMM 3B42, especially over northeastern and southeastern China. 3.2 Comparison of the diurnal cycle of summer precipitation between CMORPH and TRMM 3B42 Various studies have demonstrated the reliability of TRMM to analyze the diurnal cycle of precipitation by comparing it with observational data (Bowman et al., 2005; Dai et al., 2007; Mao and Wu, 2012). But considering the limitations of the coarse resolution of TRMM, we employ CMORPH, with its high spatiotemporal resolution, to study the diurnal cycle in detail over our study domain. Before we analyze the CMORPH data, we first compare the features of the summer precipitation diurnal cycle derived from CMORPH and TRMM 3B42. Figure 3 presents the climatological distribution of the phase of summer diurnal precipitation derived from both TRMM 3B42 (Fig. 3a) and CMORPH (Fig. 3b). Note that because the temporal resolution is 3 h, there are strip-type background color differences every 45° of longitude, and this also causes the limitation of the LST resolution. Comparing Figs. 3a and 3b, we find that the distribution pattern of the diurnal cycle phase is similar, proving

Figure 2 Distribution of the summer precipitation rate (units: mm d–1) derived from (a) Global Precipitation Climatology Project (GPCP), (b) Climate Prediction Center (CPC) MORPHing Technique (CMORPH), and (c) Tropical Rainfall Measuring Mission (TRMM 3B42).

the reliability of CMORPH data in studying the summer precipitation diurnal cycle. Yet, there are some subtle differences, especially over the Tibetan Plateau and the western Pacific Ocean. Over the central plateau, TRMM 3B42 precipitation mainly peaks during 1830–2130 LST, while CMORPH mostly peaks during 1630–1830 LST. In the region of the East China Sea and the Yellow Sea, the CMORPH precipitation maximum usually occurs during 0730–1030 LST, while TRMM has two precipitation maxima—one the same as CMORPH’s, and the other during 0430–0730 LST. We speculate that these differences

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Figure 3 Distribution of the phase of summer diurnal precipitation derived from (a) Tropical Rainfall Measuring Mission (TRMM 3B42), and (b) Climate Prediction Center (CPC) MORPHing Technique (CMORPH).

between the two precipitation estimates may come from the algorithm of the satellites to obtain the precipitation data, and there is a kind of uncertainty at the present stage. 3.3 The diurnal cycle of precipitation derived by CMORPH Next, we analyze the characteristics of the summer precipitation diurnal cycle in detail using the high resolution CMORPH data. Figure 4 shows the spatial distribution of the diurnal cycle strength and phase of summer precipitation over South Asia and East Asia, derived by CMORPH estimates. Different from in the previous subsection, the CMORPH data used here are the original data, with a high spatial resolution and 0.5 h temporal resolution, which provide greater detail and present much less LST uncertainty. For the DCS shown in Fig. 4a, 4.167% (1/24) means that the probability of maximum precipitation is the same in each LST hour throughout a whole day, and a higher DCS means a more significant diurnal precipitation cycle. Over the whole study domain, the DCS is actually over 6% everywhere, and there are a few places where DCS even exceeds 15%. Besides, over the coastal area and inshore sea, the DCS is relatively higher, meaning a remarkable diurnal cycle. However, over inland regions and

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Figure 4 Distribution of two measurements of the summer precipitation diurnal cycle: (a) strength; (b) phase.

