15 JULY 2002
GROVER AND SOUSOUNIS
1943
The Influence of Large-Scale Flow on Fall Precipitation Systems in the
Great Lakes Basin
EMILY K. GROVER*
AND
PETER J. SOUSOUNIS1
Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan
(Manuscript received 11 July 2001, in final form 20 November 2001)
ABSTRACT
A synoptic climatology is presented of the precipitation mechanisms that affect the Great Lakes Basin. The
focus is on fall because increasing precipitation in this season has contributed to record high lake levels since
the 1960s and because the causes can be synoptically evaluated. Precipitation events were identified for the
period 1935–95 from NOAA Daily Weather Maps. Precipitation days were classified as one of nine types. Trends
in the precipitation classifications, 24-h precipitation totals, and the frequency and intensity of precipitation days
and events were analyzed.
It was found that the precipitation increased 15% over the basin and 35% at Grand Rapids, Michigan, from
1935–65 to 1966–95. The increased precipitation was driven by an increase in the amount of precipitation per
day (from low pressure systems and warm, stationary, and occluded fronts) and an increase in the frequency of
precipitation days (from troughs and cold, warm, stationary, and occluded fronts). All classifications except for
isolated convection contributed to the increase. Increases from warm, stationary, and occluded fronts contributed
the most.
Analysis of precipitation mechanisms and large-scale circulation features for two 10-yr periods from 1950 to
1959 and from 1980 to 1989 revealed that higher precipitation amounts were associated with a more zonal flow
pattern that existed over the United States during 1980–89. This pattern was accompanied by more baroclinicity
and moisture over the Rockies, a stronger upper-troposphere subtropical jet, and stronger low-level flow from
the Gulf of Mexico. These features allowed a greater number of southern systems with more moisture to influence
the region. Specifically, the increased frequency of low pressure systems approaching from the south(west) and
their associated more rapid deepening rates allowed more precipitation from warm, stationary, and occluded
fronts. The similarities in the synoptic precipitation classifications and precipitation amounts between the two
10-yr periods and the two 30-yr periods examined suggest that more meridional flow was present for much of
the 1935–65 period and that more zonal flow was present for much of the 1966–95 period.
1. Introduction
The Great Lakes play an integral role in many regional aspects, from weather to environmental policy to
the economy. One of the most important ways that the
Great Lakes govern their region is through their water
levels. Record-high lake levels were set in 1973 and
1985 (Quinn 1986; Hitt and Miller 1986). High water
levels impact five major areas in the Great Lakes region:
shipping, hydropower, recreational boating, shoreline
erosion, and the environment (Changnon 1987). Damage reports on the shorelines are much more frequent
when lake levels are high (Angel 1995; Meadows et al.
* Current affiliation: International Research Institute for Climate
Research (IRI), Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.
1 Current affiliation: Weather Services International, Inc., Billerica, Massachusetts.
Corresponding author address: Dr. Peter J. Sousounis, Weather
Services International, Inc., 900 Technology Park Dr., Bldg. 9, Billerica, MA 01821-4167.
E-mail: [email protected]
q 2002 American Meteorological Society
1997). Specifically, the record-high levels set in the
1970s and 1980s, in conjunction with strong storms,
caused heavy damage to shorelines, widespread flooding, and destruction of homes (Quinn 1986). These high
lake levels and far-reaching impacts have been, in part,
a result of an increase in precipitation in the Great Lakes
Basin—especially since the mid-1960s (Quinn 1986;
Changnon 1987).
Numerous investigators have documented this increasing precipitation regime. Changnon (1987) found
a general increasing trend for the Lake Michigan basin
beginning at the turn of the century with a sharper increase beginning in the early 1970s. Lettenmaier et al.
(1994) identified a significant increase in the annual
precipitation of the Great Lakes Basin for the period
1948–88. Karl and Knight (1998) found precipitation
increases in the fall season for the Great Lakes region
as well as for many other regions in the continuous
United States beginning in 1910.
Despite the identification of an increasing precipitation trend in the Great Lakes Basin, little attention has
1944
JOURNAL OF CLIMATE
VOLUME 15
FIG. 1. The GLB (heavy line) and GLR (dashed circle). Referenced first-order stations are
indicated by their three-letter identification code.
been paid to important details such as its possible correlation with other atmospheric variables. The objectives of this study are to investigate how changes in
major precipitation-causing mechanisms (e.g., fronts,
troughs, cyclones) have played a role in the increasing
precipitation trend of the Great Lakes Basin and to explain these changes by examining the large-scale flow.
The time period 1935–95 is examined because it straddles approximately the time (mid-1960s) when precipitation and lake levels began long-term increases. A
synoptic classification scheme is implemented to determine the precipitation-forcing mechanisms in the fall
months (September–October–November). Section 2 describes the methodology. Section 3 describes the classification results. Section 4 describes how the largescale flow changed over the period. Section 5 provides
a discussion of how changes in the the large-scale flow
likely contributed to changes in precipitation characteristics. Section 6 provides a summary and conclusions.
2. Definitions, data, and methodology
For this study, a precipitation day was defined as a
day where the 24-h precipitation total was at least a
trace at Grand Rapids, Michigan (GRR). Choosing one
representative station within a circular Great Lakes region (GLR) that approximated the Great Lakes Basin
(GLB; cf. Fig. 1) facilitated the classification of a precipitation day.
