Air Mass Frequency during Precipitation Events in the United States Northern Plains
David M. Loveless, Anthony J. Baum, Richard DePasquale, Melissa L. Godek
Results and Discussion
Since 1980, numerous billion-dollar disasters have affected the Northern
Plains, including both extreme droughts and floods. The drought of
2012, for example, has been considered one of the worst droughts of
the past century. Given the importance of agriculture in the Northern
Plains region, precipitation forecasts are essential to the sustainability of
the regions economy. The ability to accurately forecast the frequency and
quantity of precipitation would provide many benefits to the agricultural
industry of the Northern Plains. The atmospheric environment present
during precipitation events are characterized by a multitude of meteorological variables at any one time over a large region. It follows therefore,
that understanding the relationship between air masses and rainfall episodes can contribute to improving both short term and long term precipitation forecasting for the region.
1. Moist air masses are most related to precipitation
Annually, moist air masses, particularly MM and MP, are most frequent during precipitation events. However, dry air masses are sometimes present during precipitation, especially the DM and DP air masses in the northwest areas of the study
region. TR is not the most prominent air mass during precipitation events despite being associated with fronts.
(Figures 1 and 2)
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2. High regional spatial variability
There is high regional spatial variability, with some cohesion between northwestern stations and southeastern stations.
The 7 southeast stations have the most spatial cohesion among air mass frequencies compared to the rest of the stations
in this study. Precipitation in the southeast stations tend to be dominated by MM, while the northwest is dominated by MP
and even DP. (Figures 1 and 2).
DM
Air mass frequencies are determined for:
A1) All annual precipitation days.
A2) All category 1-4 precipitation days.
DT
MM
MP
MT
0
TR
DM
DT
DP
MM
MP
MT
TR
Figure 2: Air mass frequency for all precipitation days for the Northwest
stations only
stations only
3. MT is most prevalent in summer
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4. TR is most frequent in spring and fall
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15
Frequency (%)
20
Frequency (%)
The region gets more tropical air masses during the summer, and is less likely to get tropical air masses in the winter
months. This finding is intuitive given the seasonal cycle. (Figures 3 and 5)
10
5
10
5
0
DP
DM
DT
MM
MP
MT
0
TR
A pattern indicative of a warm front over the Northern Plains. On this day, a DM air
mass is supported at the surface in the northern half of the Northern Plains. Despite dry
air being supported at the surface, warm moist air is being advected over the top of the
drier air. This is one way in which a dry air mass may be producing precipitation in the
region. While the SSC defines air masses based on surface conditions, a moist air mass
may be present at higher levels in the atmosphere, causing precipitation.
Image credit: SPC
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20
15
15
Frequency (%)
A typical pattern during the summer, with a large ridge over the center of the continental U.S. and overall weak upper level winds. On this day, DM air mass dominates the entire Northern Plains region. Due to the convective nature of summer time precipitation,
nearly any air mass can produce precipitation, especially high precipitation totals. However, there is a tendency for more precipitation to be caused by the moist air masses.
Image credit: SPC
DM
DP
DT
MM
MP
MT
TR
Figure 4: Spring air mass frequencies for all precipitation days
Figure 3: Winter air mass frequencies for all precipitation days
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10
5
5
0
DM
DP
MM
DT
MP
MT
0
TR
Figure 5: Summer air mass frequencies for all precipitation days
DM
DP
MM
DT
MP
MT
TR
Figure 6: Fall air mass frequencies for all precipitation days
5. DM has the most variability of all air masses
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40
8
Frequency (%)
Moist air masses are more frequent during days with higher precipitation accumulations (category 3 and 4 precipitation)
compared to the dry air masses. This pattern is seen across the entire region, including the northwest which has a higher
frequency of drier air masses during all precipitation events. This tendency is consistent seasonally, with the exception of
the summer season. (Figures 1, 2, and 9)
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60
Frequency (%)
6. Moist air masses have higher precipitation totals
Air mass frequencies from 1981-2010 are determined for precipitation
days on an A) annual basis, B) seasonal basis (defined as meteorological
seasons), C) with respect to the positive and negative phases (omitting
the neutral phase) of the PNA.
