Aircraft Observations of Rapidly Developing Wintertime Convective Clouds over the Bristol Channel, UK

In this paper we present aircraft measurements from 3 convective clouds in different stages of their evolution that formed over the Bristol Channel, UK, on January 20th 2009. These investigations were undertaken as part of the NERC APPRAISE-Clouds project (Choularton et al., AMS2010). The “Aerosol Properties, PRocesses And Influences on the Earth's climate” (or APPRAISE) programme is a UK Natural Environment Research Council (NERC) directed research programme set up to look at the science of aerosols and their effects on climate. The APPRAISE-Clouds consortium project was funded in particular to investigate the Aerosol-Cloud Interactions occurring within Mixed Phase Clouds. The consortium of UK Universities working in collaboration with the Met office and European collaborators aimed to perform, laboratory, modelling and field studies to address the question of how ice forms in clouds and in particular how this is determined by the properties of the aerosol entering into cloud. With an improved understanding of the processes occurring within these clouds, the goal is improve the treatment of ice formation in global scale models. During winters 2007-2008, and 2008-2009 a number flights were undertaken using the UK FAAM (Facility for Airborne Atmospheric Measurement) BAe146 research aircraft. The aircraft was equipped with a comprehensive range of instrumentation to measure the ice and liquid phase microphysics of the cloud and the size distribution and size resolved chemical composition of the aerosols entering cloud. Wing-borne cloud microphysics instrumentation included: SPEC CPI (cloud particle imager) and 2DS-128 (high resolution shadow) probes, DMT CAPS (includes CAS and CIP-15GS), CDP and CIP100 probes alongside a suite of heritage probes. Aerosol instrumentation included wing-borne PCASP, and internally mounted AMS, SMPS, SP2, CPC and PCASP instruments on a choice of CVI or aerosol mode inlets. Results from flight B423 carried out on 20th January 2009 will be presented. On this day an area of low pressure lay in the North Atlantic to the northwest of the UK (over Iceland), and an occluded front approached the British Isles from the west. At midday this front was still to the west of Ireland. A number of troughs were forecasted to cross the country during the day, generating convection and a number of isolated showers. In particular a number of such cells were predicted for the southwest of England and out over the Bristol channel and in the southwest approaches.. Upon arriving at the south Wales coast a number of small convective clouds could be seen ahead over the Bristol Channel, together with a smaller number of bigger turrets with developing or full anvils. The first cloud investigated was a large mature cloud. A series of 6 straight and level runs (SLRs) through the turret were carried out to investigate the microphysical properties of the cloud at different altitudes (and temperatures) above cloud base (CB). Runs 1-6 were carried out at flight levels FL180, 170, 160, 140, 120 and 100 respectively (at 505, 526, 549, 594, 643 and 695 mbar, and -35.6, -32.8, -30.2, -25.4, -19.9 and -15.8 deg C respectively) and separated by profile descents (at 1000 ft/min). Prior to entry into the cloud the 2DS -128 and CIP-15 probes saw ice crystals precipitating out from the anvil above (mode size/max size: 75/300 microns respectively). This mature cloud dissipated at lower levels before it could be investigated fully (diminishing total condensed water contents were seen at successive descending levels). The second cloud investigated was a much younger growing cloud turret which and was examined by flying a series of SLRs through it at ascending levels as the cloud itself grew in the vertical and evolved. The cloud chosen appeared to have formed over the top of a small Island (Lundy). The first two runs (R7, R8 at 1000 & 2000ft, +4.0 & +1.4 deg C respectively) were flown under the cloud and through precipitation which was mainly comprised of rain but did contain some ice. Filters were exposed briefly either side of the precipitation shafts. In-cloud SLRs; runs 9 to15, were carried out at sequentially higher levels through this growing turret, at 4, 5, 6, 7, 8, 10, and 11kft respectively (at 854, 822, 792, 763, 733, 679 and 653 mbar, and -4.2, -6.6, -8.3,-12.2, -13.3, -17.2 and -19.1 deg C respectively). Temperatures were generally colder (by about 0.4deg C) within the clouds than either side, and at lower levels the turrets contained a lot of liquid water drops particularly close to the cloud edges. Each cloud pass revealed regions dominated by supercooled droplets interspersed with regions containing ice (some particles as large as 3-4 mm). The size of cloud droplets was seen to increase with altitude, and at the -8.3deg C level as well as irregular ice, regions containing distinct columns were also seen (suggesting that the Hallett-Mossop process was active). At higher levels dendrites became the preferred habit, seen together with aggregates of dendrites, dendrites growing plate–like extensions, graupel and plates. Finally, a large mature cloud with a well developed anvil was investigated, by carrying out a series of broken profile descents down through it from the -27 deg C level (FL200, 570mbar) to the -6.2 deg C level (FL54, 830mbar). The profiles through the cloud were made mainly through the shoulders, avoiding the active region at the core (observed by the aircrafts weather radar). The shoulders consisted of a number of cells some of which appeared to consist entirely of supercooled water, and significant aircraft icing was observed. Other regions were fully mixed phase, and at various stages very heavy precipitation was also encountered. The investigation of this extremely active cloud was terminated soon after the aircraft was struck by lightning. Detailed analysis of these three case study clouds will be presented. The APPRAISE ACPIM (Aerosol-Cloud and Precipitation Interactions Model) will also be used to help to gain a better understanding of glaciation rates (particularly in case 2), which will provide an indication of the relative importance of primary nucleation of ice vs secondary ice formation.