Applying the classification scheme to numerous stations within the basin to obtain a more thorough classification climatology was certainly considered. However, it is not unreasonable to assume that the classifications at GRR are representative of the basin because
the majority of the possible classifications are synopticor meso-a-scale features and would therefore likely be
found at other stations in the region (plus or minus a
day or two) as it is only 1000 km in diameter. Grand
Rapids was chosen because of its long-term availability
of precipitation data, its central location within the GLB,
and because its precipitation trend is very representative
of that for the entire basin. This will be illustrated in
more detail in section 3.
A precipitation event was defined as a precipitation
day or a group of consecutive precipitation days that
were caused by the same surface feature. This definition
does not necessarily imply that all of the precipitation
days that constitute a precipitation event will have the
same classification. For example, a precipitation day
may be classified as being caused by a stationary front.
If this stationary front becomes a cold front and causes
precipitation on the following day, then that day will
receive a cold front classification. These two consecutive days would therefore be in the same precipitation
event but have different classifications.
The data used in this study was collected from the
Daily Weather Maps (DWMs), which are published
weekly by the National Oceanic and Atmospheric As-
15 JULY 2002
GROVER AND SOUSOUNIS
sociation (NOAA 1935–95). The DWMs were chosen
because they constituted the only single continuous and
hence most internally consistent source of information
over the entire period. The type and format of the available information on the DWMs did vary slightly through
the 1935–95 investigation period. Generally, the available information of interest included the following: sea
level pressure, surface temperature, surface dewpoint,
surface wind, cloud cover, weather, surface synoptic features, current areas of precipitation, and 500-hPa
heights—all valid at 1200 UTC; previous 24-h precipitation totals, and 24-h high and low temperatures. A
slight gap from 1941 to 1943 in the DWMs 24-h precipitation totals required the use of 24-h precipitation
totals from the National Climatic Data Center’s
(NCDC’s) Summary-of-the-Day TD-3200 data series
from the cooperative climatological network for that
period. Another source of data that was especially useful
for examining average large-scale conditions was the
National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data (Kalnay et al. 1996).
The precipitation day definition was used as a guideline for data collection. For every precipitation day in
the fall months from 1935 to 1995, classifications and
the 24-h precipitation total from GRR were recorded
from the DWMs. The investigation focused on the fall
months for several reasons. First, the record lake levels
in 1985 were largely a result of extreme precipitation
amounts in the fall and winter (Quinn 1986). Second,
the historical precipitation trend over the entire region
is impressive and distinct for the fall season (cf. Karl
and Knight 1998) and accounts for 43% of the annual
increase (winter accounts for 10% of the annual increase). Third, the mechanisms that are responsible for
precipitation in the fall are generally connected to largescale dynamical forcing mechanisms, as opposed to
small-scale convective forcing mechanisms, and would
therefore be easier to identify and be more representative
for the entire region.
All of the precipitation days in the period of interest
were classified to determine which changes in forcing
mechanisms and features are primarily responsible for
the increase in fall precipitation in the GLB. The possible precipitation day classifications are cold front,
warm front, stationary front, occluded front, low pressure, surface trough, lake effect, isolated convection,
and indistinguishable. These classifications span the
range of mechanisms that cause precipitation in the
GLR. The authors are aware that lake-effect and isolated
convection are not generally included in synoptic climatologies, but they were included here because they
are important when considering fall precipitation in the
GLR. The classifications applied in this study are similar
to those used by Shaw (1962), Smithson (1969), and
Fiorentino and Havens (1977). Fiorentino and Havens
(1977) subjectively evaluated precipitation in New England over a 5-yr period using the same frontal classi-
1945
fications as those utilized in this study and 3 other classifications: warm air mass, cold air mass, and warm
tropical (e.g., tropical cyclone).
The flow chart in Fig. 2 describes the logic used to
classify the precipitation days. The first step, and possibly the most important one, in classifying the precipitation days, was to compare the 24-h precipitation pattern with the surface features. If the pattern in the vicinity of GRR indicated that a single front as depicted
on the DWMs was responsible for the precipitation, then
the precipitation day was given the appropriate frontal
classification (e.g., cold, warm, stationary, occluded) as
determined by the fronts indicated on the DWMs (cf.
Figs. 2 and 3a–d).
If multiple fronts contributed to the precipitation on
any given day and the fronts were associated with a
closed low (e.g., identified by at least one closed isobar
at 4-hPa intervals) within or outside the region; or, if
the precipitation was not associated with any fronts but
was accompanied by a closed low where any portion1
of the closed contours surrounding the low center was
in the region for the majority of the 24-h period, then
the precipitation day was given a low pressure classification (cf. Figs. 2 and 3e,f).
If the precipitation was not frontal but was associated
with a low that was not in the basin; or if the precipitation was not frontal but was associated with a trough
in the basin, and lake-effect conditions were not present
(see below), then the precipitation day was given a
trough classification (cf. Figs. 2 and 3g).
If the precipitation was not frontal, and a closed low
was not in the basin, and lake-effect conditions were
present, then the precipitation day was given a lakeeffect classification (cf. Figs. 2 and 3h). Lake-effect
conditions included 1) the precipitation amount at GRR
being significantly higher than at surrounding leeward
stations (e.g., Detroit, Michigan—DTW, Fort Wayne,
Indiana—FWA), and especially those windward of Lake
Michigan (e.g., Milwaukee, Wisconsin—MKE; Wausau, Wisconsin—AUW; Chicago, Illinois—ORD) and
2) the GRR surface observations exhibiting a wind direction between 2108 and 3308 and a surface temperature
at least 58C less than the climatological temperature of
Lake Michigan (58–158C; cf. Saulesleja 1986).
