Evolution of cold-air-pooling processes in complex terrain

This document is the Accepted Manuscript version. The final publication is available at Springer via: https://doi.org/10.1007/s10546-013-9885-zElucidating cold-air-pooling processes forms part of the longstanding problem of parametrizing the effects of complex terrain in larger-scale numerical models. The Weather Research and Forecasting model has been set-up and run at high resolution over an idealized alpine-valley domain with a width of order 10 km, to investigate the four-dimensional variation of key cold-air-pooling forcing mechanisms, under decoupled stable conditions. Results of the simulation indicated that the total average valley-atmosphere cooling is driven by a complex balance/interplay between radiation and dynamical effects. Three fairly distinct regimes in the evolution of cold-air-pooling processes have been identified. Starting about 1 h before sunset, there is an initial 30-min period when the downslope flows are initiated and the total average valley-atmosphere instantaneous cooling is dominated by radiative heat loss. A period of instability follows, when there is a competition between radiation and dynamical effects, lasting some 90 min. Finally, there is a gradual reduction of the contribution of radiative cooling from 75 to 37 %. The maximum cold-air-pool intensity corresponds to the time of minimum radiative cooling, within the period of instability. Although, once the flow is established, the valley atmosphere cools at broadly similar rates by radiation and dynamical effects, overall, radiation effects dominate the total average valley-atmosphere cooling. Some of the intricacies of the valley mixing have been revealed. There are places where the dynamics dominate the cooling and radiation effects are minor. Characteristics of internal gravity waves propagating away from the slopes are discussedPeer reviewe

How do river nitrate concentrations respond to changes in land-use? A modelling case-study of headwaters in the River Derwent catchment, North Yorkshire, UK

A combined semi-distributed hydrological model (CASCADE/QUESTOR) is used to evaluate the steady-state that may be achieved after changes in land-use or management and to explore what additional factors need to be considered in representing catchment processes. Two rural headwater catchments of the River Derwent (North Yorkshire, UK) were studied where significant change in land-use occurred in the 1990s and the early 2000s. Much larger increases in mean nitrate concentration (55%) were observed in the catchment with significant groundwater influence (Pickering Beck) compared with the surface water-dominated catchment (13% increase). The increases in Pickering Beck were considerably greater than could be explained by the model in terms of land-use change. Consequently, the study serves to focus attention on the long-term increases in nitrate concentration reported in major UK aquifers and the ongoing and chronic impact this trend is likely to be having on surface water concentrations. For river environments, where groundwater is a source, such trends will mask the impact of measures proposed to reduce the risk of nitrate leaching from agricultural land. Model estimates of within-channel losses account for 15–40% of nitrate entering rivers