Studying the Sudden Onset and Evolution of Outer Rainband Precipitation of Hurricane Harvey (2017) Using Numerical Simulations with Data Assimilation and Cloud Initiation

A momentous Hurricane Harvey (2017) evolved over the Gulf of Mexico and greatly impacted Texas and Louisiana with strong winds and historical rainfall amounts from 26 to 30 August 2017. The hurricane is studied with emphasis on its extreme precipitation characteristics. First, a sequence of high-resolution mesoscale simulations using an advanced research version of the Weather Research and Forecasting (WRF ARW) model is conducted. A three-dimensional variational (3D-Var) assimilation of conventional observations and airborne tail Doppler radar (TDR) radial velocity and reflectivity observations are performed using the NCEP Gridpoint Statistical Interpolation (GSI) system. It is found that assimilation of radar reflectivity data with GSI cloud analysis provides the most substantive improvements in track and intensity forecasts against the control simulation. In light of the significant importance of this cloud microphysical representation, further diagnoses are made with high temporal (1 h) and spatial (3 km) High Resolution Rapid Refresh (HRRR) analysis as it is generated in a similar configuration to that of the data assimilation performed. Comprehensive investigation is conducted to examine the mesoscale environment and vortex conditions that triggered the development of this outer rainband that produced so much rainfall, beyond the fact that a large-scale upper-level blocking high pressure system caused slow movement of Harvey over Texas.

It is found that there is a relatively short period of approximately 8 h (between 0000 UTC 27 – 0800 UTC 27 August 2017) in which an outer rainband of Harvey suddenly develops and produces its largest amount of rainfall. Surrounding this timeframe, convective available potential energy (CAPE), convective inhibition (CIN) and components of diabatic heating such as sensible and latent heat fluxes are shown to be most important in characterizing the atmosphere during the development, duration, and cessation of the strong outer rainband. Surface Mesonet data is also used to examine the role that cold pools may have played in enhancing convective development within these rainbands. Results indicate that, although there are characteristics of cold pools revealed within the observation sets during rainband passage, there is not quite enough evidence to suggest that cold pools are influential enough to assist in the initiation of new convective cells within the rainband. Finally, statistical analysis through bootstrapping and stepwise multiple linear regression further prove that CAPE, a thermodynamic feature, and lower-level rain water mixing ratio (RWMR), an indication of liquid hydrometeor distribution, are the two primary variables that represent the state of the atmosphere during the extreme rainfall event. Detailed diagnosis and interpretation will be presented at the conference.