Search Results

You are looking at 31 - 40 of 105 items for

  • Author or Editor: Da-Lin Zhang x
  • All content x
Clear All Modify Search
Stefan F. Cecelski and Da-Lin Zhang

Abstract

In this study, the predictability of tropical cyclogenesis (TCG) is explored by conducting ensemble sensitivity analyses on the TCG of Hurricane Julia (2010). Using empirical orthogonal functions (EOFs), the dominant patterns of ensemble disagreements are revealed for various meteorological parameters such as mean sea level pressure (MSLP) and upper-tropospheric temperature. Using the principal components of the EOF patterns, ensemble sensitivities are generated to elucidate which mechanisms drive the parametric ensemble differences.

The dominant pattern of MSLP ensemble spread is associated with the intensity of the pre–tropical depression (pre-TD), explaining nearly half of the total variance at each respective time. Similar modes of variance are found for the low-level absolute vorticity, though the patterns explain substantially less variance. Additionally, the largest modes of variability associated with upper-level temperature anomalies closely resemble the patterns of MSLP variance, suggesting interconnectedness between the two parameters. Sensitivity analyses at both the pre-TD and TCG stages reveal that the MSLP disturbance is strongly correlated to upper-tropospheric temperature and, to a lesser degree, surface latent heat flux anomalies. Further sensitivity analyses uncover a statistically significant correlation between upper-tropospheric temperature and convective anomalies, consistent with the notion that deep convection is important for augmenting the upper-tropospheric warmth during TCG. Overall, the ensemble forecast differences for the TCG of Julia are strongly related to the processes responsible for MSLP falls and low-level cyclonic vorticity growth, including the growth of upper-tropospheric warming and persistent deep convection.

Full access
Stéphane Bélair, Da-Lin Zhang, and Jocelyn Mailhot

Abstract

In an effort to improve operational forecasts of mesoscale convective systems (MCSs), a mesoscale version of the operational Canadian Regional Finite-Element (RFE) Model with a grid size of 25 km is used to predict an intense MCS that occurred during 10–11 June 1985. The mesoscale version of the RFE model contains the Fritsch–Chappell scheme for the treatment of subgrid-scale convective processes and an explicit scheme for the treatment of grid-scale cloud water (ice) and rainwater (snow).

With higher resolution and improved condensation physics, the RFE model reproduces many detailed structures of the MCS, as compared with all available observations. In particular, the model predicts well the timing and location of the leading convective line followed by stratiform precipitation; the distribution of surface temperature and pressure perturbations (e.g., cold outflow boundaries, mesolows, mesohighs, and wake lows); and the circulation of front-to-rear flows at both upper and lower levels separated by a rear-to-front flow at midlevels.

Several sensitivity experiments are performed to examine the effects of varying initial conditions and model physics on the prediction of the squall system. It is found that both the moist convective adjustment and the Kuo schemes can reproduce the line structure of convective precipitation. However, these two schemes are unable to reproduce the internal flow structure of the squall system and the pertinent surface pressure and thermal perturbations. It is emphasized that as the grid resolution increases, reasonable treatments of both parameterized and grid-scale condensation processes are essential in obtaining realistic predictions of MCSs and associated quantitative precipitation.

Full access
Da-Lin Zhang, Ekaterina Radeva, and John Gyakum

Abstract

Despite marked improvements in the predictability of rapidly deepening extratropical cyclones, many operational models still have great difficulties in predicting frontal cyclogenesis that often begins as a mesoscale vortex embedded in a large-scale (parent) cyclone system. In this paper, a 60-h simulation and analysis of a family of frontal cyclones that were generated over the western Atlantic Ocean during 13–15 March 1992 are performed using the Pennsylvania State University–National Center for Atmospheric Research mesoscale model with a fine-mesh grid size of 30 km. Although it is initialized with conventional observations, the model reproduces well the genesis, track and intensity of the frontal cyclones, their associated thermal structure and precipitation pattern, as well as their surface circulations, as verified against the Canadian Meteorological Centre analysis and other available observations.

