In this paper we investigate the aerosol indirect effect in mixed phase layer clouds, and present an overview of a number of case studies in clouds of varying depth and temperature range. We consider the roles of the aerosol in both droplet and ice crystal nucleation and secondary ice particle production which all have a major impact on the ice crystals properties in the different clouds.
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. One project in particular within APPRAISE is funded to investigate the Aerosol-Cloud Interactions occurring within Mixed Phase Clouds.
2. THE APPRAISE CLOUD PROJECT
Objectives: To determine the nucleating ability of specific ice nuclei and the initiation and development of ice in mixed phase clouds. To determine how aerosol particles control the cloud microphysics and dynamics in mixed phase clouds To determine the type and phase partitioning of absorbing material above below and within clouds and the role of this material in ice nucleation. To reduce the uncertainty in the contribution of indirect radiative forcing by better understanding the role of aerosols in the microphysics of mixed phase cloud.
The NERC APPRAISE cloud project consists of a consortium of UK Universities working in collaboration with the Met office and European collaborators. It has the aim of performing, laboratory, modelling and field studies over the UK (and elsewhere*) that 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. Having improved the understanding of these processes, the objective is then to begin to improve the treatment of ice formation in global scale models.
The main thrust of the UK fieldwork involves the use of the UK FAAM (Facility for Airborne Atmospheric Measurement) BAe146 research aircraft flying in the vicinity of a suite radars and lidars operated at the Chilbolton Observatory in Hampshire, southern England (facilities include a scanning 3-GHz Doppler polarization radar, a 94GHz cloud radar, and a suite of lidars and radiation instrumentation). The aim is to make insitu measurements of the microphysical properties of a variety of mixed phase clouds at the same time as these clouds are being investigated remotely by the radars and lidars. The main cloud types to be studied are winter-time stratocumulus, altocumulus and deep frontal layer clouds,
3. METHOD
During winter 2007-2008 and 2009-2010 flights were made in the vicinity of Chilbolton, using the FAAM BAe146 research aircraft. This 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. The aircraft flew horizontal legs below cloud, in cloud and above cloud top on a radial towards and away from Chilbolton observatory. The vertical separation of in-clouds legs was selected so as to investigate key regions of interest for the cloud microphysics of the system, features which were determined from an initial profile through the cloud system and from the simultaneous observations of the radars and lidars. Passes below cloud base were undertaken in order to investigate the aerosol entering the cloud whilst passes above cloud top were used to investigate any ice crystal seeding that was occurring from above and for the effects of entrainment.
4. RESULTS
a. Deep Frontal clouds
On each occasion, temperatures at cloud top were significantly colder than -35deg C, and there was evidence that ice crystal formation had occurred following the freezing of haze droplets. Observations using the CPI (Cloud Particle Imager) probe and the 2D-S showed the presence of a large concentration of small pristine Bullet Rosette crystals, the preferred habit of ice growing at these ambient temperatures and humidities. Lower in these clouds the CPI showed that these ice crystals had grown to larger sizes and had developed into more complex shapes. This occurs as other crystal habits continue the growth of the crystals from their original pristine form as the ambient temperature and humidity change. At much lower levels (but at still significantly cold temperatures <-15degC) reduced concentrations of much larger crystals were observed. These were generally complex aggregates of the crystal forms seen higher up in the clouds. For these lower regions, aggregation appears to be the dominant method of crystal growth, forming snowflakes which sweep out the smaller crystals as they fall through the cloud.
Occasionally, at mid levels in the cloud there was evidence of fragmentation of crystals. These fragments were rounded and aged, which together with their rarity suggests that crystal fragmentation may be taking place as a form of secondary ice formation in certain regions.
In lower cloud layers, mixed phase conditions were observed with evidence of HM splinters growing in a narrow region bounded by regions of aggregates from aloft. However, on some occasions at the time of sampling, the lower level mixed phase cloud layers (cloud tops around -6 deg C) were separated completely from the mid/higher cloud levels by a totally cloud free region.
b. Shallow Cloud Layers
The microphysics of these clouds very much depended on the temperature range covered by the cloud. Generally the Hallett Mossop process was strongly active in clouds with tops as warm as -8C and ice was nucleated inclouds that were fed by boundary layer air. Using detailed surface measurements of aerosol composition the likely nature of the ice nuclei will be discussed. Shallow layer clouds in the free troposphere in the temperature range -10C to -30C often contained very little ice
ACKNOWLEDGEMENTS This work is supported by funding from the NERC APPRAISE directed research programme.
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