Summer rain fall duration and its diurnal cycle over the US Great Plains

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Dec 4, 2008 - ABSTRACT: By diagnosing the hourly station rain gauge data set for the 1981–1999 periods, it is found that the rainfall diurnal cycle is closely ...
INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 29: 1515–1519 (2009) Published online 4 December 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1806

Short Communication Summer rain fall duration and its diurnal cycle over the US Great Plains Haoming Chen,a,b Tianjun Zhou,a * Rucong Yuc and Jian Lic a

c

LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China b Graduate School of the Chinese Academy of Sciences, Beijing 100049, China LaSW, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081, China

ABSTRACT: By diagnosing the hourly station rain gauge data set for the 1981–1999 periods, it is found that the rainfall diurnal cycle is closely related to its duration during summer (June–August) over the Great Plains [(GP), 100–90 ° W, 35–45 ° N]. Short-duration rainfall events (an event of 1 h in duration) occur more frequently in summer, and they tend to have two diurnal maxima over the GP, with one in the early morning [0400–0600 local solar time, (LST)] and the other in the afternoon (1500–1700 LST). Long-duration rainfall events (an event that lasts longer than 3 h) contribute more to the precipitation amount, and they tend to peak from the midnight to early morning (000–0600 LST). This contrast in the diurnal cycle of different classifications of precipitation events over the GP reflects the differences in the convective processes at night and during late afternoon. Copyright  2008 Royal Meteorological Society KEY WORDS

summer rainfall; duration; diurnal cycle

Received 1 January 2008; Revised 16 September 2008; Accepted 11 October 2008

1.

Introduction

Many studies have examined the diurnal variability of precipitation over the continental United States (e.g., Wallace, 1975; Riley et al., 1987; Dai et al., 1999; Carbone et al., 2002; Liang et al., 2004; Tian et al., 2005). These studies have shown a distinctive geographical pattern of precipitation diurnal variations during summer which is characterized by a strong midnight to early morning maximum over the regions east of the Rockies and the Great Plains (GP), and a strong late afternoon maximum over the western and southeastern United States. The unique nocturnal–diurnal peak in the central United States is in contrast to the afternoon rainfall maximum over most inland regions (Dai, 2001; Dai et al., 2007), suggesting that the observed nocturnal peak over the GP is attributed to mechanisms that are not directly related to variations in static instability (Wallace, 1975; Riley et al., 1987; Dai et al., 1999). By using conventional hourly data, Wallace (1975) concluded that the central United States nocturnal maximum in convective activity was a direct consequence of the combined effects of heating over sloped terrain and changes in frictional drag. This was considered especially significant in the southern GP under southwesterly flow and was associated with the low-level jet (Bonner, 1968). Dai et al. * Correspondence to: Tianjun Zhou, LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China. E-mail: [email protected] Copyright  2008 Royal Meteorological Society

(1999) analysed diurnal variations in precipitation, surface pressure, and static energy over the United States from observations and a regional climate model. They found that the solar-driven diurnal and semidiurnal cycles of surface pressure result in large-scale convergence over most of the western United States during the day and east of the Rockies at night. The convergence suppresses daytime convection and favours nocturnal convection east of the Rockies. Higgins et al. (1997) found that the GP low-level jet transports almost one-third of the moisture that enters the continental United States with most of the influx from the low-level jet (slightly less than two-thirds of it) entering during the 12 nighttime hours. Thus, they highlight the subcontinental and large-scale regulation of diurnal convection as well as the importance of the GP low-level jet in contributing to nighttime boundary layer convergence that favours nocturnal convection over the GP region. Riley et al. (1987) discussed the role of mountain-generated storm systems, including mesoscale convective systems (MCSs) (Maddox, 1980). These storms tend to move eastward from the Rocky Mountains onto the Plains after sunset, and produce some (but not all) of the diurnal variability in the GP. Because of their relatively longer lifetime, the rainfall peaks of the MCSs usually occur in the late evening through midnight over the GP region, while non-MCS rainfall peaks in the late afternoon (McAnelly and Cotton, 1989; Nesbitt and Zipser, 2003). Carbone et al. (2002) suggested that gravity waves may contribute to the propagation speed

