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JOURNAL OF CLIMATE
VOLUME 21
Prolonged Dry Episodes over the Conterminous United States: New Tendencies
Emerging during the Last 40 Years
PAVEL YA. GROISMAN
UCAR Visiting Scientist, National Climatic Data Center, Asheville, North Carolina
RICHARD W. KNIGHT
STG, Inc., Asheville, North Carolina
(Manuscript received 24 April 2007, in final form 6 September 2007)
ABSTRACT
A disproportionate increase in precipitation coming from intense rain events, in the situation of general
warming (thus, an extension of the vegetation period with intensive transpiration), and an insignificant
change in total precipitation could lead to an increase in the frequency of a potentially serious type of
extreme events: prolonged periods without precipitation (even when the mean seasonal rainfall totals
increase). This paper investigates whether this development is already occurring during the past several
decades over the conterminous United States, for the same period when changes in frequency of intense
precipitation events are being observed. Lengthy strings of “dry” days without sizeable (⬎1.0 mm) precipitation were assessed only during the warm season (defined as a period when mean daily temperature is
above the 5°C threshold) when water is intensively used for transpiration and prolonged periods without
sizable rainfall represent a hazard for terrestrial ecosystem’s health and agriculture. During the past four
decades, the mean duration of prolonged dry episodes (1 month or longer in the eastern United States and
2 months or longer in the southwestern United States) has significantly increased. As a consequence the
return period of 1-month-long dry episodes over the eastern United States has reduced more than twofold
from 15 to 6–7 yr. The longer average duration of dry episodes has occurred during a relatively wet period
across the country but is not observed over the northwestern United States.
1. Introduction
It has been shown that with an increase or decrease
of total precipitation, disproportionate changes occur in
the upper end of the precipitation frequency distribution (Karl and Knight 1998; Groisman et al. 1999, 2001,
2004; Kunkel et al. 1999; Easterling et al. 2000; Folland
and Karl 2001; Semenov and Bengtsson 2002; Kunkel
2003). Over the conterminous United States, this feature has become prominent since circa 1970 (Soil and
Water Conservation Society 2003). In particular, upward trends in the amount of precipitation occurring in
the upper 0.3% of daily precipitation events are statistically significant for the past hundred years within the
central regions of the United States (Groisman et al.
2004). A time series of the frequency of events in the
upper 0.3% averaged for these regions shows a 22%
increase over the period since 1893 with all of this
increase occurring over the last third of the twentieth
century (Fig. 1). These upward trends are primarily a
warm season phenomenon when the most intense rainfall events typically occur.
Karl and Knight (1998) show that most (87% of the
variance nationwide for the conterminous United
States) of the increase in seasonal/annual precipitation
can be ascribed to changes in the number of days with
precipitation.1 The tendencies, which emerged during
the past 35–40 yr with a disproportional increase in
1
Corresponding author address: Pavel Ya. Groisman, National
Climatic Data Center, Federal Building, 151 Patton Avenue,
Asheville, NC 28801.
E-mail: [email protected]
DOI: 10.1175/2007JCLI2013.1
© 2008 American Meteorological Society
Karl and Knight (1998) and subsequent studies (Easterling et
al. 2000; Stone et al. 2000; Groisman et al. 2004, 2005) all show
that most of the precipitation increase over the United States and
Canada occurs due to an increase in the frequency of intense
precipitation, while the frequency of days with average and light
precipitation does not change or decreases.
1 MAY 2008
GROISMAN AND KNIGHT
1851
climate observing network of, by, and for the people.
More than 11 000 volunteers take observations on
farms, in urban and suburban areas, national parks, seashores, and mountaintops. The data are truly representative of where people live, work, and play.
b. Approach
FIG. 1. Regions of the contiguous United States (hatched)
where statistically significant annual increases in very heavy precipitation for the 1908–2002 period were reported by Groisman
et al. (2004) and very heavy precipitation (upper 0.3% of daily
rain events with a return period of 4 yr) over these regions of the
central United States and their linear trends. Linear trends for the
1893–2006 and 1967–2006 periods (solid lines) are equal to 22%
per 114 yr and 27% per 40 yr, respectively, and are statistically
significant at the 0.05 level or higher (updated from Groisman et
al. 2005).
precipitation coming from intense rain events (Groisman et al. 2004, 2005), should lead to discontinuities in
the parallel increase/decrease of both total precipitation and precipitation frequency. For the United States
this discontinuity was first reported by Sun and Groisman (2004) and for the northeastern quadrant of the
conterminous United States confirmed by Groisman et
al. (2005). Specifically, for the northeastern quadrant of
the United States, they reported an increase (or no
change) in precipitation totals but a decrease in the
number of days with precipitation. If continued, this
decrease in precipitation frequency may lead to an increase in the frequency of another potentially hazardous type of extreme event: prolonged periods without
precipitation (even when the mean seasonal rainfall totals increase). Below we investigate whether this development is already occurring during the past several decades over the conterminous United States, for the
same period when we begin observing changes in frequency of intense precipitation events (i.e., since circa
1970).
2. Methodology
a. Data
For our analyses we use the same daily precipitation
dataset of the U.S. Cooperative Observer Program
(COOP) stations described in Groisman et al. (2004)
but updated to 2006. The National Weather Service
(NWS) COOP is truly the United States’s weather and
Paleoclimatic reconstructions (e.g., Herweijer et al.
