Regional Climate Variaiblity and Historical Extreme Events

Partly following on to the initial evaluation of the 1993 Midwest Flooding, and also in working toward evaluation of MERRA and reanalyses for the National Climate Assessment, we have looked closer at US regional climate variability in reanalyses. While the Northwestern US summer precipitation  is handled quite well in all reanalyses (specifically NCEP CFSR and ERA Interim), owing to influence from ENSO teleconnections, the Midwestern region summer precipitation has substantial uncertainty across all reanalyses. In MERRA, for example, the interannual variance is noticeably low, so that droughts are not as dry and pluvial periods not as wet (see 1988 and 1993 respectively in the following figure).

The extreme summers of 1988 and 1993 have been tied to both large scale ENSO teleconnections and local land-atmosphere feedback processes. Given that the reanalyses data assimilation provides a strong reference for the large scale meteorology, the land atmosphere interactions would be a likely weak point in the models that may affect this uncertainty.

These results are discussed in further detail http://dx.doi.org/10.1175/JAMC-D-12-0291.1.

Extreme Precipitation

Some time ago, I saw a poster that showed observed extreme precipitation increasing in time along the east coast and Gulf coast of the US, suggesting increasing extremes due to land falling hurricanes (Ashouri et al, 2012). There is also some supporting analysis of increasing precipitation trends and extremes in the recent National Climate Assessment report (Figures 2.15 and 2.16). To narrow the results to potential hurricane sources, Figure 1 here evaluates the trend of maximum daily precipitation, each season from 1979-2012, where hurricane season is defined as June through November.  Significant trends are seen along the northeast US track as well as some trends along the Gulf coast track in the south east US.

Figure 1 Trend of maximum daily precipitation in each hurricane season from 1979-2012. Trends significantly different from zero at 90% confidence are outlined in white contours.

The MERRA reanalysis is able to reproduce, generally this pattern of increasing extreme precipitation (Figure 2). MERRA's increasing trends in the southeast have a wider area, and in the northeast, the strongest trends  do not extend through the New England states, as observed. Still the reproduction of the trends of such a specialized diagnostic in a relatively coarse reanalysis is noteworthy.


Figure 2 As in Figure 1, except for the precipitation produced in the MERRA reanalysis.

As a further test of these trends, we area average the observed hurricane season maximum precipitation for the North Atlantic states in MERRA and the CPC observations. The interannual variability of the extreme precipitation is well reproduced, though, MERRA's mean value tends to be less than observed. Figure 3 shows increases in time for the northeast, and not just some end point variation caused be recent very large storms (e.g. Irene). though, low anomalies can occur in the recent few years, as well.

Remnants of Tropical Storm Karen produced heavy precipitation over a substantial portion of the Northeast, so that the 2013 season in the northeast will likely also be a positive anomaly (here is some result of that storm). The southeast may not have comparable extreme precipitation in 2013, at least related to tropical storms and hurricanes. We will come back to this as the 2013 hurricane season closes and MERRA is extended through it.


Figure 3 Time series of area averaged extreme precipitation anomalies from CPC gauge observations and MERRA reanalysis. The mean value removed for comparing anomalies is presented in the legend.

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.

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.

March 1993 East Coast Snow

Recently, Midshipman S. Martin from the United States Naval Academy visited the GMAO, to learn about MERRA. The specific case study evaluated for a brief internship was the March 13, 1993 east coast snow storm (links to a recent Capital Gang discussion on the predictability of the storm). This was just a preliminary evaluation of how MERRA analyses represent the storm, in preparation for a senior paper.  As with the Feb 1979 storm (see the MERRA home page), we generated an animation (~8Mb) to get a sense of the storm track. 

Snowfall totals of 2 feet or more occurred at many observing stations. Below, the snowfall totals from Kocin et al (1995) are compared with MERRA. The northern extent of the heaviest snow seems to be a bit weak (in NY and western PA, for example) . The MERRA snow data was converted from snow water equivalent accumulated for the two days, and converted to snow depth using 10% snow/ice density.

At 12Z13MAR1993, the surface low was centered over Georgia, with the surface front extending southward through Florida. Aloft, the main part of the jet stream was North of the surface low, but a maximum in wind speed (likely a jet streak)was in the 300mb trough, lagging behind the surface front (below).

Looking closer at the vertical cross section through the trough and this wind maximum, we find a well defined tropopause fold associated with the 300 mb wind maximum. Below we compare the MERRA representation of the tropopause fold to a case study (1978) observed with aircraft measurements. The MERRA figure shows wind speed in black, potential temperature in dashed red and potential vorticity in shaded blue.

The main point here is that the MERRA analysis of the storm shows good dynamical structure of a very strong storm. More still would need done, evaluating the cyclogenesis, and how well the system physical processes through the lifecycle of the storm. However, this is one of the stronger examples of cyclogenesis in the MERRA period, and so another question is whether MERRA data can reproduce the dynamical structure of weaker storms. Ultimately it's a promising result so far.

Figures obtained from:

Keyser, Daniel. “Atmospheric Fronts:An Observational   Perspective.” In, Mesoscale Meteorology and Forecasting, 216–257.

Kocin, P., Schumacher, P., Morales, R., and Uccellini, L. (1995, February). Overview of the 12-14 March 1993 Superstorm. Bulletin of the American Meteorological Society, 76, 2, 165-182.

Feb 19, 1979

The 1979 President's Day snow storm was a significant snow event in the North eastern US. This article presents an interesting review of the impact on the DC region and the modeling capability of the time. With the 30th anniversary of this storm, an animation of the MERRA depiction has been posted on the main WWW page. Here, we just compare a snapshot of the reanalysis to GOES IR imagery. The interesting part is that there is a clear break in the cloud structures of the storm develops over the Atlantic. This is not as apparent in the visible imagery (more like a continuous comma shape). MERRA cloud cover seems to catch this aspect of the storm. This data comes from the assimilation cycle of the system, forecasts for this case have not been run, but may be interesting.

The current estimate for when MERRA will catch up to real time is Fall 2009.

The MERRA cloud data is contoured from no cloud (black) to complete cover (white), the mean sea level pressure is contoured in purple. Wind barbs are colored according to the magnitude of the wind speed, and only 1 in 4 grid points are plotted.

For a study of the event, see: Bosart (1981)