On the Impact of the 2019 Mississippi and Missouri River Valley Flooding on Boundary Layer Dynamics over the Great Plains

In a changing climate regime, ‘weather” and “climate” fluctuations are rapidly overlapping each other making the atmospheric processes a “continuum”, rather than a problem of two discrete time-scales. This calls for an improved understanding of the connections between weather and climate. In particular, understanding the kinematics and thermodynamic features of the atmospheric boundary layer (ABL) under extreme weather conditions like extreme precipitation and resultant catastrophic flooding along the Mississippi and Missouri River valleys during the spring 2019 is extremely important. For instance, the Missouri River basin has taken on more runoff in 3 months in spring 2019 than it typically gets in a year. Significant flooding conditions mainly occurred on the mainstem Mississippi due to the snowmelt in Northern Great Plains, frozen ground, rainfall in early March, and saturated soils. During spring 2019, we observed (1) more than 100 river gauges under major flooding conditions along lower and upper Mississippi River valleys, (2) more than 100 river gauges under moderate flooding conditions, (3) regions along both the Missouri and Mississippi river valleys experiencing more than 90% above normal streamflow conditions, (4) anomalously above average and, for some regions, the record wettest soil moisture conditions, (5) extreme positive departure from normal precipitation regimes over both Northern and Southern Great Plains. Overall, the 2019 spring flooding caused numerous impacts, with many roads, businesses, homes, and people affected and more than $2 billion due to losses in farming, manufacturing and navigation.

The ABL plays an important role in a number of atmospheric processes like convection initiation, turbulence mixing of tracers and pollutants, momentum exchange, aerosol-cloud-microphysics, and land-atmosphere feedback mechanisms. The role of soil moisture in modulating boundary layer dynamics over land surface has been reported in earlier studies. However, we do not have a conceptual understanding about the impact of extreme flooding conditions and “memory” effect on land-atmosphere feedback mechanisms for a fairly long-time-scale (here 2-3 months) for a broad regions like US Great Plains. Based on previous work, we hypothesize that anomalously high precipitation and consequent wet-soil conditions affects ABL depths for days. To demonstrate the “memory” effect of soil moisture conditions on ABL dynamics, we explored 35-years (1985-2019) of rawinsonde observations over 10 sites located along the Mississippi and Missouri River valleys. We will show selected results obtained from a series of analyses investigating ABL depth changes in spring 2019, and compare our findings with 3-year and 30-year climatology ABL depths over those sites. This work will help build new parameterization schemes to improve weather forecasting models where accurate simulations of land-atmosphere feedback mechanisms triggering ABL dynamics under extreme weather conditions still remains poor.