If the precipitation was isolated and accompanied by
conditions favorable for convection, then it received an
isolated convection classification (cf. Figs. 2 and 3i).
Conditions such as high/low temperatures, dewpoint
temperature, and the presence of cyclonic southwesterly
flow were considered when determining whether convection was favorable. Conditions at upper levels did
not factor into the decision. If the cause of the precipitation could not be identified as either lake-effect or
1
Because the DWMs were only available every 24 h, the positions
of the contours were determined based on interpolation between maps
with assistance from the 6-hourly positions of the low centers that
were available on most maps.
1946
JOURNAL OF CLIMATE
VOLUME 15
FIG. 2. Flow chart representing the precipitation day classification methodology. ‘‘Circled’’ letters at the bottom correspond to the panels
in Fig. 3.
isolated convection, then the precipitation day received
an indistinguishable classification (cf. Figs. 2 and 3k).
If multiple fronts were responsible for the precipitation but a closed low was not in the region, then the
precipitation day was also given an indistinguishable
classification (cf. Figs. 2 and 3j). Using the indistinguishable classification for multiple front cases that
were not associated with a closed low avoided having
to deal with a constraint in the DWMs. Analyses of the
results would be complicated by precipitation days receiving multiple classifications because it raises the
question of how much of the 24-h precipitation total
was caused by each feature. This question is unanswerable based on the information available from the
DWMs. Therefore, this problem is avoided by giving
multiple front precipitation days that are not associated
with a low, an indistinguishable classification.
There were occasional instances where a combination
of a few classifications may have offered the most correct explanation of the cause of the precipitation. For
example, if a closed low had recently moved through
the region, then the amount of resulting precipitation
could have been a result of the low initially and then
enhanced by lake-effect conditions. In such a case, the
precipitation day would receive a low pressure classi-
fication. In this study, the interest lay more in possible
large (synoptic, meso a) scale changes rather than smaller (meso b, meso g ) scale changes. Therefore, an event
such as this was given the classification that corresponded to the larger scale when two classifications were
possible. The exception to this rule was when two different types of fronts were involved, as described above.
In that case, the indistinguishable classification was
used, since nearly all fronts associated with synopticscale lows have essentially the same scale.
The classifications were made subjectively using the
above-mentioned criteria in a consistent manner. The
subjectivity facilitated the classification of difficult situations (Barry and Perry 1973; Yarnal 1993). There was
some concern regarding the consistency with which
fronts were analyzed on the DWMs over the 61-yr period because fronts have been drawn manually on the
DWMs since their appearance in the DWM series
(1941). However, because their placement on DWMs is
based on specific criteria that have not changed over the
period of interest (Petterssen 1940; National Weather
Service 1979; Kocin et al. 1991) this concern was obviated. Prior to 1941, classifications were based on frontal analyses performed by the authors using Petterssen’s
criteria (1940).
15 JULY 2002
GROVER AND SOUSOUNIS
1947
FIG. 3. Schematics of different classifications used for the study: (a) cold front,
(b) warm front, (c) stationary front, (d) occluded front, (e) low pressure (centered in
region), (f ) low pressure (centered outside
region), (g) trough, (h) lake effect, (i) isolated convection, (j) indistinguishable (multifront), (k) indistinguishable (does not fit
other classifications). Conventional surface
features are shown and are valid at the end
of the 24-h period to be classified. Hatched
areas indicate areas of active precipitation.
Arrows indicate path of surface cyclone.
3. Precipitation classifications
The trend in fall precipitation for GRR and for the
entire GLB can be seen in Fig. 4. A general increasing
trend is present beginning in the mid-1950s with steeper
rates of increase in the mid- to late 1960s at GRR and
from the mid-1970s to the early 1990s for both GRR
and the GLB. There is also a brief decreasing trend in
the early 1970s for both plots. The precipitation trends
are present in all three months of interest, though they
are slightly more prominent in September and November than in October (not shown). The overall regime at
1948
JOURNAL OF CLIMATE
FIG. 4. The 7-yr running average of fall precipitation totals for
GRR and the GLB.
GRR is remarkably representative of that for the GLB;
there is a strong positive correlation of 0.82 between
the two datasets. The strong correlation and the synoptic
(scale) nature of the classifications add credibility that
variations in the synoptic mechanisms as determined at
GRR are relevant for the entire GLR. The average fall
precipitation total at GRR was 194 mm for the 1935–
65 time period and 262 mm for the 1966–95 time period.
This is a 35% increase in seasonal precipitation and is
significant to the 5% level based on a z test. The beginning of a decreasing trend in precipitation can be
seen in the early 1990s in Fig. 4. This drop in precipitation has contributed to a decline in lake levels as well.
Table 1 illustrates the average seasonal frequency and
the precipitation amount per precipitation day (PAPPD)
from each classification. A change in the average frequencies was calculated by subtracting the average for
the 1935–65 period from the average for the 1966–95
period. Therefore, a positive change indicates an increase in the average seasonal frequency of the classification between the two time periods. The low pressure,
lake-effect, and isolated convection classifications had
negative changes while the cold front, warm front, stationary front, occluded front, and trough classifications
had positive changes. The dominant role of low pressure
and cold front classifications is evident as they account
VOLUME 15
FIG. 5. The 7-yr running average of seasonal frequency of precipitation classifications. Only the five most common classifications are
included.
for 56% of all classifications over the 61-yr period.
