Ice production in a slightly supercooled layer cloud with embedded convection

Stratiform clouds have a strong impact on the Earths' radiation budget, predominantly through the reflection of incoming solar radiation back into space. The radiative properties (effective radius, optical depth etc) and lifetime of clouds are affected by their microphysical properties (cloud droplet number concentration etc). Changes in the microphysical properties of stratiform clouds can have significant radiative impacts due to the large fraction of the Earths surface which has overlying stratus at any given time. The formation and development of the ice phase in supercooled stratiform clouds can lead to precipitation which removes the condensed phase from the cloud. Also, the presence of ice in clouds alters the radiative properties of the cloud. Therefore, detailed measurements of the important microphysical processes which occur in clouds which lead to changes in bulk/radiative properties need to be characterised.

In-situ measurements of mixed phase cloud properties were obtained during the winter months (December-March) of 2008-2010 as a part of the APPRAISE-Clouds project. The in-situ measurements were obtained from probes mounted on the UK BAe146 Facility for Airborne Atmospheric Research (FAAM). Cloud microphysical properties were measured using a variety of optical sizing instrumentation, such as Droplet Measurement Technologies (DMT) Cloud Droplet Probe (CDP) and Cloud, Aerosol and Precipitation Spectrometer (CAPS), and the Stratton Park Engineering Company Incorporated (SPEC Inc) 2D-128 Optical Array Probe (OAP). Most of the measurements were collected in the region of the Chilbolton Facility for Atmospheric and Radio Research (CFARR), which is located in the South of England (51.1445 degrees north, -1.4370 degrees west). Various remote sensing instruments were deployed at the Chilbolton Observatory including the 3 GHz Chilbolton Advanced Meteorological Radar (CAMRa) mounted on a 25m steerable dish, and a vertically pointing 94 GHz cloud Radar.

On the 18th February 2009 the UK was under the influence of a stationary front which was aligned roughly north-south (warm air to the west, cold air to the east) and was situated close to the Chilbolton Observatory. Low level cloud covered a large portion of the UK and was sampled by the BAa146 aircraft while being scanned by the Chilbolton Radars. High pressure in the area meant that the cloud layer was capped by relatively warm and dry air which inhibited any convective activity, with no clouds above providing ice as input. Cloud top height of this cloud was around 3.8 km (-13 deg C) in the Chilbolton region. In-situ measurements from the BAe146 aircraft show that the low level cloud mainly consisted of supercooled water drops, but also had a significant number of large stellar ice crystals in it. Convective activity was observed on the radars on several occasions at locations which approximately coincide with the location of the stationary front. This convective activity led to significant amounts of precipitation which reached the surface according to the CAMRa reflectivity. The convection could not penetrate the temperature inversion which capped the supercooled layer cloud at 3.8 km altitude. In-situ measurements from the BAe146 showed that the region of convective activity below the supercooled layer cloud consisted of a combination supercooled drops and ice crystals. The ice crystals consisted of small pristine columns and larger rimed crystals at around -4.5 deg C, what appear to be rimed columns at around -6.8 deg C, and large rimed crystals at around -9.5 deg C. Ice concentrations in the convective region below the supercooled layer cloud are far higher than at cloud tops in the supercooled layer cloud itself.

The presence small and pristine columns at around -4.5 deg C in the vicinity of supercooled water from convection suggest that the ice is formed via ice splinter ejection from riming (Hallett-Mossop process). This appears to be occurring when the ice from the supercooled layer cloud falls through the supercooled column of liquid water from the convective activity (which spans the Hallett-Mossop zone) and results in secondary ice formation. As most of the precipitation appears to be occurring in the region of convective activity, and the precipitation is in the form of rimed ice crystals, the Hallett-Mossop process may influence the formation of precipitation in this particular system. This process of secondary ice formation only occurs as the supercooled layer cloud contains some ice which must be nucleated via some primary mechanism at temperatures greater than -14 deg C.