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CONTENTSREPORT ON THE 11TH CEOS PLENARY
News From the Working Group on Information Systems and Services
CEOS Working Group on Calibration and Validation
IGOS Strategic Implementation Team - Progress Report
The Global Ocean Data Assimilation Experiment
The Upper Air Measurements ---This page
Long-Term Continuity of Ozone Measurements
Ocean Biology
Global Observation of Forest Cover
Disaster Management Support
The Global Terrestrial Observing System
ISRO takes over CEOS Chair for 1998
The Upper Air Measurements
Paul Menzel NOAAReduction of radiosonde and omega sonde observations of the upper atmosphere has raised concerns about possible degradation in weather forecasting. NOAA, in cooperation with the WMO and under the auspices of the Integrated Global Observing Strategy (IGOS), is leading an Upper Air (UA) Measurements Project to assess the impact of the sonde reduction on numerical weather prediction (NWP) models, to explore near term mitigation with satellite data, to evaluate satellite capabilities for satisfying NWP user requirements, and to plan for future space-based upper air measurements. In 1997 the IGOS UA Project has enlisted global modelers to conduct sonde denial studies and initiated enhanced use of satellite remote sensing data.
- Initial studies have revealed that loss of radiosonde observations
(raobs) will have a negative impact on global forecasts. Fortunately, raob
reduction has not been as widespread as had been feared. However, satellite
sounding data have not been utilized over land where the raobs are being
removed. A preliminary study using sounder total precipitable water vapor
and cloud heights over land reduced the negative impact of raob denial
appreciably. The UA Project thus recommended that the global weather prediction
centers collaborate with satellite data providers to further assess the
impact of removing designated ground based upper air observations as well
as including satellite observations over land. It was suggested that for
a test period of ten days, wind estimates from all operational geostationary
satellites would be assimilated at high density, four times daily, in both
the control and the trial runs. Both runs would also include the direct
assimilation of polar orbiter sounder radiances (or retrieved temperature
and moisture profiles) over land as well as ocean. Scatterometer and aircraft
winds would also be used in control and trial runs. The control would include
all available ground based observations and the trial would remove the
observations from the designated sites. The Strategic Implementation Team
agreed to study action on this recommendation.
Evaluation of satellite observing capabilities versus NWP user requirements were guided by reports from the International Winds and TOVS working groups. Satellite derived estimates of upper tropospheric motion (both cloud and water vapor motions) are generally produced every six hours at 100 km resolution (mostly in cloudy regions) with root mean square (RMS) vector differences with respect to radiosondes between 3 m/s for low and 7.5 m/s for high levels; the speed biases at all levels are less than 1.0 m/s. Wind vector production every three hours or better is envisaged in the near future. However, the stated NWP optimum requirements for hourly winds every 10 km with an accuracy of 1 m/s is far from being met by the existing remote sensing system. It is expected that the advent of satellite borne lidars (both backscatter and Doppler) will greatly improve the accuracy and coverage of global motion estimates. Satellite derived temperature profiles are produced twice daily every 100 kilometers in clear skies and compare within 2.0 to 2.5 K RMS of radiosonde temperature profiles; relative humidity profiles are within 20%. Significant progress has recently been realized from direct assimilation of the TOVS radiances over oceans in the global numerical weather prediction (NWP) models. In addition, more frequent soundings from geostationary orbit are expected to enhance NWP performance through 4-d variational assimilation of the hourly radiance measurements. However, the stated optimum NWP requirements are hourly soundings every 10 to 50 km with 1.0 K RMS temperature and 10% relative humidity with 1 km vertical resolution. Improvements in sounding are expected from high spectral resolution infrared sounders for better vertical resolution, advanced microwave humidity sounders for all weather soundings, and occultation sensors for tropopause and upper atmospheric definition. Preliminary findings of this evaluation (capabilities versus requirements) include: (a) the present suite of satellite observations do not meet median or optimum NWP requirements, with wind profiles needing the most help; (b) optimum global NWP requirements suggest the need for a global geostationary sounding capability; and (c) full capabilities of current satellite remote sensing have not been realized suggesting that more resources need to be focused on better usage.
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