Multimethod intercomparisons

Multimethod intercomparisons have produced the most fruitful measurements of mixed- phase clouds in the Arctic ([14]). This is partly because each measurement location has its

own strengths. While aircraft observations are clearly better suited to making in-situ, direct measurements of cloud properties, remote sensing by lidar and radar observations have the advantage of long time series at high temporal resolution with good vertical coverage. At mid-latitudes and during the sunlit portion of the Arctic year, satellite measurements can also be of help. The importance of multimethod intercomparisons is also because a mixed- phase cloud is a vastly complicated system which has many different properties to under- stand; each of these properties is best investigated by a different tool. To date, there have been many individual measurements made of specific properties of mixed phase clouds, and large-scale cooperative multi-instrument projects, but very few campaigns in the Arctic have extended longer than one season. A brief description of some recent campaign-based projects follows:

• SHEBA: Surface Heat Budget of the Arctic Ocean. 15 Sept 1997—1 Nov 1998. SHEBA was a year long ice camp including an icebreaker frozen into the drifting arctic ice pack. Most measurements were oceanographic in nature, but lidar measure- ments were made between 1 Nov 1997 and 8 Aug 1998 [28].

• FIRE-ACE: First ISCCP regional Experiment Arctic Cloud Experiment . April—July 1998 in conjunction with the SHEBA experiment [38]. FIRE-ACE focused on all aspects of Arctic clouds, flying four research aircraft over the SHEBA ground sites [39].

• MPACE: Mixed-Phase Arctic Clouds Experiment. 27 Sept—22 Oct 2004. MPACE included four surface sites in Alaska with radiosonde, tethered balloon, lidar, and radar capability. An aircraft component of the campaign added cloud microphysics and radiation measurements both in situ and from above ([40], [41]). One example of comparisons with campaign data made at a later date is the 2008 validation compari- son by Turner and Eloranta [42], in which ground-based high spectral resolution lidar is used to validate the optical depth retrievals in mixed-phase clouds from measure- ments by Atmospheric Emitted Radiance Interferometer (AERI) during MPACE.

• ASTAR: Arctic Study of Tropospheric Aerosol, Clouds and Radiation. March and April 2007. ASTAR was conducted in Ny-Alesund, Svalbard with two ground-based lidars (one micro pulse lidar and one Raman lidar) and one airborn elastic lidar [5].

• SEARCH: Study of Environmental Arctic Change. 2005—2010. Since 2005, the National Atmospheric and Oceanographic Administration (NOAA) and the Canadian Network for the Detection of Atmsopheric Change (CANDAC) have collaborated through SEARCH to make measurements at Eureka, Nunavut. This is the longest data set of its kind in the Arctic, and includes a high spectral resolution lidar (HSRL), a millimeter cloud radar and radiosonde measurements [13], [43]. The Eureka site has many co-located instruments and is an ideal location for continued measurements. Eureka is the location of the work in this thesis.

Because these campaigns were made at different times of year in different regions of the Arctic, their data are not always comparable. As an example, the results show that liquid water fraction increases with increasing temperature (MPACE, vertically integrated), de- creases with increasing temperature (MPACE, in situ measurements), and does not depend strongly at all on temperature (SEARCH) [13]. Clearly, this would be difficult to param- eterize in a model. Similarly, several experiments were carried out below -35o_{C to figure}

out rate of ice crystal formation. Lohmann 2002, in the lab, varied saturation with respect to ice, with the parameterization of ice crystal size found by Ou and Liou 1995, in which ice crystal effective radius is a function of temperature. In the field, Kristjansson et al. (2000, [44]), Ivanova (2001, [45]) and Boudala (2002, [46]) measured the same properties, but these relationships deviate quite substantially from each other for certain temperature intervals [23]. Again, this data is obtained from field campaigns in vastly differing envi- ronments.

It is desirable to continue with long-term measurements at specific locations to reduce similar uncertainty in the data products of this and other microphysical properties. To date, various independent measurements have been made of ice water content, liquid water

content, ice particle effective radius, particle number density, cloud optical depth, particle phase and other complementary measurements. Getting an idea of the processes within these clouds is more difficult because each of these properties depends on all the others.

One must know or make assumptions about crystal shape before an effective size can be calculated [23]. That is not complicated for warm clouds where every particle is a spherical liquid droplet, but becomes orders of magnitude more difficult to ascertain in an ice or mixed-phase cloud.

The influence of aerosols is also not to be neglected, and there is little agreement on their effect on the radiative impact of the clouds in which they reside. Storelvmo, Krist- jansson and Lohmann [23] points out that lab experiments by Pruppacher and Klett (1977, [47]) show that lower temperatures are required to freeze the small aerosol-nucleated cloud droplets, while Lohmann and Karcher (2002, [48]) suggests that the increase in cloud droplet number concentration as a result of those same aerosols could promote freezing. Again, more measurements are required in order to definitively understand these compet- ing effects.