ZeYu Chen

Simulation of the stratospheric gravity waves generated by the Typhoon Matsa in 2005

The generation of stratospheric gravity waves (GWs) due to typhoon is simulated by using a meso-scale model (WRF) with a typhoon case, the Matsa in 2005. An 8-day model run that covers the major stages of the Matsa’s development reproduces the key features of the typhoon. For example, good agreements in the typhoon’s track, the intensity, and the spiral clouds, as well as mean state of stratosphere, are seen between the simulation results and the observation. Simulation results clearly show that with typhoon propagates northwestward, pronounced stratospheric GWs are generated continuously in the vicinity of Matsa. The GWs exhibit the typical curve-like wave fronts away from the Typhoon Matsa, and propagate preferentially in the upstream of the background winds. These characteristics reflect that the stratospheric GWs are closely associated with the typhoon, and thus the GWs are referred to as Tropical Cyclone related Gravity Waves (TC-GWs). The results also show that these waves should have a rather large horizontal scale so that the outmost wave fronts can be seen at the distance of ∼1000 km to the typhoon center in the horizontal plane of 20 km. This is consistent with the phenomenon of stratospheric TC-GWs with ∼1000 km horizontal scale disclosed by the previous observational analysis results.

Simulation of the stratosphere-troposphere exchange process in a typical cold vortex over Northeast China

A mesoscale weather research and forecasting (WRF) model was used to simulate a cold vortex that developed over Northeast China during June 19–23, 2010. The simulation used high vertical resolution to reproduce the key features of the cold vortex development. Characteristics of the associated stratosphere-troposphere exchange (STE), specifically the spatiotemporal distribution of the cross-tropopause mass flux (CTF), were investigated using the Wei formula. The simulation results showed that the net mass exchange induced by the cold vortex was controlled by stratosphere-to-troposphere transport (STT) processes. In the pre-formation stage of the cold vortex (i.e., the development of the trough and ridge), active exchange was evident. Over the lifecycle of the cold vortex, STT processes prevailed at the rear of the trough and moving vortex, whereas troposphere-to-stratosphere transport (TST) processes prevailed at the front end. This spatial pattern was caused by temporal fluctuations of the tropopause. However, because of the cancellation of the upward flux by the downward flux, the contribution of the tropopause fluctuation term to the net mass exchange was only minor. In this case, horizontal motion dominated the net mass exchange. The time evolution of the CTF exhibited three characteristics: (1) the predominance of the STT during the pre-formation stage; (2) the formation and development of the cold vortex, in which the CTF varied in a fluctuating pattern from TST to STT to TST; and (3) the prevalence of the STT during the decay stage.

Spatiotemporal spectrum and momentum flux of the stratospheric gravity waves generated by a typhoon

The simulation results of Typhoon Matsa (2005) by using the Weather Research and Forecasting (WRF) model show that pronounced stratospheric gravity waves (GWs) are generated in the vicinity of the typhoon. Using the model output, we investigate the spatial structures and the temporal variations of the GWs through a three dimensional (3-d) spectral analysis, i.e. the spectrum with respect to two horizontal wavenumbers and frequency. We further derive the momentum flux carried by the GWs. Spectral investigation results show that the power spectral density (PSD) of the GWs exhibits a single-peaked spectrum, which consists primarily of a distinct spectrum at horizontal wavelength of ∼1000 km, time period of 12–18 h, and vertical wavelength of 7–9 km. This spectrum is different from the spectra of GWs generated by deep convections disclosed by the previous researches. Both the PSD and momentum flux spectrum are prominent in positive kh portion, which is consistent with the fact that the GWs propagate in the upstream of mean flow. Large momentum flux is found to be associated with the GWs, and the net zonal momentum flux is 0.7845×10−3 Pa at 20 km height, which can account for ∼26% of the momentum flux that is required in driving the QBO phenomenon.