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VOLUME 26
GPS Precipitable Water as a Diagnostic of the North American Monsoon
in California and Nevada
JAMES D. MEANS
Scripps Institution of Oceanography, University of California, San Diego, San Diego, California
(Manuscript received 4 April 2012, in final form 4 September 2012)
ABSTRACT
Precipitable water derived from archived global positioning system (GPS) zenith travel-time delays is used
to describe the seasonal and interannual variation of the North American monsoon in California and Nevada.
A 3-hourly dataset of precipitable water from 2003 to 2009, for over 500 sites in California and Nevada using
temperature and pressure interpolated from the North American Regional Reanalysis (NARR), is constructed to study the temporal and spatial extent of the North American monsoon in the desert regions of
California and Nevada. The statistical distribution of precipitable water values is shown to delineate the
region that is most often affected by the monsoonal influence. A normalized precipitable water index is
employed to indicate when the monsoon starts and stops and to investigate spatial variability. The GPS
network provides much higher spatial resolution than other meteorological networks using surface-based
methods, such as dewpoint criteria and rainfall, and is seen to contain comparable ability in capturing
temporal variations. This dataset reveals the northwestward propagation of the monsoon onset both
synoptically and seasonally. The GPS observations indicate that in the mean the decay of the monsoon is
less well defined than the onset. Seven-year reanalysis 700-mb geopotential height composites for the
monsoon onset and 3 days prior indicate that the onset of the monsoon is associated with a shift in the
synoptic pattern characterized by upper-level high pressure building from the east and offshore troughing
retreating to the northwest.
1. Introduction
In recent decades it has become apparent that the
North American monsoon system is one of the largest
seasonal weather systems to affect the continent. It
brings widespread thunderstorms to the southwestern
United States and northwestern Mexico, typically during the months of July, August, and early September.
It is an important rain producer, bringing a significant
portion of the annual precipitation to the region, albeit
with significant year-to-year variability.
The scope of terminology of the monsoon has expanded over time, with early studies referring to the
‘‘Arizona singularity’’ (Bryson and Lowrey 1955) and
the Arizona monsoon, then the Mexican monsoon
(Douglas et al. 1993), Southwest monsoon, and now the
North American monsoon (Adams and Comrie 1997;
Corresponding author address: James D. Means, Scripps Institution of Oceanography, Mail Code 0224, UCSD, 9500 Gilman
Dr., La Jolla, CA 92093.
E-mail: jmeans@ucsd.edu
DOI: 10.1175/JCLI-D-12-00185.1
Ó 2013 American Meteorological Society
Higgins et al. 1997), but with few exceptions the primary
studies have been over Arizona and Mexico. However,
as explored here, the monsoon exerts its influence regularly up to the Peninsular Range of Southern and Baja
California, where thunderstorms are a regular occurrence during the months of July, August, and early
September (Campbell 1906; Tubbs 1972). While the
actual amount and fraction of the total annual rainfall is
smaller west of the Colorado River, it still is an important factor in the summertime climate of the Southern
California deserts and adjacent mountains. When monsoonal moisture arrives on a ‘‘Gulf surge’’ it is not uncommon to see the dewpoints in southeastern California
reach levels more typical of places like Key West or
Corpus Christi, belying the adage that desert heat is a
‘‘dry heat’’ (Brenner 1974; Fuller and Stensrud 2000;
Hales 1972).
In many parts of the world, such as the Indian subcontinent, it is standard to use observed rainfall as a
criterion for monsoon onset. However, in the southwestern deserts it is problematic to use such an approach
because the localized nature of the convection and the
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TABLE 1. Meteorological stations and monsoon diagnostics for Southern California and adjacent regions.
Location
Lat (8)
Lon (8)
Elev (m)
Jun (mm)
Jul (mm)
Aug (mm)
Jul O Jun (—)
Aug O Jun (—)
Anza
Anza-Borrego
Baker
Barstow
Bishop
Blythe
Brawley
Campo
Cuyamaca
El Centro
Idyllwild
Las Vegas
Laughlin
Mojave
Needles
Mountain Pass
Ocotillo
Palm Springs
Palomar (Mt.)
