New insight on the Plains' biggest rains

June 15, 1999

Rocky Mountains affect Midwest flooding

BOULDER--Until now scientists have found it hard to predict which summer storms forming over the Rocky Mountains would produce giant, flood-prone storm systems in the Great Plains to the east. Now Andrew Crook (National Center for Atmospheric Research, or NCAR) and Donna Tucker (University of Kansas) may have found the key: the strength of intense downdrafts that emerge from the mountain storms and stir up severe weather downstream. Computer modeling to track these downdrafts and the cloud-level ice crystals that help produce them may eventually give forecasters the edge in predicting severe storm systems, and possibly flooding, over the plains. Crook and Tucker (the lead author) are publishing their results in the June issue of Monthly Weather Review. NCAR's primary sponsor is the National Science Foundation.

Most summertime floods across the Great Plains are caused by mesoscale convective systems (MCSs). These giant complexes often emerge from showers and thunderstorms that form over the Rocky Mountains. Tucker and Crook used the Pennsylvania State University/NCAR mesoscale model to simulate convection (showers and thunderstorms) and to test how different modes of mountain convection affect the likelihood of MCS formation downstream. In the model, they found that an MCS was most likely to form when a mass of rain-cooled air descended from the mountains, colliding with moist air on the plains and forcing it upward.

Although forecasters have seen this process unfold many times, it is still unclear whether a given day's mountain storms will be the right kind to trigger an MCS. Sometimes the initial storms lead to an MCS that can travel as far as Illinois; other times, the storms dissipate shortly after they move off the mountains. Tucker and Crook's modeling suggests that the strength of the rain-cooled outflow from the mountain storms is critical to downstream MCS development. Several factors play into the outflow strength, including the fall speed of ice crystals within the mountain storms.

Fine-scale modeling for better prediction

Even today's most sophisticated forecast models cannot peg mountain convection well enough to assess how it might trigger storm complexes downstream. However, under a new NSF grant, Tucker and Crook are using a finer-scale model built by NCAR scientist Terry Clark to look more closely at mountain convection and how it relates to the larger-scale atmospheric flow. Since the large-scale flow is routinely forecast by computer models, this new work could allow forecasters to better pinpoint a given day's mountain convection and where it might trigger large storm complexes on the plains. Tucker and Crook's work is supported by the University of Kansas and NSF.

One downpour leads to another: NCAR team pinpoints culprit

A typical MCS peaks in strength during the overnight hours and dissipates the next day. However, it may be followed by a second MCS the following night. Sometimes a slow-moving sequence of MCSs will extend over several days, causing torrential rains over a large area. If such a multiday sequence could be forecast, valuable lead time might be gained on flooding threats.

NCAR scientists Christopher Davis, Stanley Trier, and colleagues have gained new insight on a type of low-pressure center that connects one MCS to the next. This low is called a mesoscale convective vortex (MCV). With a core only 30 to 60 miles wide and 1 to 3 miles deep, an MCV is often overlooked in standard weather analyses. But Davis and Trier have found that MCVs play a key role in helping storms regenerate over two or more days.

Looking closely at satellite, upper-air, and radar observations from 1998, Davis and Trier found evidence of 17 separate MCVs over the central and eastern United States. Previous studies had found only two or three MCVs per year. The vortices appear most likely to persist when lower- and upper-level winds are relatively light. This allows the circulation to maintain its integrity for up to 12 hours after the storms dissipate. If other conditions are favorable, a new round of storms may cluster around the vortex. For example, one MCV triggered heavy rains in Texas on May 27, 1998; flooding in Arkansas early on the 28th; and additional flooding the following night in Mississippi. An MCV that moves into tropical waters, such as the Gulf of Mexico, can serve as the nucleus for a tropical storm or hurricane.

Currently, it's difficult to spot and track mesoscale convective vortices from upper wind observations alone, due to their small size. However, a technique developed by NCAR's John Tuttle calculates winds using cloud movements observed by satellite in order to spot MCVs and other features. This promising technique, along with better observations and models, could make it practical for forecasters to use MCVs as a guide to predict locations of heavy rain. Davis and Trier's work is supported by NASA and the U.S. Weather Research Program, which is examining forecast tools for heavy precipitation.

NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.
-end-
Writer: Bob Henson

Note to Editors:
Visuals: Images are available at ftp://ftp.ucar.edu/communications.
Filename(s): mcsradar.tif, downdraft.tif, sat.tif, satwind.tif. Captions are at the Web address below.

UCAR and NCAR news: http://www.ucar.edu/publications/newsreleases/1999.

To subscribe via e-mail send name, title, affiliation, postal address, fax, and phone number to butterwo@ucar.edu.

National Center for Atmospheric Research/University Corporation for Atmospheric Research

Related Flooding Articles from Brightsurf:

Coastal flooding will disproportionately impact 31 million people globally
Indiana University researchers analyzed these geographic regions, which include cities like New Orleans, Bangkok, and Shanghai, using a new global dataset to determine how many people live on river deltas, how many are vulnerable to a 100-year storm surge event, and the ability of the deltas to naturally mitigate impacts of climate change.

New woodlands can help reduce flooding risk within 15 years
New research by the University of Plymouth suggests the planting of more trees could have a significant and positive effect in preventing flash flooding.

Land use change leads to increased flooding in Indonesia
While high greenhouse gas emissions and biodiversity loss are often associated with rapid land-use change in Indonesia, impacts on local water cycles have been largely overlooked.

Climate change: Coastal flooding could threaten up to 20% of global GDP
Coastal flooding events could threaten assets worth up to 20% of the global GDP by 2100, a study in Scientific Reports suggests.

River plants counter both flooding and drought to protect biodiversity
'Water plants are a nuisance in streams, blocking the flow.

Scientists predict dramatic increase in flooding, drought in California
California may see a 54 percent increase in rainfall variability by the end of this century, according to research from a UC Davis atmospheric scientist.

Multiple flooding sources threaten Honolulu's infrastructure
In a study published in Scientific Reports, researchers at the University of Hawai'i at Mānoa, found in the next few decades, sea level rise will likely cause large and increasing percentages of land area to be impacted simultaneously by the three flood mechanisms.

Climate change: Extreme coastal flooding events in the US expected to rise
Extreme flooding events in some US coastal areas could double every five years if sea levels continue to rise as expected, a study published in Scientific Reports suggests.

Study find delta helps to decrease the impact of river flooding
Most coastal cities and ports face a double threat from storm surge and river flooding.

Texas A&M researchers develop flooding prediction tool
By incorporating the architecture of city drainage systems and readings from flood gauges into a comprehensive statistical framework, researchers at Texas A&M University can now accurately predict the evolution of floods in extreme situations like hurricanes.

Read More: Flooding News and Flooding Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.