A SIMPLE NONLINEAR MODEL OF INERTIAL OSCILLATION OF ATMOSPHERE IN LOW-LATITUDES

In this paper,by simplifying the governing equation in low latitudes a nonlinear model which takes into consideration the equatorial β-plane approximation and describes the natural oscillation of the atmosphere has been set up.By applying this model the following results are shown: (1)There exists the pure inertial oscillation only in u_0>0(westerly current),the angular frequency of linear oscillation of y is ω_0~*=(β_0u_0~*)^(1/2),the corresponding oscillatory period is 1—2 weeks.There are two kinds of angular frequencies under the nonlinear condition,the one is ω_0=(β_0u_0)^(1/2)the other is ω_1= 1/2 β_0y_0.When a soliton oscillator occurs(ω_1=ω_0),the oscillatory period increases rapidly,and T→∞. (2)When the pressure field is considered,the oscillation exists not only in u_0>0(westerly current) but also in u_0<0(weak easterly current).However this weak pressure field has slight effect on the oscillatory period. (3)The stability of inertial oscillation depends on the linear inertial parameter μ.As the parameter μ changes sign from negative to positive,the supercritical bifurcation takes place in b<0.

On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10–18 August 2020

A sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020,showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values,except on the first day.Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet(LLJ)in and to the southeast of the basin,which favored continuous water vapor transport and abnormally high precipitable water in the basin.The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing.The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present.The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south.It was the slantwise lifting of the warm,moist airflow above the cold air,often facilitated by southwest vortices and quasi-geostrophic ascent,that released the convective instability and produced heavy rainfall.

Evolution and Frontogenesis of an Imbalanced Flow- the Influence of Vapor Distribution and Orographic Forcing

If the initial fields are not in geostrophic balance, the adjustment and evolution will occur in the stratified fluid. and the frontogenesis will occur under suitable conditions. The evolution is studied here with a nonhydrostatic fully compressible meso-scale model (Advanced Regional Prediction System, ARPS). Four cases are designed and compared: (i) control experiment: (ii) with different initial temperature gradient; (iii) with vapor distribution; (iv) with orographic forcing. The results show that: (1) there is an inertial oscillation in the evolution of the imbalanced flow with the frequency of the local Coriolis f, and with its amplitude decreasing with time. The stationary balanced state can only be approached as it cannot be reached in the limit duration of time. The energy conversion ratio varies in the range of [0, 1; 3]; (2) the stronger initial temperature gradient can make the final energy conversion ratio higher. and vice versa; (3) suitable vapor distribution is favorable for the frontogenesis. It will bring forward the time of the frontogenesis, strengthen the intensity of the cold front, and influence the final energy conversion ratio; (4) the orographic forcing has an evidently strengthening effect on the frontogenesis. The strengthening effect on the frontogenesis and the influence on the final energy conversion ratio depend on the relative location of the mountain to the cold front.

The oceanic responses to Typhoon Rananim on the East China Sea

Many typhoons pass through the East China Sea(ECS)and the oceanic responses to typhoons on the ECS shelf are very energetic.However,these responses are not well studied because of the complicated background oceanic environment.The sea surface temperature(SST)response to a severe Typhoon Rananim in August 2004 on the ECS shelf was observed by the merged cloud-penetrating microwave and infrared SST data.The observed SST response shows an extensive SST cooling with a maximum cooling of 3°C on the ECS shelf and the SST cooling lags the typhoon by about one day.A numerical model is designed to simulate the oceanic responses to Rananim.The numerical model reasonably simulates the observed SST response and thereby provides a more comprehensive investigation on the oceanic temperature and current responses.The simulation shows that Rananim deepens the ocean mix layer by more than 10 m on the ECS shelf and causes a cooling in the whole mixed layer.Both upwelling and entrainment are responsible for the cooling.Rananim significantly deforms the background Taiwan Warm Current on the ECS shelf and generates strong Ekman current at the surface.After the typhoon disappears,the surface current rotates clockwise and vertically,the current is featured by near inertial oscillation with upward propagating phase.