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of major convective episodes over the GP region. Jiang et al. (2006) also illustrated that the eastward propagation of convection systems from the Rockies to the GP could be the dominant factor for the observed nocturnal rainfall peak over the GP. Similarly, Yu et al. (2007a) found that summer precipitation peaks at midnight over the eastern periphery of the Tibetan Plateau. This feature is revealed by both rain gauge observations and satellite measurements (Zhou et al., 2008). Further analyses show that the nocturnal precipitation peak mainly comes from long-duration rainfall events (Yu et al., 2007b). Discussing the characteristics of surface precipitation is important, because it can help to understand the underlying processes of the diurnal cycle. Since barely any effort has been devoted to the discussion of duration characteristics of rainfall over the continental United States in previous studies, we aim to reveal the relation between summer rainfall duration and diurnal variation over the central United States in this study. The results show that long-duration rainfall events play an important role in the formation of the nocturnal precipitation maxima over the central United States, whereas short-duration ones contribute to both the early morning and the afternoon peaks. The rest of the paper is organized as follows. The data and analysis methods will be briefly documented in Section 2. The main results are shown in Section 3. Section 4 contains our conclusions and discussion.

2.

Data and analysis methods

The quality-controlled hourly and daily rain gauge records between 1981 and 1999 are obtained from National Center for Atmospheric Research (http:// dss.ucar.edu/datasets/ds505.0/). These records include more than 2000 stations over the contiguous United States, consisting of primary, secondary, and cooperative observer stations operated by the National Weather Service and the Federal Aviation Agency (Hammer and Steurer, 1997). At each station, the classification and diurnal analysis are performed, and the calculation results are interpolated onto a 1° longitude by 1° latitude grid before drawing the plots. Interpolating station data into regular grid may prolong the rainfall duration because of the spatial and time smoothing effects introduced by interpolating. Therefore, the calculations are all based on the original rain gauge data. The hourly rainfall events, defined as the ones with more than 0.1 mm precipitation accumulation during the hour (Dai et al., 1999), are classified according to their continuous durations, i.e. by the hours between the beginning and the ending events. Only after two continuing non-precipitation hours appear, we judge a rainfall event to be over. For example, if there is precipitation over 0.1 mm at 0100 LST but not at 0200 and 0300 LST, this event is defined as a 1-h event. The rainfall event beginning at 0100 LST and ending at 0300 LST is defined as a 3 h event, i.e. there is precipitation Copyright  2008 Royal Meteorological Society

over 0.1 mm from 0100 LST to 0300 LST but not at 0400 and 0500 LST. In this study, short-duration events are defined as 1-h events that begin and end with the same hour, and long-duration events are defined as lasting more than 3 h. Our results are insensitive to the choice of a lower limit of 3 or greater for the long-duration events, with longer time periods serving only to reduce the sample size. More details about the duration-sorting methods used in this paper can be found in Yu et al. (2007b). A diurnal analysis with harmonic decomposition (Dai and Wang, 1999) is performed on each classification of the rainfall event. The diurnal amplitudes are then normalized using the JJA mean for the corresponding rainfall classification. Since the diurnal cycle of precipitation is relatively weak in cold seasons (Dai et al., 1999), we only show the results of summer (June–August).

3.