2007) provide a large-scale picture of drought frequencies during the past millennium, and in situ observations (e.g., Dai et al. 2004; Andreadis et al. 2005) can
now deliver quite detailed information about the dry
conditions during the past 100 years. If not driven by a
large-scale storm system, a precipitation mosaic from
summer storms leaves numerous dry spots across the
country (cf. http://www.drought.unl.edu/dm/monitor.
html). It is not our intention to study this mosaic or
individual spectacularly dry years. Instead we focus on
systematic changes of dry conditions on the nationwide
scale during the past 40 yr using only precipitation information from our station network. We are looking for
large-scale changes in the annual areal summation of
the duration of nonrain episodes over the nation and
are not very much interested in a particular pattern of
these dry episodes. The rationale for this selective interest is a potential of linking these changes with
changes in global-scale processes (e.g., global warming), which might affect continental dryness (Manabe et
al. 1981, 2004). The GCM simulations forced by different scenarios of changes in atmospheric composition
(e.g., Manabe et al. 2004; Kharin et al. 2007; McAvaney
et al. 2001) hint at the potential for significant changes
in “summer” dry conditions but are not yet able to
produce a detailed picture of these changes.
c. Focus on the warm season and prolonged
no-rain episodes
We know that the number of rainy days over the
country has increased during the past 100 years and that
a tendency toward opposite trends shows up in some
regions during the last 35–40 yr (Karl and Knight 1998;
Groisman et al. 2001, 2005). A decrease in the number
of rainy days itself does not represent an obvious hazard for society and ecosystems and cannot be considered as an extreme event per se. However, an increase
in the frequency (and length) of prolonged periods
without sizable rainfall during the warm season when
water is intensively used for transpiration may represent a hazard for both terrestrial ecosystem health and
agriculture. Therefore, we select the warm season only
(here we define it as a period when mean daily temperature remains persistently above the 5°C threshold)
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JOURNAL OF CLIMATE
and count strings of dry days defined as the days when
daily precipitation is absent or is below 1 mm. We consider the string as “broken” when its growth is stopped
on the day when the station daily rainfall is 1 mm or
above. The string growth is suspended (but the tally
saved) at a given day when the daily temperature occasionally falls below 5°C. This dry day is not counted,
but the string can continue growing when temperature
rises. In other words, a brief late spring or early fall cold
spell will not break the “season.” As a result of this
procedure, for each year at each station we calculate
(i) the number of days in the “warm” season, (ii) the
number of strings with dry days, and (iii) the string
lengths (in days). Our focus is not on the short strings
but only those that are longer than given thresholds, X.
The number of days in these long strings is counted and
a percentage of the warm season with thus defined prolonged dry periods with duration of X days or longer is
calculated. The number of days above a chosen threshold (X ) of dry day strings is tallied. The count is then
divided by the season day length, which becomes a percent of the warm season length with prolonged dry day
strings. This approach gives us a chance to express in
one number the severity of dry periods above the X day
threshold (later described by the notation DryX⫹, e.g.,
Dry20⫹, Dry30⫹, Dry60⫹, etc.) during a specific warm
season at a given station and allows for area averaging
of these quantities, first over 1° grid cells and second
over the large areas of the conterminous United States
and, finally, to consider their trends for the past several
decades.
d. Regional averaging routine used
We are focusing on the last few decades and are hoping to reveal late changes in the dry conditions on the
background of large weather variability and geographic
“noise.” Therefore, we are using area averaging so as to
suppress this noise (Groisman et al. 2005). Meteorological stations are not uniformly distributed and missing years are present in most of the records. Both factors had to be addressed to properly represent regional
averages of (i) the frequency and/or duration of prolonged dry episodes, (ii) duration of the warm season
(DWS), and (iii) frequency of very heavy (very light)
precipitation derived from in situ observations. Areaaveraged calculations presented in this paper all use the
same method. First, we selected a reference period with
the greatest availability of data to estimate the longterm mean values for each quantity studied. The period
selected was 1961–90, but the use of other reference
periods (1951–2005 and 1967–2006) was also tested to
ensure that this selection does not affect our results.
For each station, for each quantity (very heavy precipi-
VOLUME 21
tation, warm season duration, duration of the dry episodes above X thresholds, and for the dry episode percentage within the warm season), we determined their
climatological mean values during the reference period.
For each quantity, region, and year we calculated the
anomalies from the long-term mean value of these
quantities for each station and then arithmetically averaged these anomalies within 1° ⫻ 1° grid cells. These
anomalies were regionally averaged with the weights
proportional to their area. Data from the large regions
use the regional area weights to form a national average
when those analyses are presented (e.g., for the warm
season duration and days with very heavy and very light
precipitation). The long-term mean values (normals
from reference period years 1961–90) were area averaged in a similar fashion and used to restore actual
regional quantity values from regional anomalies. This
approach emphasizes underrepresented parts of the region/country because a region, even with a relatively
low percentage of grid cells with data, will receive the
full weight comparable to the region’s area relative to
other regions. It also allows preservation of the regional
time series unaffected by the changing availability of
data with time.2
e. Interpretation of linear trend estimates
We used (in addition to the linear trend assessment)
a nonparametric test to check for a monotonic change
of the time series. Once a statistically significant trend
has been discovered, we characterize it by the mean rate
of change. A linear trend estimate is an essential characteristic in this case. We tested the presence of systematic change in the time series using two standard
methods: least squares regression (Draper and Smith
1966; Polyak 1996) and a nonparametric method based
on the Spearman rank order correlation (Kendall and
Stuart 1967). We used two-tailed tests at the 0.05 or
higher significance level (except in Table 2 where a
one-tailed test was also used). We tested for autocorrelation of the detrended time series, but the residu2
We used this area-averaging routine during the past decade
for various climate variables (e.g., Groisman and Legates 1995;
Karl and Knight 1998; Groisman et al. 2001, 2004, 2005) after
extensive testing regarding the robustness of the algorithm. The
results of its implementation are close to those based on areaaveraging procedures built on optimal interpolation and optimal
averaging with normalizing weights (Kagan 1997). “Optimal” procedures (i.e., those that deliver the minimal standard error of area
averaging) are much more computationally expensive and preserve their optimal properties only when specifics of the statistical
structure of the meteorological field to be averaged are well
known. This is not the case for many of the quantities we analyzed
and, thus, we opted not to use them.
