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Cold air pools over complex terrain

Jemmett-Smith, Bradley Colin (2014) Cold air pools over complex terrain. PhD thesis, University of Leeds.

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Abstract

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.

Item Type: Thesis (PhD)
Academic Units: The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds)
Identification Number/EthosID: uk.bl.ethos.605538
Depositing User: Repository Administrator
Date Deposited: 23 Jun 2014 15:45
Last Modified: 03 Sep 2014 10:49
URI: http://etheses.whiterose.ac.uk/id/eprint/6414

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