Study Of Generation And Propagation Of Gravity Waves Induced By Tropical Cyclone

Atmospheric gravity waves (GWs) are believed to be important processes in the lower and middle atmosphere by transporting momentum and energy to the global mid-dle atmosphere circulation. Deep convection is one of the most important sources of GWs, particularly in the tropics. Recent research has indicated that diabatic forcing and overshooting within a deep convective system can excite a broad spectrum of GWs on large temporal and spatial scales. In the northwestern region of the Pacific Ocean, a typhoon (equivalent to a tropical cyclone in the South Pacific) is a well-organized, large-scale severe convective system that can induce a series of GWs with horizon-tal wavelengths of several hundred kilometers long and durations that last for multiple hours. Typhoon-induced (or Tropical Cyclone induced) GWs (TGWs) are generated in the troposphere along the typhoon track and propagate horizontally and vertically. TGWs play an important role in the momentum budget of the middle atmosphere and affect global atmospheric circulation. In recent decades, observations and numerical models are sparing no effort to explore the GWs, including wave sources, characteris-tics and physical processes. Despite the TGWs are thought to be important to the global middle atmosphere circulation, it is not possible to observe and quantify the wave field in sufficient temporal and spatial detail. Due to the limitations of TCs’synoptic envi-ronment and each instrument’s particular restriction, current observational techniques (e.g., lidar, radiosonde, airglow and satellite) are still poorly quantify the TGW charac-teristic.In this study, we investigate the GWs generated by Typhoon Mindulle (2004), which formed in June 2004 near Guam (13°N,144°N), using Atmospheric Infrared Sounder (AIRS) observations and the Weather Research and Forecasting (WRF) model. Stratospheric gravity waves induced by Typhoon Mindulle (2004) were detected by the AIRS. Semicircular GWs with horizontal wavelengths of 100-400 km were found over Taiwan through an inspection of AIRS radiances at 4.3 μm. Characteristics of the strato-spheric gravity waves generated by Typhoon Mindulle were investigated using the WRF model. The initial and boundary data were determined by the high-resolution European Center for Medium-Range Weather Forecasts (ECMWF) re-analysis data. The WRF simulation reproduces the main features of Typhoon Mindulle and the significant GWs. The simulated GWs with horizontal wavelengths of 100-400 km match the AIRS obser- vations:they propagate upward and eastward, and the westward components are mostly filtered in the stratosphere.We also focus on the conditons for orographic gravity waves (OGWs) propagation in this paper. A large number of observational and modeling studies have reported that the occurrence of OGWs varies significantly with season and latitude. Because prevail-ing wind patterns prevent OGW propagation, a limited number of OGWs are detected and investigated at low latitudes during the summer. By comparing the measured waves with a WRF simulation in the absent of orography (WRF-FLAT), we find that the OGWs generated by the flow of Typhoon Mindulle over the Central Mountain Range (CMR) in Taiwan account for approximately 50% of the total wave momentum flux in the tro-posphere. The dominant orientation of the OGW wave fronts is parallel to the CMR rideline. When entering into the stratosphere, OGW propagation is determined by the position of the typhoon center relative to the CMR. We find that the OGWs propagate upward prior to the typhoon landing, when the background wind is easterly from the surface to high altitudes without the existence of the critical level, but are filtered by the critical level in the troposphere after typhoon landing. Because dozens of cyclones and typhoons form in the Pacific every year, it is important to examine the OGWs associated with these cyclones.In recent years, with the continuous increase of computing power availability, Liu, et al. (2014) has developed a new whole atmosphere general circulation model (GCM), which uses the National Center for Atmospheric Research Whole Atmosphere Com-munity Climate Model (WACCM) with-0.25° horizontal resolution, and is targeted to resolve mesoscale motions. They captured mesoscale features such as a tropical cy-clone (TC) at 167°E and 20°S east of Australia, and concentric GWs excited by the TC over the convective system region were also reported. To understand the scale interac-tions between global and regional climate models and evaluate the necessity of higher resolution in GWs resolving, a dynamical downscaling approach is adopted to simulate the TC induced GWs with the products of the high-resolution WACCM, the reginal cli-mate model (RCM) integrated here is the WRF model version 3.6.1. Semicircular GWs excited by the TC are found with similar structures and locations in WACCM and WRF with 25 km grid spacing. In this study, the sensitivity of GW structure to the choice of model horizontal resolutions (25 km,15 km,10 km, and 4 km) is examined, in order to examine the necessity and feasibility of resolving down to a finer-scale (a few kilome-ters) of GWs in GCMs. Although WRF is successful in generating semicircular GWs in each case, the simulation reveals more power and shorter horizontal wavelengths of GWs at finer resolutions. By comparing the GWs with a WRF simulation with explic-itly resolved convection (i.e., no convective parameterization) at the same horizontal resolution scale (4 km), we find that the resolved convection, however, plays a negli-gible role in the GWs generated by the TC on the order of 4 km horizontal resolution. As far as we know, this is the first time that a dynamic downscaling approach from a mesocale-revoling GCM is applied to further investigate TGWs.