Brown Ocean Effect on the Louisiana August 2016 Extreme Flooding Event

An extreme flooding event, caused by a weather system resembling a tropical depression, occurred over southern Louisiana in mid August of 2016. This event resulted in over $10 billion in property damage and several fatalities. An unusual aspect of this event was that the depression was not the remnants of an oceanic tropical disturbance, but one that formed and persisted over the land. We tested the hypothesis that the “brown ocean effect,” namely high moisture availability over land, played a key role in the evolution of the system. Numerical modeling simulations captured key aspects of the system and enabled a set of experiments to evaluate sensitivity to surface moisture availability. To test, the hypothesis that surface moisture contributed to the extreme precipitation event, wetlands were converted to open water, wet croplands, and dry croplands within the land surface model. These scenarios are end members of land cover changes occurring in the region. In the numerical model experiments, the storm evolution that most resembles tropical lows that form and intensify over oceans occurred when wetlands are converted to open water. The storm evolution in the control experiment best matched the open water experiment. Conversion of wetlands to open water reduced the storm intensity irrespective of soil moisture conditions, and in addition, drier soils reduced rainfall totals by 20%. Sensitivity experiments show that projections of future conversion of wetlands to open water will result in a persistent ingredient that favors intensification of tropical systems in the area.