Cold-Air Pools in Mountain Valleys
nights with calm winds, the ground cools rapidly. Air in contact with the
colder ground cools by conducting heat to the ground. This process can continue
this cooling process occurs along mountain slopes, the cooling air becomes
colder and denser than the air away from the slopes, which causes the cold air
to sink downslope. The dense cold air flows downslope in streams (called
katabatic winds) following the steepest slopes. When the cold air flows into a
relatively flat area (a mountain or river valley, for example), the streams of
cold air slow down. This causes the valley to fill with cold air, much like
streams filling a lake.
valleys around the world often fill with cold air a few hundred to thousands of
feet deep, depending on the depth of the valley and the shape of the valley
allowing cold air to flow out of the valley. Above this “cold air pool,” the
air remains warmer because is not in contact with the ground. This means that
low elevations in mountain valleys are regularly exposed to cold temperatures
at night and higher elevations above the cold air pool remain in warmer air
when weather conditions are clear and calm.
observations of cold air pool formation and evolution were taken in the
mountain valley of Hubbard Brook Experimental Forest, New Hampshire in November
2015. We deployed a tethered weather balloon to 150 meters (~500 feet) with
temperature sensors tied to the nylon string every 5 meters (~16 feet; Fig. 1)
to record the thermal evolution and depth of the cold pool overnight and its
dissipation the next morning. We also measured wind, temperature and relative
humidity on the north and south facing slopes surrounding the valley. We worked
in pairs all night taking weather observations on the mountain slopes and
measuring the balloon elevation angle – an indication of wind at the balloon’s
Figure 1:. Hubbard Brook Experimental Forest site map with weather stations. Wind data recorded at Site 1, SCAN site, Kineo tower, MWS, and Pierce Lab (HQ). HBEF base map courtesy of Mary Martin.
we recorded revealed interesting characteristics of the cold air pool that
formed that night. The temperature profile in Figure 2 reveals several
different processes that impacted the evolution of the cold pool. A rapid
cooling throughout the depth of the profile and especially near the ground
(lowest lines) in the late afternoon signaled the formation of the cold pool (2100-2300
UTC). The separation of the lines indicates a cold air pool is present and that
the air warms going up through the cold air pool – a typical characteristic.
The data suggest that the top of the cold air pool was at about 145 meters
above the ground – near the top of the tethered balloon – and remained there
through the first half of the night.
Figure 2: Time series of temperatures at
all tethered data loggers. Temperatures are filtered using a 10-minute moving
the night, warmer air began to move in above the ridge tops. This warm air was
first observed at the Kineo Tower along the south ridge; winds increased in
association with the warm air between 0600-0800 UTC (Fig. 3). At about 0800 UTC
(4:00 am EDT), this warm air mixed down past the height of the balloon, to the
135-meter temperature sensor (evidenced by the rapid increase in temperature of
the top several lines just after 0800 UTC). This mixing down of warm air
continued through 1000 UTC to about the 85-meter sensor – an erosion of 60
meters (~200 feet) of the cold air pool. (The cold air pool is gone where the
upper temperature lines are nearly on top of each other.) Kineo Tower wind
speed and temperature both decreased after ~0800 UTC revealing the pulse of
warm air advection subsided. The undulating temperatures through 1200 UTC
suggest a possible seiche (i.e., sloshing of cold air in the valley, like water
in a bathtub); the decreasing wind speeds above the cold air pool (e.g., Fig.
3) supports this possibility.
Figure 3: Mount
Kineo 15-minute averaged wind speed (solid black line), maximum wind speed
(dashed line), and temperature (red line). Triangles along the horizontal axis
represent time of sunset (filled; 2135 UTC) and sunrise (outlined; 1127 UTC).
data we collected suggest that the top of the cold air pool was maintained at
~145 meters (~480 feet) deep before the warm air began to erode it. We think
that this is a typical maximum depth because of the shape of the orography.
Just north of the tethered balloon the land rises, then plateaus at about 145
meters (~480 feet) above the valley (Fig. 4). In order for the cold air pool to
deepen further, it would need to fill a much bigger volume above the plateau. In
addition, while katabatic winds are supplying the valley with cold air, the
cold air is also draining to the east into the Pemigewasset Valley, which
balances the katabatic winds. If the cold air pool were to deepen, it would
drain faster across a wider channel, especially over the low hills on the south
side. This would reduce the cold air pool depth and drainage rate until the
katabatic flow and drainage are back in balance.
Figure 4: Schematic diagram of drainage
flows during maximum observed cold air pool height in HBEF. Small blue arrows
(lower panel) represent katabatic wind flow. Large blue arrows (upper panel)
represent mountain wind (drainage flow) of the cold air pool; the large orange
arrow represents the increased drainage that would occur to the
southeast if cold air pool height were to increase above the ridge height.
Vertical black line shows location of vertical cross-section. Vertical transect
of elevation generated from Google Earth (2017). HBEF base map courtesy of Mary
these dynamics helps advance understanding of where inversions typically occur
on mountain slopes, energy and water cycling in the forest ecosystem, and how
forest health may be impacted by these atmospheric conditions.
Meteorologically, this research helps us improve forecasts of freezing rain,
dense fog, and other hazardous conditions related to cold air pools.
observations are needed to definitely measure cold air pool height along the
full west-to-east length of the valley. These will include using an array of
temperature data from around the entire valley and more vertical profiles. Observations
of cold air flow at more locations around the valley will help quantify
katabatic inflow and mountain drainage outflow rates. Numerical modeling will
be used to compare with observations and develop new hypotheses of how cold air
moves within the valley and how disturbances, such as warm air advection,
impact the stability and depth of cold air pools.
The ability of forecasters to have "real-time" data of moisture at the upper-levels, instead of the current system of twice daily soundings via balloon, will dramatically increase forecasting accuracy.