Phoenix
San Diego
Thermal
Yuma
33.56
33.23
35.27
34.88
37.37
33.62
32.95
32.62
32.98
32.77
33.75
36.07
35.17
35.05
34.76
35.47
32.75
33.83
33.38
33.43
32.73
33.63
32.65
2116.68
2116.41
2116.07
2117.02
2118.37
2114.60
2115.55
2116.47
2116.58
2115.57
2116.72
2115.16
2114.58
2118.16
2114.62
2115.54
2116.00
2116.50
2116.83
2112.02
2117.17
2116.17
2114.62
1193.3
245.4
287.1
707.1
1253.0
81.7
230.5
801.6
1414.3
29.1
1639.8
649.5
184.4
833.6
278.6
1441.7
125.0
129.5
1691.6
337.1
4.0
234.1
64.9
1.8
0.5
1.8
1.3
5.3
0.8
0.3
2.3
6.9
0.3
5.8
2.0
1.8
1.3
1.0
6.4
2.0
1.3
4.6
2.3
2.3
0.5
0.3
11.7
8.4
4.8
5.8
4.3
4.6
1.8
7.4
10.9
1.5
19.8
11.2
6.9
4.1
8.1
24.6
8.4
4.8
10.7
25.1
0.8
4.8
5.8
14.7
15.0
12.2
5.6
3.3
16.5
10.7
14.0
23.6
8.1
24.1
11.4
11.2
6.9
17.8
30.2
11.9
10.2
23.1
23.9
2.3
9.4
13.5
6.6
16.5
2.7
4.6
0.8
6.0
7.0
3.2
1.6
6.0
3.4
5.5
3.9
3.2
8.0
3.9
4.1
3.8
2.3
11.0
0.3
9.5
23.0
8.3
29.5
6.9
4.4
0.6
21.7
42.0
6.1
3.4
32.0
4.1
5.6
6.3
5.4
17.5
4.8
5.9
8.0
5.1
10.4
1.0
18.5
53.0
relatively sparse observations make it difficult to ensure that rainfall is being adequately sampled. Nevertheless, that was the approach of Bryson and Lowrey
(1955) in their seminal paper on the monsoon. They
defined a ‘‘raininess index,’’ which was simply the
fraction of climatological stations in Arizona that received a trace or more of precipitation on a given day.
Their work showed a rapid increase in the raininess
index after 1 July. Such an approach is more useful for
Arizona than it is for California because the monsoon
rainfall is generally greater there. This can be seen
in Table 1, which shows monsoon (June–August) rainfall over stations in Southern California, Nevada, and
Arizona. Monsoon precipitation at Phoenix is greater
than at any California site except Mountain Pass, which
is at a much higher elevation. Nevertheless, it is clear
that monsoonal influence is important in California
also, as can be seen by the ratio of rainfall during
July and August, during the monsoon, to June rainfall,
prior to the monsoon onset. The California desert and
mountain stations have ratios greater than one, indicating monsoonal influence, while San Diego (a coastal
site) and Bishop (a high desert site farther north than
the others) have ratios less than or equal to one, indicating that the monsoon is weak or essentially not
present in those areas.
Later researchers (e.g., Skindlov 2007) used a dewpoint criterion for monsoon onset instead of a raininess
index. Dewpoint data from the few available Southern
California desert stations indicate that they fall within
the monsoon region. Figure 1 shows a comparison of
the dewpoints in Phoenix, Arizona, and El Centro,
California, over the monsoons of 2006–09. Despite the
measurements being taken over 300 km apart, the two
curves are remarkably similar, with many signature
peaks and valleys corresponding to monsoon moisture
waxing and waning. Generally the dewpoints are higher
at El Centro than they are at Phoenix, a strong indication that the monsoon influence extends into California, even if monsoon rainfall is more prevalent in
Arizona.
2. Methodology
Travel-time delays of global positioning system (GPS)
signals between satellites and ground-based receivers
can be used to determine precipitable water at the receiver site, if the temperature and pressure at the
site are known (Bevis et al. 1992; Duan et al. 1996). A
database of GPS-derived precipitable water at 3-hourly
intervals has been constructed for over 500 sites in
California and Nevada (see Fig. 2), covering the period
2003–09. The precipitable water was calculated using
hourly GPS zenith delays archived by the Scripps Orbit
and Permanent Array Center (SOPAC) and temperature and pressure at the GPS sites obtained from spatial
interpolation of the 32-km resolution North American
Regional Reanalysis (NARR) data, using a method
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FIG. 1. Plot showing daily averaged dewpoints for El Centro and Phoenix during monsoon time periods of 2006–09.
described in Means and Cayan (2013). An hourly database was also constructed in a similar fashion by interpolating the 3-hourly NARR values to intermediate
hours.
