Modelling of radiation fog and understanding the processes which impact its life-cycle

Smith, Daniel (2019) Modelling of radiation fog and understanding the processes which impact its life-cycle. Doctoral thesis, University of East Anglia.

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Fog has a large impact on human life including on ground transport, aviation and human health. The numerical weather prediction (NWP) of fog remains a challenge with an accurate forecast relying on the representation of many interacting physical processes. The recent local and non-local fog experiment (LANFEX) has provided a new, comprehensive and detailed observational dataset creating a unique opportunity to further our understanding of the processes which impact fog and improve the NWP of fog events.

One challenge for numerical models is predicting the development of the boundary-layer, which often undergoes a transition from statically stable to weakly unstable, during the life-cycle of a fog event. The effect of the humidity of the residual layer and wind speed on this stability transition has been investigated through idealised single column modelling. A high sensitivity was found; an increase in wind speed delays the stability transition through the modification of the boundary-layer temperature and humidity. Similarly, a drier residual layer delays the stability transition.

The performance of the Met Office Unified Model (MetUM) with three horizontal grid-lengths; 1.5 km, 333 m and 100 m, is compared against the LANFEX observations for four case studies. In general, the sub-km scale MetUM outperforms the 1.5 km version, but all the configurations show high sensitivities to a number of poorly-constrained processes, such as soil thermal conductivity, aspects of the boundary-layer scheme and domain size.

The impact of different physical processes in valleys of different geometry has been examined for two of the generally well-simulated case studies. Orographically-driven processes including valley sheltering, lee waves, drainage flows, advection of fog onto hills, warm air advection from hills over fog and anabatic flows were all found to be key to the life-cycle of fog events.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Chris White
Date Deposited: 14 Apr 2021 13:58
Last Modified: 14 Apr 2021 13:58

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