It is shown that each frontal cyclone is initiated successively to the southwest of its predecessor in the cold sector, first appearing as a pressure trough superposed on a baroclinically unstable basic state in the lowest 150–300 hPa. Then, it derives kinetic energy from the low-level available potential energy as it moves over an underlying warm ocean surface (with weak static stability) toward a leading large-scale frontal zone and deepens rapidly by release of latent heat occurring in its own circulations. One of the frontal cyclones, originating in the cold air mass, deepens 44 hPa in 42 h and overwhelms the parent cyclone after passing over the warm Gulf Stream water into the leading frontal zone. These cyclones have diameters ranging from 500 to 1100 km (as denoted by the last closed isobar) and are spaced 1000–1400 km apart (between their circulation centers) during the mature stage. They begin to establish their own cold/warm frontal circulations once their first closed isobars appear, thus distorting the leading large-scale frontal structures and altering the distribution and type (convective versus stratiform) of precipitation.

It is found that the frontal cyclones accelerate and experience their central pressure drops as they move from high to low pressure regions toward the parent cyclone center, and then they decelerate and fill as they travel away from the parent cyclone. Their spatial and temporal scales, vertical structures, as well as deepening mechanisms, are shown to differ significantly from those typical extratropical cyclones as previously studied.

Full access
Chanh Q. Kieu and Da-Lin Zhang

Abstract

In this study, a series of sensitivity simulations is performed to examine the processes leading to the genesis of Tropical Storm Eugene (2005) from merging vortices associated with the breakdowns of the intertropical convergence zone (ITCZ) over the eastern Pacific. This is achieved by removing or modifying one of the two vortices in the model initial conditions or one physical process during the model integration using the results presented in and as a control run. Results reveal that while the ITCZ breakdowns and subsequent poleward rollup (through a continuous potential vorticity supply) provide favorable conditions for the genesis of Eugene, the vortex merger is the most effective process in transforming weak tropical disturbances into a tropical storm. The sensitivity experiments confirm the authors’ previous conclusions that Eugene would not reach its observed tropical storm intensity in the absence of the merger and would become much shorter lived without the potential vorticity supply from the ITCZ.

It is found that the merging process is sensitive not only to larger-scale steering flows but also to the intensity of their associated cyclonic circulations and frictional convergence. When one of the vortices is initialized at a weaker intensity, the two vortices bifurcate in track and fail to merge. The frictional convergence in the boundary layer appears to play an important role in accelerating the mutual attraction of the two vortices leading to their final merger. It is also found from simulations with different storm realizations that the storm-scale cyclonic vorticity grows at the fastest rate in the lowest layers, regardless of the merger, because of the important contribution of the convergence associated with the boundary layer friction and latent heating.

Full access
Tong Zhu, Da-Lin Zhang, and Fuzhong Weng

Abstract

Due to the lack of meteorological observations over the tropical oceans, almost all the current hurricane models require bogusing of a vortex into the large-scale analysis of the model initial state. In this study, an algorithm to construct hurricane vortices is developed using the Advanced Microwave Sounding Unit (AMSU-A) data. Under rain-free atmospheric conditions, the temperature profile could be retrieved with a root-mean-square error of 1.5°C. Under heavy rainfall conditions, measurements from channels 3–5 are removed in retrieving temperatures. An application of this algorithm to Hurricane Bonnie (1998) shows well the warm-core eye and strong thermal gradients across the eyewall.

The rotational and divergent winds are obtained by solving the nonlinear balance and omega equations using the large-scale analysis as the lateral boundary conditions. In doing so, the sea level pressure distribution is empirically specified, and the geopotential heights are calculated from the retrieved temperatures using the hydrostatic equation. The so-derived temperature and wind fields associated with Bonnie compare favorably to the dropsonde observations taken in the vicinity of the storm. The initial moisture field is specified based on the AMSU-derived total precipitable water.

The effectiveness of using the retrieved hurricane vortex as the model initial conditions is tested using three 48-h simulations of Bonnie with the finest grid size of the 4-km, triply nested version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). It is found that the control run captures reasonably well the track and rapid deepening stage of the storm. The simulated radar reflectivity exhibits highly asymmetric structures of the eyewall and cloud bands, similar to the observed. A sensitivity simulation is conducted, in which an axisymmetric vortex is used in the model initial conditions. The simulated features are less favorable compared to the observations. Without the incorporation of the AMSU data, the simulated intensity and cloud structures differ markedly from the observed. The results suggest that this algorithm could provide an objective, observation-based way to incorporate a dynamically consistent vortex with reasonable asymmetries into the initial conditions of hurricane models. This algorithm could also be utilized to estimate three-dimensional hurricane flows after the hurricane warm core and eyewall are developed.