This database of GPS precipitable water is similar to
that provided by the SuomiNet network of GPS receivers, and indeed some of the same GPS sites are used.
The ability to use NARR data to provide the station
pressure and temperature needed to calculate the precipitable water from the station delay, rather than to rely
on meteorological station data, opens up many more
sites to the analysis. If we had restricted our study to
SuomiNet sites we would have had only about 50 sites
in California and Nevada that we could have used, and
our spatial coverage would have been much less dense.
A direct comparison between nine overlapping sites
showed that root-mean-square (RMS) differences between the SuomiNet values for precipitable water and
those calculated using our technique ranged from 1.5
to 2.1 mm (Means and Cayan 2013).
These derived precipitable water values provide a
regular progression of ‘‘snapshots’’ of atmospheric water vapor at higher temporal and geographic resolution
than can be obtained from radiosonde data. The insight
that can be obtained into the monsoon onset from this
database is illustrated in Figs. 3a and 3b, two precipitable water images of the region from 18 h apart.
The first image, Fig. 3a, shows dry conditions over the
entire region that were present before the onset, while
Fig. 3b, the later image, shows moisture increasing over
the Imperial Valley in the southeastern portion of the
region as the monsoon begins. Figure 3c shows the situation 18 days later, with the monsoon fully in place over
the region.
3. Precipitable water statistical distribution
Many sites around the world show a lognormal distribution in their precipitable water values (Foster et al.
2006). This is presumably a consequence of being under
the influence of a single air mass for a majority of the
year. In contrast, monsoonal regions exhibit distributions of precipitable water that are bimodal. This is understandable as being due to a seasonal variation in the
dominant air mass, with different moisture characteristics for each. Figure 4 shows the contrast between a site
that is nearly always subject to a midlatitude maritime
air mass, GPS site farb in the Farallon islands off of San
Francisco, and Mumbai, India, a classic monsoon location. The precipitable water for farb is drawn from the
GPS dataset, while that for Mumbai is from 10 years of
40-yr European Centre for Medium-Range Weather
Forecasts (ECMWF) Re-Analysis (ERA-40) data (Uppala
et al. 2005). An excellent fit to a lognormal distribution
is seen at farb while the distribution for Mumbai is
clearly bimodal.
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FIG. 2. GPS sites for which precipitable water was derived. The
U.S. states of California, Nevada, and Arizona are labeled CA, NV,
and AZ; the Mexican states of Baja California Norte and Sonora
are labeled BC and SO; and the Gulf of California (Sea of Cortez)
is labeled GC.
No previous studies have been done on the precipitable water distributions associated with the North
American monsoon, so it was not clear at the outset
whether bimodal characteristics would be seen. A visual
examination of the 3-h frequency distribution from 2003
to 2009 from more than 500 GPS sites in California and
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Nevada revealed that sites in the monsoon had either a
clear bimodal nature or at least an enhancement of
the high-end tail of the distribution (a ‘‘shoulder’’) (see
Fig. 4). Of the 513 sites examined, 100 of them, or 19.5%,
were found to deviate from a simple lognormal distribution by having an enhancement of the tail of the distribution. Every one of those sites fell into the ‘‘South
Coast Drainage’’ or ‘‘Southeast Desert Basin’’ climate
divisions of California, or the ‘‘Extreme Southern’’ climate division of Nevada. Each of these are from the
southernmost climate divisions, presumably ones that
most come under the influence of the North American
monsoon during the summer months. The shape of the
precipitable water probability distribution appears to be
an effective way of identifying monsoonal regions.
To be more useful as a criterion for distinguishing
between monsoonal and nonmonsoonal distributions
it would be good to have a quantity that may be objectively calculated. Although bimodality is a concept of
widespread interest in all areas of science, there is no
single accepted objective measure of bimodality, but
instead many different measures that appear in the literature (Knapp 2007). The reason that so many different
measures of bimodality have been constructed may be
that none is universally useful—they all have regions of
applicability where they characterize the bimodal nature of a distribution well, but they fail as a measure of
the bimodality of other distributions. This characteristic
means that a bimodality measure that works well for
describing obvious monsoonal distributions like that for
Mumbai (Fig. 5) fails to detect the subtle variations that
FIG. 3. GPS PWV images. (left) 13 Jul 2007, prior to the onset of the monsoon. (middle) Image is 18 h later at the onset of the monsoon.
