Improvement of Microphysical Processes in HARMONIE

Abstract

HARMONIE, the operational weather forecast model of the KNMI, has some weakenesses regarding its microphysical scheme. One example is 5 January last year during the freezing rain event in the Northern part of the Netherlands. HARMONIE predicted solid precipitation (snow and graupel) instead of this freezing rain, which influences the decision making for issuing a weather alarm (code orange or red) directly and affects therefore the safety of people. I propose an adjusted version of the subroutine rain_ice.f90, which accounts for the conversion of liquid to solid precipitation, to overcome some of the weaknesses HARMONIE has. The adjustments are varying from a precipitation conversion to adding a concentration and temperature dependency to several processes in the microphycical scheme. The temperature dependency of processes is combined with a threshold of the vertical velocity in the atmopshere, to add a condition that accounts for the presence of condensation nuclei in the atmosphere. In this study an overview will be given of the weaknesses and proposed adjustments in HARMONIE and its effects on the weather forecast. It will become clear that the adjusted subroutine captures this freezing rain event well and has no negative effects on other weather events investigated so far. HARMONIE, the operational weather forecast model of the KNMI, has weaknesses regarding the microphysics during situations with complex hydrometeor interactions. One example studied is the freezing rain event in the northern part of the Netherlands on 5 January 2016. HARMONIE predicted solid precipitation (snow and graupel) instead of freezing rain, which influenced the decision for issuing a weather alarm. In these type of situations, it is essential that HARMONIE predictions can be trusted to warn the Dutch population accordingly. In this study, a corrected version is proposed that accounts for the conversion of liquid to solid precipitation. The adjustments include a changing precipitation conversion by adding a minimum hydrometeor concentration and temperature dependency to several key processes in the microphysical scheme. The temperature dependency is combined with a threshold value of the vertical velocity in the atmosphere, to add a condition that accounts for the presence of ice condensation nuclei in the atmosphere. This study presents an overview regarding the weaknesses and proposed adjustments in HARMONIE, and how it affects the weather forecast. The adjustments enable the model to predicts the freezing rain event and introduces no negative effects on other weather events investigated so far.