1 MAY 2008
GROISMAN AND KNIGHT
1853
FIG. 2. (a)–(c) Climatology of the mean percentage of the warm season included in the string lengths of dry days of
(a) ⱖ30 days, (b) ⱖ60 days, and (c) ⱖ90 days in the strings. When no such strings have been observed during the past 55
yr (1951–2005), the area is left blank. A few 1° ⫻ 1° grid cells in the western United States (e.g., in Nevada) are also left
blank due to insufficient long-term station data coverage. (d) Climatology of the warm season duration (days with mean
daily temperature above 5°C) over the conterminous United States.
als of the regional warm season duration, frequencies of
prolonged dry episodes, and very heavy precipitation
were never found to be significantly autocorrelated. All
area averaging and trend estimation procedures are linear and allow transposition: We can calculate linear
trends at each station and then area average them or we
can construct area-averaged time series of the regional
percentage of the days with prolonged dry day strings
and then calculate the trends. We selected the latter
way to present our results in this paper. Peculiarities of
processing the missing observations, the effects of selection of the X threshold, the 1-mm threshold for inclusion of a day with “sizeable” precipitation, and the
beginning year for trend analyses are discussed in the
appendix.
3. Climatology and regional partition
a. Long-term mean values of the duration of the
warm season and percentage of prolonged
no-rain episodes
Figure 2 shows the long-term mean percentage of
days with strings of different length as calculated for the
1951–2005 period. The station mean values were arithmetically area averaged within 1° ⫻ 1° grid cells. If no
one station within the grid cell has a single string with
dry days during the entire 1951–2005 period, the grid
cell was left blank. For example, the blue dots in Fig. 2a
(⬍0.8%) indicate that during the past 55 yr in these
1° ⫻ 1° grid cells there were from one to four 30-daylong or longer, no rain strings observed. Figure 2d
shows the climatology of the warm season duration
(with mean daily temperatures above 5°C) for the same
period. These periods vary from 103 days (in the North
and in the mountainous West) to 365 days in southern
Florida and California. The duration of this period has
systematically changed during the past 100 yr with
nationwide and global warming (cf. Easterling 2002;
Shein 2006). While not a subject of this study, we, nevertheless, estimated trends in this quantity for the past
century (no significant trends were found) and during
the past 40 yr.
The Upper Great Lakes region (Minnesota, Wisconsin, and Michigan) is a very humid region with frequent
rainfall in the warm season. We observe there a very
small percentage of month-long dry strings (only 0.5%).
Therefore, we separated this region as a special entity
and generally excluded it from further analyses of dry
episodes. Thereafter, we partitioned the rest of the contiguous United States into several regions as shown in
Fig. 3. In the eastern United States (the states east of
95°W without Minnesota, Wisconsin, and Michigan),
we looked for month-long dry strings. West of 95°W we
looked for 60-day-long dry strings (in the southwestern
United States that includes Texas, Oklahoma, New
Mexico, Arizona, California, and Nevada) and for both
30- and 60-day dry strings in the northwestern quadrant
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JOURNAL OF CLIMATE
VOLUME 21
FIG. 3. Station map and the regional partition selected for this study.
against the idea of looking at them for extremes. Therefore, in addition to assessing the frequency of Dry60⫹
strings over the entire southwestern United States (with
the mean occurrence of dry strings of 15% during the
past 55 yr), we analyzed the abridged southwestern region (without California and Nevada). Here, the mean
occurrence of such strings is more rare (7%) and its
changes, therefore, qualify for consideration as changes
in extreme events.
of the country that includes the Great Plains, Central
Rockies, and northwestern United States. We divided
the western United States into southern and northern
parts using a well-known anticorrelation between rainfall in the northwest and southwest of the country (e.g.,
Groisman and Easterling 1994) due to variations in annual rainfall anomalies associated with ENSO-imposed
latitudinal variations of the winter storm tracks
(Ropelewski and Halpert 1996).
Figures 2c and 2d show that the California climate
stands apart from the national climate: strings of dry
days 60 days and longer constitute more than 15% (actually ⬃40% on average) of days during the warm season. However, consideration should be made that in the
valleys where most of the COOP stations are located
the warm season can last the entire year. The mean
statewide percentage of Dry90⫹ strings in this state is
28% and in some years nearly half of the warm season
lies within these 3-month-long strings. Considering percentages of these frequently occurring events goes
b. The driest warm seasons during the past 99 years
The seven driest warm seasons during the past 99
years over the eastern United States (since 1908) were
1963, 1924, 1953, 2000, 1908, 1939, and 2001 with more
than twofold exceedance of the average regional percentage of Dry30⫹ episodes (Table 1). While the norain episode of 1963 was the most extended during the
past 99 yr, only once during this period (in 2000 and
2001) did we observe a sequence of two extraordinary
TABLE 1. Seven driest warm seasons during the past 99 yr over the conterminous United States. Partition was defined using
percentages of 1-month or longer dry episodes (Dry30⫹) for the eastern United States and 2-month or longer dry episodes (Dry60⫹)
within the warm season for western regions of the country.