(right) Image is 18 days later when the monsoon is fully developed.
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FIG. 4. Plot of precipitable water distribution at southern California
desert site iid2. Deviation of distribution from lognormal form at
the high end is indicative of monsoon.
distinguish the degree of influence of the North American monsoon. To this end we have tested a number of
different measures of bimodality with real and synthetic
data and have settled on the ‘‘bimodality coefficient’’ of
Sarle (Knapp 2007; SAS 2008) as the most useful for our
data. It can be written in terms of the kurtosis k, skewness s, and number of observations n as (SAS 2008)
b5
k2 1 1
3(n 2 1)2
s1
(n 2 2)(n 2 3)
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VOLUME 26
If this easily calculated quantity allows us to distinguish between monsoonal and nonmonsoonal stations
just by a simple calculation based on the statistical distribution of precipitable water values for a location,
it can be thought of as a possible ‘‘monsoon index.’’
Higher values should be obtained for stations where
the monsoon exhibits a greater influence, and lesser
values for stations away from the monsoon region.
Figure 6 (left) is a plot of b and shows that the highest
values (greater than 0.4) are indeed found in the
monsoon regions, with values decreasing away from
the monsoon region. To test whether the monsoon is
responsible for these higher values of b, the monsoon
time period can be excluded from the distributions and
b calculated again; Fig. 6 (right) shows the results of
doing this, using mid-June to mid-September as an approximate time span for the monsoon. The bimodality
index is greatly decreased for the sites in the monsoon
region, with about 90% of all sites showing a decrease,
and no sites with values above 0.35. The only sites
showing an increase are those in the area north of 388N,
and with an absolute magnitude change that is not significant. Thus, it can be concluded that b serves as a
useful measure of the monsoonal nature of a particular
station, at least in the region under study.
4. Monsoon onset and retreat
Experts have debated the best way to characterize the
onset and retreat of the North American monsoon.
Various criteria have been proposed (Skindlov 2007),
FIG. 5. (left) PW distribution from the GPS site farb, in the Farallon Islands off of San Francisco. The plot from this site is an excellent fit
to a lognormal distribution and displays no monsoonal influence. (right) The distribution of precipitable water values for Mumbai, India,
as derived from 10 yr of ERA-40 data. Mumbai is a classic monsoon location and clearly shows a bimodal distribution of precipitable
water values.
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FIG. 6. (left) The bimodality index calculated for the entire year. (right) The bimodality index with the prime monsoon period excluded.
The large decrease demonstrates that it is effective as a monsoon index.
including ones based on rainfall, dewpoint, and integrated
column water vapor or precipitable water. A criterion
based on rainfall is perhaps most practical from a forecasting viewpoint, since the public has a strong interest
in knowing when summer thunderstorms may occur for
business planning, recreation, and safety, as well as just
general interest. There are impracticalities associated
with such an approach, however, since it is dependent on
rainfall catch in gauges that may be very sparse in the
harsh desert environment.
Dewpoint criteria have proven very useful, and this
is the approach that was taken by forecasters in the
Phoenix and Tucson offices of the National Weather Service for a number of years, where the criteria has been
three consecutive days of mean dewpoint of 12.88C (558F)
or greater at Phoenix. One problem with dewpoint criteria
is that they are dependent on the elevation of the observing
station, since higher stations typically have lower dewpoints. In fact the National Weather Service recognized
this and set the monsoon onset criterion slightly differently
for Tucson, which is at a higher elevation than Phoenix, at
3 days with dewpoints of 12.28C (548F) or higher.
Another shortcoming of dewpoint criteria is that they
may not reflect the presence (or lack of) deeper moisture. It is not uncommon to see very shallow intrusions
of high dewpoint air into the Imperial Valley without
accompanying thunderstorm activity over nearby mountains, because the moist layer is too shallow. Ellis et al.
(2004) have combined dewpoint and precipitation into a
single monsoon criterion that offers a regional approach
to monsoon onset and retreat, but may be less useful when
applied locally.
