Ensemble-based data assimilation for thermally forced circulations Academic Article uri icon

abstract

  • The effectiveness of the ensemble Kalman filter (EnKF) for thermally forced circulations is investigated with simulated observations. A twodimensional, nonlinear, hydrostatic, nonrotating, and incompressible sea breeze model is developed for this purpose with buoyancy and vorticity as the prognostic variables. Model resolution is 4 km horizontally and 50 m vertically. Forcing is maintained through an explicit heating function with additive stochastic noise. Pure forecast experiments reveal that the model exhibits moderate nonlinearity. The strongest nonlinearity occurs along the sea breeze front at the time of peak sea breeze phase. Considerable smallscale error growth occurs at this phase for vorticity, while buoyancy is dominated by largescale error as the direct result of the initial condition uncertainty. In the EnKF experiments, simulated buoyancy observations (with assumed error of 103 ms2) on land surface with 40km spacing are assimilated every 3 hours. As a result of their resolution, the observations naturally sample the largerscale flow structure. At the first analysis step, the filter is found to remove most of the largescale error resulting from the initial conditions and the domainaveraged error of buoyancy and vorticity is reduced by about 83% and 42%, respectively. Subsequent analyses continue to remove error at a progressively slower rate and the error ultimately stabilizes within about 24 hours for both variables. At later model times, while mostly largescale buoyancy errors due to the stochastic heating uncertainty are effectively removed, the filter also performs well at reducing smallerscale vorticity errors associated with the sea breeze front. This is an indication that observations also contain useful smallscale information relevant at the scales of frontal convergence.

published proceedings

  • JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES

author list (cited authors)

  • Aksoy, A., Zhang, F. Q., Nielsen-Gammon, J. W., & Epifanio, C. C.

citation count

  • 14

complete list of authors

  • Aksoy, A||Zhang, FQ||Nielsen-Gammon, JW||Epifanio, CC

publication date

  • August 2005