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Russ Schumacher - Understanding and Predicting Extreme Precipitation

Precipitation extremes, including severe droughts and periods of excessive rainfall, impact society in numerous ways: the agricultural industry is extremely sensitive to both too much and too little precipitation, water supplies may dwindle and wildfires are more likely during dry seasons, and both short-term flash floods and longer-term river floods cause great damage and injury. Predicting excessive precipitation in the midlatitude warm season is a particular challenge, because most of the rainfall is associated with deep, moist convection, which has inherently limited predictability.
This deep convection typically organizes into clusters larger than individual updrafts; these clusters are known generally as mesoscale convective systems (MCSs). The timing, location, and duration of MCSs are often determined by small-scale atmospheric features, but MCSs also have impacts at larger scales.
Russ Schumacher's research uses observations and numerical models on a variety of temporal and spatial scales to better understand the physical processes that lead to heavy precipitation, and to potentially improve predictions of these high-impact weather and climate events.

One of Russ's research projects involves assessing the ability of global ensemble prediction systems to forecast widespread heavy rainfall that occurs over multiple days. These rain events often lead to flash floods and river flooding over large areas, such as the floods observed in June 2007 in the Southern Plains of the United States, and in June 2008 in the Midwest. Although global ensembles provide very skillful forecasts far in advance for heavy rainfall caused by strongly-forced weather systems, the lead time for successful forecasts is much shorter for events associated with convection in the warm season (Fig. 1).

Figure 1
Probability of 50 mm of rain in the 120 hours between 1200 UTC 25 June and 1200 UTC 30 June 2007 from the ECMWF ensemble prediction system. The upper left panel shows the ensemble run initialized at 1200 UTC 25 June (the start of the event), the next panel shows the run initialized 12 hours earlier, and so on. The purple contours are ensemble probabilities, with contours at 4%, 10%, and every 10% above that. Probabilities greater than 50% are colored as shown. The green line shows the observed 50-mm rainfall contour. (click image for larger view)

Considering their limited predictability, Russ is exploring the atmospheric processes that are important in prolonged warm-season rain events. One case of particular interest occurred in the Southern Plains in late June and early July 2007, and caused destructive flooding in Kansas, Oklahoma, and Texas. Observations and numerical simulations of this event show the importance of processes interacting on multiple scales, from the organization of deep convection, to the maintenance of a mesoscale vortex in the mid-troposphere, to the development of a large-scale anticyclone aloft, all contributed to persistent heavy rains over a period of several days.

Another of Russ's research projects, in collaboration with scientists at NCAR and the University at Albany, is investigating the effects of landfalling tropical cyclones on precipitation far away from the cyclone itself. One notable example of this took place in August 2007, as Tropical Storm Erin made landfall along the Texas coast and moved slowly northward. As Erin came ashore, it brought with it a plume of deep tropical moisture that then moved poleward within a low-level jet. As this very moist air interacted with a front in the Midwestern US, record rainfall and flooding occurred in Minnesota and Wisconsin. Fig. 2 shows the results of two WRF model simulations: a control run that includes the moisture attributable to Erin, and one in which that moisture was removed. Although the stage was set for heavy rainfall even without the tropical moisture, the effects of Erin transformed it into a record-breaking, high-impact event.

Figure 2
Rainfall from the predecessor rain event associated with Tropical Storm Erin. The left panel shows the oberved precipitation between 1200 UTC 18 August and 1200 UTC 19 August 2007. The center panel shows the control WRF simulation, which closely resembles the observations. The right panel shows the simulation with the tropical moisture from Erin removed. (click image for larger view)

ASP Spotlight April 2009
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