An improvement over dewpoint may be the integrated water vapor or ‘‘precipitable water.’’ In recent
years GPS delay measurements of precipitable water
have greatly increased the availability, both spatially
and temporally, of precipitable water data. This has
prompted some researchers to suggest that precipitable
water can be used instead of dewpoint, with one suggestion being that onset be associated with the first of at
least three consecutive days with precipitable water over
30 mm. In fact, such a criterion yields results, when applied to central Arizona, that are very similar to those of
the dewpoint criterion, with the added guarantee that
when the precipitable water criterion is met there will
certainly be sufficient atmospheric moisture to generate
convection—whether or not instability is present or precipitation is observed in what may be sparsely distributed gauges.
Precipitable water, being the amount of water vapor
in the air above the observation point, is subject to one
of the same kind of shortcomings as is dewpoint—its
value depends upon the elevation of the observation
point, as higher elevations will necessarily have less atmosphere, and hence less water vapor, above them. A way
around this shortcoming has been suggested by (Zeng
and Lu 2004), who define a Normalized Precipitable
Water Index (NPWI):
NPWI 5
PW 2 PWmin
,
PWmax 2 PWmin
where PWmax and PWmin are the maximum and minimum
of the daily averaged precipitable water at a location. This
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FIG. 7. (a) Time series of GPS precipitable water at four sites in southeastern California for July and August 2007. (b) The Normalized
Precipitable Water Index (NPWI) at the same sites. NPWI has smaller spread than actual precipitable water.
removes some of the dependence of precipitable water
on location and elevation, since it normalizes the range
of the variable from a minimum of about zero (it can be
negative on exceptionally dry days) to a maximum on
the order of one (values of one can be exceeded for
particular days that exceed the maximum daily average
value). Figure 7 compares the actual precipitable water
value and the NPWI for four sites along a southeast–
northwest transect across the Imperial Valley during the
months of July and August 2007. The similarity between
the precipitable water curves at the different sites is
striking, and is made even more so by looking at the
NPWI rather than the actual value of precipitable water.
Such an approach has been found to be extremely
useful for defining monsoon onset and retreat, because
NPWI provides a measure that is independent of elevation and position for a particular geographic area.
Additionally, the same criterion based on NPWI has
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FIG. 8. Daily averaged dewpoint at El Centro and NPWI at nearby GPS station P496 for 2006–09. Plots are scaled vertically so the
horizontal gridlines showing the threshold for monsoon onset are coincident in terms of dewpoint (512.88C) on left axis and NPWI
(50.618) on right axis.
been found to be useful for defining monsoon onset and
retreat in vastly different basins, from the prototypical
Indian monsoon, with a mean value of precipitable water of 77 mm, to the much drier monsoon in Arizona,
with a mean value of precipitable water of 45 mm (Zeng
and Lu 2004). The actual monsoon onset criterion suggested by Zeng and Lu, because it proved useful and also
as something of an intuitive leap, was one where the
NPWI exceeded the ‘‘golden ratio’’ of 0.618 for three
consecutive days, with the first such day defining the onset.
While there may have been a touch of whimsy in basing
the definition on the golden ratio—anything around 0.6
would probably have worked just as well—the golden
ratio appears quite often in nature where things are
being divided, and in a sense the year is being divided up
into a wet season and dry season. The monsoon retreat is
defined similarly, as the first of three consecutive days
with an NPWI less than the golden ratio, but with the
further condition that if there are multiple ‘‘onsets’’
(monsoons often exhibit dry breaks), then the retreat
is the first such retreat date that occurs after the final
onset. This ensures that the retreat actually occurs at the
end of the monsoon, and is not just a sequence of three
dry days that has occurred after a particularly moist
synoptic situation (such as an atmospheric river storm),
which may temporarily increase the NPWI up to monsoon levels.
Since the Zeng and Lu onset definition based on the
NPWI has proven to be remarkably robust worldwide, this
is examined here, using the database of precipitable water
values, but first it is appropriate to determine whether it
gives similar results to a criterion based on dewpoint.
To evaluate this, a GPS site, P496, is compared with El
Centro, where hourly dewpoint readings are available.
FIG. 9. GPS sites (widc, cact, glrs, and iid2) used to study monsoon onset propagation. The U.S. states of California and Arizona
are labeled CA and AZ; the Mexican states of Baja California
Norte and Sonora are labeled BC and SO; the Pacific Ocean is
labeled PO and the Gulf of California is GC.
