DOI: 10.5285/c5cc491e-7119-4500-b05a-e5e34274438f
title: Kilometre and sub-kilometre scale atmosphere-only model simulations using the Met Office Unified Model of a foehn wind event over the Larsen C Ice Shelf, Antarctic Peninsula on 27 January 2011

description: Firstly, simulations of surface pressure, 10 m zonal wind speed, 10 m meridional wind speed, 1.5 m air temperature, and 1.5 m specific humidity over the Larsen C Ice Shelf for the duration of the OFCAP (Orographic Flows and the Climate of the Antarctic Peninsula) field campaign from 8 January 2011 to 8 February 2011 were conducted using the regional atmosphere-only configuration of the Met Office Unified Model (MetUM) at 4 km grid spacing by the British Antarctic Survey, Cambridge, UK. The datasets produced were necessary to compare with corresponding measurements derived from five Automatic Weather Stations (AWSs) distributed across the Larsen C Ice Shelf to evaluate the main biases in the simulations. Secondly, further MetUM simulations at grid spacings of 1.5 and 0.5 km of a foehn wind event that occurred on 27 January 2011 were conducted, with the datasets produced used to compare results at 4, 1.5 and 0.5 km grid spacing and examine whether the added benefit of sub-kilometre scale grid spacing improves the model representation of foehn winds. Thirdly, a simulation of the foehn wind event on 27 January 2011 using the MetUM at 4 km grid spacing but replacing the 'sharp' stability function used by the boundary layer scheme with the 'long-tail' stability function were also conducted, with the dataset produced used to examine the impact of stronger turbulent mixing for statically stable conditions on the model representation of foehn winds. Funding was provided by NERC grant NE/G014124/1.
description: We carried out a high-resolution atmosphere-only model simulation of the Larsen C Ice Shelf and surrounding area from 8 January 2011 to 8 February 2011, using the nested configuration of version 11.1 of the UK Met Office Unified Model (MetUM). Three nested model domains at horizontal resolutions of 4, 1.5 and 0.5 km are used. The outer 4 km model domain covers the Antarctic Peninsula and the Larsen C Ice Shelf (domain size of 220 × 220 grid points), while the two additional (one way nested) 1.5 and 0.5 km model domains cover the northern Antarctic Peninsula and Larsen C Ice Shelf (domain size of 450 × 450 and 1000 × 1000 grid points, respectively). Boundary conditions for the 1.5 km (0.5 km) model are derived from the 4 km (1.5 km) model. All three models employ 70 vertical levels (going up to 40 km), which are terrain-following near the surface, with 16 levels below 1000 m and a lowest model level at ~5 m above the surface. The orography used by the 4, 1.5 and 0.5 km models is derived from the high-resolution (200 m) Radarsat Antarctic Modelling Project (RAMP) digital elevation model. Due to the very steep terrain, the 4 and 1.5 km (0.5 km) orography was smoothed by convolution with a normalised 2-d Gaussian kernel with standard deviation of 0.6 grid points (0.8485 grid points). The coastline and ice shelf extent are taken from the Scientific Committee on Antarctic Research (SCAR) Antarctic Digital Database. The science configuration of the dynamics and physics schemes used by the nested models was the mid-latitude regional atmosphere configuration (RA1-M). The 4 km model is nested within a 12 km version of the model (domain size of 150 × 150 grid points), which has a much larger limited area domain encompassing the whole of the Antarctic Peninsula and the surrounding ocean (not shown). The required start data and boundary conditions for the 12 km model are provided by the global model configuration of the MetUM at N320 resolution (domain size of 640 × 480 grid points, equivalent to a horizontal resolution of ~40 km), which is initialised by ERA-Interim reanalysis. The MetUM 4 km model was run twice daily at 00 UTC and 12 UTC from 8 January 2011 to 8 February 2011 for 24 h, with output saved at T+15, T+18, T+21 and T+24 h. Earlier output was discarded as spin-up, with the remaining part of the forecasts concatenated together to form a continuous time-series from 15 UTC 8 January 2011 to 24 UTC 8 February 2011. The computationally expensive MetUM 1.5 km and 0.5 km models were only run to obtain output for 27 January 2011, i.e., coinciding with the foehn event that occurred at this time. A further sensitivity experiment was conducted for 27 January using the MetUM 4 km model but replacing the 'sharp' stability function with the 'long-tail' stability function for the boundary layer scheme, i.e., to examine the impact of stronger turbulent mixing for statically stable conditions.
description: Examination of the model output showed that: i) the features commonly associated with foehn winds, such a leeside hydraulic jump, and enhanced leeside warming, were comparatively insensitive to resolution in the 4 to 0.5 km range, although the 0.5 km simulation shows a slightly sharper and larger hydraulic jump, ii) during the foehn wind event, the simulated near-surface air temperature over the Larsen C Ice Shelf was characterised by a both a large nocturnal cold bias of around 10°C and a much too strong low-level temperature inversion, with neither representation improving with increased resolution, and iii) the simulation of fine-scale foehn jets over the Larsen C Ice Shelf during the foehn event showed considerable dependence on grid spacing, although no evidence of convergence at higher resolution.
description: The data are provided in the form of CF-compliant netCDF-4 files, with one file containing each data variable. The horizontal coordinate system is defined using CF-compliant rotated_latitude_longitude grid mapping variables, and the true latitude and longitude coordinates are provided in each file as auxiliary coordinates. The wind variables are provided as both grid-directed wind components (x_wind and y_wind) and as zonal and meridional wind components (eastward_wind and northward_wind). This dataset also includes the MetUM surface altitude and land/sea binary mask fields. Scalar variables are provided on the rotated lat/lon grids on which they were output by the MetUM; wind variables have been interpolated onto the grid used for the scalar variables by bilinear interpolation. The data files are named according to the following file naming convention: OFCAP_MetUM_[model]_[frequency_or_duration]_[quantity]_[level]_[date/time_range].nc, where the file name components in square brackets are as follows: [model]: one of "4.0_km", "1.5_km", "0.5_km" or "4.0_km_long-tail", corresponding to the model grid spacing, and in the case of 4.0_km_long-tail, the use of the 'long-tail' stability function. [frequency_or_duration]: for instantaneous data, this indicates the frequency with which the data are provided ("3-hourly" or "6-hourly"); for temporally aggregated (mean or total) data, this indicates the length and type of the aggregation ("hourly_mean" or "hourly_total"); for surface altitude, which is invariant, this file name component is omitted. [quantity]: a description of the quantity, based on the CF convention standard name for the quantity (where available). [level]: for data relating to a specific height, this describes the relevant height ("at_1.5_m" or "at_10.0_m"); for data on model levels, this is "on_model_levels"; for surface or other single level data this is omitted (as the [quantity] component already includes the relevant level description). [date/time_range]: for data provided for multiple periods of time, this is [first_period_date/time]-[last_period_date/time], with each date/time expressed in the form YYYYMMDDhhmm; for data provided for a single period, -[last_period_date/time] is omitted; for surface altitude, which is invariant, this file name component is omitted. The data files are organised into a two-level folder hierarchy by model, frequency/duration and date/time range. Using the above naming convention, the first level of the folder hierarchy is named [model]; the second is named [frequency_or_duration]_data_[date/time_range].