Importance of Midlevel Moisture for Tropical Cyclone Formation in Easterly and Monsoon Environments over the Western North Pacific

Abstract

This study explores the importance of mid-level moisture for tropical cyclone (TC) formation in monsoon and easterly environments over the western North Pacific in regional simulations (15-km resolution). The Weather Research and Forecasting (WRF) model is used to simulate 22 TCs that form in monsoon environments (MTCs) and 13 TCs that form in easterly environments (ETCs) over the period 2006–2010. To characterize the moisture contribution, simulations with mid-level moisture improved through assimilation of global positioning system (GPS) radio occultation (RO) data (labeled as EPH) are compared to those without (labeled as GTS). In general, the probability of TC formation being detected in the simulations is higher for MTCs than ETCs, regardless of GPS RO assimilation, especially for the monsoon trough environment. Fifty-four percent of ETC formations are sensitive to the mid-level moisture patterns, while only 18% for MTC formations are sensitive, indicating the importance of mid-level moisture is higher for ETC formations. Because of a model dry bias, the simulation of TC formation in an observed environment with lower vorticity but higher moisture is sensitive to the moisture increase through GPS RO data. Sensitivity experiments show that if the moisture in GTS is replaced by that in EPH, the TC formation can be detected in the GTS simulations. In turn, the TC formation cannot be detected in the EPH simulations with GTS moisture. The mechanism causing the difference in simulation performance of TC formation is attributed to more diabatic heating release and stronger positive potential vorticity tendency at mid-levels around the disturbance center caused by the higher moisture magnitudes.

Landfalling tropical cyclone characteristics and their multi‐timescale variability connected to monsoon and easterly formation environments over the western North Pacific

“…Monsoon and easterly systems provide different thermodynamic and dynamic conditions for TC formation (Teng et al ., 2021). These differences may alter TC landfalling characteristics.…”

Potential Impacts of Radio Occultation Data Assimilation on Forecast Skill of Tropical Cyclone Formation in the Western North Pacific

“…The definition of TC formation in WRF forecasts is based on Hsieh et al (2017), Chen et al (2020), andTeng et al (2021). An initial disturbance with the maximum 850-hPa vorticity center within 5° radius of the observed TCC location at the initial time of forecast is tracked and recorded as the pre-TC disturbance in each forecast.…”

“…To systematically calculate and compare the objective skill scores, the observed TCC formation time is regarded as the time reference for all experiments ( t = 0 hr; Figure 1c). To enhance the impact of GNSS RO DA, a 3‐day cycling DA procedure is used in regional models for TC forecasts (WRF model; Chen et al., 2020; Teng et al., 2021). GNSS RO data can considerably impact winds and vorticity through cycling DA procedure and model integration.…”

The Potential Value of Combining Assimilation of Water Vapor Radiances and Cloud Liquid/Ice Water Path From ABI on Analyses and Forecasts for Hurricane Irma (2017)

“…The limited intensity predictability is attributed to the complex interplay among multiscale dynamics, including but not limited to environmental flow and TC vortex interaction, boundary layer flux, radiative processes, and water vapor species microphysical conversions (Emanuel, 2018; Fu et al., 2012; Gray, 1968, 1998; Kerns & Chen, 2013). The predictability of TCs is highly related to moist convection and vortex flow (Teng et al., 2021). Therefore, initial condition errors of moisture may cause significant variations in convection distribution and thus carry a considerable inherent uncertainty in predicting TC intensity (Emanuel & Zhang, 2017; Tao & Zhang, 2014).…”