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The two sites are located 10 km apart. The four plots of
Fig. 8 show the daily averaged dewpoint at El Centro
and normalized precipitable water index at the nearby
GPS station P496 for 2006, 2007, 2008, and 2009. The
plots are scaled so that the horizontal gridline that shows
the threshold for monsoon onset in terms of Td 5 12:88C
(on left axis) and the horizontal gridline that shows
the precipitable water threshold for monsoon onset,
NPWI 5 0:618 (on right axis), are coincident. If these
criteria are equivalent, the dewpoint curve and precipitable water curve will cross the gridline at the same
time (each threshold requires 3 days of exceedance).
Despite measuring somewhat different quantities, with
different sampling frequencies, and at slightly different
locations, the curves are very similar. This demonstrates
that surface dewpoint and NPWI yield approximately
equivalent measures of monsoon onset. Examination
of a set of dewpoint and NPWI records before and
during the monsoon yields a similar degree of correspondence, so it is concluded that NPWI is an appropriate measure of monsoon activity and is adopted here
for that purpose.
5. Propagation of monsoon onset
The rather sudden onset of the monsoon suggests that
the moisture front associated with onset must propagate
fairly rapidly from south to north. To resolve a rapidly
propagating moisture front between nearby sites, the
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TABLE 2. Imperial Valley GPS sites used in study of monsoon onset
propagation.
Site
Lat (8)
Lon (8)
Elev (m)
iid2
glrs
cact
widc
32.71
33.27
33.66
33.93
2115.03
2115.52
2115.99
2116.39
44.2
214.85
551.2
477.03
Distance between
sites (km)
77.3
61.6
47.7
NARR 3-hourly data (Mesinger et al. 2006) are interpolated to every hour and an hourly database of precipitable water is calculated based on the hourly GPS
zenith delays. While the NARR interpolation could lead
to problems in rapidly changing synoptic situations, the
amount of error in precipitable water that errors in
pressure and temperature contribute is reasonably
small (Bevis et al. 1994; Means and Cayan 2013), and
there is no reason to expect the errors associated with
changing values of these quantities to be large during the
monsoon.
If the moisture front moves slowly enough and if there
is enough distance between the sites, it should be possible to detect the monsoon onset propagating northward.
A map of isochrones of an advancing gulf surge given in
Brenner (1974) suggests that the speed may vary depending on influences of the terrain—speeding up in low
friction areas while slowing down in more complex terrain. The surge studied by Brenner was first observed at
FIG. 10. The 2003, 2004, 2006, and 2007 monsoon onset for four sites in the Imperial Valley, southeastern California.
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FIG. 11. Geostationary Operational Environmental Satellite (GOES) visible image from 4
Jul 2008 showing cumulus and cumulonimbus forming over the Peninsular Range of Southern
and Baja California associated with the onset of the monsoon in that region. Extensive cumulus
field and large thunderstorm complexes can also be seen farther east over Arizona and northern
Mexico.
Yuma, then at Phoenix and Thermal 3 h later, indicating
a relatively fast passage across the lower Colorado River
Valley, while the isochrones north of Thermal are much
more closely packed, suggesting that the surge had
slowed down. Even the faster propagation seen by
Brenner farther south suggests that it should be possible
to observe the moisture advance with the 1-h time resolution used here.
To examine the space–time development of the monsoon in southeastern California, four GPS sites were
chosen along a southeast–northwest transect through
the Imperial Valley (the starred stations in Fig. 9) and
the precipitable water time series for each station that
contains the monsoon onset is plotted. The monsoon
onset time along the transect can then be examined for
systematic variations associated with the propagation
of the monsoon moisture front. The total length of the
transect is about 186 km. Table 2 gives the location
information for the sites and the distance between adjacent sites.
Data from all these sites are not available for every
year of the record, so only those years were included for
which each of the four sites contained the monsoon
onset: 2003, 2004, 2006, and 2007. The results are shown
in Fig. 10.