Eastern
United States
Southwestern
United States
California
and Nevada
Arizona, New Mexico,
Texas, and Oklahoma
Northwestern
United States
Year
Dry30⫹ (%)
Year
Dry60⫹ (%)
Year
Dry60⫹ (%)
Year
Dry60⫹ (%)
Year
Dry60⫹ (%)
1963
1924
1953
2000
1908
1939
2001
Reference period
7.1
5.0
5.0
4.2
4.1
3.8
3.6
1.6
2002
1924
1942
2000
1956
1909
1929
24.2
22.7
22.3
22.2
22.1
22.0
20.7
15.3
1924
2002
1942
1929
1928
1911
1932
59.2
52.5
51.9
51.9
50.1
49.9
49.4
34.6
2000
1956
1909
2002
1998
1945
1996
18.1
16.0
15.7
13.3
13.3
12.4
12.3
7.5
1969
1922
1974
1917
1929
1931
1988
9.0
6.9
6.8
6.7
6.6
6.5
6.4
2.8
1 MAY 2008
GROISMAN AND KNIGHT
dry warm seasons over the eastern United States. In the
southwestern United States, the driest in the century
was the warm season of 2002, which also was observed
to a slightly lesser degree in the region extending from
California through Texas. Year 1969 was characterized
as a year with the most expansive no-rain episode in the
northwestern part of the United States, including the
Great Plains. It is interesting that the instrumental summer Palmer drought severity index (PDSI) (available
online at http://www.ncdc.noaa.gov/paleo/pdsiyear.
html) does not report any unusual dry conditions in this
region. Generally, the durations of prolonged dry episodes (or their percentage in the warm season) represent new indices of summer dryness and are related to
agricultural droughts, and, therefore, it was instructive
to compare these episodes with the most popular indices that characterize droughts. PDSIs (Willeke et al.
1994) accumulated in archive (NCDC 2007) were compared (to the extent possible) to the extreme characteristics of Dry30⫹ and Dry60⫹ indices (including
those shown in Table 1). We found that, while in most
cases there is a reasonable correspondence, however,
these indices are not congruent and may at times be
quite different (e.g., in 1969 in the northwestern United
States). For example, this could happen when unusually
heavy precipitation is followed by a prolonged no-rain
period and thereafter by another heavy rain event. This
kind of situation would not trigger a drought condition
in some of the PDSI calculations. These situations may
be of interest in a follow-up study.
4. Results
a. The eastern United States
Changes in the percentage of the warm season occupied by prolonged (–one month or longer) dry periods
over the eastern United States for the past 40 yr are
shown in Fig. 4. One-month-long dry intervals are infrequent in the region and on average constitute only
1.5% of the warm season duration (over Florida and
Louisiana about 3%). During the past 100 years this
duration has not been substantially changed and in the
first half of the twentieth century it was even somewhat
higher than in the second half (although the differences
were insignificant). However, the situation changed
during the past 40 yr. Figure 4 shows a systematic neartwofold (by 1.0% per 40 yr) increase in the duration of
prolonged (one month and more) no-rain episodes. To
more clearly describe this change, let us assume that
these no-rain episodes occur in the area with the 200day-long warm period (e.g., Washington, D.C., area)
and that the duration of dry episodes is exactly 30 days.
Then a change from 1% to 2% of the duration of the
1855
FIG. 4. Percentage of the dry day episodes with 1-month or
longer duration during the warm season during the past 40 yr area
averaged over the eastern United States. Linear trend (dashed
line; 1.1% per 40 yr) is statistically significant at the 0.05 level.
no-rain 30-day-long or longer episodes means that instead of a return period of 15 to 20 yr of such episodes
(2–3 of them per 40 yr) we are now facing a return
period of 6 to 8 yr for such episodes (5–6 of them per 40
yr). Our analyses show that during the past 99 yr, we
have not encountered systematic changes in the dry
days frequency over the entire eastern United States,
but the strings of dry days (in particular month-long
strings of dry days) became more frequent during the
past 40 yr. Furthermore, for this 40-yr period, the selections of the beginning of the trend as well as the
string length itself are not of crucial importance for this
conclusion (Table 2).
b. The southwestern United States
Changes in the duration (in percent of the warm season) of prolonged dry periods 60 or more days without
rain during the past 40 yr over the southwestern United
States are shown in Fig. 5. The entire region was additionally partitioned into two parts (i) California and
Nevada and (ii) Arizona, New Mexico, Oklahoma, and
Texas because of very different frequencies of prolonged dry periods (Fig. 2). Taking into account the
scale of frequency of occurrence of dry episodes of this
kind, a regionwide increasing trend of 3.6% per 40 yr in
duration of 2-month or longer periods without rain is
observed in both parts of the southwestern United
States. This change constitutes different relative
changes in California (where on average 40% of the
warm season, or approximately 100 days statewide, belong to dry episodes 60 days or more) and Oklahoma
(⬃1% of the warm season belonging to such episodes,
which occurred on average once in 25 yr during the
twentieth century). Significance of the trends for the
past 40 yr for the southwestern United States is quite
robust regarding the change of the beginning year for
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VOLUME 21
TABLE 2. Indicators of statistically significant positive (1) and insignificant (0) linear trend estimates for the period beginning
year–2006 at the 0.05 level using a one-tailed t test (in italic letters) and a two-tailed t test (in bold letters) for the frequency of dry days
and dry-day strings with length above the X threshold for the eastern and southwestern United States. This was the only place
throughout the paper when a one-tailed t test was also used.
