Deployments of microwave and millimeterwave radiometers in the Arctic
ABSTRACT Measurement of water vapor and cloud liquid during very cold (-20 to -40 Deg. C) and dry (precipitable water vapor < 5 mm) conditions is a very important, but difficult task. Starting in 1999, three radiometric experiments were conducted at the U. S. Department of Energypsilas Atmospheric Radiation Measurement (ARM) Programpsilas North Slope of Alaska (NSA) field site near Barrow, Alaska. Principal results from the first two experiments are summarized. Most recently, the Radiative Heating in Underexplored Bands Campaign (RHUBC) was conducted in February-March 2007. The millimeter- and submillimeter-wave channels of the ground-based scanning radiometer (GSR) are very sensitive to low water vapor and cloud contents and allow for accurate observations in the extremely dry and cold conditions typical of the Arctic. Moreover, window channels (e.g., at 90 and 340 GHz, both with horizontal and vertical polarizations) show a high sensitivity to Artic clouds over an extended range of liquid water path (LWP). For RHUBC, during conditions when the precipitable water vapor (PWV) was less than 2 mm, these radiometers were supplemented by frequent Vaisala RS92 radiosonde observations at the ARM site. In this paper, representative PWV and LWP retrievals from the GSR and several ARM instruments are compared. Based on comparisons with 87 RS92 RAOB launches, GSR PWV retrievals achieved almost unprecedented accuracy of 0.1 mm, or about 6 % of the mean PWV during the operating period.
- SourceAvailable from: David D. Turner[show abstract] [hide abstract]
ABSTRACT: Ground-based two-channel microwave radiometers have been used for over 15 years by the Atmospheric Radiation Measurement (ARM) program to provide observations of downwelling emitted radiance from which precipitable water vapor (PWV) and liquid water path (LWP) – twp geophysical parameters critical for many areas of atmospheric research – are retrieved. An algorithm that utilizes two advanced retrieval techniques, a computationally expensive physical-iterative approach and an efficient statistical method, has been developed to retrieve these parameters. An important component of this Microwave Retrieval (MWRRET) algorithm is the determination of small (< 1K) offsets that are subtracted from the observed brightness temperatures before the retrievals are performed. Accounting for these offsets removes systematic biases from the observations and/or the model spectroscopy necessary for the retrieval, significantly reducing the systematic biases in the retrieved LWP. The MWRRET algorithm provides significantly more accurate retrievals than the original ARM statistical retrieval which uses monthly retrieval coefficients. By combining the two retrieval methods with the application of brightness temperature offsets to reduce the spurious LWP bias in clear skies, the MWRRET algorithm provides significantly better retrievals of PWV and LWP from the ARM 2-channel microwave radiometers compared to the original ARM product.IEEE Transactions on Geoscience and Remote Sensing 01/2007; 45:3680-3690. · 3.47 Impact Factor
- IEEE T. Geoscience and Remote Sensing. 01/2007; 45:2759-2777.
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ABSTRACT: We show that observed biases in retrievals of temperature and water vapor profiles from a 12-channel microwave radiometer arise from systematic differences between the observed and model-calculated brightness temperatures at five measurement frequencies between 22 and 30 GHz. Replacing the value for the air-broadened half-width of the 22-GHz water vapor line used in the Rosenkranz absorption model with the 5% smaller half-width from the HITRAN compilation largely eliminated the systematic differences in brightness temperatures. An a priori statistical retrieval based on the revised model demonstrated significant improvements in the accuracy and vertical resolution of the retrieved temperature and water vapor profiles. Additional improvements were demonstrated by combining the MWRP retrievals with those from the GOES-8 sounder and by incorporating brightness temperature measurements at off-zenith angles in the retrievals.IEEE Transactions on Geoscience and Remote Sensing 01/2005; 43:1102-1108. · 3.47 Impact Factor