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

Abstract

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 the 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 nuclei (IN) concentration, secondary ice multiplication and the advection of surface ice crystals into orographic clouds. We find that increasing IN 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 surface flux of 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.

Impact of surface and near-surface processes on ice crystal concentrations measured at mountain-top research stations

“…18). Thus, additional processes, i.e., ice-multiplication (Field et al, 2017) as well as surface and near-surface processes, have to contribute significantly to the ICNC. The contribution of ice crystals from the surface is on the order of several hundreds of ice crystals per liter, which is estimated from the measurements after 19:10 UTC on 4 February 2017.…”

“…In situ measurements are important to further improve our understanding of the microphysical properties and fundamental processes of orographic MPCs (Baumgardner et al, 2011) and are frequently conducted at mountain-top research stations. Despite an improved understanding of the origin of ice crystals from nucleation (DeMott et al, 2010;Hoose and Möhler, 2012;Murray et al, 2012;Boose et al, 2016) and secondary ice-multiplication processes (Field et al, 2017), the source of most of the ice crystals observed at mountaintop stations and their impact on the development of the cloud remains an enigma (Lohmann et al, 2016).…”

Observations of fluorescent aerosol–cloud interactions in the free troposphere at the High-Altitude Research Station Jungfraujoch

“…proteins, tryptophan and Nicotinamide adenine dinucleotide (NADH); the latter is related to cell metabolism) such that they can be discriminated from non-biological, non-fluorescent particles (Kaye et al, 2005). Due to detector sensitivity and background fluorescence within the WIBS-4 optical chamber, the fluorescence of aerosol with diameters D p < 0.8 µm cannot be accurately measured, and the counting efficiency decreases (Gabey et al, 2011). Therefore, the analysis presented here is limited to aerosols with diameters greater than 0.8 µm, unless otherwise stated.…”

Response to RC1 - Anonymous Referee #1

“…ice crystals from nucleation (DeMott et al, 2010;Hoose and Möhler, 2012;Murray et al, 2012;Boose et al, 2016) and secondary ice-multiplication processes (Field et al, 2017), the source of most of the ice crystals observed at mountaintop stations and their impact on the development of the cloud remains an enigma .…”