Search Results

You are looking at 1 - 10 of 54 items for

  • Author or Editor: Yuh-Lang Lin x
  • Refine by Access: All Content x
Clear All Modify Search
Yuh-Lang Lin

Abstract

A two-dimensional, linearized problem in a stratified shell flow with either isolated heating or differential heating is investigated. In response to isolated heating with the heating top below the wind reversal height, the low-level vertical velocity near the heating center may be positive or negative depending upon the Richardson number associated with the basic flow and the depth of the heating layer. The phenomenon can be explained by the advection effect of the basic flow. In response to differential heating associated with the low-level temperature contrast, a sharp contrast of upward and downward motion is produced near the center of the differential heating. Similar to the isolated heating case, the low-level upward motion may be located over the warm or cold side. Convection may be triggered or maintained if the upward motion is located over the side where the supply of moisture is abundant in a mesoscale circulation. In response to thermal forcing in the vicinity of the critical level, the vertical velocity is almost positive near the heating center in the heating layer for a wide range of Richardson number. The consistency of the vertical motion and the heating at the heating base is important in supporting the existing convection. The disturbance is mainly a superposition of the gravity waves generated separately by the heating from below and above the critical level. The condensational heating in the vicinity of the critical level may play an important role, in the interaction of the flow below and above the critical level in a moist convection.

Full access
Yuh-Lang Lin

Abstract

The stably stratified airflow over a three-dimensional elevated heat source is investigated using the linearized equations of motion. A low-level upward motion can be produced for airflow over a prescribed, isolated heat source for a wide variety of mean wind speeds. Above the heating layer, a V-shaped region of upward displacement is formed by the action of the mean wind on the upward propagating waves. The horizontal pattern of the heat source is important in determining the formation of the V-shaped region of upward displacement A high-pressure region is produced in the vicinity of the heat source at the top of the heating layer. The response of a hydrostatic airflow to a transient heating is a V-shaped region of upward displacement with an embedded region of downward displacement above the heated layer. The whole system advects downstream with a slower speed than the mean wind and eventually disperses. A region of strong divergence is associated with the region of upward displacement above the heated layer. In relation to the thunderstorm generated V-shaped clouds, the cold (warm) can be explained by the adiabatic cooling (warming) associated with the upward (downward) displacement. In addition, the upwind displacement of the cold area in the upper level may be explained as a gravity wave type phenomenon.

Full access
Yuh-Lang Lin

Abstract

A linear quasi-geostrophic theory of coastal cyclogenesis proposed by Lin has been extended by a semigeostrophic model. The response of an east–west backsheared, quasi-geostrophic baroclinic flow over an isolated heat source is a low pressure near the heating center and a weaker high pressure downstream, as found in part I of the theory. With the inclusion of nonlinear geostrophic advection, the low is weakened slightly and becomes asymmetric, while the high remains about the same strength. With the inclusion of nonlinear ageostrophic advection, the low is strengthened significantly by the warm air advection and becomes more asymmetric.

With the addition of uniform friction, the cyclogenesis process is weakened. However, the cyclone is strengthened slightly by differential friction. It appears that the primary source of the cyclonic vorticity of the coastal cyclone is the hydrostatic response of the diabatic heating modified by the baroclinicity. With a mountain ridge included, there exists an inverted pressure ridge (trough) over (downstream of) the mountain. A damming effect is evidenced by a pool of cold air located upslope of the mountain ridge, which is formed by the cold air advection and upslope adiabatic cooling. The inverted pressure ridge between the mountain and the heat source is strengthened by the surface heating and is shifted farther upstream of the mountain.

When the semigeostrophic model is applied to East Coast cyclogenesis with a northeasterly surface wind, a cyclone develops near the western boundary of the Gulf Stream. The low is skewed with a larger gradient located to the southeast corner. The mountain-induced high pressure is relatively weak since the surface wind is almost parallel to the mountain ridge. The cold-air damming is negligible and the Appalachians play a minor role in this type of coastal cyclogenesis. With an easterly surface wind, the cold-air damming is more pronounced. The inverted ridge or the damming area is shifted further upstream and more widespread in the region between the southern part of the Appalachians and the Gulf Stream. A confluent–diffluent couplet is produced to the northwest and southwest of the heat source. The results are consistent with observations.

