Articles | Volume 16, issue 8
Research article
21 Apr 2016
Research article |  | 21 Apr 2016

Comparing model and measured ice crystal concentrations in orographic clouds during the INUPIAQ campaign

Robert J. Farrington, Paul J. Connolly, Gary Lloyd, Keith N. Bower, Michael J. Flynn, Martin W. Gallagher, Paul R. Field, Chris Dearden, and Thomas W. Choularton

Abstract. This paper assesses the reasons for high ice number concentrations observed in orographic clouds by comparing in situ measurements from the Ice NUcleation Process Investigation And Quantification field campaign (INUPIAQ) at Jungfraujoch, Switzerland (3570 m a.s.l.) with the Weather Research and Forecasting model (WRF) simulations over real terrain surrounding Jungfraujoch. During the 2014 winter field campaign, between 20 January and 28 February, the model simulations regularly underpredicted the observed ice number concentration by 103 L−1. Previous literature has proposed several processes for the high ice number concentrations in orographic clouds, including an increased ice nucleating particle (INP) concentration, secondary ice multiplication and the advection of surface ice crystals into orographic clouds. We find that increasing INP concentrations in the model prevents the simulation of the mixed-phase clouds that were witnessed during the INUPIAQ campaign at Jungfraujoch. Additionally, the inclusion of secondary ice production upwind of Jungfraujoch into the WRF simulations cannot consistently produce enough ice splinters to match the observed concentrations. A flux of surface hoar crystals was included in the WRF model, which simulated ice concentrations comparable to the measured ice number concentrations, without depleting the liquid water content (LWC) simulated in the model. Our simulations therefore suggest that high ice concentrations observed in mixed-phase clouds at Jungfraujoch are caused by a flux of surface hoar crystals into the orographic clouds.

Short summary
This paper assesses the reasons for high ice number concentrations observed in orographic clouds by comparing observations with model simulations over Jungfraujoch, Switzerland. The results suggest that ice nuclei do not significantly contribute to the high concentrations and that a surface source of ice crystals is responsible for the witnessed ice number concentrations.
Final-revised paper