Eastern United States
Southwestern United States
X threshold (days)
X threshold (days)
Beginning
year
Dry
days
20
25
30
35
40
Beginning
year
Dry
days
30
55
60
65
70
75
80
85
90
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
1
1
0
0
0
0
0
1
1
0
1
1
0
1
1
1
0
1
0
0
0
1
1
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
1
1
1
1
0
0
0
1
1
1
1
1
1
1
1
1
0
0
0
1
1
1
1
1
1
0
0
1
1
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
1
1
1
0
0
0
1
1
1
0
1
0
0
0
1
1
1
1
1
1
1
1
1
1
the trend evaluation as well as regarding the X threshold selection (Table 2). However, if we were looking for
the 3-month-long dry episodes in the southwestern
United States, the formal results would be similar, but
not representative for Oklahoma and most of Texas
and New Mexico, where 3-month-long dry periods have
been extremely rare or nonexistent during the past century.
c. The northwestern United States
For the northwestern United States, for any duration
of the dry episodes considered, we did not find statistically significant trends at the 0.05 level (using two- or
one-tailed t tests) in the percentage of the warm season
consumed by these episodes.
d. Duration of the warm season and dry episodes
FIG. 5. Percentage of the dry day episodes with 2-month or
longer duration during the warm season during the past 40 yr area
averaged over the southwestern quadrant of the United States
(dots) and the two groups of states that constitute it, California
and Nevada (triangles) and Texas, Oklahoma, New Mexico, and
Arizona (squares). For the entire region, linear trend (dashed
line; 4.3% per 40 yr) is statistically significant at the 0.05 level. For
the region encompassing four states (Arizona, New Mexico, Oklahoma, and Texas) an increase (by 3.1% per 40 yr) is not statistically significant at the 0.05 level.
The century-long warming over the contiguous
United States that became most pronounced during the
post–World War II period (Groisman et al. 2005)
caused a general increase in the duration of the warm
season (Fig. 6, Tables 3 and 4). The increase was especially strong over the southwestern part of the
country and is supported here by earlier snowmelt
(cf. Groisman et al. 2001; McCabe and Clark 2005),
increase in the frost-free period length (Easterling
2002; Kunkel et al. 2004), and earlier dates of flower
blooming (Cayan et al. 2001). However, only over the
1 MAY 2008
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GROISMAN AND KNIGHT
TABLE 3. Duration of the warm season defined as the period
with mean daily temperatures above 5°C over the conterminous
United States and its southwestern part. Mean values and trend
estimates are presented for the 1967–2006 period. All linear trend
estimates are statistically significant at the 0.05 or better levels;
R 2 is the percent of time series variance described by linear trend
and d is days.
Region
FIG. 6. Warm season duration over the conterminous United
States (dots), southwestern United States (filled diamonds), and
northwestern United States (squares). Linear trends (dashed
lines) for the conterminous United States and its southwestern
part are statistically significant at the 0.05 level or above.
southwestern United States (and nationwide) were
these increases statistically significant.3 If, instead of
considering the percentage of the warm season consumed by dry episodes, we take into account their actual duration, the result will not be different. In Table 4,
for 1- and 2-month durations of dry episodes during the
warm season, we present statistics for time series of the
product of mean regional percentage of the dry season
episodes, mean regional duration of the warm season,
and (in parentheses) direct estimates of regionally averaged absolute durations of such episodes.4
3
The Web site “Global Climate at a Glance” (http://www.ncdc.
noaa.gov/gcag/gcag.html) indicates that over the entire conterminous United States we observe a systematic increase (e.g., April
through October) in temperatures during the past 40 yr. However,
if one is considering only statistically significant results (at 0.1 or
0.05 levels), these results will be restricted only to the southwestern corner of the United States.
4
Mean regional durations of prolonged dry periods estimated
directly and as products of DWS ⫻ Dry30⫹ (or Dry60⫹) were not
expected to be exactly the same because (i) durations of the extended dry periods and of the warm period can be correlated and
(ii) the day counts having different weights in these two areaaveraging procedures. By area averaging the percentages of the
vegetation period with prolonged dry episodes, we assigned
smaller weights to lowland and southernmost sites compared to
the mountains and northernmost sites. When area averaging of
actual duration of prolonged dry episodes is made, the sites with
a longer vegetation period receive “an advantage” of both a
longer vegetation period and a decline in orographic-induced
rainfall. Both approaches are legitimate. In this paper, we have
chosen the first approach trying to estimate more carefully the
regional percentage of time when evapotranspiration might be
suppressed by prolonged dry episodes. The regional duration of
extended dry episodes estimated using the second approach is also
presented in Table 4 in parentheses (see also section b of the
appendix).
