Targeting Optimal Use
TOUGH | WP6000 | ...
Knowledge of the atmospheric distribution of water vapour is of key importance in weather prediction and climate research. It is tightly coupled to processes like energy transfer, precipitation, and is an important greenhouse gas. However, currently there is lack of knowledge about the actual humidity field, both due to a shortage of observations and a sub-optimal handling of humidity in the data assimilation systems, which are used to make estimates of the actual atmospheric field. Such fields are used to start numerical weather prediction and for climate monitoring. Global Positioning System (GPS) signals are particularly sensitive to water vapour. The TOUGH project is to develop and refine methods enabling the optimal use of GPS data from existing European GPS stations in numerical weather prediction models and to assess the impact of such data upon the skill of weather forecasts.
The scientific and technical work of the project has been divided into 7 basic workpackages:
The main focus of KNMI and TUD in TOUGH, together with our partners at FMI and DMI, will be on the retrieval of slant delays and the development of methods for assimilation of slant delays.
Participants: KNMI (WP leader, 17), TUD (5), FMI (9), DMI (3)
Instead of deriving zenith quantities, GPS signal delay and integrated water vapour can also be measured along slant paths from ground-based receivers to GPS satellites. By using not only the zenith delay of a receiver but also the slant delays the number of observations will increase by roughly a factor ten. By applying variational algorithms a three-dimensional water vapour field can be retrieved from slant observations, at least from a dense network of receivers. Furthermore, the horizontal resolution of the retrieved water vapour field will also profit from this larger amount of observations.
Participants: TUD (WP leader, 5), KNMI (3)
The derivation of zenith and slant GPS delays from GPS observations involves several assumptions about the atmospheric structure. In particular, assumptions about atmospheric homogeneity and receiver multipath when observing satellites are at low elevation angles (close to the horizon) influence the results. The multipath must be carefully modelled as a function of receiver environment while the atmospheric model used for the mapping must be carefully chosen in cases of atmospheric inhomogeneities. Even when estimating only slant delays, mapping functions are still needed in order to separate receiver clock errors from atmospheric delays. Traditionally, mapping functions are empirical functions derived from multi-year averages of radiosonde data. A new approach is to derive the mapping function directly from NWP model output. This could result in a significant improvement of IWV measurements for low elevations. Pre-processing of raw slant delays before assimilation will be investigated, using additional input from NWP analysis. This will help to discriminate site dependent effects (multipath, antenna phase center variations) and receiver clock errors from atmospheric delays. It can also be used to derive intermediate quantities such as ZTD, horizontal gradients, scale height and or timing information, which could be used as an alternative to assimilating slant delays. Currently used software will be modified, if necessary, and additional modules to estimate slant delays and model multipath will be developed. Furthermore, mapping procedures based on forecast model input will be developed and tested by one weather service, KNMI, and a geodetic institute, TUD.
Participant: KNMI (6)
In order to obtain realistic results the error biases and correlations of the GPS slant measurements must be modelled. Observations for a network of ground-based receivers will be simulated from a 3-D water vapour field and used for assimilation trials. The goal of these simulations is to test our software and to estimate the capability of a network of GPS receivers to reconstruct refractivity field inhomogeneities at different scales. In addition we need to determine an optimal discretisation and interpolation scheme of the refractivity field to be used for the processing of observational data. The retrieved fields will be validated against water vapour radiometer measurements during the CLIWANET campaign.
Participants: KNMI (WP leader, 4), FMI (9)
The natural first step towards using slant-delay measurements in NWP assimilation is to properly evaluate them against the model counterparts. For this task an appropriate observation operator is needed. The zenith delay observation operator is simple to develop, as the observation geometry is relatively straightforward and similar to the NWP model geometry. The slant-delay observation operator, in contrast, requires a model profile along a slanted path with unknown intersections with the model levels. Once the problem of interpolating the model variables on a slanted path is solved, the associated delay calculation problem can be fairly easily solved.
Participants: KNMI (WP leader, 4), DMI (3)
A demonstration version of a GPS slant delay observation operator will be developed by FMI and KNMI in co-operation, and this observation operator will be adapted to the HIRLAM three-dimensional variational data assimilation system. The operational NWP model of KNMI will be used for impact studies with a resolution of at least 10 km x 10 km. The performance of the assimilation of these slant delays will be investigated by conducting observation system simulation experiments (OSSE). Impact studies will be performed with the analysed water vapour fields, obtained from the GPS data of a dense GPS network (Observation System Experiment, OSE). DMI will perform assimilation tests using the software developed by KNMI and FMI.
The following institutes participate in TOUGH WP6000:
TOUGH is a shared-cost project (contract EVG1-CT-2002-00080) co-funded by the Research DG of the European Commission within the RTD activities of the Environment and Sustainable Development sub-programme (5th Framework Programme)
The official web site for TOUGH is tough.dmi.dk. For project members there is restricted access to restricted web server and restricted ftp server.