Full access
Yuh-Lang Lin

Abstract

A quasi-geostrophic theory of cyclogenesis forced by a low-level diabatic heating in a backsheared baroclinic flow is proposed. Existence of wind reversal in one direction of the basic flow is an essential criterion to obtain a forced baroclinic wave in the vicinity of the heating region. It was found that an analogy exists for a quasi-geostrophic flow over a mountain and a region of steady-state diabatic heating. The relationship is described by h(x, y) = (− g/T0 N 2)T(x, y), where h(x, y) is the mountain shape and T(x, y) is the temperature anomaly.

The response of a backsheared baroclinic flow over a region of two-dimensional diabatic heating (cooling) is a coupled low–high (high–low) pressure pair located in the vicinity and on the downstream side of the heating (cooling), respectively. Physically, the growth of the pressure perturbation can be explained by a group velocity argument. The disturbance remains locally in the vicinity of the forcing due to zero phase speed of the forced baroclinic wave. The upshear phase tilt indicates that the disturbance is a baroclinic wave generated by diabatic forcing.

The response of an east–west backsheared baroclinic flow over an isolated region of diabatic heating with circular contours is a growing cyclone located near the center of the heat source. A coupled high pressure forms downstream of the diabatic heating. The disturbance is confined in a shallow layer. The forced low resembles the geometry of the heat source in the early stage. One interesting finding is that an inverted ridge forms downstream of the low.

When applied to East Coast cyclogenesis, a cyclone develops near the center of the region of maximum diabatic heating, i.e., near the western boundary of the Gulf Stream. The cutoff low remains in the vicinity of the diabatic heat source. Two regions of weaker high pressure form to the southeast and northwest corners of the low. To the south of the low, there exists a strong anticyclonic circulation. A confluent zone forms to the northeast of the low, while a diffluent zone forms to the southwest of the low. The genesis region and the flow pattern of the cyclone predicted by the theory are consistent with observations. With an easterly wind at the surface, the inverted trough–ridge couplet is more pronounced than with a northeasterly wind. The low starts to decay as it moves out of the concentrated heating region. The cyclone is produced hydrostatically by the less dense air above the heating region with the modification of the baroclinic effects.

Full access
Yuh-Lang Lin

Abstract

No abstract available.

Full access
Yuh-Lang Lin

Abstract

Inertial and frictional effects on stratified hydrostatic airflow past an isolated warm region are investigated by linear theories. For an inviscid quasi-geostrophic flow, there exists in the lower layer upward motion upstream and downward motion downstream of the warm region. The vertical velocity field is in phase with the diabatic heating and cooling. As expected, regions of high buoyancy, low pressure and positive vorticity are produced in the vicinity of the warm region. Strong vortex stretching occurs near the center of the warm region, accompanied by two regions of weak vortex compression upstream and downstream. With the inertial effects included, the vertical motion and the vorticity are strengthened. The horizontal wind experiences a much stronger cyclonic circulation near the diabatic source.

For a flow with a larger Rossby number O(1), the advection effect of the basic flow is dominant. U-shaped patterns of disturbance are pronounced, which are associated with the upward propagating inertia-gravity waves. The wind is deflected cyclonically around the region of positive relative vorticity and is advected downstream of the center of the warm region, rather than around the region of the low pressure.

The frictional effects are investigated by the addition of an Ekman friction layer to a quasi-geostrophic flow. There are three significant features of the disturbance: (i) an upstream-downstream asymmetry, (ii) an upstream phase tilt in the lower layer, and (iii) weakening of the positive relative vorticity and low pressure. Items (i) and (ii) are explained by the upward motion and vorticity and the advection of the basic flow on the disturbance induced by the Ekman friction. The weakening of the positive relative vorticity and the low pressure can be explained as the spindown process of the interior flow to the Ekman friction.

