Large Scale Dynamic Flood Forecasting

Development of a Dynamic Flood Forecasting Model Based on
Large Scale Climatic Information

Various climate change projections suggest an intensification of precipitation in the future, in terms of both frequency and magnitude. The intensity of extreme precipitation is projected to increase under global warming in many parts of the world. However, these arguments are driven largely by considerations of the moisture holding capacity as a function of temperature, as indicated by the Clausius-Clapeyron equation. One also needs to consider the attendant atmospheric circulation and moisture transport dynamics that lead to persistent extreme precipitation and subsequent flooding as evidenced in recent major floods.

Although the climate mechanisms governing precipitation vary by location, extreme precipitation events in the mid-latitudes are typically associated with anomalous atmospheric moisture from warmer tropical or subtropical oceanic areas. Enhanced Integrated Water Vapor (IWV) bands, also known Atmospheric Rivers (ARs), are associated with direct poleward transport of tropical moisture along the IWV bands from the Tropics all the way to the extratropics. For meridional transport at middle latitudes, ARs account for a substantial part of the moisture transport. The AR concept indicated a direction to track the moisture from warmer oceanic source to the heavy precipitated regions. Stationary Rossby waves account for a substantial fraction of summertime monthly mean surface temperature and precipitation variability over a number of regions of the Northern Hemisphere middle latitudes. Tropical moisture exports (TMEs) to the Northern Hemispheric extratropics are an important feature of the general circulation of the atmosphere and link tropical moisture sources with extratropical precipitation and occasionally with explosive cyclogenesis.

TME tracks were calculated based on 6-hourly ERA-Interim data for 42 flood events from 1989 – 2010 in the Northeastern United States

Tropical Moisture Sources
There are four tropical regions that were identified as major moisture
sources for the northern hemisphere:

  1. “Great Plains” (GP), A locally very confined maximum occurs between
    the Rocky and the Appalachian Mountains; The inflow of moist air masses
    from the Gulf of Mexico (0-20°N, 100°W-90°W)into the central parts of
    the United States.
  2. “Pineapple Express” (PE),(0-20°N, 160°W-150°W) connects the tropics
    near Hawaii with the North America.
  3. “Gulf Stream”, (GS), (0-20°N, 60°W-40°W) located over the western inflow
    region of the North Atlantic storm track along the east coast of North
    America.
  4. “West Pacific” (WP), (0-20°N, 130°E-150°E) presumably closely related to
    the mei-yu-baiu front over East Asia during Summer time.

Summary
Contributions of the moisture born in the four hotspots have distinctive seasonal patterns. The “Great Plains” Hotspot is the major year-around moisture source to the Northeast region, corresponding to the moisture born in Gulf of Mexico. The trajectories of the tracks have seasonal variations, suggesting the role of atmospheric circulation in bring the moisture from the tropics. There is also a strong association between TME tracks and flood events year-around, with the least association during Summer, and the strongest association in Winter.

Next Steps

A complementary analysis on the whole country is being performed to understand the role of large scale transport of moisture associated with floods. Research on associating stationary waves and transients with these tracks and their convergence is in progress, both for understanding the antecedent and concurrent dynamics and for prognostic modeling.