Laura Slivinski develops reanalysis at NOAA

"Reanalysis" is a method through which observations are combined with numerical model output over a long period of time (generally, on the order of decades) via data assimilation, to create a historical record of weather fields covering the entire globe. These records can then be used to investigate how weather and climate are changing over time.  A research group headed by NOAA and CIRES scientists has created the 20th Century Reanalysis (20CR; Compo et al 2011), which assimilates only surface pressure data and therefore can extend back 150 years before the present.  The data assimilation method utilized is the ensemble Kalman filter, and therefore not only provides estimates of 6-hourly weather around the globe, but also provides uncertainty estimates in the form of ensemble spreads.  I am assisting in the development of the newest version of this reanalysis, which uses a higher-resolution model and an improved covariance inflation algorithm, as well as observations extending back almost 200 years.  Among other things, researchers will be able to use the output of this reanalysis to investigate specific weather events, to look at weather and climate trends, and to generate probability distributions from samples of weather taken from this long time series.

Below are some weather maps from the reanalysis project I described. The top row is from the year 1922 when there were relatively few observations (shown by blue dots), leading to relatively high uncertainty (darker colors).  Conversely, the bottom row is from 1972, when (as shown by the blue dots) there were many more observations, and thus much lower uncertainty (much lighter shading.)  Overall, this figure demonstrates how assimilating more observations leads to higher certainty in the reanalysis estimates.

Figure caption: Synoptic charts of the ensemble mean analysis and ensemble uncertainty for (a) sea-level pressure and (b) 500 hPa geopotential mean and spread at 0000 UTC on 29 January 1922. (c, d) are as (a, b), but for 29 January 1972. Line contours indicate the analysis and shading indicates the uncertainty measured as the ensemble standard deviation (or spread) at each location. The line contour interval in (a,c) is 4 hPa, with the 1000 hPa contour thickened, and in (b,d) it is 50 m, with the 5600 m contour thickened. The shading interval in (a,c) is 0.25 hPa and in (b,d) is 5 m.
Figure Reference:  Compo, G. P., et. al. (2011), The Twentieth Century Reanalysis Project. Q.J.R. Meteorol. Soc., 137: 1–28. doi:10.1002/qj.77



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