High resolution modelling of valley cold pools

Abstract

The Met Office Unified Model has been adapted to run with a 100 m horizontal grid length for a region of complex terrain whose valleys are too narrow to be represented in the operational forecast (1.5 km grid length) model. Forecasts of temperatures are compared with observations from the COLd air Pooling EXperiment. The model provides a realistic representation of valley cold pools which form during stable nights, although the predictions are shown to be sensitive to vertical resolution. Reducing the vertical grid spacing near the surface results in lower night-time temperatures and larger, more realistic, valley-hill temperature contrasts.

Assessment of valley cold pools and clouds in a very high-resolution numerical weather prediction model

“…Vosper et al (2013a) described the modeling setup in detail, so only a brief summary is given here. The innermost 100m domain uses a non-uniform horizontal grid with the resolution decreasing from 100 m to 1.5 km at the boundaries (Tang et al, 2012).…”

“…The 4 km model uses a 1-D boundary-layer scheme, while the 1.5 km uses a 2-D Smagorinsky turbulence parametrization for horizontal mixing and a 1-D boundary-layer scheme for vertical mixing, and the 100 m model uses a 3-D Smagorinsky scheme (Lock et al, 2000). Finally, the vertical profile of critical relative humidity (RH crit ) necessary for cloud formation is increased, to take into account the reduction in grid box size (see Vosper et al, 2013a). The critical relative humidity threshold approach allows non-zero cloud cover to be diagnosed when grid box relative humidity is below saturation.…”

“…Sheridan et al (2013) analyzed the observed cold pool structure and related the frequency and strength of cold pools to previous idealized studies of cold pool formation (Vosper and Brown, 2008). Modeling work has used a nested version of the Met Office Unified Model (MetUM), with a horizontal grid spacing of 100 m. Over the lowest 1 km, the vertical resolution is on average 27 m. The basic model configuration and initial comparison with observations is given by Vosper et al (2013a). The results show that the high horizontal and vertical resolution is required in order to sufficiently resolve the orography and obtain a good level of agreement between the simulated and observed temperature variations across the cold pools in clear-sky cases.…”

“…The difference between hilltop (Spring Hill) and valley bottom (Duffryn) screen temperature gives a measure of the strength of cold air pooling in the Clun Valley Vosper et al (2013a). Using this difference, Fig.…”

“…In addition to these, an extended simulation has been completed, for the period 17 August to 15 October 2009 (the long-term simulation). This period was chosen because it includes multiple IOP cases, two of which (9-10 September, IOPs 4 and 5) have already been investigated in detail (Price et al, 2011;Vosper et al, 2013a mechanisms involved in cold pool formation it is appropriate to focus on IOPs where the model performs well; however, when the more general performance of the model is of interest it is important to consider a wider range of conditions over the whole 2-month period.…”