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...Fig. 5), as compared to the instantaneously calibrated observations. In addition to using the nominal quasi-statistical retrieval of PWV based upon the Liebe87 model as described in section 2c, a physical retrieval was also developed. The radiosonde profile was used to specify the atmospheric structure, and the mixing ratio profile was scaled in an iterative manner until the brightness temperature at 23.8 GHz calculated by the absorption model agreed with the observed brightness temperature to within 0.1%. Two contemporary absorption models were used in this process: Liebe87 and Rosenkranz98 (Rosenkranz 1998). A final set of MWR data was also computed. Unscaled radiosonde profiles were input into the absorption models to compute the 23.8-GHz brightness temperature, and the observed minus computed brightness temperature residuals were used to determine a zero PWV bias. This bias was approximately 0.6 K for the Rosenkranz98 model. The cause of this small bias is currently under investigation and is attributed to a combination of small offset calibration errors in the MWR and uncertainties in the dry component of the microwave absorption models, such as oxygen absorption. Without removal of this bias...

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...eval accuracy can be reduced by properly choosing the channels. The optimal channel combination is obtained by performing regression analysis on the simulated AMSU measurements. Of all the AMSU channels, the 23.8 and 31.4 GHz channels generally perform the best. As discussed next, the coefficients in (5a) and (5b) are obtained by performing regression analysis on the simulated AMSU-A2 channel measurements with Ts set to 285 K. Simulated brightness temperatures at 23.8 and 31.4 GHz were obtained using the radiative transfer equation with the latest absorption models for oxygen and water vapor [Rosenkranz, 1998]. Brightness temperatures are calculated using radiosonde profiles of temperature and water vapor from the midlatitudes to the tropics so that the TPW varied from 5 to 70 mm. For each profile, nonprecipitating clouds were introduced at low, middle, and high levels, and the corresponding CLW varied from 0 to a maximum of 0.5 mm for warm clouds. Also, for each data set the sea surface emissivity was varied using winds at 0, 5, 10, and 15 m/s. To cover the range of AMSU-A scan positions, each set of simulations were performed at seven local zenith angles, from 08 to 608 with 108 steps. At each a...

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...) This version of MPM, has 34 water vapour lines between 20–1000 GHz, defined in a slightly different manner from MPM89. The 183 GHz line is 8.5 % wider and 5 % stronger than in MPM89. The water vapour continuum absorption is formulated as a “pseudo-line” near 2 THz, and has a different temperature dependence, based on newer observations. Like its predecessors, MPM93 includes 44 oxygen lines with the same line strengths, but 5 % greater widths and 15 % stronger mixing than MPM89. The non-resonant nitrogen absorption is essentially the same as MPM89 at the frequencies in this study. 2.4 Ros98 (ROSENKRANZ, 1998) Ros98 uses 15 water vapour line parameters, which are very similar to the strongest lines used in MPM89. The other half of the lines have been omitted as they were judged to have negligible impact. ROSENKRANZ’s investigations suggested a range of observations could be best modelled by using a water vapour continuum with a combination of MPM87’s foreign-broadened component, and MPM93’s self-broadened component. However, the water vapour lines used were truncated at ±750 GHz, so the foreign- and self- broadened parts of the water vapour continuum were increased 15 % and 3 %, respectively to com...

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...6.s24-hour time series of GSR and MWR Tbs.sNote the 1 K drop at 23.8 GHz arounds91.5 UTC; the corresponding gradient shown by submillimeter-wave channels is of the order of 30 K.sSimulations based on =-=Rosenkranz 1998-=- model are indicated with black stars.s11sFifteenth ARM Science Team Meeting Proceedings, Daytona Beach, Florida, March 14-18, 2005sFigure 7.sComparison between simulated and measured Tb during elevat...

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...e discretization and model approximations (see Section IV). The spectroscopic component was estimated as the covariance of the difference in zenith Tb, which was calculated using two absorption codes =-=[7]-=-, [8]. The other terms were calculated as the covariance of the difference between Tb, which was calculated using the full line-by-line model at high vertical resolution and the approximations. F cont...

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...ARCHAND ET AL.: MICROWAVE ABSORPTION AND CLOUD WATER the radiometers and has been corrected for in the SGP data sets. Barnard and Powell were able to calculate the downwelling direct and diffuse shortwave fluxes to better than 9 W/m2 on average, lending additional credence to our claim that the cases examined here are clear of clouds. 5. Results [27] In this section we examine in detail the results for one of the case study days. After this we present tables that summarizes the results for all 10 cases. As described in section 3, we use three different microwave absorption models: Liebe 1987, Rosenkranz 1998, and MonoRTM. We also examine two configurations, one in which the atmospheric column is not permitted to have any liquid water, we will refer to this as the clear-sky solution, and one in which it may have liquid water, but only in the region between 1 and 2 km above ground level. [28] In the clear-sky or no-liquid-water solutions, the iterative solver is required to find the total column precipitable water vapor which minimizes the difference between the measured and a forward model calculation of the 23.8 GHz brightness temperature. In principle, the iterative solver can make this differen...