Results

The long-term (1981–1999) mean occurrence frequency and rainfall amount as a percentage of all hourly events (with rainfall >0.1 mm/h) for the short- and longduration events are shown in Figure 1. The percent frequency here refers to occurrence of a classification of event compared to all rainfall events, and the corresponding amount is defined as the percent amount of a classification of event to total rainfall amount. For example, a 3-h event is counted as one event in computing the percent frequency and amount. As the short- and long-duration events generally reflect different convective processes, the results of the rainfall events lasting 2–3 h are omitted because they only contribute to a small part (less than 30%) of total rainfall. The short-duration (1 h) events dominate over all of the GP in summer, and they account for over 48% of all rainfall events (Figure 1(a)). The long-duration (>3 h) events generally occur much less frequently compared to the short-duration ones, e.g. only 14–22% events occur as the long-duration events (Figure 1(c)). A smaller percent (48–50%) of the rainfall events occur as the short-duration events together with slightly more (20–22%) long-duration events over the southeast of the GP (around 95 ° W, 35° N). In contrast, the less frequent long-duration events contribute 40–50% to the total rainfall amount, while the short events only contribute less than 40% (Figure 1(b), (d)). The phase (local solar time of the maximum, LST) of the 1981–1999 mean diurnal cycle (determined from the 24-h harmonic estimated from the composite curve) of precipitation amount for the short- and long-duration events are compared in Figure 2. There exist considerable differences between these two classifications. The shortduration events show noontime maxima (0900–1500 LST) over most part of the GP, except that they tend to peak in the early morning (0600–0900 LST) over northwestern GP (Figure 2(a)). The long-duration events exhibit a nearly uniform nocturnal and early morning peaks (000–0600 LST) in this key region, except that Int. J. Climatol. 29: 1515–1519 (2009) DOI: 10.1002/joc

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Figure 1. Long-term (1981–1999) mean occurrence frequency (left column) and rainfall amount (right column) expressed as a percent of all hourly rainfall events (with > 0.1 mm/h) during June–August for short- (< 1 h, top row)and long- (>3 h bottom row) duration events. Values greater than 50% are shaded.

they tend to reach the maxima near the noon over the southeastern GP, similar to the short-duration ones (Figure 2(b)). Quantitative analysis shows that, for shortduration rainfall events, 242 stations (53.5% of 452 stations in this key region) have maximum rainfall between 0900 and 1500 LST, whereas for long-duration ones, 314 stations (69.5%) have the maximum rainfall between 000 and 0600 LST. In Figure 2(b), the afternoon diurnal phases delay eastward, e.g. rainfall peaks in the late afternoon around 105 ° W, while peaks in the morning around 95 ° W. The prominent feature of nocturnal–diurnal peaks in the GP mainly results from long-duration rainfall events (Figure 2). To further reveal this feature, the Hovm¨oller diagrams of normalized (using the daily mean) rainfall diurnal variations averaged between 35 and 45° N are shown in Figure 3. To facilitate comparison, here we mark both the coordinated universal time (UTC) and LST (LST = UTC − 6 in 90 ° W) in the ordinate of Figure 3. Similar to the results of previous studies (e.g. Carbone et al., 2002; Liang et al., 2004; Jiang et al., 2006), summer convective systems generated over the Rockies propagate eastward to the Central Plains (Figure 3(a)). The short-duration rainfall events show no propagating feature west of 105 ° W (Figure 3(b)). The convective systems tend to generate locally at 1600–0200 UTC (e.g. 1000–2000 LST) and do not propagate downward. Different from short-duration events, long-duration ones depict strong eastward propagation as the total summertime precipitation (Figure 3(c)). Copyright  2008 Royal Meteorological Society

Figure 2. Spatial distributions of JJA mean diurnal cycle for rainfall events of (a) short-duration (< 1 h) and (b) long-duration (>3 h). Vectors denote the local solar time (LST) of the maximum of the 24 h harmonic (see phase clock).

There is only one nocturnal peak (0400–0900 UTC, i.e. 2200–0300 LST) from 100 to 95 ° W (Figure 3(a)), Int. J. Climatol. 29: 1515–1519 (2009) DOI: 10.1002/joc

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Figure 4. The normalized diurnal variations of precipitation averaged over two selected regions for total summertime precipitation (solid line), durations of 1 h (dashed line with filled circles), and more than 3 h (dot-dashed line with open squares).

Figure 3. Time-longitude distribution of normalized rainfall diurnal variations averaged between 35 and 45° N for (a) averaged precipitation amount, (b) short-duration rainfall events and (c) long-duration rainfall events. The western and eastern parts of the GP are drawn by blue lines.