However, the dominance of the low pressure classifications decreases as it contributes 29% of all classifications in the 1935–65 period and 23% of all classifications in the 1966–95 period. Note that the 0% precipitation contribution shown for isolated convection
classifications in Table 1 is due to the overwhelming
number of isolated convection classified days that were
trace events (precipitation days that received only trace
amounts of precipitation) and offered no measurable
precipitation. The combined change in frequencies for
all classifications of nearly five precipitation days per
season per time period is indicative of an increasing
precipitation regime throughout the 61-yr investigation
period (not shown) and is significant to the 5% level.
Figure 5 illustrates that the changes in the synoptic frequencies of the five most precipitation significant categories in Table 1 are not the result of smooth (linear)
trends for all classifications over the 61-yr period. Some
classifications exhibit more of a linear trend (warm
front, stationary front) than others (low, trough) and no
two classifications are highly correlated for the 61-yr
period.
The dominance of low pressure and cold front classifications and the relatively low frequency of the other
TABLE 1. Average precipitation day (PD) frequency and PAPPD for mechanisms per fall season.
1935–65
Low pressure
Cold front
Warm front
Stationary front
Occluded front
Trough
Lake effect
Isolated convection
Indistinguishable
1966–95
Recent–Historical
PD (days)
PAPPD (mm)
PD (days)
PAPPD (mm)
DPD (days)
DPAPPD (mm)
12.84
12.48
3.13
3.68
1.35
4.10
2.19
0.71
3.55
5.68
6.06
3.77
4.22
1.80
2.21
0.65
0.13
1.44
11.20
15.17
4.20
5.27
2.23
6.23
1.37
0.07
3.03
7.90
5.66
5.61
5.15
5.30
2.13
1.33
0.00
3.46
21.64
12.69
11.07
11.59
10.88
12.13
20.82
20.64
20.52
12.22
20.40
11.84
10.93
13.50
20.08
10.68
20.13
12.02
15 JULY 2002
GROVER AND SOUSOUNIS
classifications described above is understandable. For
example, the relatively low number of warm front classifications exists because a cyclone typically must have
moved in a sharp northeastward direction (from southwest of the region) in order for a precipitation day to
receive a warm front classification (cf. Fig. 3b). This is
uncommon given that the primary cyclone track still
lies to the north of the region through October (Reitan
1974). Stationary fronts are also uncommon in the GLR
in the fall. In order for a stationary front to develop,
geopotential height and thickness (temperature) contours must be parallel to each other. Fall is a transition
season in the GLR and is characterized by baroclinic
zones that make this alignment of height and thickness
lines relatively uncommon. Trough classifications are
uncommon in the fall in the GLR because sharp temperature contrasts during that season mean that a trough
will likely have a cold front and, therefore, such a case
would receive a cold front classification.
The classification frequencies are understandably different from those of Fiorentino and Havens (1977). Differences in methodology and geographical locations explain the different results. For example, they did not
consider one of the most important mechanisms in this
study, extratropical low pressure systems, as a separate
category. Additionally, they found that 60% of all precipitation was warm frontal in winter as a result of the
strong land–ocean temperature contrasts.
Table 1 also illustrates the average PAPPD or intensity for each classification. Intensities for most classifications increased: low pressure, warm front, stationary
front, occluded front, lake-effect, and indistinguishable
classifications had significant positive changes in intensity while cold front, trough, and isolated convection
classifications had insignificant positive or negative
changes in intensity. These changes are likely due to
changes in the duration or the hourly rate of precipitation, which in turn may be due to changes in the
strength, speed, or the track of the synoptic feature.
Possible explanations for these increases are provided
in section 5 although detailed evaluations are beyond
the scope of the current study and are left for future
research.
The overall precipitation contribution by each of the
five most dominant classifications, which represents the
combined effects of both changes in frequency and intensity, is illustrated in Fig. 6. Note that while the relative contributions from some classifications increased
(warm front and stationary front) and some decreased
(low pressure and cold front) between 1935–65 and
1966–95, the absolute contributions from all five classifications increased. The large precipitation contribution by lows and cold fronts is due to their dominance
in both frequency and intensity (cf. Table 1). However,
while they remain the dominant contributors of precipitation to the seasonal total, their precipitation contribution decreases from 75% of the fall precipitation in
1935–65 to 67% of the fall precipitation in 1966–95.
1949
FIG. 6. The 7-yr running average of precipitation contributed by
precipitation classifications to seasonal precipitation total. Only the
five highest contributing classifications are included.
This decrease in relative precipitation contribution by
low pressure and cold front mechanisms is coincident
with a relative increase from 15% to 23% between the
two time periods by warm, stationary, and occluded
(occluded not shown in Fig. 6) fronts combined. Seasonal precipitation contributions from these three categories increased from 30 to 63 mm per season between
1935–65 and 1966–95.
The increase from 44.03 precipitation days per season
in the 1935–65 period to 48.77 precipitation days per
season in the 1966–95 period is also representative of
an increase in the number of precipitation events per
season from 27.4 events during the 1935–65 time period
to 28.6 events during the 1966–95 time period. While
this increase in event frequency is not statistically significant, it should be noted that Karl and Knight (1998)
demonstrated that an increase in the frequency of precipitation events was responsible for a portion of the
increase in annual precipitation. An increase in the average length of precipitation events from 1.60 to 1.70
days (not statistically significant) occurred gradually
and also contributed to the increased precipitation over
the investigation period. This increase in event length
may have come from larger and/or slower weather systems.
