Validation using AWS observations

Figure 5.2: A time series comparing both resolutions of the MetUM to observations from Bear Peninsula AWS: 2-m temperature (top), Relative Humidity w.r.t water (middle), Wind speed and direction (bottom). In the wind direction plot the symbols represent the following: black stars = AWS observations, blue circles = MetUM high resolution simulation, blue crosses = MetUM coarse resolution simulation with low flow blocking drag coefficient, red crosses = MetUM coarse resolution simulation with default flow blocking drag coefficient (high).

There are two coarse resolution MetUM runs shown in Fig. 5.2 — the red dashed line shows the coarse resolution simulation with the high flow blocking drag coefficient whereas the blue dashed line shows the simulation with the updated (lower) flow blocking drag coefficient value (see chapter 2 for details). The difference between the MetUM global simulations with ‘high’ and ‘low’ drag coefficients shows that by updating the flow blocking drag coefficient in the coarse resolution model the mean temperature bias at Bear Peninsula (BP) AWS decreases from -6.5^◦C (‘high drag’) to -4.7^◦C (‘low drag’). However, it does remain larger than the temperature bias in the high resolution model (-1.4^◦C). The coarse resolution model temperature bias is greatest between 0300 and 1100 UTC each day which corresponds to the ‘overnight’

period (i.e. the sun is close to or below the horizon) in the ASE at this time of year.

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Analysis of the radiation budget at Evans Knoll (EK) AWS (not shown) revealed that the net upwelling longwave radiation term was larger in the coarse resolution simulation during these ‘overnight’ time periods and hence the near-surface air temperature was colder. There is also a difference between the observed and modelled wind direction which may be contributing to the temperature bias. For much of the case study the observed winds at BP AWS come from the northeasterly direction (bringing relatively mild conditions) whereas both resolutions of the model show cooler southerly or southeasterly winds at this location (Fig.5.2). Such wind direction biases are often linked to model temperature biases in simulations over the Amundsen Sea region (Deb et al.,2016).

The wind speed time series in Fig.5.2shows that both resolutions of the MetUM are in reasonable agreement with the observations. While wind speeds were generally quite light at BP AWS both resolutions of the model struggle to capture stronger winds that were observed at 0800-1200 UTC 18th February. Both resolutions of the model have higher values of relative humidity than observed for much of the time series (Fig.5.2). The high resolution model relative humidity is on average 4% higher than observed, while the coarse resolution model is 13% higher than observed (in the updated ‘low drag’ simulation). While this bias can be greater than 20% at times, it is not overly concerning as observations of relative humidity in Antarctica are known to contain inaccuracies, particularly when air temperatures are low, so there could be observational bias as well as model bias (Renfrew and Anderson,2002).

Fig.5.3compares the observations from New York University (NYU) AWS on PIG to both resolutions of the MetUM. Note that during this period the AWS humidity sensor was not operational so the humidity panel of Fig.5.3shows only model values.

The high resolution model accurately reproduces the observed temperatures with mean modelled 2-m temperatures only 0.3 ^◦C colder than observed. The coarse resolution model contains a persistent cold bias and is on average 2.8 ^◦C colder than the observations. In the wind speed and direction comparison in Fig.5.3both resolutions of the model do a reasonable job of reproducing the observed conditions with light to moderate south to southeasterly winds.

Figs. 5.2 and 5.3 are generally representative of the other AWS sites too. The magnitude of the coarse resolution model temperature bias varies at different sites, ranging between -1.6^◦C and -6.9^◦C, it is always larger than the high resolution model

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Figure 5.3: A time series comparing both resolutions of the MetUM to observations from New York University AWS: 2-m temperature (top), Relative Humidity w.r.t water (middle), Wind speed and direction (bottom). In the wind direction plot the symbols represent the following:

black stars = AWS observations, blue circles = MetUM high resolution simulation, blue crosses

= MetUM coarse resolution simulation with low flow blocking drag coefficient, red crosses = MetUM coarse resolution simulation with default flow blocking drag coefficient (high).

temperature bias. In the high resolution model the range of temperature biases is -0.2^◦C to -2.9 ^◦C. The temperature comparison in Fig.5.2is representative of those seen at EK and TI AWS where the coarse resolution model shows an overly large drop in temperature during the overnight period when observed wind speeds are light. Modelled wind speed and direction at all four AWS are generally in reasonable agreement with the observations, typically wind speed biases compared with AWS observations are less than 2 m s⁻¹. The models do however tend to miss short term increases in the wind speed such as those seen at BP AWS at 1000 UTC 18th February (Fig.5.2) and that at ∼ 1700 UTC 18th February at NYU AWS (Fig.5.3).

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Figure 5.4: A time series comparing the MetUM to meteorological observations collected onboard RRS James Clarke Ross: Temperature (top), Relative Humidity w.r.t water (middle), Wind speed and direction (bottom). In the wind direction plot the symbols represent the following: black stars = AWS observations, blue circles = MetUM high resolution simulation, blue crosses = MetUM coarse resolution simulation with low flow blocking drag coefficient, red crosses = MetUM coarse resolution simulation with default flow blocking drag coefficient (high).