Data Applications

VALIDATION OF THE NORTH AMERICAN LAND DATA ASSIMILATION SYSTEM (NLDAS) USING DATA FROM OKLAHOMA MESONET OASIS SITES

A team of federal agencies and universities which includes the NCEP Environmental Modeling Center, NASA Goddard Space Flight Center (GSFC), NWS/OHD, NESDIS/ORA, Princeton University, Rutgers University, the University of Washington, University of Maryland, and the University of Oklahoma is developing a Land Data Assimilation System (LDAS) for use at both North American (NLDAS) and global scales (GLDAS). The NLDAS infrastructure consists of uncoupled land surface models forced with precipitation observations, output from the Eta model data assimilation system (EDAS), solar radiation from the GOES satellites, and radar precipitation estimates. It is being developed to initialize and improve the simulation of land surface states and energy fluxes in coupled and uncoupled land model components of numerical forecast models. The NLDAS system is expected to reduce forecast errors by providing better initial condition for the land surface component in NWP and climate models.

Retrospective simulation data sets for the Mosaic and Noah land surface models (LSMs) were provided in collaboration with NLDAS Group for the 1999-2000 period. The model simulated energy fluxes were compared with the OASIS surface flux data set, which includes net radiation, downwelling shortwave radiation, upwelling shortwave radiation, downwelling longwave radiation, upwelling longwave radiation, latent heat flux, sensible heat flux, ground heat flux, and skin temperature for the following days: 23 May 2000, 29 May 2000, 30 May 2000, 10 July 2000, 13 August 2000, 14 August 2000, 25 August 2000, 29 August 2000. The time-averaged hourly model output is being compared to the hourly averaged flux observations at each OASIS Super Site.

Mosaic vs. OASIS:

The components of the surface energy balance for Mosaic and all OASIS Super Sites are plotted in Figure 1. Overall, the net radiation results indicate good agreement between the Mosaic model and the OASIS observations. The latent heat flux comparison reveals differences between the Mosaic model and the observations, with Mosaic producing larger flux values than those measured. Sensible heat fluxes compared less favorably. Significant differences were found between the Mosaic model and observed values. Finally, the ground heat flux results indicate a strong bias. The Mosaic land surface model consistently overestimated ground heat flux compared to the observed values.

Noah vs. OASIS

The results of the comparison between Noah and OASIS fluxes are shown in Figure 2. Overall, the preliminary analyses show good agreement between the observed and modeled values of net radiation and are consistent throughout the data set for each Super Site. In terms of a regional approach, these preliminary results show that net radiation is captured very well, sensible heat flux is overestimated, latent heat flux is underestimated, and ground heat flux was slightly overestimated.  

The preliminary validation of the Noah and Mosaic modeled surface energy fluxes using OASIS data provides a blueprint for future work. Further investigations of additional candidate days and closer inspection of individual sites will allow for a more complete validation effort. In turn, model performance will be improved.