In looking at some land hydrology in the southern US, a question came up on the effect of the 2005 hurricane season on the hydrology time series. So, we started looking around at the evolution of Katrina. This animation shows the MERRA version of Katrina (13Mb gif) moving over the southern tip of Florida and through the Gulf of Mexico. The colors show precipitation in mm/day and the white contours are sea level pressure (contour interval 2mb). The Best Track location is plotted every six hours. Firstly, the MERRA closed low pressure follows the best track fairly well throughout the evolution. This is notable only in that while MERRA does assimilate observations every six hours, there is no relocating or bogusing routines involved with the analysis/forecast cycles.
The MERRA resolution (1/2 degree) is not fine enough to get at the mesoscale structures in hurricanes, and we see that in MERRA where the surface winds (not shown) only reach Category 2 on the Saffir-Simpson scale (observations and estimates of Category 5 occurred during Katrina). Likewise, the rainbands at a distance from the central low are not well defined. The animation shows a curious shift of the main rain fall from the southern quadrant to the north, as landfall occurs (see also the figure below). In trying to validate this, we found radar data at NCDC, presented is in the second plot below, which agrees with the MERRA distribution. However, and also likely related to the resolvable scales in the MERRA grid, the heaviest precipitation in MERRA is a larger distance away from the central low than observed.
While this seems like it is a reasonable representation of the real system, and likely useful, users must consider carefully the limitations in MERRA or any reanalysis data set when applying it to a project.
Figure 1. MERRA precipitation (color, mm/day) and sea level pressure (mb) at 12Z29AUG2005 with the complete NHC best track path for Hurricane Katrina.
Figure 2 Nexrad radar rainfall at 12:32Z29AUG2005.
