INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. (2008)
Published online 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
H. CHEN ET AL.
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. (2008)
DOI: 10.1002/joc
SUMMER DURATION RAINFALL OVER THE UNITED STATES GREAT PLAINS
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 Hovmöller
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. (2008)
DOI: 10.1002/joc
H. CHEN ET AL.
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. (2008)
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
Copyright  2008 Royal Meteorological Society
suggestions for this work. Helpful comments from two
anonymous reviewers are also gratefully acknowledged.
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Int. J. Climatol. (2008)
DOI: 10.1002/joc