The development of a powerful Mongolian cyclone on 14-15 March 2021:Eddy energy analysis

Intense and extensive dust,caused by a strong Mongolian cyclone,hit Mongolia and northern China on 14-15 March 2021.In this study,the development process of this cyclone is analysed from the perspective of highfrequency eddy energetics.During the low-frequency circulation field of early March of 2021,an amplified polar vortex intruding towards central Asia and a ridge straddling eastern and northeastern Asia worked in concert to comprise a strong baroclinic zone from central Asia to Lake Baikal.Under these favourable conditions,on 13 March,a migratory trough triggered the Mongolian cyclone by crossing over the Sayan Mountains.The downwards transfer of kinetic energy from the eddy at 850 hPa played a key role in the intensification and mature stage of the cyclone.This mechanism was primarily completed by the cold air sinking behind the cold front.The frontal cyclone wave mechanism became crucial once the cyclone started to rapidly develop.The authors emphasize that the anomalously large growth of high-frequency available potential energy,which characterized this super strong cyclone,was obtained by extracting energy first from the time-mean available potential energy and then from the low-frequency available potential energy.The interannual temperature anomaly pattern of"north cold south warm"facilitated the additional time-mean available potential energy,and the temperature anomaly pattern of"northwest cold southeast warm"conditioned the extra low-frequency available potential energy.The analysis results suggest that the interaction between high-and low-frequency waves was also important in the development of the intense cyclone.

Effects of surface waves and sea spray on air–sea fluxes during the passage of Typhoon Hagupit

Air–sea exchange plays a vital role in the development and maintenance of tropical cyclones（TCs）. Although studies have suggested the dependence of air–sea fluxes on surface waves and sea spray, how these processes modify those fluxes under TC conditions have not been sufficiently investigated based on in-situ observations.Using continuous meteorological and surface wave data from a moored buoy in the northern South China Sea,this study examines the effects of surface waves and sea spray on air–sea fluxes during the passage of Typhoon Hagupit. The mooring was within about 40 km of the center of Hagupit. Surface waves could increase momentum flux to the ocean by about 15%, and sea spray enhanced both sensible and latent heat fluxes to the atmosphere,causing Hagupit to absorb 500 W/m^2 more heat flux from the ocean. These results have powerful implications for understanding TC–ocean interaction and improving TC intensity forecasting.

Impact of Cyclone Nilam on Tropical Lower Atmospheric Dynamics

A deep depression formed over the Bay of Bengal on 28 October 2012, and developed into a cyclonic storm. After landfall near the south coast of Chennai, cyclone Nilam moved north-northwestwards. Coordinated experiments were conducted from the Indian stations of Gadanki（13.5？N, 79.2？E） and Hyderabad（17.4？N, 78.5？E） to study the modification of gravity-wave activity and turbulence by cyclone Nilam, using GPS radiosonde and mesosphere–stratosphere–troposphere radar data. The horizontal velocities underwent large changes during the closest approach of the storm to the experimental sites. Hodograph analysis revealed that inertia gravity waves（IGWs） associated with the cyclone changed their directions from northeast（control time） to northwest following the path of the cyclone. The momentum flux of IGWs and short-period gravity waves（1–8 h） enhanced prior to, and during, the passage of the storm（±0.05 m2s-2and ±0.3 m2s-2, respectively）, compared to the flux after its passage. The corresponding body forces underwent similar changes, with values ranging between ±2–4m s-1d-1and ±12–15 m s-1d-1. The turbulence refractivity structure constant（C2n） showed large values below 10 km before the passage of the cyclone when humidity in the region was very high. Turbulence and humidity reduced during the passage of the storm when a turbulent layer at ～17 km became more intense. Turbulence in the lower troposphere and near the tropopause became weak after the passage of the cyclone.

POTENTIAL VORTICITY STRUCTURE OF SUMMER CYCLONES OVER THE CHANGJIANG-HUAIHE VALLEY

Based on the theory of Ertel potential vorticity,the isentropic potential vorticity maps and vertical pro- files of potential vorticity for two summer cyclones over the Changjiang-Huaihe Valley are analysed.After discussing a possible mechanism for the genesis and development of such systems and their differences from typical extratropical cyclones,a conceptual model for their activities is proposed:A weak disturbance in the mid- level of troposphere originated from around the Qinghai-Xizang Plateau may cause heavy precipitation under favourable conditions and latent heat release in the mid-troposphere leads to downward extension of cyclonic circulation and a wave on the quasi-stationary front.This weak cyclone can develop substantially and become a typical extratropical cyclone only when air from the lower stratosphere flows downslope along isentropic sur- faces into the region of interest.

Adjoint Sensitivity Study on Idealized Explosive Cyclogenesis

The adjoint sensitivity related to explosive cyclogenesis in a conditionally unstable atmosphere is investigated in this study.The PSU/NCAR limited-area,nonhydrostatic primitive equation numerical model MM5 and its adjoint system are employed for numerical simulation and adjoint computation,respectively.To ensure the explosive development of a baroclinic wave,the forecast model is initialized with an idealized condition including an idealized two-dimensional baroclinic jet with a balanced three-dimensional moderateamplitude disturbance,derived from a potential vorticity inversion technique.Firstly,the validity period of the tangent linear model for this idealized baroclinic wave case is discussed,considering different initial moisture distributions and a dry condition.Secondly,the 48-h forecast surface pressure center and the vertical component of the relative vorticity of the cyclone are selected as the response functions for adjoint computation in a dry and moist environment,respectively.The preliminary results show that the validity of the tangent linear assumption for this idealized baroclinic wave case can extend to 48 h with intense moist convection,and the validity period can last even longer in the dry adjoint integration.Adjoint sensitivity analysis indicates that the rapid development of the idealized baroclinic wave is sensitive to the initial wind and temperature perturbations around the steering level in the upstream.Moreover,the moist adjoint sensitivity can capture a secondary high sensitivity center in the upper troposphere,which cannot be depicted in the dry adjoint run.