On each plot the golden ratio onset criterion is plotted
as a horizontal line. There are several examples of
‘‘false’’ onsets, during which the NPWI exceeded the
golden ratio but failed to sustain that level for the 3-day
minimum requirement. For example, the plot from 2003
shows such a false onset at all the stations, which is followed a few days later by the true onset. Some of the
onsets are quite dramatic, showing a large increase in
normalized precipitable water index over a short time
span. Each plot shows the southeasternmost station
experiencing the onset first—in fact, this holds for the
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FIG. 12. The NARR 700-mb height composites (m) for 7 yr of monsoon onsets. (left) The composite of 3 days prior to monsoon onset in
California with flow generally from the south and southwest. (right) The composite of day of monsoon onset showing a high building in from
east with flow shifting to south to southeasterly across the region. Images provided by the National Oceanic and Atmospheric Administration’s
Earth System Research Laboratory, Physical Science Division, Boulder, Colorado, from their web site at http://www.esrl.noaa.gov/psd/.
false onsets also. Generally the onset can be seen to
occur later at the more northwestward stations, more or
less in the order iid2, glrs, cact, and widc (site abbreviations are given in Tables 2 and 3) but at times it is
difficult to see any time difference between nearby stations, suggesting that there is geographical complexity to
the moisture field, and also that this is probably near the
time resolution of the onset passage for nearby stations.
In some cases the monsoon onset may be associated
with the northern extension of a tropical easterly wave
passing through the area. This may be the case for the
monsoon onset of 2008, shown in the visible satellite
image of Fig. 11. Cumulus and cumulonimbus can be
seen forming over the Peninsular Range of Southern
California and Baja California, while farther east over
Arizona large thunderstorm complexes and an extensive
cumulus field can be seen.
6. Climatology of monsoon duration
From the database sites were extracted that are
contained within the National Climatic Data Center’s
Southeast Desert climate division, and then from those
sites a subset was selected using only those sites that had
a record of greater than 95% temporal coverage over the
period. This yielded 40 GPS sites with very complete records of observations, which were further culled down to
24 sites by examining the gaps in each site’s record, and
rejecting sites with large gaps during the summer season.
For each of these sites, the monsoon onset, retreat,
and duration (time between onset and retreat) were
calculated for each year from 2003 to 2009, and the results are given in Table 3.
The first thing to notice about the monsoon onset date
is how small the variation is from site to site, only varying
between day of year 188 and 191 for every site except for
cccc. The site-to-site variation is much smaller than the
variance from year to year at a single site. There is a similar homogeneity in the mean retreat days, which vary
from 255 to 261 across all sites, with the single site variances generally being somewhat smaller for the retreat
days than for the onset days. In general from this data
there does not appear to be much evidence for a geographical variation in the onset and retreat, indicating that
in each year the onset and retreat are synoptic-scale
events that affect the entire region.
The monsoon onset and retreat can also be calculated
in a somewhat different sense by forming the daily mean
precipitable water at each site over the full 7 years and
using that to determine the monsoon onset and retreat
days. Since this method involves averaging across the 7
years under study, it smooths out the precipitable water
variations that often herald the onset in a particular
year, delaying the onset and hastening the retreat of the
monsoon. The time period defined by the onset and
retreat calculated in this fashion can be thought of as
the ‘‘core’’ time period of the monsoon. The core onset
calculated from the 7-yr mean has an even smaller
15 FEBRUARY 2013
1443
MEANS
TABLE 3. GPS sites used in this study and their 2003–09 monsoon index, mean monsoon onset day, retreat day, and duration determined
from 7-yr GPS precipitable water data. (S.D. 5 standard deviation.)
