Analysis on Tropical Storm Erin (2007) and the Diurnal Cycle in Post-Landfall Tropical Cyclones

Tropical Storm Erin (2007) was an unusual tropical cyclone (TC) that intensified over the state of Oklahoma. Prior research uncovered multiple factors responsible for Erin’s strange re-intensification, such as abundant surface moisture producing a ”brown ocean” effect, but no investigation into the diurnal cycle has been conducted. The rare case of an overland intensification in addition to technological improvements in modeling capabilities render it a suitable case to test diurnal variability in a post-landfall TC. Previous studies on the diurnal cycle in TCs have established a nocturnal preference for rapid intensification and deeper convection. However, research into the effects of the diurnal cycle on TCs as they make landfall and beyond has been relatively scarce in comparison. Even further, the necessity for understanding the behavior of TCs once they make landfall is extremely important due to impacts on humans and population centers on land. As a result, we chose to investigate the diurnal cycle in Tropical Storm Erin (2007) using the Weather Research and Forecasting (WRF) model through sensitivity tests by modifying the diurnal cycle. We hypothesized that the diurnal cycle will affect the timing of intensification. Through our research, we were able to develop a control run with extremely similar structure, rainfall patterns, and storm track using the WRF Model. The clock was offset by 12 hours to investigate if similar changes occurred with different solar radiation through comparing it with a thoroughly- tested control run using the WRF model. Our results show that the diurnal cycle played a major role in the timing of intensification for Tropical Storm Erin through an earlier initiation. Additionally, the altered rainfall patterns mimic those that occurred 12 hours later on the control run, indicating rainfall patterns in large scale MCSs have a similar link to the diurnal cycle and timing of rapid intensification. Ongoing research into the Great Plains Low Level Jet (LLJ) is being conducted between the altered run and the control run to understand the importance of the LLJ in TC Erin’s intensification.