Abstract. We have developed a tunable temperature profiler based on a highly stable synthesizer that can observe at multiple frequencies on the shoulder of the 60-GHz atmospheric oxygen feature. We are developing a similar radiometer to obtain the vertical distribution of water vapor by making observations on the pressure-broadened water vapor line from 22 to 29 GHz. Information on cloud liquid water profiles is also contained in these two wave bands. Various mathematical retrieval methods for temperature, water vapor, and cloud liquid water profiles were tested based on these radiometer designs. These include neural networking, Newtonian iteration of statistically retrieved profiles, and Bayesian "most probable" retrievals. On the basis of realistic radiometer errors and performance, very good retrieval capability is demonstrated. The performance of the various retrieval methods are presented and compared. Examples of profile retrievals are also presented. Data were not binned into seasons to reduce computer time; better retrieval results for all methods would be expected with binning. Copyright 1998 by the American Geophysical UnionPaper number 97RS03656.0048-6604/98/97RS-03656 $11.00 393 ments. They also have the capability to profile cloud liquid water, a capability absent in RAOBs and all other systems except for in situ aircraft devices. Applications for this passive radiometric profiling include weather forecasting and nowcasting; detection of aircraft icing and other aviation-related meteorological hazards; determination of density profiles for artillery trajectory and sound propagation; refractivity profiles for radio ducting prediction; corrections to radio astronomy; satellite positioning and Global Positioning System (GPS) measurements; atmospheric radiation flux studies; estimation and prediction of telecommunication link degradation; and measurement of water vapor densities as they affect hygroscopic aerosols and smokes. Microwave Profiling MethodsThe microwave profiling methods discussed here make use of atmospheric radiation measurements in the 20-to 75-GHz region. The zenith path atmospheric absorption spectrum at sea level for a typical midlatitude atmosphere with a 1-km-thick, 0.5 g/m 3 cloud in this frequency band is shown in Figure 1. The feature at 22.2 GHz is a water vapor resonance that is pressure broadened according to the pressure altitude

Abstract. We investigated a variety of factors that influence the determination of precipitable water vapor (V) and integrated cloud liquid (L) by dual-channel microwave radiometers (MWRs). These factors include radiometric calibration; dry, water vapor, and liquid absorption coefficients; and physical versus statistical retrieval methods. We then applied the analysis to the MWR that was operated by the Atmospheric Radiation Measurement Program (ARM) during the Surface Heat Budget of the Arctic Ocean project. In the work reported here, MWR data taken from April 1 to July 31, 1998, were analyzed. Data acquired in situ did not always agree with the original MWR liquid retrievals, with MWR estimates at times being too large by perhaps a factor of 2. These differences led us to examine in detail several of the assumptions that go into V and L retrievals. The radiometer was carefully examined and found to be well calibrated with a 0.3 K RMS error. The predicted accuracy in the L retrievals, for this 0.3 K RMS radiometric error and a statistical retrieval, was 25 g m -2 RMS. This accuracy improves to 10%, if we use the improved knowledge of cloud temperature, as can be obtained using radiosondes and cloud radar/lidar measurements. We also studied the degree to which different clear air and cloud liquid models have an effect on V and L retrievals. The most significant changes from the original ARM retrievals were due to the dry opacity and the cloud liquid dielectric model. Although nothing was found in the original ARM data that was grossly incorrect, application of these models reduced the original ARM retrievals by roughly 20 to 30%. The change of clear-air absorption model from the original to a more recent one has little impact on V retrievals except when V is <0.5 cm. with MWR estimates at times being too large by perhaps a factor of 2. These differences led us to examine in detail several of the assumptions that go into V and integrated cloud liquid (L) retrievals. As a preliminary to our analysis, we first outline the steps taken in moisture retrieval. We then examine several contemporary absorption algorithms for both clear and cloudy skies, and then evaluate differences in moisture retrieval using these algorithms. Finally, we apply these algorithms to the SHEBA MWR data and compare both the original and revised retrievals with each other and with aircraft in situ measurements. Retrieval MethodsThe MWR that was operated during SHEBA was one of the operational units from the Atmospheric Radiation Measure-32,019