DP
Figure 1: Air mass frequency for all precipitation days for the southeast
DM has the most variability, both annually and seasonally across the entire region. DM has highest variability in the winter and summer months. Annually, the frequency of DM during precipitation events ranges from around 9% to as high as
34%. This air mass is most prominent with precipitation in the northwest part of the region compared to the southeast
part of the region. (All Figures)
Precipitation categories are defined
as follows:
Trace < Category 1 < 0.10”
0.10” ≤ Category 2 < 0.49”
0.50” ≤ Category 3 < 1.00”
1.00” ≤ Category 4
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10
0
Frequency (%)
For the purposes of this study, the 7 air masses defined by the SSC (Sheridan 2002)
United States Northern Plains are
defined within the boundaries of
93-104˚N, and 37-49˚N. The Spatial Synoptic Air Mass Classification System (SSC) defines air masses based on meteorological surface observations. 15 different SSC
stations in the Northern Plains are
selected to establish spatial coverage of the study region. Each SSC
air mass station is paired with a
nearby United States Historical Climatology Network (USHCN) station,
which provides daily precipitation
amounts. The distribution of these
stations are displayed in the map
on the right. Precipitation categories (used by the Midwest Regional
Climate Center) are used in order
to gain an additional understanding of the total amount of precipitation that falls on each day.
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10
TR is fairly consistent in its frequency in precipitation events throughout all seasons. However, TR becomes most frequent
during precipitation in the spring and fall. (Figures 4 and 6)
Methods
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Frequency (%)
Introduction
Frequency (%)
Department of Earth and Atmospheric Sciences, State University of New York College at Oneonta
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20
4
10
0
DM
7. Summer tends to be the season of greatest variability
The summer season has the largest variability in terms of which air masses are present during precipitation events. This
is likely caused by the frequent occurrence of air mass thunderstorms, allowing nearly any air mass boundary to produce
precipitation and occasionally heavy precipitation, as well. (Figure 5)
DP
DT
MM
MP
MT
0
TR
DM
DP
DT
MM
MP
MT
Figure 7: Air mass frequency for winter precipitation days
Figure 8: Air mass frequency for winter precipitation days
(Categories 1 and 2)
(Categories 3 and 4)
TR
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8
8. The winter season is dominated by polar air masses DP and MP
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There does not appear to be a clear spatial pattern regarding the PNA’s influence on an annual basis. (Figure 10)
Frequency (%)
C1) All positive and negative PNA precipitation days
9. Frequency does not change with regard to the PNA
6
Frequency (%)
B1) All seasonal precipitation days
B2) All category 1-4 seasonal precipitation days
Precipitation in the winter season is dominated by polar air masses. DP and MP dominate the category 1 and category 2
winter precipitation days. However, the MM and MP air masses are most frequent during category 3 and category 4 precipitation days. (Figures 7 and 8)
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15
10
5
2
0
0
DM
DM
DP
DT
MM
MP
MT
Figure 9: Air mass frequency for all precipitation days
(Category 4 frequencies only)
DP
DT
MM
MP
MT
TR
TR
Figure 10: Air mass frequency for all precipitation days within each phase of
the PNA
Future Research
Figure A: Annual air mass frequency for precipitation days at KMSP
(Minneapolis/St. Paul, Minnesota)
Figure B1: Air mass frequency for all precipitation days within each season at
KMSP (Minneapolis/St. Paul, Minnesota)
Figure B2: Air mass frequency for summer precipitation days within each precipitation category at KMSP (Minneapolis/St. Paul, Minnesota)
Figure C: Air mass frequency for annual precipitation days within positive and
negative phase of the PNA at KMSP (Minneapolis/St. Paul, Minnesota)
Future work will involve identifying why transitional air masses are not very prominent during precipitation events. Additionally, it will be necessary to separate the TR category in order to distinguish between dry transitions, moist transitions, and moist-dry transitions. Also, further investigation is required to determine why DM and DP would produce precipitation. Preliminary investigation revealed that this could be caused by the presence of mid-level moisture. Future work will also investigate the lack of influence that the PNA has on air mass frequency during precipitation
events. This would include an investigation of a potential lag time between the PNA signal and its
effect on the Northern Plains region.