Site
Lat (8)
Lon (8)
Elev (m)
Monsoon
index
Mean
onset 6 S.D. (DOY)
Mean
retreat 6 S.D. (DOY)
Mean duration
(days)
crrs
mvfd
cotd
dssc
bemt
msob
bmhl
sdhl
ldes
opbl
tabl
opcl
avry
pbpp
agmt
ldsw
hcmn
troy
rstp
bsry
phlb
cccc
shos
ryan
33.07
33.21
33.73
33.73
34.00
34.23
34.25
34.26
34.27
34.37
34.38
34.43
34.47
34.51
34.59
34.70
34.75
34.84
34.88
34.92
34.93
35.57
35.97
36.32
2115.74
2116.53
2116.39
2116.71
2116.00
2117.21
2116.05
2116.28
2116.43
2115.92
2117.68
2116.31
2117.15
2117.92
2116.43
2116.21
2116.43
2116.53
2118.19
2117.01
2117.69
2117.67
2116.30
2116.65
248
1222
60
1692
1405
1765
754
866
1009
1258
2259
1344
920
934
1369
672
600
645
745
645
918
844
612
1315
0.426
0.431
0.424
0.414
0.436
0.406
0.434
0.436
0.429
0.435
0.416
0.43
0.398
0.384
0.426
0.424
0.415
0.411
0.363
0.386
0.374
0.377
0.409
0.415
190 610
188 611
191 610
188 612
190 69
188 612
191 69
191 69
189 612
188 612
188 612
188 612
188 612
188 612
188 612
189 612
189 612
188 612
188 612
188 612
188 612
184 614
189 612
189 612
249 65
249 611
246 69
246 69
251 68
246 612
247 69
247 69
247 69
247 69
245 613
247 69
246 68
247 69
247 69
252 67
251 68
251 68
249 67
251 68
247 69
246 69
248 612
245 611
60
62
55
59
62
59
59
56
58
59
57
58
59
60
59
63
63
63
61
63
60
62
59
56
variation, and is day 195 or 196 at all sites. The small
variation could be an accident of the short duration of
the dataset (only 7 yr), but the same is not seen for the
date of the monsoon core retreat, which varies from day
213 to day 233. This large variation in end date results in
a similarly large variation in monsoon duration. Generally, the monsoon retreat is seen earlier in sites that
are farther north and west, resulting in longer monsoon duration to the south and east, as might be expected with the moisture source lying in that direction.
The duration of the monsoon core dates has a weak but
statistically significant correlation with latitude (decreasing to the north) and longitude (decreasing to the
west).
The results indicate that monsoon onset and retreat
are synoptic-scale events that affect most of the monsoon region within a relatively short time span, with
monsoonal moisture flooding northward in a single
surge, while the retreat may be associated with an early
cool-season passage of a baroclinic disturbance.
To better understand the dynamics of the monsoon
onset, a NARR 700-mb geopotential height composite
was created from 7 yr of monsoon onset dates at the GPS
site crrs, as well as a composite of 3 days previous to the
monsoon onset dates (see Fig. 12). In the premonsoon
onset composite the 700-mb winds are generally out of
the southwest, becoming more southerly at the eastern
side of the region. Upper-level high pressure is present
over Arizona and a trough is offshore from California.
This situation would bring in stable, dry air that has
descended off the east Pacific high and would inhibit
convection.
The composite of the monsoon onset days shows a
strikingly different synoptic situation. The upper-level
high over Arizona is stronger and has built westward
and northward, and the offshore trough has retreated
to the northwest, with a col (an area of weak pressure
gradients between two highs and two lows) now appearing
southwest of the region. This configuration results in
upper-level winds backing to a south to southeasterly
direction. This would bring in air that had previous
contact with the Gulf of California (and possibly the
Gulf of Mexico and tropical eastern Pacific) and has a
much higher moisture content that would make it more
susceptible to convection when lifted by heating and
orographics.
7. Conclusions
GPS precipitable water measurements for the period
2003–09 show that the Southern California deserts experience a large increase in atmospheric water vapor
1444
JOURNAL OF CLIMATE
associated with the North American monsoon. The
monsoonal character of the late summer season is easily
recognizable in the statistical distribution of precipitable
water, and can be quantified using an index of distribution bimodality. The Normalized Precipitable Water
Index has been demonstrated to be a very useful indicator of monsoon onset and retreat and can be used
to study the propagation of the monsoon onset front
across the region. The relatively short 7-yr climatology
of precipitable water indicates that the monsoon starts
and stops at approximately the same time across the
desert region, perhaps associated with particular synoptic events, such as the passage of a tropical easterly
wave across the region for the onset and a cool-season
baroclinic disturbance bringing about the retreat. During the onset the moisture front can be seen propagating
northwestward in the precipitable water data over a time
period of a day or less. In contrast, the retreat of the
monsoon is more gradual, with the monsoonal moisture
leaving northern and western regions first and southern
and eastern regions days or weeks later.
Acknowledgments. I’d like to thank my thesis adviser,
Dr. Daniel Cayan, for suggesting GPS precipitable water
as the topic for my dissertation research, and allowing
me to follow that research in several disparate directions, including study of the North American monsoon.
The Scripps Orbit and Permanent Array Center (SOPAC)
provided the zenith delays, and Drs. Yehuda Bock and
Peng Fang provided advice and computing assistance.
I would like to thank Dr. Thomas Knapp for advice and
assistance with bimodality calculations. The work was
supported in part by the California Energy Commission and through a Blasker grant from the San Diego
Foundation.
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