whereas there are two weak diurnal peaks over 90–95 ° W, one in the nocturnal to early morning (0600–1200 UTC, i.e. 000–0600 LST), and the other one in the late afternoon (2100–0100 UTC, i.e. 1500–1900 LST). Comparing Figure 3(b)–(c) with 3(a), it is found that the early morning maxima around 100 ° W–90 ° W come mainly from the long-duration events, while the short-duration events occur in both morning and afternoon over the GP. For further analyses, we divided the GP into two subregions, which exhibit different characteristics associated with different rainfall events. Reg1 refers to the western part of the GP (100 ° W–95 ° W), where short-duration rainfall events show two diurnal maxima while longduration rainfall events peak in midnight. Reg2 refers to the eastern part of the GP (95 ° W–90 ° W), where the semidiurnal cycle is obvious. These features are then verified by taking regional average as shown in Figure 4. Over the western part of the GP (Reg 1), the total precipitation shows large diurnal variation, with a strong peak around 2200 LST (Figure 4(a)). The short-duration rainfall events in this region show two weak diurnal peaks, with one in the late afternoon (1700 LST), and the other in the early morning (0400 LST). When the duration Copyright  2008 Royal Meteorological Society

time increases, the afternoon diurnal phase delays, e.g. the diurnal maxima of rainfall events lasting for 1–3 h occur much later than that of events lasting for only 1 h (Figure omitted). Comparing the diurnal cycle of the total summertime precipitation with long-duration events reveals a close resemblance, suggesting that the midnight diurnal maxima in Reg1 mainly come from the longduration rainfall events. Over eastern part of the GP where the semidiurnal cycle is significant (Reg 2), the total precipitation has two diurnal peaks. A strong peak is evident in early morning (0500 LST) and a weaker one is in late afternoon (1700 LST) (Figure 4(b)). The shortduration rainfall events also show two diurnal peaks in this region, with a sharp diurnal peak in the late afternoon and a much weaker peak in the early morning (0600 LST). The long-duration rainfall events exhibit only one strong diurnal peak in early morning (0300 LST), which are consistent with the results shown in Figure 3(c). Therefore, long-duration events dominate the nocturnal peak in Reg1. In Reg2, both long- and short-duration events contribute to the early morning maxima, but the secondary afternoon peak is only resulted from the shortduration events.

4. 4.1.

Summary and discussion Summary

Through analyses of the long-term (1981–1999) United States hourly rain gauge data set, we find that the diurnal variation of summer precipitation over the GP is closely related to rainfall duration time. In summer, the shortduration rainfall events occur more frequently than the Int. J. Climatol. 29: 1515–1519 (2009) DOI: 10.1002/joc

SUMMER DURATION RAINFALL OVER THE UNITED STATES GREAT PLAINS

long-duration events. The short-duration events mainly show two diurnal peaks, one in the morning and the other in the afternoon. The long-duration ones dominate the precipitation amount in the GP region, and they exhibit a strong midnight to early morning maxima. The diurnal variations of precipitation in the western and eastern part of the GP are different. In the western part (100 ° W–95 ° W), the nocturnal–diurnal peak of precipitation amount mainly comes from long-duration rainfall events. In the eastern part (95 ° W–90 ° W), both the long- and short-duration events contribute to the main early morning peak, and the secondary afternoon maxima are mainly caused by short-duration ones. 4.2. Discussion The late afternoon peak of short-duration rainfall events may be explained by the diurnal variation of lowlevel atmospheric instability due to thermal heating generated by the sun (Wallace, 1975; Dai et al., 1999). However, the mechanisms for midnight to early morning precipitation maxima are more complex. As mentioned in the introduction, numerous studies have discussed the physical processes of the prevailing nocturnal and early morning peaks. Our results also suggest the importance of eastward propagating convection systems on this phenomenon. As shown in Figure 3, the early morning maxima of the long-duration rainfall events in the GP may be caused by the eastward propagating convections generated over the Rockies in the previous afternoon. The long-duration rainfall events are closely related to the organized MCSs that have long nocturnal life cycle (Wallace, 1975). Nesbitt and Zipser (2003) suggested that the nocturnal rain is often caused by MCSs rather than isolated convection, and the MCSs are strongest after midnight, presumably from the upscale growth of late afternoon convection. A long duration may be a necessary condition for moisture accumulation and for each isolated convection cell to grow into well-organized MCSs before the maximum rainfall. After the MCSs form, they may propagate eastward and contribute to the nocturnal peak over the GP. Nevertheless, the physical processes behind long-duration rainfall events still warrant further study. Acknowledgements This work was jointly supported by National Natural Science Foundation under grant No. 40523001, 40625014, the 973 Program (2006CB403603), and Chinese COPES project (GYHY200706005). We also wish to express our appreciation to Dr. Aiguo Dai of NCAR to help us to obtain the rain gauge data and give many constructive

