Observations of ice-nucleating particles during deep convective cloud development in New Mexico, USA

Deep convective clouds produce severe weather and their reflective anvil tops play a role in climate feedbacks. As mixed-phase clouds, they can be greatly affected by the concentration and activity of ice-nucleating particles (INPs) (Hawker et al., 2021). INP are aerosol that initiate primary ice production in mixed-phase clouds and in-situ measurements of their concentrations are scarce. Here we present analysis of INP data from aerosol filter sampling done on the FAAM aircraft the DCMEX (Deep Convective Microphysics Experiment) field campaign in New Mexico, USA, during July-August 2022. The project aims to improve the representation in climate models of microphysical processes in deep convective clouds. This location was selected because clouds formed and developed almost daily in the same location over the Magdalena Mountains during the summer monsoon season, which allowed observation of the first primary ice particles and also repeated sampling of INP in the air that that flowed into the cloud bases.

Offline INP sampling consisted of filter sampling using the FAAM Bae-146 aircraft platform along circuits around the Magdalena Mountains (16 flights). Filters were analysed during the campaign for INP using microlitre droplet freezing assays and then combined with total air flow to determine INP concentrations. We found that airborne-derived INP concentrations were higher than the upper range of past observations from continental precipitation samples (Petters and Wright, 2015) but within those of primary ice-crystal concentrations observed in clouds (Cooper, 1986). Airborne sampling runs typically saw 1-10 L^-1 at -15 °C with INP active above -10 °C and up to -5 °C frequently observed both below cloud base (>200 m above ground level) and also through a substantial portion of the free troposphere, typically to up to an altitude of 4,000-6,000 m (2,500-4,500m above ground level). INP concentrations decreased above this level by an order of magnitude. INP concentrations were normalised to aerosol surface area measurements derived from aircraft Passive Cavity Aerosol Spectrometer Probe data to derive spectra for number of active sites per aerosol surface area (n_s(T)). This revealed that the ice-nucleating activity of the aerosol consistently was higher than that described by parameterisations for desert dust, inferring a biological origin for the INP population.

Further analysis including relating INP concentrations to aerosol composition and particle size distribution, comparison of INP number to cloud ice-crystal concentration are in progress to ultimately determine relationships between primary production via INP and ice concentrations in cloud. This will help us better understand the properties and development of and ultimately improve modelling of convective clouds.

References

Cooper, W. A.: Ice Initiation in Natural Clouds, in: Precipitation Enhancement—A Scientific Challenge,  American Meteorological Society, 29-32, 1986.

Hawker, R. E., Miltenberger, A. K., Wilkinson, J. M., Hill, A. A., Shipway, B. J., Cui, Z., Cotton, R. J., Carslaw, K. S., Field, P. R., and Murray, B. J.: The temperature dependence of ice-nucleating particle concentrations affects the radiative properties of tropical convective cloud systems, Atmos. Chem. Phys., 21, 5439-5461, 10.5194/acp-21-5439-2021, 2021.

Petters, M. D., and Wright, T. P.: Revisiting ice nucleation from precipitation samples, Geophysical Research Letters, 42, 8758-8766,