Full access
Stefan F. Cecelski and Da-Lin Zhang

Abstract

While a robust theoretical framework for tropical cyclogenesis (TCG) within African easterly waves (AEWs) has recently been developed, little work explores the development of low-level meso-β-scale vortices (LLVs) and a meso-α-scale surface low in relation to deep convection and upper-tropospheric warming. In this study, the development of an LLV into Hurricane Julia (2010) is shown through a high-resolution model simulation with the finest grid size of 1 km. The results presented expand upon the connections between LLVs and the AEW presented in previous studies while demonstrating the importance of upper-tropospheric warming for TCG.

It is found that the significant intensification phase of Hurricane Julia is triggered by the pronounced upper-tropospheric warming associated with organized deep convection. The warming is able to intensify and expand during TCG owing to formation of a storm-scale outflow beyond the Rossby radius of deformation. Results confirm previous ideas by demonstrating that the intersection of the AEW's trough axis and critical latitude is a preferred location for TCG, while supplementing such work by illustrating the importance of upper-tropospheric warming and meso-α-scale surface pressure falls during TCG. It is shown that the meso-β-scale surface low enhances boundary layer convergence and aids in the bottom-up vorticity development of the meso-β-scale LLV. The upper-level warming is attributed to heating within convective bursts at earlier TCG stages while compensating subsidence warming becomes more prevalent once a mesoscale convective system develops.

Full access
Chanh Q. Kieu and Da-Lin Zhang

Abstract

In this study, the roles of merging midlevel mesoscale convective vortices (MCVs) and convectively generated potential vorticity (PV) patches embedded in the intertropical convergence zone (ITCZ) in determining tropical cyclogenesis are examined by calculating PV and absolute vorticity budgets with a cloud-resolving simulation of Tropical Storm Eugene (2005). Results show that the vortex merger occurs as the gradual capture of small-scale PV patches within a slow-drifting MCV by another fast-moving MCV, thus concentrating high PV near the merger’s circulation center, with its peak amplitude located slightly above the melting level. The merging phase is characterized by sharp increases in surface heat fluxes, low-level convergence, latent heat release (and upward motion), lower tropospheric PV, surface pressure falls, and growth of cyclonic vorticity from the bottom upward. Melting and freezing appear to affect markedly the vertical structures of diabatic heating, convergence, absolute vorticity, and PV, as well the production of PV during the life cycle of Eugene. Results also show significant contributions of the horizontal vorticity to the magnitude of PV and its production within the storm.

The storm-scale PV budgets show that the above-mentioned amplification of PV results partly from the net internal dynamical forcing between the PV condensing and diabatic production and partly from the continuous lateral PV fluxes from the ITCZ. Without the latter, Eugene would likely be shorter lived after the merger under the influence of intense vertical shear and colder sea surface temperatures. The vorticity budget reveals that the storm-scale rotational growth occurs in the deep troposphere as a result of the increased flux convergence of absolute vorticity during the merging phase. Unlike the previously hypothesized downward growth associated with merging MCVs, the most rapid growth rate is found in the bottom layers of the merger because of the frictional convergence. It is concluded that tropical cyclogenesis from merging MCVs occurs from the bottom upward.

Full access
Yali Luo, Yu Gong, and Da-Lin Zhang

Abstract

The initiation and organization of a quasi-linear extreme-rain-producing mesoscale convective system (MCS) along a mei-yu front in east China during the midnight-to-morning hours of 8 July 2007 are studied using high-resolution surface observations and radar reflectivity, and a 24-h convection-permitting simulation with the nested grid spacing of 1.11 km. Both the observations and the simulation reveal that the quasi-linear MCS forms through continuous convective initiation and organization into west–east-oriented rainbands with life spans of about 4–10 h, and their subsequent southeastward propagation. Results show that the early convective initiation at the western end of the MCS results from moist southwesterly monsoonal flows ascending cold domes left behind by convective activity that develops during the previous afternoon-to-evening hours, suggesting a possible linkage between the early morning and late afternoon peaks of the mei-yu rainfall. Two scales of convective organization are found during the MCS's development: one is the east- to northeastward “echo training” of convective cells along individual rainbands, and the other is the southeastward “band training” of the rainbands along the quasi-linear MCS. The two organizational modes are similar within the context of “training” of convective elements, but they differ in their spatial scales and movement directions. It is concluded that the repeated convective backbuilding and the subsequent echo training along the same path account for the extreme rainfall production in the present case, whereas the band training is responsible for the longevity of the rainbands and the formation of the quasi-linear MCS.

