Katrina Quick Look

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.

MERRA, MAC and LandFlux

In a recent paper, Bosilovich et al. (2009) evaluate 8 operational analyses to better assess the uncertainty of the physical fields (not the assimilated states) derived form (re)analysis systems. During the first phase of the Coordinated Enhanced Observing Period (CEOP) 8 international meteorological analyses were collected for a common period, along with supporting satellite and in situ observations. The model data is called the Multi-model Analysis for CEOP (MAC). In general, it was found that the global precipitation and outgoing long wave radiation derived from an ensemble of analyses provided fields that more closely resemble available observations than any one of the members.

For certain physical quantities, such as surface evaporation, a reliable method of observation is not available, and international projects such as SeaFlux and LandFlux are being developed to best fill the gap. In some research, data from analyses and reanalyses are taken as a substituted for observations. The figure below shows the MAC ensemble members compared with the ensemble mean. In places, there is as much as +/- 75 W/m^2 differences among systems. Comparison with such a data set can clearly identify outlying systems. Also, for certain research, using the physical fields from a single system may not be adequate.

Caption: MAC version 2 systems Latent Heat Flux differenced from the ensemble mean for July 2004. Version 2 includes the MERRA and ECMWF ERA Interim reanalyses in addition to the version 1 data. Mean and standard deviation of the difference fields are provided in each panels title.

More information on MAC and the data download are available.

Bosilovich, M.G., D. Mocko, J.O. Roads, and A. Ruane, 2009: A Multimodel Analysis for the Coordinated Enhanced Observing Period (CEOP). J. Hydrometeor., 10, 912–934.

Status and some recent analysis

Data at the download site are now continuous from 1979 through December 2006. To date, the data volume is approaching 70Tb, and in September alone, 5.7 million files totaling 91Tb of data were served.

Production continues toward real time, and June 2007 is halfway complete. The throughput is approximately 1 year every 6 weeks, so the production should catch up to real time in early 2010.

An overview of MERRA, including some recent results were presented at Purdue University Department of Earth and Atmospheric Sciences. This includes some evaluation of the global water and energy clycles and processes, however, the analysis continues and a peer review manuscript is in preparation. So, use these results accordingly.

AMS Annual Meeting, Jan 2010, Atlanta

The absttract deadline for the 24th Conference on Hydrology has been extended to Aug 10. Of note, abstracts regarding "Hydrometeorological representation and applications of reanalyses" are sought. The full list of conferences, themes and topics can be found at the AMS WWW page.

Further Production Update

Stream 2 has past the end of Dec 1997 and stream 3 has past the end of Dec 2005, so that we now have a time series from Jan 1979 through Dec 2005. The 1997 and 2005 data are still being assessed for quality, and should be posted at the MDISC site by then of the month or no later than early August.

Production update

Stream 1 has completed Jan 1979 through Dec 1988, catching up to the beginning of Stream 2. The reamining Stream 1 data will be released to the the download site within a week or so. Further, we are conituing Stream 1 for two additional years in order to study the initialization of Stream 2, and evaluate the transition of the data between the two streams.

Stream 2 should catch up to the begining of Stream 3 by the end of July. We we conduct a similar evatuation of the transtion between streams there as well.

Stream 3 production is on hold, pending a fix to allow for a format change in some of hte input data. It will resume ASAP.

Land Interaction Processes

MERRA Production has been moving along steadily, still on track for a continuous time teries (1979-2006) to be available in early August. Data (missing 1988, 1997, 2005) is presently available for download, see the MERRA home page for access information.

With much of the record available, comparisons to previous studies of reanalyses are possible. For example, there has been a lot of work on the land interaction processes in NCEP and ERA40. Betts and Viterbo (2005) has defined cloud albedo as an observable diagnostic of the all-sky radiative forcing of the surface (Acld = -{SWDNsfc-SWDNsfcclr}/SWDNsfcclr). Below is an example of the cloud albedo comparison for ISCCP and ERA 40 over the Madeira basin, followed by a similar figure for MERRA. ERA40 tends to overestimate the cloud albedo, while MERRA underestimates it. This indicates that for the Madeira, the shortwave at the surface is

too close to the clear sky values. This bias is especially pronounced during the austral winter and less so in summer.

In evaluating the coupling processes, the links between the surface, surface layer and boundary layer relate to precipitation. Below, Betts (2009) shows the relationships of boundary layer (through LCL height in pressure thickness), surface layer (EF, evaporative fraction) and soil wetness (SMI). While lower wetness is often and indicator of higher LCL and lower precipitation, there is a range in the characteristic values. EF results are similar. The LCL - wetness connection in MERRA appears stronger and more linear than that of ERA40, where the MERA LCL height is almost double that determined from ERA40. (Note that the soil moisture index in Betts 2009 is not the same as the MERRA surface soild wetness, the SMI cannot be easily reproduced in MERRA data). The MERRA relationship between LCL and EF is much tighter than that shown for ERA40.

In the figures above, the colors indicate the amount of daily preciitation that occurs relative to the LCL and Wetness/EF values. For the MERRA figures, the dots are each daily mean showing the range in the data, and for ERA40, the range it demonstrated by the error bars.

Betts, A. K., 2009: Land surface coupling in observations and models. J. Adv. Model. Earth Sys. Articles in Press.

Betts, A. K. and P. Viterbo, 2005: Land-surface, boundary layer, and cloud-field coupling over the southwestern Amazon in ERA-40, J. Geophys. Res., 110, D14108, doi:10.1029/2004JD005702