Full access
Mark S. Kulie
and
Yuh-Lang Lin

Abstract

The structure and evolution of a high-precipitation (HP) supercell thunderstorm is investigated using a three-dimensional, nonhydrostatic, cloud-scale numerical model (TASS). The model is initialized with a sounding taken from a mesoscale modeling study of the environment that produced the 28 November 1988 Raleigh tornadic thunderstorm. TASS produces a long-lived convective system that compares favorably with the observed Raleigh tornadic thunderstorm. The simulated storm evolves from a multicell-type storm to a multiple-updraft supercell storm. The storm complex resembles a hybrid multicell-supercell thunderstorm and is consistent with the conceptual model of cool season strong dynamic HP supercells that are characterized by shallow mesocyclones. The origin of rotation in this type of storm is often in the lowest levels.

Interactions between various cells in the simulated convective system are responsible for the transition to a supercellular structure. An intense low-level updraft core forms on the southwest flank of the simulated storm and moves over a region that is rich in vertical vorticity. The stretching of this preexisting vertical vorticity in the storm’s lowest levels is the most important vertical vorticity production mechanism during the initial stages of the main updraft’s development. Interactions with an extensive cold pool created by the storm complex are also important in producing vertical vorticity as the main updraft grows. Overall, the development of vorticity associated with the main updraft appears similar to nonsupercellular tornadic storms. However, classic supercell signatures are seen early in the simulation associated with other updrafts (e.g., formation of vortex couplet due to tilting of ambient horizontal vorticity, storm splitting, etc.) and are deemed important.

In the storm’s supercell stage, rotation is sustained in the lowest levels of the storm despite large amounts of precipitation located near and within the main mesocyclone. Pulsating downdrafts periodically invigorate the storm and the gust front never occludes, thus allowing the main updraft to persist for a prolonged period of time. The storm’s intensity is also maintained by frequent updraft mergers.

Full access
Lian Xie
and
Yuh-Lang Lin

Abstract

This paper presents the results from a numerical investigation of the responses of stratified airflow to prescribed near-surface mesoscale axisymmetric (circular) and elongated (elliptical) heat sources under uniform basic wind conditions using a simple three-dimensional model. Model results indicate that the structure of the response depends on the Froude number (Fr = V/NH) and the Rossby number (Ro = V/fL) associated with an axisymmetric thermal forcing, where H and L are the vertical and horizontal scales of the heat source, V the wind speed, N the Brunt-Väisälä frequency, and f the Coriolis parameter. However, the response to an elongated thermal forcing depends not only on the Froude and Rossby numbers, but also on the ratio of the alongstream scale (Ls ) to the cross-stream scale (Ln ) of the heat source, which are determined by the horizontal shape of the heat source and the direction of the basic flow. When Ls /Ln is less than 1, that is, the heat source is elongated in the cross-stream direction, the response resembles that of two-dimensional solutions. In this case, the heat-induced vertical motion is in phase with the heating field when Fr < 1 and is out of phase with the heating field when Fr > 1. Meanwhile, vertically propagating inertia-gravity waves are induced. However, when Ls /Ln > 1, that is, the heat source is elongated in the alongstream direction, the response appears quite different. In this case, the heat-induced vertical motion is primarily upward but confined to the lower atmosphere. The phase relationship between the vertical motion and the heating cannot be determined by the Froude number.

The horizontal distribution of surface sensible heat fluxes observed off the Carolina coast often appears to be elongated along the Gulf Stream front. These surface heat sources can induce coastal frontogenesis. In order to explain the effect of surface sensible heat fluxes on Carolina coastal frontogenesis, the shape factor of the heat source and the basic wind direction need to be taken into account.

