Two assimilation experiments making use of 1DVAR TCWV were run for a 15-day period. The "Rain-1'' experiment only assimilates 1DVAR retrievals where the observed rain rate is nonzero while the "Rain-2'' experiment assimilates all 1DVAR TCWV estimates. The period selected includes Hurricane Bonnie, which was well sampled by TRMM (late August 1998).

Results show a positive impact on the humidity analysis of assimilating 1DVAR TCWV in 4DVAR. The model rain rates at the analysis time are closer to the TRMM observations showing a posteriori the consistency of the two-step approach chosen to assimilate rain-rate information in 4DVAR. The modification of the humidity analysis induces changes in the wind and pressure analysis. In particular the analysis of the track of Hurricane Bonnie is noticeably improved for the early stage of the storm development for both the Rain-1 and Rain-2 experiments. When Bonnie is in a mature stage the influence of the 1DVAR TCWV assimilation is to intensify the hurricane. Comparison with Clouds and the Earth's Radiant Energy System (CERES) measurements also show a neutral impact on the radiative fluxes at the top-of-the atmosphere when using 1DVAR TCWV estimates.

The impact on the forecasts is a slight reduction of the model precipitation spindown over tropical oceans. Objective scores for the Tropics are improved, particularly for wind and for upper-tropospheric temperature.

Analysis and forecast results are generally better for the Rain-2 experiment compared to Rain-1, implying that the 1DVAR TCWV estimates retrieved where no rain is observed provide useful information to 4DVAR.

Consequently, such merging of data must take into account the intrinsic time dynamics of measured parameters. In this paper, the impact of atmospheric water vapour dynamics on the merging of satellite-based retrieval of precipitable water estimates is investigated by comparing independent measurements obtained at different spatial resolutions from the High Resolution Infrared Sounder (HIRS) and the Advanced Very High Resolution Radiometer (AVHRR). Correlations are used to infer optimal merging parameters depending on the observational conditions. The authors conclude that the merging technique reproduces HIRS-based retrievals in cloud-free and partly cloudy locations from AVHRR soundings.

Verification of the retrieved products in particular the liquid water path is a great problem, a few ideas will be discussed. Results will be shown for the total precipitable water W and liquid water path LWP over the Atlantic Ocean for different time scales. The structure of the W field is very similar for the same month in different years. But LWP is very variable, even for monthly means (October 1987 and 1989) the differences can be larger than 0.1 kg/m(2).

Therefore, a new method is developed which allows the detection of the atmospheric total water vapour content from aircraft or satellite with the aid of backscattered solar radiation in the near infrared above land surfaces.

The Matrix-Operator-Method has been used to simulate backscattered solar radiances, including various atmospheric profiles of temperature, pressure, water vapour, and aerosols of various types, several sun zenith angles, and different types of land surfaces. From these calculations it can be concluded, that the detection of water vapour content in cloudless atmospheres is possible with an error of < 10 % even for higher aerosol contents.

In addition to the theoretical results first comparisons with aircraft measurements of the backscattered solar radiances are shown. These measurements have been carried out with the aid of OVID (Optical Visible and near Infrared Detector), a new multichannel array spectrometer, in 1993.

Both liquid and ice clouds impact the T-B's in a variety of ways. The T-B's at 23.8 and 89 GHz are more strongly affected by altostratus liquid clouds than by cirms clouds for equivalent water paths. In contrast, channels near 157 and 183 GHz are more strongly affected by ice clouds. Higher clouds have a greater impact on 157- and 183-GHz T-B's than do lower clouds. Clouds depress T-B's of the higher-frequency channels by suppressing, but not necessarily obscuring, radiance contributions from below. Thus, T-B's are less closely associated with cloud-top temperatures than are IR radiometric temperatures. Water vapor alone accounts for up to 89% of the total attenuation by a midtropospheric liquid cloud for channels near 183 GHz. The Rayleigh approximation is found to be adequate for typical droplet size distributions; however, Mie scattering effects from liquid droplets become important for droplet size distribution functions with modal radii greater than 20 mu m near 157 and 183 GHz, and greater than 30-40 mu m at 89 GHz. This is due mainly to the relatively small concentrations of droplets much larger than the mode radius. Orographic clouds and tropical cumuli have been observed to contain droplet size distributions with mode radii in the 30-40-mu m range. Thus, as new instruments bridge the gap between microwave and infrared to frequencies even higher than 183 GHz, radiative transfer modelers are cautioned to explicitly address scattering characteristics of such clouds.