Abstract
Abstract. In situ high resolution aircraft measurements of cloud microphysical properties were made in coordination with ground based remote sensing observations of a line of small cumulus clouds, using Radar and Lidar, as part of the Aerosol Properties, PRocesses And InfluenceS on the Earth's climate (APPRAISE) project. A narrow but extensive line (~100 km long) of shallow convective clouds over the southern UK was studied. Cloud top temperatures were observed to be higher than −8 °C, but the clouds were seen to consist of supercooled droplets and varying concentrations of ice particles. No ice particles were observed to be falling into the cloud tops from above. Current parameterisations of ice nuclei (IN) numbers predict too few particles will be active as ice nuclei to account for ice particle concentrations at the observed, near cloud top, temperatures (−7.5 °C). The role of mineral dust particles, consistent with concentrations observed near the surface, acting as high temperature IN is considered important in this case. It was found that very high concentrations of ice particles (up to 100 L−1) could be produced by secondary ice particle production providing the observed small amount of primary ice (about 0.01 L−1) was present to initiate it. This emphasises the need to understand primary ice formation in slightly supercooled clouds. It is shown using simple calculations that the Hallett-Mossop process (HM) is the likely source of the secondary ice. Model simulations of the case study were performed with the Aerosol Cloud and Precipitation Interactions Model (ACPIM). These parcel model investigations confirmed the HM process to be a very important mechanism for producing the observed high ice concentrations. A key step in generating the high concentrations was the process of collision and coalescence of rain drops, which once formed fell rapidly through the cloud, collecting ice particles which caused them to freeze and form instant large riming particles. The broadening of the droplet size-distribution by collision-coalescence was, therefore, a vital step in this process as this was required to generate the large number of ice crystals observed in the time available. Simulations were also performed with the WRF (Weather, Research and Forecasting) model. The results showed that while HM does act to increase the mass and number concentration of ice particles in these model simulations it was not found to be critical for the formation of precipitation. However, the WRF simulations produced a cloud top that was too cold and this, combined with the assumption of continual replenishing of ice nuclei removed by ice crystal formation, resulted in too many ice crystals forming by primary nucleation compared to the observations and parcel modelling.
Highlights
Clouds and their interaction with radiation play an important part in the earth’s climate
No filter samples were taken on the aircraft during this case study, nucleopore filters were exposed at the ground site, and these were analysed off-line using an automated scanning electron microscope (SEM) and an energy dispersive X-ray (EDX) analysis technique
Using the set-up described in Sect. 3.1, an initial 24 h simulation was performed with WRF, using the Morrison microphysics scheme and a fixed droplet number concentration of 150 cm−3 based on the peak concentration from the in situ measurements
Summary
Clouds and their interaction with radiation play an important part in the earth’s climate. There has been extensive work over the past few years to explain the observed numbers of ice crystals within clouds (Phillips et al, 2008; Connolly et al, 2009; DeMott et al, 2010) These studies have been motivated partially by observations of ice in clouds from field campaigns and by the need to develop parameterisations within atmospheric models. One field study in particular, which was based in the Florida region, noted the glaciation of an altocumulus cloud at −5 ◦C and correlated this to dust from the Sahara which had advected into the region by long range transport in easterly winds over the Atlantic (Sassen et al, 2003) They postulated that the dust may have been acting as an effective ice nucleus at these high temperatures. Aerosol measurements, made at a ground based site at the Chilbolton Facility for Atmospheric and Radio Research (CFARR, 1.44◦ W, 51.14◦ N), and onboard the BAe146 Facility for Airborne Atmospheric Measurement (FAAM) aircraft are used to investigate the role of aerosols on the microphysics of a shallow convective cloud in conjunction with radar and lidar measurements
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