An increase in the PAPPD for all mechanisms combined from 4.57 mm day 21 for the period 1935–65 to
5.08 mm day 21 for the period 1966–95 is indicative of
the temporal trend shown in Fig. 7. A comparison of
Figs. 4 and 7 illustrates a strong relationship (correlation
of 0.93) between the PAPPD and the overall precipitation trend for the region, demonstrating that the
PAPPD played a very important role in the overall precipitation trend. In order to further investigate this important role, each precipitation day was categorized
based on its 24-h precipitation amount. The categories
and the number of days in each catagory for the two
30-yr periods are presented in Table 2. While the number
1950
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VOLUME 15
TABLE 2. Average number of precipitation days falling in the following 24-h precipitation total categories per fall season: cat 1 (0.25–
1.27 mm), cat 2 (1.52–6.10 mm), cat 3 (6.35–12.45 mm), cat 4 (12.4–
25.15 mm), cat 5 ($25.40 mm). Precipitation totals of trace are not
included. Therefore, the sum of each time period’s number of precipitation category days is less than the average number of total
precipitation days for that period.
1935–65
1966–95
Change
FIG. 7. The 7-yr running average of average amount of
precipitation per precipitation day at GRR.
of days increased in each catagory from 1935–65 to
1966–95, the fractional increases in the number of category-1, -2, and -3 (light precipitation) days has remained relatively small compared to those for catagory4 and -5 (heavy precipitation) days.
Figure 8 indicates how each of the precipitation categories contributed to the seasonal precipitation total.
The amount of precipitation contribution to the seasonal
total by category-4 days starts to increase significantly
in the early 1960s. There is a similar trend for category5 days beginning in the late 1970s. Both of these increases, when compared between the two time periods,
are significant to the 5% level. Despite a small difference
in the definition of extreme precipitation days, these
increases in the contribution toward the seasonal precipitation total by extreme precipitation days are consistent with those found by Karl and Knight (1998). (In
this study, category-4 and -5 days are considered to be
extreme precipitation days, corresponding to precipitation amounts greater than or equal to 12.70 mm, while
Karl and Knight’s extreme precipitation days are those
falling in the upper 10% of all 24-h precipitation
amounts.)
The decrease in the dominance of low pressure and
cold front classifications is also exhibited in their contribution to the number of extreme precipitation days.
Low pressure and cold fronts composed 78% of all category-4 and -5 days in the 1935–65 time period. This
contribution to extreme precipitation days decreased to
66% in the 1966–95 time period. As was the case with
total precipitation, the contribution to extreme precipitation days from warm, stationary, and occluded fronts
increased the most, leading to a large jump in extreme
precipitation days and seasonal precipitation totals.
Cat 1
(days)
Cat 2
(days)
Cat 3
(days)
Cat 4
(days)
Cat 5
(days)
10.55
10.57
0.02
10.87
11.97
1.10
4.94
5.57
0.63
2.58
5.00
2.42
1.52
2.07
0.55
1996; Robertson and Ghil 1999). Here too, the higher
precipitation regime, as well as some of the changes in
precipitation contributions from the different classifications during the 1966–95 period, can be explained in
terms of several key differences in the synoptic-scale
flow. To this end, it is useful to compare the large-scale
conditions for a 10-yr period (1980–89) within the wet
30-yr period (1966–95) to those from a 10-yr period
(1950–59) within the dry 30-yr period (1935–65). The
10-yr periods were selected to have enough years to be
statistically meaningful but not so many that differences
between the wet and dry ones were obscured. Comparison of Tables 3 and 1 shows that the average precipitation characteristics over the 10-yr periods are
somewhat representative of those for the 30-yr averages.
The correlations for the synoptic classifications and for
the precipitation amounts per precipitation day between
the 10- and 30-yr periods were 0.49 and 0.51, respectively. The increase of 5.2 precipitation days between
the two decades agrees well with the increase of 4.8
days between the two 30-yr periods.
The large-scale comparisons were made using NCEP–
NCAR reanalysis data. Figure 9 shows selected largescale features during the 1980s, and differences for those
features relative to the 1950s. A comparison of 500-hPa
geopotential heights (Fig. 9b) shows that the 1980s were
characterized by a more zonal flow pattern than the
1950s, when 500-hPa heights were 20–25 m higher over
4. Large-scale flow influences
The influence of the large-scale circulation on regional precipitation has been demonstrated by many
studies (e.g., Harman et al. 1980; Rodionov 1994; Mock
FIG. 8. The 7-yr running average of precipitation amount contributed
by precipitation categories to seasonal precipitation total.
15 JULY 2002
1951
GROVER AND SOUSOUNIS
TABLE 3. Similar to Table 1 but for two 10-yr periods.
