A modification of the Smith cloud scheme to allow supersaturation with respect to ice in the ARPEGE NWP global model

This work presents a modification of the Smith (1990) cloud scheme used in the ARPEGE (Action de Recherche Petite Echelle Grande Echelle) NWP global model in order to improve the forecast of relative humidity with respect to ice, with particular attention paid to supersaturation, a necessary condition for the persistence of aviation contrails. The modeling extends the Smith cloud scheme used in the operational ARPEGE by reworking the statistical concepts of Sommeria and Deardorff (1977) while including a temperature-based parametrization for the representation of homogenous nucleation. A notable point is that this modification can be implemented without major changes and does not require additional computational effort. Furthermore, it allows for extensions to other atmospheric models using a similar framework. The new forecasts are verified using in situ humidity observations made by IAGOS program aircraft and compared to ARPEGE operational forecasts, resulting in a better description of supersaturated regions. Further impacts on other general parameters (wind, temperature) are also presented in this study.