Retrieval of cloud phase over the Arctic using MODIS 6.7-12 micron data

A polar cloud phase model is developed using Moderate Resolution Imaging Spectroradiometer (MODIS) data taken onboard the Terra and Aqua satellites which passed over Barrow, AK during the years 2000-2004. The model utilizes 3 water vapor and 2 infrared channels in the 6.7-12 micron wavelength range. To develop the model, a wide range of cloud systems was sampled where the brightness temperature (BT) data from MODIS was compared to surface-based cloud phase retrievals at the Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) Barrow site. The main feature of the ARM MODIS Phase Model (AMPHM) is its capability to discriminate mixed-phase clouds from those having an all ice or all liquid phase. For the first step, the MODIS data is partitioned using the difference in the absorption coefficient from 8.5-12 microns for liquid water compared to ice. The second and primary part of the AMPHM is a water vapor and 11-micron BT parameterization of the cloud phase and works because each cloud system (and phase) has a unique atmospheric water vapor and thermal structure associated with it. The model also has some capability to detect thin cirrus clouds when they overlie boundary layer mixed or liquid phase clouds. Results from the AMPHM are compared to the surface-based phase retrievals over the ARM-NSA Barrow site and to in-situ data from the University of North Dakota Citation aircraft which flew during the Mixed-Phase Arctic Cloud Experiment. Since the AMPHM uses only water vapor and infrared MODIS channels, it can be applied equally well to daytime and nighttime scenes with no discontinuities in the output cloud phase. The model should be applied to high-latitude regions only and even there, it is unclear how it will perform for scenes containing highly elevated terrain.