1950–59
Low pressure
Cold front
Warm front
Stationary front
Occluded front
Trough
Lake effect
Isolated convection
Indistinguishable
1980s 2 1950s
1980–89
PD (days)
PAPPD (mm)
PD (days)
PAPPD (mm)
DPD (days)
DPAPPD (mm)
12.20
13.40
2.80
5.20
1.80
3.50
3.90
3.60
3.60
6.30
4.42
3.48
3.43
2.56
1.42
1.17
0.85
0.74
14.10
16.00
4.80
5.20
1.60
5.90
0.70
3.40
3.50
6.93
6.32
6.25
4.45
7.24
2.85
0.74
2.86
2.84
11.90
12.60
12.00
10.00
20.20
12.40
23.20
20.20
20.1
22.22
10.40
21.84
20.93
23.50
10.08
20.68
10.13
22.02
the Rockies and 10–15 m lower over the southeastern
United States. This result compares favorably with those
from Rodionov (1994), who analyzed the influence of
large-scale circulation on winter precipitation in the
Great Lakes by implementing a modified Pacific–North
American (PNA) index. He showed that a negative in-
dex, corresponding to zonal flow, was associated with
cyclones that originated over the high plains and generated above-normal precipitation. A positive index was
associated with cyclones that originated in Alberta, Canada (e.g., Alberta Clippers), and generated below-normal precipitation. In this study, standard monthly PNA
FIG. 9. Average conditions for 1980–89 fall seasons (heavy contours) and differences between 1980–89 and 1950–59 fall seasons (thin
contours) for (a) 250-hPa wind speeds (heavy contours every 2.5 m s 21 and thin contours every 1 m s 21 ), (b) 500-hPa geopotential heights
(heavy contours every 6 dam and thin contours every 5 m); (c) 850-hPa temperatures (heavy contours every 48C and thin contours every
0.58C); (d) 850-hPa y -component winds (heavy contours every 1 m s 21 and thin contours every 0.5 m s 21 ). Shaded regions indicate significant
positive [in (a) and (d)] or negative [in (b) and (c)] differences. [Provided by the NOAA–CIRES Climate Diagnostics Center, Boulder,
Colorado, at their Web site at http://www.cdc.noaa.gov/.]
1952
JOURNAL OF CLIMATE
indices (Wallace and Gutzler 1981) were averaged over
the fall season to obtain values of 20.21 for the wet
period (1980s) and 0.16 for the dry period (1950s).
The lower 500-hPa heights over the Rockies during
the 1980s were associated with lower temperatures
throughout the lower troposphere in that region as well.
Temperature differences were as large as 22.58C at 850
hPa over the southern Rockies (Fig. 9c) and were part
of a larger area of lower temperatures that extended
southwestward over the eastern Pacific. The lower temperatures and negative height differences translated to
increased low-level baroclinicity over the Rockies,
which likely contributed to (lee) cyclogenesis.
The increased baroclinicity over the southern Rockies
during the 1980s also likely contributed to the stronger
winds in the upper troposphere over the Great Lakes
Basin. Figure 9a shows that wind speeds at 250 hPa
were about 4 m s 21 higher directly over the region than
they were during the dry period. These stronger wind
speeds were embedded within a more extensive region
of higher wind speeds that stretched from the southwestern United States northeastward to New England.
The 250-hPa wind speeds that existed during the dry
period indicated a weaker jet entrance region in the
vicinity of the GLR and one that was also shifted farther
north by several hundred kilometers. As a result, the
axis of strongest wind speed differences was situated
just south of GRR so that most of the region was on
the cyclonic side of the jet, which is a favorable location
for receiving significant precipitation. Farther to the
north and west, evidence for a weaker polar jet exists.
Wind speeds were ;5 m s 21 weaker in the area just
south of the Gulf of Alaska during the 1980s than during
the 1950s. This pattern suggests that the subtropical jet
was stronger and the polar jet was weaker during the
wet period than during the dry period, which suggests
that storms may have developed farther south and hence
been able to draw additional moisture from the Gulf of
Mexico rather than be limited to just the moisture from
the Pacific.
Other large-scale differences include enhanced lowlevel moisture in the southwestern United States (not
shown). Relative humidity values at 700 hPa were higher by nearly 10% during the 1980s as compared to the
1950s. The moister conditions may have been a result
of the stronger subtropical jet. Farther to the north, in
Canada, relative humidities were about 10% less. The
enhanced moisture, combined with the increased lowlevel baroclinicity in that region, was likely responsible
for the increase in precipitation from all of the mechanisms except isolated convection. This is understandable since isolated convection is a mechanism that is
not likely to be affected by enhanced synoptic forcing
of large-scale precipitation mechanisms.
Finally, the 1980s were characterized by stronger
low-level southerly flow over the region (Fig. 9d). This
stronger flow likely contributed to enhanced moisture
transport from the Gulf of Mexico and warm advection
VOLUME 15
(cf. Figs. 9c and 9d), both of which contribute to precipitation generation. The surface signature of this stronger southerly flow was in the form of a weak trough
located just to the west of the GLR and extended from
Texas northeastward to Hudson Bay (not shown). During the wet period, the trough separated a high pressure
system over the Rockies to the west from a high pressure
system over the western Atlantic to the east. During the
1950s, this trough was even weaker and had a more
southwest–northeast flow rather than a south–north flow
on the eastern side.
Although it is beyond the scope of this study to examine the specific causes for the different large-scale
flow patterns, there is some evidence that these patterns
and the precipitation amounts from them during the
1950s and 1980s were influenced by the Pacific Decadal
Oscillation (PDO). Published literature (e.g., Mantua et
al. 1997) indicates that the PDO index was strongly
negative during the 1950s and strongly positive during
the 1980s. Given that the effects from PDO are similar
to but less intense than those from corresponding El
Niño–Southern Oscillation (ENSO) phases (Zhang et al.