over the ocean far from land, the diurnal cycle is weak. Taking into account the precipitation rates in Fig. 2b, we perform a comparative analysis. For those areas with a mean precipitation rate over 4 mm d−1, such as Southeast China, the Sichuan Basin, Hainan Province, Taiwan Province, the Philippines, and Indonesia, the DCS is higher than 11%, which means there is a remarkable diurnal cycle over these humid regions in summer. However, over the south of Northeast China, central East China, Yunnan Province, the central Indian Peninsula, and most oceanic areas, although the precipitation rate is over 4 mm d−1, the DCS is lower than 8%, which means the diurnal cycle over these areas is not obvious. As shown in Fig. 4b, consistent with previous studies, over most continental areas, the precipitation peaks in the afternoon, but over most oceanic areas, it peaks in the morning (Dai, 2001; Bowman et al., 2005; Yang and Smith, 2006). However, there are remarkable regional differences over the selected domain, which is also different from in previous studies. Over Northeast China, a less coherent phase pattern is shown. The result does not correspond to previous studies that showed late afternoon precipitation prevailing over most of northern China (Dai et al., 2007; Yu et al., 2007b; He and Zhang, 2010). The main reason might be that the DCS over Northeast China is lower than 8%, a relatively

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weak diurnal cycle. Over Northwest China, precipitation mostly peaks during 0400–0600 LST over the Junggar Basin, Qaidam Basin, and Tarim Basin, with low altitude and dry climate. In contrast, over regions with high altitude, such as the Tianshan Mountains, Qilian Mountains, and the eastern Inner Mongolian Plateau, precipitation peaks during 1500–1700 LST. Besides, over the arid region of western Inner Mongolia, the precipitation also peaks during 0200–0400 LST. This suggests that the diurnal cycle of summer precipitation is closely associated with the ground type and topography. Due to the basin-and-range terrain pattern, it tends to induce a valley wind circulation near mountains, leading to the afternoon precipitation peak over mountains and evening peak over valleys. Over North China, the maximum precipitation usually occurs during 1400–1600 LST, especially over the Taihang Mountains. However, over the Circum-Bohai Sea region, it occurs during 0000–0300 LST and the DCS is low. Over western China, the DCS is higher than 11%. Precipitation peaks during 1600–1800 LST over the central Tibetan Plateau, but during 2200–0200 LST over the eastern periphery of the plateau and the western Sichuan Basin, and during 0600–0800 LST over the eastern Sichuan Basin and most of Chongqing and Guizhou Province. The characteristics of the precipitation diurnal cycle over the plateau and its vicinity may result from the diurnal variation of local circulation forced by the complex terrain. Compared with adjacent areas, the plateau absorbs more solar radiation during the day, which results in maximum low-level atmospheric instability and moist convection, and thus the rainfall peaks in the late afternoon. Coupled with the influence of the westerly, thermal convection activities propagate eastward, leading to a nocturnal precipitation maximum over the western Sichuan Basin. Over Southeast China, the DCS is also higher than 11%, showing an obvious diurnal cycle. The precipitation peak is also less coherent, and the precipitation diurnal cycle phase decreases outward from the center. Over Jiangxi and Fujian provinces, precipitation mostly peaks around 1800–2000 LST. Over southern Hunan Province, northern Guangzhou Province, Fujian Province, and Zhejiang Province, precipitation mainly peaks around 1600–1800 LST. Over eastern Guangxi Province and southern Guangzhou Province, precipitation peaks around 1400–1600 LST. These conclusions are also more specific than the general conclusion of previous studies that the precipitation maximum occurs in the late afternoon over South China. Over the Indochina Peninsula, the Indian Peninsula, the Philippines, and Indonesia, the phase of the summer precipitation diurnal cycle shows a similar pattern to that in Southeast China, decreasing outward from the center. The major factor of influence may be the sea and land breeze circulation, resulting from both the diurnal cycle of solar heating and the land-sea contrast. Over most oceanic areas, the precipitation maximum occurs mainly during 0000–0800 LST, but over the cen-