Full access
William Miller, Hua Chen, and Da-Lin Zhang

Abstract

The impacts of the latent heat of fusion on the rapid intensification (RI) of Hurricane Wilma (2005) are examined by comparing a 72-h control simulation (CTL) of the storm to a sensitivity simulation in which the latent heat of deposition is reduced by removing fusion heating (NFUS). Results show that, while both storms undergo RI, the intensification rate is substantially reduced in NFUS. At peak intensity, NFUS is weaker than CTL by 30 hPa in minimum central pressure and by 12 m s−1 in maximum surface winds. The reduced rate of surface pressure falls in NFUS appears to result hydrostatically from less upper-level warming in the eye. It is shown that CTL generates more inner-core convective bursts (CBs) during RI, with higher altitudes of peak vertical motion in the eyewall, compared to NFUS. The latent heat of fusion contributes positively to sufficient eyewall conditional instability to support CB updrafts. Slantwise soundings taken in CB updraft cores reveal moist adiabatic lapse rates until 200 hPa, where the updraft intensity peaks. These results suggest that CBs may impact hurricane intensification by inducing compensating subsidence of the lower-stratospheric air, and the authors conclude that the development of more CBs inside the upper-level radius of maximum wind and at the higher altitude of latent heating all appear to be favorable for the RI of Wilma.

Full access
Da-Lin Zhang and J. Michael Fritsch

Abstract

The interaction between internal gravity waves and a squall line that developed early in the evolution of the 1977 Johnston flood event is studied based on available surface observations and a three-dimensional model simulation of the flood-related mesoscale convective systems (MCSs). Several experimental simulators are carried out to investigate the mechanisms whereby gravity waves form and obtain energy. Both observations and model simulators of the wave/convection interaction fit certain theories of gravity wave propagation. Following the formation of the squall line, subsequent deep convection typically initiates behind a pressure trough associated with the lint and ahead of or along the axis of the trailing ridge. The zero contours of vertical motion correspond closely to the axis of the surface pressure trough. Positive potential temperature perturbations correspond with descending motion occurring ahead of the trough while negative perturbations occur with increasing ascending motion towards the approaching ridge axis. Model airflow trajectories show that the simulated gravity wave surface pressure perturbations (with amplitudes of about 1 mb) correspond to vertical parcel displacements of more than 30 mb.

The model simulations indicate that the gravity waves am initiated by a super-geostrophic low-level jet with strong horizontal wind shear over an area where an explosive convective development occurs, and then are enhanced by intense convection. The waves propagate at a speed significantly faster than a meso-α scale quasi-geostrophic wave that is partly responsible for the initial explosive development and that later plays a key role in controlling the evolution of a mesoscale convective complex (MCC). The fag moving gravity waves help the squall line accelerate eastward and separate from a trailing area of convection that later develops into the MCC. It appears that the waves and the squall line interact with each other constructively prior to the squall line's mature stage. Specifically, the line of deep convection seems to provide the waves with energy through enhancing mass convergence/divergence in a deep layer and acting as an “obstacle” to the sheared flow. The waves tend to help organize convective elements into a line structure and turn the line a little clockwise. After the squall line moves into a convectively less favorable environment, it slows down, whereas the accompanying gravity waves continue their eastward movement. Then the convection and gravity waves gradually become out of phase and interact with each other destructively. Because of the absence of low-level inversions and critical levels to duct the wave propagation, the gravity waves quickly diminish as they move away from the energy source region. Free-wave experimental simulations show many wave characteristics similar to the control simulation, indicating that the gravity waves determine the orientation, propagation and structure of the squall line. A sea breeze circulation and mountain waves associated with the Appalachians also occur in the model simulation, but do not seem to have a significant effect on the evolution of the daytime deep convection.

The results indicate that physical interaction between deep convection and internal gravity waves can be simulated by numerical models if a compatible grid resolution, proper model physics and good initial conditions are incorporated. In particular, the apparent relationship between the gravity waves and the squall ling suggests that preserving the components of layered internal gravity waves in the model initial conditions may be very important for successful model prediction of the timing and location of wave-related MCSs.

Full access