Detecting ice in Arctic mixed-phase clouds by airborne spectral reflectivity measurements

Airborne spectral measurements of cloud top reflectivity have been conducted during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) in spring 2007 over the Greenland Sea in the presence of boundary layer mixed-phase clouds. The data have been utilized to retrieve information on the ice phase by analysing the spectral pattern of the cloud top reflectance in the wavelength range dominated by liquid water and ice absorption (1400-1700 nm). A new algorithm to derive an ice index which distinguishes pure ice, liquid water and mixed-phase clouds was developed. The calculation of the ice index uses, other than common two-wavelength approaches, the full spectral information of the measurements via principle component analysis (PCA). The horizontal distribution of the ice index observed during ASTAR 2007 agrees with in situ and lidar measurements showing patches of glaciated clouds at the edges of the investigated mixed-phase cloud fields. Information on the vertical distribution of ice crystals in mixed-phase clouds was derived by comparing the measured cloud top reflectivity in the wavelength band 1400-1700 nm to radiative transfer simulations. To interpret the data the vertical weighting of the measurements was calculated. In the investigated wavelength range the weightings differ according to the spectral absorption of ice and liquid water. With the high absorption of ice at 1400 nm the cloud reflectivity at this wavelength is affected strongly by cloud layers close to cloud top. Accordingly, the weighting of the cloud reflectivity is shifted slightly to lower cloud layers at 1700 nm wavelength where the absorption is weaker. From the observed spectral cloud reflectivity with low values in the ice absorption maximum (1400 nm) and higher values at the liquid water absorption maximum (1700 nm) it was concluded that ice crystals were present in the otherwise liquid dominated upper most cloud layers. Although in situ measurements (limited due to vertical resolution and detection limits) did confirm these findings only in certain limits, the retrieved vertical structure is in agreement with published ground based remote sensing measurements.