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[1] The multifilter rotating shadowband radiometer (MFRSR) measures direct and diffuse irradiances in the visible and near-infrared spectral range. In addition to characteristics of atmospheric aerosols, MFRSR data also allow retrieval of precipitable water vapor (PWV) column amounts, which are determined from the direct normal irradiances in the 940-nm spectral channel. The HITRAN 2004 spectral database was used in our retrievals to model the water vapor absorption. We present a detailed error analysis describing the influence of uncertainties in instrument calibration and spectral response, as well as those in available spectral databases, on the retrieval results. The results of our PWV retrievals from the Southern Great Plains (SGP) site operated by the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program were compared with correlative standard measurements by microwave radiometers (MWRs) and a global positioning system (GPS) water vapor sensor, as well as with retrievals from other solar radiometers (AERONET's CIMEL, AATS-6). Some of these data are routinely available at the SGP's Central Facility; however, we also used measurements from a wider array of instrumentations deployed at this site during the water vapor intensive observation period (WVIOP2000) in September-October 2000. The WVIOP data show better agreement between different solar radiometers or between different microwave radiometers (both groups showing relative biases within 4%) than between these two groups of instruments, with MWR values being consistently higher (up to 14%) than those from solar instruments (especially in the large PWV column amount range). We also demonstrate the feasibility of using MFRSR network data for creation of 2D data sets comparable with that of the MODIS satellite water vapor product.

A dual frequency ground‐based radiometer operating in the 1 to 1.4 cm wavelength range can provide continuous measurements of integrated water vapor and cloud liquid water. Using climatological data, the accuracy of the vapor and liquid determinations is estimated as a function of cloud amount. Limiting factors in the water determination are uncertainties in water vapor absorption coefficients and, during cloudy conditions, uncertainties in cloud temperature. For integrated water vapor content greater than 10 mm, the accuracy of the vapor determination is better than 15% for a wide range of cloudy conditions.

Ground-based microwave radiometer profilers in the 20-60-GHz range operate continuously at numerous sites in different climate regions. Recent work suggests that a 1-D variational (1-DVAR) technique, coupling radiometric observations with outputs from a numerical weather prediction model, may outperform traditional retrieval methods for temperature and humidity profiling. The 1-DVAR technique is applied here to observations from a commercially available microwave radiometer deployed at Whistler, British Columbia, which was operated by Environment Canada to support nowcasting and short-term weather forecast-ing during the Vancouver 2010 Winter Olympic and Paralympic Winter Games. The analysis period included rain, sleet, and snow events (∼235-mm total accumulation and rates up to 18 mm/h). The 1-DVAR method is applied "quasi-operationally," i.e., as it could have been applied in real time, as no data were culled. The 1-DVAR-achieved accuracy has been evaluated by using simultaneous radiosonde and ceilometer observations as reference. For atmospheric profiling from the surface to 10 km, we obtain retrieval errors within 1.5 K for temperature and 0.5 g/m 3 for water vapor density. The retrieval accuracy for column-integrated water vapor is 0.8 kg/m 2 , with small bias (−0.1 kg/m 2 ) and excellent correlation (0.96). The retrieval of cloud properties shows a high probability of detection of cloud/no cloud (0.8/0.9, respectively), low false-alarm ratio (0.1), and cloud-base height estimate error within ∼0.60 km.

[1] Passive microwave radiometers have a long history in the remote sensing of atmospheric liquid and water vapor. Retrievals of these quantities are sensitive to variations in pressure and temperature of the liquid and water vapor. Rather than use a statistical or climatological approach to account for the natural variability in atmospheric pressure and temperature, additional information on the atmospheric profile at the time of the radiometer measurements can be directly incorporated into the retrieval process. Such an approach has been referred to in the literature as a ''physical-iterative'' solution. This paper presents an assessment of the accuracy of the column liquid water path that can be expected using such an iterative technique as a result of uncertainties in the microwave emissions from oxygen and water vapor. It is shown that the retrieval accuracy is influenced by the accuracy of the instrument measurements and the quality of the atmospheric profiles of temperature and pressure, as one would expect. However, also critical is the uncertainty in the absorption coefficients used in the underlying microwave radiative transfer model. The uncertainty in the absorption coefficients is particularly problematic in that it may well bias the liquid water retrieval. The differences between three absorption models examined in this paper are equivalent to a bias of 15 to 30 g/m 2 , depending on the total column water vapor. An examination of typical liquid water paths from the Southern Great Plains region of the United States shows that errors of this magnitude have significant implications for shortwave radiation and retrievals of cloud effective particle size. Citation: Marchand, R., T. Ackerman, E. R. Westwater, S. A. Clough, K. Cady-Pereira, and J. C. Liljegren, An assessment of microwave absorption models and retrievals of cloud liquid water using clear-sky data,

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