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suggestions for this work. Helpful comments from two anonymous reviewers are also gratefully acknowledged. References Bonner WD. 1968. Climatology of the low-level jet. Monthly Weather Review 96: 833–850. Carbone RE, Tuttle JD, Ahijevych DA, Trier SB. 2002. Inferences of predictability associated with warm season precipitation episodes. Journal of the Atmospheric Sciences 59: 2033–2056. Dai A. 2001. Global precipitation and thunderstorm frequencies. Part II: Diurnal variations. Journal of Climate 14: 1112–1128. Dai A, Giorgi F, Trenberth KE. 1999. Observed and model simulated diurnal cycles of precipitation over the contiguous United States. Journal of Geophysical Research 104: 6377–6402. Dai A, Lin X, Hsu K-L. 2007. The frequency, intensity, and diurnal cycle of precipitation in surface and satellite observations over lowand mid-latitudes. Climate Dynamics 29: 727–744. Dai A, Wang J. 1999. Diurnal and semidiurnal tides in global surface pressure fields. Journal of the Atmospheric Sciences 56(22): 3874–3891. Hammer GR, Steurer PM. 1997. Data set documentation for Hourly Precipitation Data, NOAA/NCDC TD3240 Documentation Series, Asheville, NC, 18. Higgins RW, Yao Y, Yarosh ES, Janowiak JE, Mo KC. 1997. Influence of the Great Plains low-level jet on the summertime precipitation and moisture transport over the central United States. Journal of Climate 10: 481–507. Jiang X, Lau N-C, Klein SA. 2006. Role of eastward propagating convection systems in the diurnal cycle and seasonal mean of summertime rainfall over the U.S. Great Plains. Geophysical Research Letters 33: L19809, DOI: 10.1029/2006GL027022. Liang X-Z, Li L, Dai A, Kunkel KE. 2004. Regional climate model simulation of summer precipitation diurnal cycle over the United States. Geophysical Research Letters 31: L24208, DOI: 10.1029/2004GL021054. Maddox RA. 1980. Mesoscale convective complexes. Bulletin of the American Meteorological Society 61: 1374–1387. McAnelly RL, Cotton WR. 1989. The precipitation life cycle of mesoscale convective complexes over the central United States. Monthly Weather Review 117: 784–808. Nesbitt SW, Zipser EJ. 2003. The diurnal cycle of rainfall and convective intensity according to three years of TRMM measurements. Journal of Climate 16: 1456–1475. Riley GT, Landin MG, Bosart LF. 1987. The Diurnal Variability of Precipitation across the Central Rockies and Adjacent Great Plains. Monthly Weather Review 115: 1161–1172. Tian B, Held IM, Lau N, Soden BJ. 2005. Diurnal cycle of summertime deep convection over North America: A satellite perspective. Journal of Geophysical Research 110: D08108, DOI: 10.1029/2004JD005275. Wallace JM. 1975. Diurnal variations in precipitation and thunderstorm frequency over the conterminous United States. Monthly Weather Review 103: 406–419. Yu R, Xu Y, Zhou T, Li J. 2007b. Relation between rainfall duration and diurnal variation in the warm season precipitation over central eastern China. Geophysical Research Letters 34: L13703, DOI:10.1029/2007GL030315. Yu R, Zhou T, Xiong A, Zhu Y, Li J. 2007a. Diurnal variations of summer precipitation over contiguous China. Geophysical Research Letters 34: L01704, DOI: 10.1029/2006GL028129. Zhou T, Yu R, Chen H, Dai A, Yang P. 2008. Summer precipitation frequency, intensity, and diurnal cycle over China: A comparison of satellite data with raingauge observations. Journal of Climate 21: 3997–4010.

Int. J. Climatol. 29: 1515–1519 (2009) DOI: 10.1002/joc