Mean
value
(d)
Conterminous
United States
Southwestern
United States
California
and Nevada
Texas, Oklahoma,
New Mexico,
and Arizona
Linear trend during
the 1967–2006 period
[d (40 yr)⫺1]
[% (40 yr)⫺1]
R 2 (%)
205
5.5
2.7
10
245
10.5
4.3
19
238
15.4
6.5
21
252
8.9
3.5
10
5. Discussion
In the above presentation, we focus on the statistical
significance of the results. But, how practically significant are they? Climatology (Table 4) indicates that
Dry30⫹ episodes have been quite infrequent in the
eastern United States (with a return period of approximately 15 yr forty years ago). But analysis of trend
results in Table 4 shows that “now” the return period of
such an event is reduced to 6–7 yr. At the same time,
Dry30⫹ episodes in the southwestern United States
and especially in its westernmost parts, such as California, occur each year several times. Therefore, a 20-day
increase in duration of these events in California and
Nevada means a 15% increase of the duration of the
events that occupy more than half (55%) of the warm
period in this part of the nation. For the southwestern
United States, Dry60⫹ episodes are also not rare.
These episodes materialize every second year regionwide and each year in California and Nevada. The observed 11- to 12-day increase in duration of these episodes for the Southwest consumes the entire increase in
the warm season duration and causes their return period to change from 2 yr (40 yr ago) to 1.25 yr “now”
(which means every 4 of 5 years).
Contemporary GCM projections of the climate
change in various scenarios of atmospheric composition
changes over the conterminous United States (e.g.
McAvaney et al. 2001; Hegerl et al. 2004; Kharin et al.
2007) show (i) a gradual increase in surface air temperature with the rate close to that of the Northern
Hemisphere, (ii) no significant total precipitation
changes during the next several decades, and (iii) a
significant increase in intense (extreme) precipitation.
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JOURNAL OF CLIMATE
VOLUME 21
TABLE 4. Summary of the linear trend assessment for the conterminous United States from 1967 to 2006: duration of the warm season
(DWS) percentages of 1-month or longer dry episodes (Dry30⫹) and 2-month or longer dry episodes (Dry60⫹) within the warm season,
and duration of the dry episodes estimated as product of DWS ⫻ Dry30⫹ (or Dry60⫹) and directly by area averaging of these durations
at individual stations (estimates are given in parentheses). Trend estimates that are statistically significant at the 0.05 level or better are
shown in bold; d is days.
DWS
DWS ⫻ Dry30⫹
Dry30⫹
Mean
(d)
Trend
[d (40
yr)⫺1]
Mean
(%)
Eastern United States
221
4.7
1.5
Upper Great Lakes
154
0.8
0.5
Southwestern United States
245
10.5
31
5.1
California and Nevada
238
15.4
54
Northwestern United States
163
2.7
13
Region
Trend
[% (40
yr)⫺1]
Mean
(d)
Trend
[d (40
yr)⫺1]
1.1
3.3
(3.8)
—
0.8
(0.8)
DWS ⫻ Dry60⫹
Dry60⫹
Mean
(%)
Trend
[% (40
yr)⫺1]
2.6
(2.9)
0.0
—
—
0.0
76
(79)
16.1
(12.6)
13.9
4.3
34
(38)
12.3
(11.2)
5.4
129
(134)
21.3
(14.7)
31.6
7.5
75
(85)
23.1
(18.7)
⫺1.5
21
(21)
⫺2.1
(⫺1.4)
2.7
⫺1.3
All of this together could lead to higher stress on water
supply/demand during the warm season between the
extreme rain events. For the northeastern quadrant of
the United States, there are model indications (e.g.,
Semenov and Bengtsson 2002; Groisman et al. 2005)
that in the last decades of the twentieth century this
scenario has already materialized (both in the model
run and in observations). Our analysis shows that during the past several decades a similar scenario was observed over a significant part of the country. It would be
of interest (although beyond the scope of this paper) to
assess the existing composite of prognostic GCM runs
for their ability to reproduce this feature of the warm
season rainfall distribution dynamics over the nation.
6. Summary
Analyzing the duration of prolonged periods without
sizeable rainfall during the vegetation period (roughly
approximated by the threshold ⫹5°C) over the conterminous United States, we found the following:
• During the past four decades the warm season dura-
tion has significantly increased nationwide and over
the southwestern United States (by 3%–4%, respectively). The largest increase in the warm season duration was observed over California and Nevada
where it comprised 15 days (or a 6.4% increase during the past 40 yr).
• During the past four decades the duration of prolonged dry episodes has significantly increased over
Mean
(d)
Trend
[d (40
yr)⫺1]
—
0.0
(0.1)
—
—
—
0.0
(0.0)
—
4.4
(4.7)
⫺2.1
(⫺1.8)
the eastern and southwestern United States in absolute numbers (counting the number of days within
such dry periods) and as a percentage of the warm
“vegetation” period. The changes are interesting because they are observed on the background of the
relatively “wet” period around the nation but do not
cover the entire country. They are consistent with the
notable change in rainfall rate distribution over the
country (increase in intense rainfall frequencies while
mean precipitation grows slower) and with modern
GCM projections (and nowcast, cf. Groisman et al.
2005), of a warmer climate caused by the increase in
greenhouse gases concentration in the atmosphere.
• We presented results based on observations during
the past several decades only. When we looked for
century-long tendencies in the quantities described in
this study, we did not see statistically significant
systematic changes toward increase/decrease of the
prolonged dry episodes over both the eastern and
southwestern United States, and the first 60 yr of the
twentieth century were even somewhat drier than the
last 40 yr (Andreadis et al. 2005). The situation is
somewhat similar to discovered trends in intense
rainfall frequency (Soil and Water Conservation Society 2003; Groisman et al. 2005). We first constructed the century-long time series that led us to
discover that the main (and the only) signal containing systematic changes in the frequency of prolonged
dry episodes is provided by the last decades of observations (cf. section f of the appendix).
