April 2011 Precipitation Extremes
During last April, record or near-record precipitation occurred across a large section of the United States (from the Midwest through the Ohio Valley). This rain, and likely snow melt from the Northern Great Plains, are contributing to the current prolonged and severe flooding along the Mississippi River and its delta. In addition, a prolonged drought of varying degrees persists across the Gulf Coast states.
MERRA precipitation (color shaded) with CPC gauge observations (black contour) time averaged for April 2011. (units: mm/day)This week, MERRA data for April 2011 was released at the MDISC, roughly two weeks behind real time. Preliminary comparison with the CPC gauge data shows that MERRA precipitation is generally weaker than observed especially in southern Missouri, though the maximum in Pennsylvania is an overestimate. Because the reanalysis system is strongly constrained by observations, the weather systems that produce the rain, and hence the occurrence of rain events, are faithfully reproduced. The physical process of producing the precipitating water then leads to the error in the data product (assuming that the rain gauges capture the extent of the precipitating mass). It is worthwhile to note that MERRA does not assimilate precipitation observations over land, as in NARR.
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A user did a precipitation analysis of the GPCP data, which we posted on our Giovanni User Images page:
http://disc.sci.gsfc.nasa.gov/giovanni/giovanni_user_images#april_2011_precip
What exactly does it mean that MERRA doesn't assimilate precipitation?
Data assimilation is the process of integrating the observations with the background model to produce analyzed fields for the initialization of the next model forecast. The GEOS5 data assimilation system, used to produce MERRA, assimilates data like radiosnde temperature and wind, as well as satellite radiances among a lot of other sources of data. However, observed precipitation over land is not used in the data assimilation. Some reanalysis systems, such as NARR, do assimilate precipitation. In this way, the precipitation comparison here is more of a validation of the models physics, than a result of the data assimilation.
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