Modelling of air flow and cloud fields over the northern Pennines

Numerical modelling of air how and the clouds evolving in this air flow are performed to better understand the dynamical and thermodynamical conditions which prevailed during the main observational episodes of the EUROTRAC-ground based cloud experiment 1993 on Great Dun Fell (England). A nonhydrostatic cloud model with two nested model domains of increasing grid resolution is used, to zoom from the mesoscale environment of northern England, into the cloud forming relief of the northern Pennines and finally into the small-scale terrain of the held experiment. The vertical and horizontal extensions of the modelled surface cloud fields are mainly determined by the initial profile of temperature and humidity in the first 1500 m. Initial wind profile and topography also contribute to the structure of the orographic surface clouds. A trajectory analysis for the neighbouring observational points in the field site show that, also on scales of a few kilometres, air parcels with quite different cloud life histories can be expected. Numerical tracer experiments indicate that for the prevailing northeasterly wind conditions SO, plumes from the New Castle environment can affect the measuring site. Furthermore, the methods providing the trajectories and streamlines for the chemical and mircophysical evaluations of the experiment by parcel models are presented. (C) 1997 Elsevier Science Ltd.