1 MAY 2008
GROISMAN AND KNIGHT
1859
• This study provides observational evidence only: it
shows what has happened during the past decades.
Extrapolation of trends based on empirical estimates
is not a legitimate procedure unless theoretical considerations support it. In the discussion, we cite some
examples of expected developments actually observed during the past few decades. Speaking about
the future, we can state only that the tendencies reported in this study do not contradict and (to some
extent) support the contemporary GCM projections
of ongoing climatic changes.
Acknowledgments. NOAA/Climate and Global
Change Program (Climate Change and Detection Element) provided support for this study. The thoughtful
recommendations of three anonymous reviewers
helped us to significantly improve the manuscript.
APPENDIX
Description of Technical Details and
Supplementary Experiments
This appendix contains descriptions of technical details of the study as well as some justifications for
choices that we made in the design of the processing
system. Testing different ways to handle missing data
and our decision to use a minimum precipitation
threshold are examples of the choices tested. We solidified the design when it was shown that precautions and
additional testing and experiments were generally in
vain and did not change the results presented in the
paper.
a. Handling of missing days
A missing day (days) may create havoc in statistics of
uninterrupted strings of dry days. To mitigate this problem, we selected only 4165 stations from the entire set
of more than 8000 currently operational U.S. cooperative stations network according to the following criteria: both station temperature and precipitation must
have a minimum of 83% (25 yr) of nonmissing data
during the reference period 1961–90. For precipitation
we used a very strict missing day tolerance: if the total
annual missing day count was greater than five, the year
was not used. Thereafter, we eased this requirement
and repeated our calculations keeping the station data
for each year if the total annual missing day count was
less than 20. However, in these relaxed conditions of a
notable amount of missing data we allow the count processing of no-rain day duration to be suspended tem-
FIG. A1. Southwestern United States. Dry60⫹ episode durations estimated using area-averaged absolute values of day counts
in these episodes directly and using the product of area-averaged
percentage of the dry days within these episodes and the warm
season duration: R 2 ⫽ 0.95. Systematic difference (bias) ⫽ 3.4 days.
porarily if one missing day is encountered and thereafter continue the tally until the next rain day. This
modification allowed us to (i) substantially increase the
number of years with valid data at the stations used in
our analyses, thus reducing the noise level of the areaaveraged time series, and (ii) receive practically identical results when we used strict and relaxed missing day
tolerance criteria. By definition, accumulated data will
always be preceded by one or more missing days in the
data file. This could cause a detrimental effect by eliminating many years. However, in a separate study using
a similar dataset we found that out of the 153 million
observations checked, only 0.29% were accumulations.
b. Summary of mean duration of dry day episodes
versus a percentage of dry day episodes in the
warm season
To characterize regional dry day episodes extent, we
area averaged the percentages of these episode durations within the individual warm seasons (Dry30⫹ and
Dry60⫹) and thereafter multiplied them by the regionally averaged duration of the warm season. But, it was
possible to area-average absolute values of day counts
in these episodes directly. We used both approaches
and compared them (Fig. A1). The resulting time series
correlated extremely well with R2 ⫽ 0.98 for the eastern
United States and 0.95 for the southwestern United
States. In both regions, we found only some systematic
biases that arise due to different weights of the area
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JOURNAL OF CLIMATE
FIG. A2. Mean number of days with nonzero very light daily precipitation over the conterminous United States. Days with nonzero daily precipitation below 0.5 mm (dots) and days with daily
precipitation in the range of 0.5 and 1 mm (filled diamonds).
averaging (see footnote 4). Except for this difference,
the results after area averaging brought us to the same
conclusions.
c. Selection of the “sizeable rain” threshold
One of the physical reasons to ignore small daily
rainfall totals in assessment of the dry episode length is
that a small amount of rain either does not reach the
soil (stays on and evaporates from the vegetation) or
does not replenish soil moisture. In the calculation of
the most popular index of forest fire danger, the
Keetch–Byram drought index (KBDI), the first 5 mm
(0.2 in.) of each rain event are ignored and do not affect
the KBDI values (Keetch and Byram 1968). However,
there also exists an “observational” reason to skip the
daily “drizzle totals”: throughout the history of many
national networks these totals are not observed consistently. This was observed first at the Norwegian network, when letters of appreciation sent by the National
Meteorological Service to observers caused a doubling
of the number of 0.1-mm precipitation reports nationwide (Groisman et al. 1999). In the countries where
accuracy of observations was changed throughout time
(e.g., Russia and Canada), the lowest “nonzero” precipitation amounts might be assigned to “traces” or to
measurable amounts depending upon changes of the
gauge resolution and/or observational practice (Stone
et al. 2000; Groisman et al. 1999). Ignoring this matter
may lead to “discoveries” of nationwide “statistically
significant” trends in each part of the country in frequency of precipitation that are not real but are a result
of switching—let’s say from British to metric rainfall
reporting (e.g., Frich et al. 2002; Vincent and Mekis
VOLUME 21
2006). During the past 100 years, the U.S. cooperative
network has measured precipitation with the same
gauges, 8-in. nonrecording rain gauges with an accuracy
of (and increments in) observations equal to 0.01 inch
(0.254 mm). However, even this network was impacted
throughout time because the percentage of ignored
small precipitation events changed with time (Fig. A2).