1997) and given that fall is drier than normal in the
Great Lakes region during the negative ENSO phase
and wetter than normal during the positive phase (Climate Prediction Center 2001), it is likely that the lower
amounts of fall precipitation in the 1950s and the higher
amounts in the 1980s were at least partly the result of
the PDO.
5. Discussion
The characteristic differences in the synoptic flow
between the 1980s and 1950s help provide a conceptual
explanation for understanding how large-scale flow influenced precipitation event frequency and intensity and
also the responsible mechanism (e.g., front). The 1980s
were characterized by lower geopotential heights over
the Rockies that resulted in enhanced low-level baroclinicity, which led to stronger winds near the tropopause. The enhanced winds at upper levels and the enhanced baroclinicity at low levels were both likely more
favorable for the development of ascent, clouds, and lee
cyclogenesis over the southern Rockies. These more
favorable features are reflected in the fact that the 1980s
were characterized by a southward shift in the latitude
from which cyclones approached the region (cf. Fig.
10). Other features regarding cyclone frequency in Fig.
10 are consistent with the large-scale flow differences
between the 1950s and the 1980s. For example, the
increased cyclone frequency along the U.S. east coast
during the 1950s is consistent with the presence of a
more prominent large-scale trough over the eastern half
of the United States during that time.
Differences in the large-scale flow pattern(s) between
the 1950s and 1980s are also consistent with many of
the changes in warm, cold, occluded, and stationary
fronts and the respective amounts of precipitation that
15 JULY 2002
GROVER AND SOUSOUNIS
FIG. 10. Number of cyclones per 2.58 3 2.58 box found from
NCEP–NCAR reanalysis daily data during Sep–Nov for (a) 1950–
59 and (b) 1980–89.
fell over the GLB. The greater number of cyclones that
developed farther south during the 1980s (wet zonal
flow) not only resulted in more cyclones going through
the region (GRR) so precipitation was classified as ‘‘low
pressure,’’ but also resulted in more cyclones tracking
just west of the region (cf. Fig. 10)—so precipitation
was classified as ‘‘frontal’’ (e.g., instead of ‘‘low pressure’’). The type of front depended on the relative position of the low with respect to GRR, its track, and
intensity.
Warm fronts typically extend (south)eastward from
the low center and typically advance north(east)ward.
The precipitation can usually extend for several hundred
kilometers ahead (e.g., north) of the actual surface warm
front. Thus, cyclones tracking from the southwest and
passing just west of the region are more likely to influence GRR with a warm front than cyclones tracking
from the northwest and passing just east of the region
(cf. Fig. 11). Comparison of Figs. 11a and 11b indicates
that the mean frontal position the day before the event
(e.g., day T-1) during the 1980s was located several
hundred kilometers to the south of the corresponding
1950s position—the result of nearly twice as many
warm fronts coming from southern cyclones. The southwesterly flow that was associated with these southern
cyclones at upper levels had more moisture associated
with it so frontal precipitation was also more intense.
A similar explanation holds for cold fronts, which
typically extend south(west)ward from a low center and
1953
typically advance (south)eastward. Thus, both cyclone
types—those tracking from the northwest and passing
just east of the region and those tracking from the southwest and passing just west of the region—can influence
GRR with cold frontal precipitation. Figures 11c and
11d reflect the fact that even during the 1950s there
were cyclones of both types that generated cold front
precipitation at GRR—albeit fewer from the south. During the 1980s, there were more of both types. The increase in cold fronts (and precipitation) from southern
cyclones is more easily understood than the increase
from northern cyclones. One possible explanation for
these latter increases is that for developing low pressure
systems (e.g., troughs), the warm southerly flow ahead
of the low center or trough axis is associated with more
moisture when the system is embedded in zonal flow
rather than northwesterly flow—even when the system
(e.g., low center) is located north of the region. The
latent heat release ahead of a developing system then
contributes to an enhanced horizontal temperature gradient (e.g., cold front) and hence to increased precipitation. The increase in cold fronts from northern cyclones also explains the increase in troughs and trough
precipitation (not shown).
Although changes in the frequencies of occluded and
stationary fronts from the 1950s to the 1980s are not
statistically significant, differences in the locations and
tracks of the cyclones to which these fronts were attached also influenced precipitation intensity. For example, for occluded fronts, Figs. 12a and 12b illustrate
how more cyclones approached from the southwest during the 1980s than during the 1950s. These systems were
associated with more moisture and hence generated
more precipitation. The lack of any significant change
in the frequency of occluded fronts along with an increase in the frequency of lows from the southwest suggests that these lows in general may be less mature at
the time of approach than those that influence the region
from the northwest. Stationary fronts that influenced the
Great Lakes during the 1950s and 1980s were usually
linked to two cyclones that straddled the region from
southwest to northeast. Figures 12c and 12d suggest that
during the 1950s, the northern low was the more prominent of the two, and during the 1980s the southern low
was the more prominent of the two. The fact that the
stationary fronts that influence the region tend to be
supported by two low pressure systems on either end
(with one usually being the dominant one) may explain
the lack of any significant change in frequency.
Figures 11 and 12 illustrate that for all frontal types,
high pressure at the surface along the east coast of the
United States was more influential (e.g., stronger, closer
to the region, and more meridional in shape) during the
1980s, which is consistent with the positive height perturbations at 500 hPa (cf. Fig. 9a) as well as with the
reduced storm activity (cf. Fig. 10b) in that region. This
feature likely contributed to the stronger southerly flow
and to the increased precipitation during the 1980s.
