Remote Sensing of the Polar Troposphere: Total Water Vapor and Liquid Water Path

Climate projections suggest the Arctic Ocean as the location of the most rapid and dramatic climate changes in the 21st century, with the potential of major repercussions on mid-latitude climate and the freshwater balance of the North Atlantic. Accurate initial state of Arctic ocean and atmosphere represent a large potential to improve numerical weather predictions and climate forecast both in seasonal and decadal scales. While reliable data about the sea ice cover are available since over three decades from sataellite observations, our knowledge about the Arctic troposphere is much less complete. Two atmospheric parameters for which data in the Arctic are most needed are water vapour and cloud liquid water.

The standard satellite remote sensing methods for water vapour sounding (based on, e.g., microwave radiometer data) fail in dry and cold Arctic conditions. Instead, total water vapour (TWV, vertically integrated water vapour, also called integrated water vapour, or precipitable water vapour), can be derived from microwave radiometer data (Miao et al., 2001; Wang and Manning, 2003; Melsheimer and Heygster, 2008). Daily TWV data over sea ice from 2003 to date based on sounding data from AMSU-B will be presented.

While over open ocean the cloud liquid water path (LWP, vertically integrated cloud liquid water) has been observed for over 30 years with passive microwave instruments on satellites (e.g., Wentz and Schabel, 2000), over sea ice, up to now no reliable observations are available. LWP has a much lower signal in passive microwave observations than TWV, and there are much less in situ data available for calibration and validatation. The situation has improved with the ASCOS (Arctic Summer Cloud Ocean Study) campaign in August and September 2008 when the RV Oden over six week among other took data with a ship-based microwave radiometer to determine LWP. This dataset is used for comparisons with LWP retrievals from two satellite sensors:

1. Cloudsat, launched April 2006: The Cloud Profiling Radar (CPR) of CloudSat is a 94-GHz nadir-looking radar which measures the power backscattered by clouds as a function of distance from the radar. The height resolution is about 500m, the footprint size 1.5 km, taken once each 1.1 km along the flight track. Several versions of Cloud Liquid Water (CLW) and Cloud Ice Water (CIW) data product available, but the quality over sea ice has not been assessed. 2. MODIS, (MODerate resolution Imaging Spectrometer) launched 1999 on the TERRA and 2002 on the AQUA spacecraft, is a visible and IR radiometer scanning the earth in a 2000 km wide swath in 36 channels ranging from 0.4 to 14 μm. Daily the complete globe is covered. Comparisons of the routinely produced LWP data with the AMSR-E LWP product over open ocean are now available (Torre Juarez et al. 2009, Greenwald 2009). However, over sea ice the MODIS LWP has not yet been investigated.

LWP retrieval results from these two sensors will be compared with the ground truth data of the ODEN campaign. While the horizon of CloudSat restricted to latitudes lower than +- 82° strongly limits the comparison data base, the daily global coverage of MODIS allows much more and much narrower co-locations. Comparison results will be presented in detail.