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tral Bay of Bengal and the South China Sea, precipitation peaks in the afternoon, which may be caused by the Asian monsoon and the land-sea contrast. To intuitively analyze the regional features of the summer precipitation diurnal cycle, we select eight relatively typical sub-regions based on the composite analysis in Fig. 2 and Fig. 4: Region 1—southern Jilin Province and northern Korea; Region 2—Beijing, Tianjin, and central Hebei Province; Region 3—southern Shandong Province and Northern Anhui Province; Region 4—northeastern Sichuan Province; Region 5—the eastern periphery of the Tibetan Plateau; Region 6—southern Jiangxi Province and western Fujian Province; Region 7 —southern India; and Region 8—central India. Given that the summer precipitation diurnal cycle is closely related to the terrain, the size of each sub-region is fixed at 4° × 3° to limit the terrain variability. Figure 5 shows the diurnal cycle of region-averaged precipitation. It reflects the regional difference of the summer precipitation diurnal cycle well. In addition, there is an obvious semidiurnal cycle over some areas, e.g. sub-regions 1 and 2. For the other six sub-regions, the diurnal cycle is dominant and the peaks are consistent with the previous phase results. In brief, the main pattern of the summer precipitation diurnal cycle is consistent with previous studies. However, benefiting from the high resolution of the CMORPH data used here, more significant regional differences are shown in our study compared to previous studies, such as a less coherent phase pattern over certain regions. The remarkable spatial dependence in the precipitation diurnal cycle over our study domain is clear evidence of orographic and heterogeneous land-surface impacts on convective development. The surface temperature differences resulting from the diurnal variation of solar heating and the inhomogeneous underlying surface may induce local circulation through thermal and dynamic effects, thus ultimately generating the diurnal cycle of summer precipitation. However, there are many other factors affecting the regional features of the diurnal cycle of precipitation, such as the regional conditions of water vapor. Thus, its underlying mechanism remains to be further investigated.

4

Summary

This paper analyzes the characteristics of the summer precipitation diurnal cycle over South Asia and East Asia during 2001–13. Firstly, the differences between CMORPH and TRMM 3B42 estimates are compared. Then, the features of diurnal summer precipitation variations are investigated in detail based on CMORPH data with a high spatiotemporal resolution. The major conclusions can be summarized as follows: CMORPH and TRMM 3B42 estimates exhibit remarkable accordance with GPCP data in terms of their precipitation patterns. Precipitation maxima mainly exist over three regions: the western coast of the Indian Peninsula, the Indochina Peninsula, and the Philippines. However, the magnitude and extent of the maxima of CMORPH rainfall are smaller than those of TRMM rainfall. Besides,

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Figure 5 Summer precipitation diurnal variation over the eight sub-regions shown in Fig. 1. The abscissa represents the diurnal cycle phase (LST), and the ordinate represents the region-averaged precipitation rate (units: mm d–1).

the features of the diurnal cycle of summer precipitation captured by CMORPH and TRMM 3B42 estimates are similar over most regions, but there are differences over some areas (e.g., the Tibetan Plateau). Summer precipitation over our study domain possesses diurnal cycles with obvious regional differences. Over coastal areas, the plateau, and high mountain areas, summer precipitation peaks in the late afternoon, while over low areas such as the valleys and basins near the plateau or high mountains, and ocean close to the land, it peaks

around midnight to early morning. This suggests that the diurnal cycle of summer precipitation is closely related to the local circulation resulting from solar radiation and the inhomogeneous underlying surface. In addition to these general features that are consistent with previous studies, the high resolution CMORPH data can depict finer regional details, such as a less coherent phase pattern over a few regions. Besides, through a comparative analysis of the frequency of the diurnal cycle phase and mean precipitation rate fields, we find that, for humid areas, the

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summer precipitation diurnal cycle is especially significant over Southeast China, the Sichuan Basin, Hainan Province, Taiwan Province, the Philippines, and Indonesia, but not so obvious over the south of Northeast China, central East China, Yunnan Province, the central Indian Peninsula, and most oceanic areas. Acknowledgements. This work was supported by the National Basic Research Program of China (Grant No. 2013CB430201) and the China Meteorological Administration Special Fund for Scientific Research in the Public Interest (Grant No. GYHY201206008).

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