1954
JOURNAL OF CLIMATE
VOLUME 15
FIG. 11. Average sea level pressure (solid—hPa), surface temperature (dashed—8C), and daily precipitation (see legend for shading
definitions) for (a) warm front cases during the 1950s, (b) warm front cases during the 1980s, (c) cold front cases during the 1950s, and
(d) cold front cases during the 1980s at day T-1 from NCEP–NCAR reanalyses. Small circles indicate locations of corresponding low centers.
Small stars indicate locations of significant high centers associated with the fronts.
Construction of figures similar to Figs. 11 and 12 but
for the entire 61-yr period is tedious but is one way to
partially circumvent the nonexistence of upper-air data
prior to 1948 in order to deduce some relevant information about what the large-scale flow must have been
like and how frontal characteristics were influenced over
the entire period of study. A less tedious method may
be to combine the results from the two decades that
have been evaluated comprehensively with a long-term
evaluation of cyclone activity in the Great Lakes Basin.
For example, Angel (1996), Angel and Isard (1997,
1998), and Isard et al. (2000) have shown that strong
cyclone activity from the 1920s to the 1940s was relatively low in the Great Lakes Basin. Because strong
cyclones approach from the southwest, it may be deduced that the large-scale flow during this time was
more like that during the 1950s than it was during the
1980s. The fact that the frontal activity during the
1930s–40s was more like that of the 1950s than it was
like that of the 1980s is consistent with this deduction.
Still, complications (discrepancies) exist between the
10- and 30-yr comparisons, which suggest that addi-
tional study is needed to understand how other aspects
of the large-scale flow influence fronts and other precipitation mechanisms in the Great Lakes Basin.
6. Summary and conclusions
This study was conducted to understand the cause of
the increase in fall precipitation in the Great Lakes Basin
since the mid-1960s. It incorporated data from September–November for the period 1935–95 and utilized data
from Grand Rapids, Michigan, to represent conditions
over the Great Lakes Basin. Precipitation was identified
and assessed based on NOAA Daily Weather Maps. Precipitation days were classified according to the synopticforcing mechanism that was responsible for the precipitation.
The increasing precipitation trend was documented at
both local (GRR) and regional (GLB) levels. Increases
of 35% and 15% in fall precipitation totals were exhibited, respectively. All precipitation classifications, with
the exception of isolated convection, produced more
precipitation in the 1966–95 time period than in the
15 JULY 2002
GROVER AND SOUSOUNIS
1955
FIG. 12. Average sea level pressure (solid—hPa), surface temperature (dashed—8C), and daily precipitation (see legend for shading
definitions) for (a) occluded front cases during the 1950s, (b) occluded front cases during the 1980s, (c) stationary front cases during the
1950s, and (d) stationary front cases during the 1980s at day T-1 from NCEP–NCAR reanalyses. Small circles indicate locations of
corresponding low centers.
1935–65 time period. The relative dominance of the low
pressure and cold front classifications decreased as
warm, stationary, and occluded fronts became more important contributors to fall precipitation. This increase
in precipitation production by warm, stationary, and occluded fronts played an important role in the increasing
precipitation trend.
Trends in the precipitation day variables were also
analyzed. It was found that the frequency of precipitation days as well as the length of precipitation events
increased. These increases were accompanied by an increase in the average 24-h precipitation total regardless
of classification. The frequency of extreme precipitation
days also increased significantly, which was driven by
increases from warm, stationary, and occluded fronts.
An evaluation of precipitation and large-scale flow
was performed using the NCEP–NCAR reanalysis data.
Comparison of wet and dry 10-yr periods within each
30-yr period illustrated that higher precipitation
amounts were associated with a more zonal 500-hPa
pattern, an enhanced upper-level subtropical jet, an in-
crease in moisture and low-level baroclinicity, and
stronger low-level southerly flow. These features contributed to an increase in southern cyclones and hence
to an increase in the frequency of and the precipitation
from various frontal types. Existing long-term cyclone
analyses suggest that the 30-yr periods had large-scale
flow characteristics similar to the corresponding 10-yr
periods.
It is concluded that significant changes in the largescale flow between 1935–65 and 1965–95 caused significant changes in the characteristics of precipitation
mechanisms, which increased the frequency and intensity with which precipitation fell over the Great Lakes
Basin during the fall months. Although this study identified a significant change in the large-scale flow pattern
as a cause for the change in precipitation characteristics,
it did not address the reasons for the change in the flow
pattern and it did not address other possible causes.
Additional studies that focus on the other seasons, examine more fully the reasons for changes in the largescale flow (e.g., from various planetary-scale oscilla-
1956
JOURNAL OF CLIMATE
tions), and other causes (e.g., global warming, increased
cloud condensation nuclei) in the Great Lakes Basin are
needed.
Acknowledgments. The authors would like to thank
Dr. Frank Quinn from the Great Lakes Environmental
Research Laboratory in Ann Arbor, Michigan, for his
motivation to conduct this study. This work was sponsored by the Cooperative Institute of Limnology and
Ecosystems Research (CILER) under Cooperative
Agreement F000575 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The U.S. Government is authorized to produce
and distribute reprints for the governmental purposes
notwithstanding any copyright notation that may appear
herein.
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