The Potential for Estimating Cloud Liquid Water Path over Sea Ice from Airborne Passive Microwave Measurements

Arctic clouds have been historically difficult to detect with standard remote sensing techniques due to various limitations imposed by the meteorological and surface conditions in the region. The primary problem is that of low contrast between the clouds and surface over the range of frequencies typically used for remote sensing. In the microwave portion of the spectrum, for example, sea ice has a high emissivity, so arctic clouds containing low liquid water amounts do not have a large effect on the signal. The comprehensive data set collected during the FIRE Arctic Clouds Experiment (FIRE-ACE) and the Surface Heat Budget of the Arctic (SHEBA) experiment enables us to examine in detail the conditions under which liquid water path retrievals over sea ice may be feasible. Airborne passive microwave measurements at frequencies ranging from 37 GHz to 220 GHz are available along with simultaneous measurements of cloud microphysical properties and sea ice characteristics.

Microwave measurements have been used successfully for LWP retrievals over the ocean due to the low emissivity and horizontal uniformity of the surface. Calculations of sea ice emissivity from the airborne radiometers during clear sky conditions demonstrates that multi-year sea ice emissivities at 90 and 150 GHz may be as low as water emissivities at those frequencies in certain conditions. Furthermore, arctic clouds containing substantial amounts of liquid water are easily observed in brightness temperature images over sea ice. Thus it seems reasonable to attempt LWP retrievals in the Arctic.

Radiative transfer model simulations of the conditions encountered during FIRE-SHEBA demonstrate that the LWP signal is most significant at 90 GHz. Surface emissivity at nadir angles of 50 degrees and above is less for horizontal polarizations than vertical, so the contrast between surface and clouds is greater when horizontally polarized data is used. Variations in surface emissivity produce brightness temperature changes comparable to the changes induced by a cloud with LWP less than 50 gm-2. Uncertainties in surface temperature, cloud temperature, and relative humidity have much smaller effects.

A physically-based LWP retrieval algorithm is adapted for use with the airborne radiometer and applied to several FIRE-SHEBA cases. Retrieved LWP values show good agreement with aircraft in situ measurements when the cloud contains 100 gm-2 or more of liquid water. Accuracies are not good for clouds with LWP less than 50 gm-2. Examples of retrievals will be shown at the conference, and the frequency of occurrence of conditions where accurate retrievals can be performed will be discussed.