This figure presents the nationally averaged counts of
precipitation events between 0 and 0.5 mm and between 0.5 and 1 mm during the past 99 yr (since the
beginning year when we are comfortable presenting
such averages, 1908). It shows that the reporting of the
lowest nonzero precipitation (0.01 in. ⫽ 0.254 mm) has
never been stable during the entire twentieth century
(their number steadily grows from 6 to 13 yr⫺1). Furthermore, the reporting of the next nonzero increments
(0.02 and 0.03 in.) stabilized only after 1948, when the
first digital archives were introduced, but prior to these
years they were also reported less frequently. During
the 1961–90 period on average, there were 88 days with
nonzero precipitation nationwide and ⬃19 of them
(22%) reported from 0.01 to 0.03 in. of precipitation. In
the beginning of the twentieth century, these numbers
were close to 11 days and 100 years later exceed the
20-days threshold, that is, nearly doubled. Therefore,
when focusing on the changes in precipitation frequency, we (as in Groisman et al. 1999) selected 1 mm
as a safe threshold to start our data analyses.
d. Experiments with different X thresholds and
beginning year of the trend estimates
There were no magic numbers, for example, 30 and
60 days, when we selected X thresholds to present the
results for strings of days without sizeable rain. Therefore, we varied X values in broad boundaries. Also, we
selected 40 yr (instead of 37, which is the start year
1970) just for convenience of rounding. But, we also
tested the robustness of our results against these arbitrary choices. Table 2 presents the summary of these
tests and shows that an exact selection of X and/or the
beginning year for the linear trend calculations is not so
important for the statements about increases in duration of prolonged dry episodes during the past several
decades over the eastern and southwestern United
States to hold. There are quite broad windows of X
thresholds and “beginning” years when the trends in
the duration are positive and statistically significantly
different from zero at the 0.05 level in two-tailed and/or
one-tailed statistical testing. For the past 50 yr, statistically significant negative trends were never encountered in prolonged dry episodes over the conterminous
United States. Table 2 also shows that the trends in
1 MAY 2008
GROISMAN AND KNIGHT
1861
FIG. A3. Distribution function of prolonged dry episode durations ⬎30 (eastern United States) and ⬎60 days
(southwestern United States).
counts of dry days have never been statistically significant at the 0.05 level during the past 50 yr for both
regions, the eastern and southwestern United States, no
matter which beginning year was selected. This remains
the case for the northwestern United States too.
e. Distribution of mean duration of dry day
episodes above the X threshold
If the X threshold is sufficiently high and the region
where we are analyzing dry episodes is spatially homogeneous, the duration of dry day episodes above the X
threshold should be distributed according to “generalized Pareto” distribution (Coles 2001). However, the
regions that we are assessing cannot be considered as
“homogeneous” (cf. Oklahoma and California or
Maine and Florida) and it is difficult to select a “sufficiently high” X threshold in the southwestern United
States. Therefore, the distribution of duration of extended dry day episodes above the X threshold can
(and do) behave differently. Figure A3 shows these distributions for the eastern United States (30-day or
longer dry episodes) and for the southwestern United
States (60-day or longer episodes). This figure shows
that, while it is extremely rare to encounter a 2-monthlong no-rain episode at the stations of the eastern
United States (only 5 cases during the past 40 yr in the
sample of ⬃1400 stations), at the 750 stations of the
southwestern United States, 4-month-long, no-rain episodes number in the hundreds.
f. Assessment of century-long time series of
prolonged dry episodes
The dense network of meteorological stations across
the conterminous United States allowed us to construct
a near-century-long time series of Dry30⫹ and Dry60⫹
episodes across the country for the 1908–2006 period
(not shown). Analyses of these time series clearly show
that
• on a century time scale there were no systematic
changes in the duration of these episodes,
• there were years (cf. Table 1) when the prolonged
no-rain intervals were more widespread across the
eastern United States prior to 1967 than in the 2000s,
and
• in the southwestern and northwestern United States
on average in the pre-1967 period Dry60⫹ episodes
were slightly (i.e., statistically insignificant at the 0.05
level) more widespread than in the past 40 yr.
But, the trend assessment of these century-long time
series shows that (i) whichever 40-yr-long period during
the period of instrumental observations since 1908 and
(ii) whichever interval YB-2006 (where YB is a beginning year) is selected, only in the last periods of 40⫹
years are there statistically significant systematic
changes within two of these regions (cf. Table 2). Will
these changes continue and, if so, what is their attribution? These are legitimate questions but they should be
addressed to climate modelers. We simply show that
they have happened and would strongly argue that the
quantity that we have assessed is of real practical importance, its increase may result in hazardous hydrometeorological conditions and, therefore, tendencies
revealed in the past several decades are worthy of interest. We see a systematic change that coincides in
time with a change in frequency of intense precipitation
and with an increase in mean precipitation. It is not
contradictory with GCM projections of changes in rainfall frequencies [e.g., Semenov and Bengtsson (2002)
for the northeastern quadrant of the United States] but,
because of a simultaneous increase in precipitation [7%
per 100 yr across the conterminous United States
(Groisman et al. 2004)], the observed tendencies are
1862
JOURNAL OF CLIMATE
not as prominent as with the discovered earlier increases in intense precipitation, which were enhanced
by the growing mean precipitation (Groisman et al.
2005).
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