Assessing the impact of different liquid water permittivity models on the fit between model and observations
Permittivity models for microwave frequencies of liquid water below 0 °C (supercooled liquid water) are poorly constrained due to limited laboratory experiments and observations, especially for high microwave frequencies. This uncertainty translates directly into errors in retrieved liquid water paths of up to 80 %. This study investigates the effect of different liquid water permittivity models on simulated brightness temperatures by using the all-sky assimilation framework of the Integrated Forecast System. Here, a model configuration with an improved representation of supercooled liquid water has been used. The comparison of five different permittivity models with the current one shows a small mean reduction in simulated brightness temperatures of at most 0.15 K at 92 GHz on a global monthly scale. During austral winter differences occur more prominently in the storm tracks of the southern hemisphere and in the Intertropical Convergence Zone with values of around 0.5 K to 1.5 K. For most permittivity models the fit to observations is slightly improved compared to the default one. The permittivity model by Rosenkranz (2015) is recommended to be used inside the observation operator RTTOV-SCATT for frequencies below 183 GHz.