Cold air pools over complex terrain

Cold air pools (CAPs) over complex terrain have rarely been investigated in hilly regions
that are typical across the UK. This thesis gives a detailed account of CAP observations
gained during the COLd air Pooling EXperiment (COLPEX), conducted in the Clun Valley
region of Shropshire, England.
A short 9–month climatology study reveals that weak CAPs, defined as temperature inversions
larger than 1◦C across a valley depth of ∼170m, occur 45% of all nights. Strong
CAPs, defined as temperature inversions larger than 4◦C, occur 12% of all nights. Strong
CAPs are found to occur when the following “ideal” conditions are met: (1) mean sea level
pressures >1029 hPa, (2) pressure gradients <1.5 Pa km−1, (3) mean night-time ambient
wind speeds <3 m s−1, (4) mean night-time ambient wind directions from the N, (5) low
values of Flw, i.e., <0.80, where Flw is the ratio of incoming to outgoing LW radiation.
Using this criteria a case study investigation is conducted. The case study highlights the
sensitivity of CAPs to nocturnal phenomena, which have rarely been documented before.
The CAP is disturbed by; a gravity wave, an acceleration of the ambient wind (∼4.6 x
10−4 m s−2) and by an increase in the ambient wind speed associated with a developing
nocturnal low level jet (NLLJ). The final breakup of the CAP occurred some 3.5hrs after
local sunrise and the NLLJ appears to play a role. Further investigations indicate that
NLLJs occur during other CAP nights also.
A wind climatology study is conducted investigating the relationship between ambient
winds and valley winds. Four forcing mechanisms for valley winds are proposed by Whiteman
& Doran (1993), and these are; forced channeling, downward momentum transport,
pressure driven channeling and thermally driven flows. Downward momentum transport
preferentially occurs in less sheltered regions and forced channeling in narrow valley regions.
The valley wind behaviour is notably different from day to CAP nights, with thermally
driven down-valley winds prominent during CAP nights. Pressure driven channeling
and daytime thermally driven flows (anabatic winds) are not seen.
High resolution model simulations of five CAP nights, show that drainage flows develop
frequently. However, the timing, structure and strength of drainage flows differs from case
to case. Two regimes stand out: (1) For low ambient wind speed nights the development
of strong drainage flows leads to increased mixing in valley bottom regions, resulting in
weaker stability. (2) For high ambient wind speed nights, weaker drainage flows form
initially, which result in stronger stability and stronger temperature gradients in valley
bottom regions. This result suggest a negative feedback on near surface stability caused by
stronger drainage flows that preferentially form during strong CAP nights. This highlights
a potential misunderstanding of CAP characteristics in valleys. Here it is shown that
stronger CAPs do not have stronger near surface temperature gradients.
An investigation into the affects of ambient wind on local sheltering/decoupling in valley
bottom regions is conducted. Results suggest that quantifying the amount of sheltering/
decoupling of valley bottom regions can be a useful indicative tool for understanding
the timing and amount of cooling that is occurring during CAP formation. However, future
attempts to use NH/U as a downscaling tool should endeavour to integrate other
factors, such as; changes in the ambient wind, drainage flows, gravity waves and NLLJs.