Full access
Yuh-Lang Lin
and
I-Chun Jao

Abstract

In this study, the authors have conducted a series of numerical experiments to investigate the flow circulations in the Central Valley of California and the formation mechanisms of the Fresno eddy. The authors have found the following:

  1. Under an adiabatic northwesterly, low–Froude number flow over the Central Valley, two cyclonic vortices form in the basin. One is located on the lee slope of the northern Coastal Range, while the other is located to the south of the San Joaquin Valley. The first may be identified as the Sacramento eddy, while the second may be identified as the Fresno eddy, although the Fresno eddy is located slightly farther to the south. The formation of the Sacramento eddy may be explained by either the generation of potential vorticity (Smith) or the generation of vorticity due to baroclinicity (Smolarkiewicz and Rotunno) on the lee slope in a low–Froude number flow. The Sacramento eddy may also be classified as a lee mesocylone since it is collocated with a lee mesolow (Lin et al.). In addition, a northwesterly jet forms at the gap of the Coastal Range due to the channeling effect.

  2. The Fresno eddy forms when the low–Froude number northwesterly flow meets the return flow from the Tehachapi Mountains in a rotating fluid system and is strengthened and expands farther to the north due to the effects of nocturnal radiative cooling. The northwesterly jet in the Central Valley, the southeasterly wind from the foothills of the Sierra Nevada, and the blocking effect due to the Tehachapi Mountains all play important roles in the formation of the Freano eddy. The Sacramento eddy moves eastward to the foothills of the Sierra Nevada, while the jet at the gap of the Coastal Range is suppressed when nocturnal radiative cooling is present.

  3. The nocturnal drainage flow over the western slope of the Sierra Nevada is weakened by the southerly return flow from the Tehachapi Mountains. The simulations indicate that in the absence of nocturnal radiative cooling the Fresno eddy still forms but is weaker and is located new the southern end of the San Joaquin Valley.

  4. The Fresno eddy will form in an environment characterized by low–Froude number northwesterly wind. Suitable incoming flow speed and direction are among the major factors in determining the formation and strength of the Fresno eddy.

  5. The return flow from the southern boundary of the San Joaquin Valley plays an important role in the formation of the Fresno eddy.

  6. The Fresno eddy does not form in the absence of planetary rotation.

  7. The β effect plays a negligible role in the formation of the Fresno eddy.

Full access
Sen Chiao
and
Yuh-Lang Lin

Abstract

An orographic rainfall event that occurred on 6–7 August 1999 during the passage of Tropical Storm (TS) Rachel over Taiwan is investigated by performing triply nested, nonhydrostatic numerical simulations using the Naval Research Laboratory's (NRL) Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. By examining both observational data and numerical model output, it is found that this orographic rainfall event may be separated into three distinct stages. During the first stage (0000–1200 UTC 6 August), TS Rachel was located in the South China Sea and tracked northeastward to Taiwan. Meanwhile, TS Paul was steered by the subtropical high over southwest Japan. During the second stage (1200 UTC 6 August–0000 UTC 7 August), the southwesterly monsoon current as well as the circulation of TS Rachel over southwest Taiwan strengthened and formed a low-level jet (LLJ) with high equivalent potential temperature when TS Rachel moved closer to Taiwan. In comparing the control and sensitivity (without orography) experiments, it was found that the strong LLJ triggered convective systems in the concave region of the southwest Central Mountain Range (CMR), which then produced the first episode of heavy rainfall. The second episode of heavy rainfall, which occurred during the third stage (0000–1800 7 August), was attributed to the modification of TS Rachel's own rainbands by the mountains as well as the strong southwesterly flow impinging on the mountains. The low-level convergence was propagated upstream over the sea, and the impinging flow from southwest Taiwan produced new convective cells. The orographic vertical moisture flux, which is a product of low-level horizontal velocity, mountain steepness, and moisture content, is calculated based on the fine-resolution model output. The regions of maximum moisture flux roughly coincide with the heavy-rainfall regions over the island during this event, while the regions of the general vertical moisture flux coincide with the heavy-rainfall regions over the ocean. Hence, the orographic vertical moisture flux may serve as an index for predicting this type of upslope orographic heavy rainfall. Overall, the model is able to predict the storm track, rainbands, and period of rainfall reasonably well over southern Taiwan, although the maximum rainfall may be slightly overpredicted. Nevertheless, the model results also suggest that a finer-resolution domain or vortex bogusing might be needed for the simulation of precipitation in association with a tropical storm over complex terrain.

Full access