Impacts of US deforestation on rainfall from mesoscale convective systems

Abstract

This study investigates the effects of land use land cover (LULC) change on the 1993 and 2008 US Midwest flood events using the Weather Research and Forecasting model with the Noah-MP land surface model. Deforestation in the United States has led to large changes in LULC since the year 1850, in which forests have been replaced by croplands. This study examines the overall changes to the atmosphere caused by changing LULC from the present-day to the 1850 vegetation cover, and particularly the response of precipitation amounts and extremes. The LULC experiments caused a change in surface latent heat flux through the change in effective leaf area index. In the Noah-MP model, the latent heat flux over agricultural land was often greater than that over forests. Surface sensible heat flux and temperature increased with increasing forest cover in the 1850 experiments due to a lower albedo of forests. The increased temperatures in the 1850-vegetation experiment led to a decrease in pressure gradients across the domain, leading to a weakened great plains low level jet. The weakened low level jet weakened the dynamical forcing along the front and decreased the transport of warm, moist air. The decreased surface latent heat flux in the 1850-vegetation experiment led to decreased available moisture at the surface. It was also found that precipitable water decreased on average across the United States with 1850-vegetation. These changes led to a decrease in overall precipitation in the 1850-vegetation experiments. Although precipitation decreased, the atmospheric supply of water at the surface (precipitation minus evaporation) did not significantly change due to a decrease in surface evaporation in the 1850-vegetation experiment. This essentially offset the change in precipitation.