Vortex genesis over the Bay of Bengal in spring and its role in the onset of the Asian Summer Monsoon

Physical processes associated with onset of the 1998 Asian summer monsoon were examined in detail using multi-source datasets. We demonstrated that strong ocean-atmosphere-land interaction in the northern Indian Ocean and tropical Asian area during spring is a fundamental factor that induces the genesis and development of a monsoon onset vortex over the Bay of Bengal （BOB）, with the vortex in turn triggering onset of the Asian summer monsoon. In spring, strong surface sensible heat- ing over India and the Indochina Peninsula is transferred to the atmosphere, forming prominent in situ cyclonic circulation, with anticyclonic circulations over the Arabian Sea and northern BOB where the ocean receives abundant solar radiation. The corresponding surface winds along the North Indian Ocean coastal areas cause the ocean to produce the in situ offshore cur- rents and upwelling, resulting in sea surface temperature （SST） cooling. With precipitation on the Indochina Peninsula in- creasing from late April to early May, the offshore current disappears in the eastern BOB or develops into an onshore current, leading to SST increasing. A southwest-northeast oriented spring BOB warm pool with SST 〉31℃forms in a band from the southeastern Arabian Sea to the eastern BOB. In early May, the Somali cross-equatorial flow forms due to the meridional SST gradient between the two hemispheres, and surface sensible heat over the African land surface. The Somali flow overlaps in phase with the anticyclone over the northern Arabian Sea in the course of its inertial fluctuation along the equator. The con- vergent cold northerlies on the eastern side of the anticyclone cause the westerly in the inertial trough to increase rapidly, so that enhanced sensible heat is released from the sea surface into the atmosphere. The cyclonic vorticity forced by such sensible heating is superimposed on the inertial trough, leading to its further increase in vorticity strength. Since atmospheric inertial motion is destroyed, the flow deviates from the inertial track in an intensified cyclonic curvature, and then turns northward to- ward the warm pool in the northern BOB. It therefore converges with the easterly flow on the south side of the anticyclone over the northern BOB, forming a cyclonic circulation center east of Sri Lanka. Co-located with the cyclonic circulation is a generation of atmospheric potential energy, due to lower tropospheric heating by the warm ocean. Eventually the BOB mon- soon onset vortex （MOV） is generated east of Sri Lanka. As the MOV migrates northward to the warm pool it develops quickly such that the zonal oriented subtropical high is split over the eastern BOB. Thus, the tropical southwesterly on the southern and eastern sides of the MOV merges into the subtropical westerly in the north, leading to active convection over the eastern BOB and western Indochina Peninsula and onset of the Asian summer monsoon.

Responses of Guangdong coastal upwelling to the summertime typhoons of 2006

The effects of typhoon intrusion on the Guangdong coastal upwelling system were investigated on the basis of in situ CTD （conductivity-temperature-depth） cruise observations and especially upward-looking ADCP （Acoustic Doppler Current Profilers） measurements obtained from a comprehensive survey of the Guangdong coastal region carded out by the Chinese Off- shore Investigation and Assessment Project in the summer of 2006. It was found that northeastward geostrophic advection driven by the summer monsoon has a significant near-seabed onshore component adjacent to Shantou, which in conjunction with upper-level offshore Ekman flow, constitutes the canonical Guangdong coastal upwelling system. Further analyses suggested that the Guangdong coastal upwelling system is sensitive to subtle changes in the typhoon intensity and migration pathway. On one hand, as a typhoon approaches from north of the upwelling system （e.g. Typhoon 0604 （Bilis） and Typhoon 0605 （Kaemi）） in the early phase of inmasion, the enhanced southwesterly leads to exceptional enhancement of the onshore flow; i.e., enhanced upwelling. Afterward, irrespective of the forced ocean responses resulting from the stronger local winds （Typhoon 0604） or the moderate typhoon-induced inertial oscillations （Typhoon 0605）, the situation is not conducive to sustaining a stable, persistent upwelling system. On the other hand, when there is typhoon intrusion south of the upwelling system （e.g. Typhoon 0606 （Prapiroon））, the favorable southwesterly tends to be substituted by an anomalous northeasterly, which destroys the traditional coastal upwelling pattern. However, the canonical upwelling system tends to recover within 1-2 days of the typhoon passing.

Unsteady Hydromagnetic Couette Flow within Porous plates in a Rotating System

Unsteady hydromagnetic Couette flow of a viscous incompressible electrically conducting fluid in a rotating system is studied when the fluid flow within the channel is induced due to the impulsive movement of the one of the plates of the channel.The plates of the channel are considered porous and the magnetic field is fixed relative to the moving plate.Exact solution of the governing equations is obtained by Laplace transform technique.The expression for the shear stress at the moving plate is also obtained.Asymptotic behaviour of the solution is analyzed for small as well as large values of time t to highlight the transient approach to the final steady state flow and the effects of rotation,magnetic field and suction/injection.It is found that suction has retarding influence on the primary as well as secondary flow where as injection and time have accelerating influence on the primary and secondary flows.