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Da-Lin Zhang, Yubao Liu, and M. K. Yau

Abstract

Despite considerable progress in understanding the hurricane vortex using balanced models, the validity of gradient wind balance in the eyewall remains controversial in observational studies. In this paper, the structure and development of unbalanced forces and flows in hurricanes are examined, through the analyses of the radial momentum and absolute angular momentum (AAM) budgets, using a high-resolution (i.e., Δx = 6 km), fully explicit simulation of Hurricane Andrew (1992).

It is found from the radial momentum budgets that supergradient flows and accelerations, even after temporal and azimuthal averaging, are well organized from the bottom of the eye center to the upper outflow layer in the eyewall. The agradient accelerations are on average twice greater than the local Coriolis force, and caused mainly by the excess of the centrifugal force over the pressure gradient force. It is shown by the AAM budgets that supergradient flows could occur not only in the inflow region as a result of the inward AAM transport, but also in the outflow region through the upward transport of AAM. The eyewall is dominated by radial outflow in which the upward transport of AAM overcompensates the spindown effect of the outflow during the deepening stage. The intense upper outflow layer is generated as a consequence of the continuous outward acceleration of airflows in the eyewall updrafts. In spite of the pronounced agradient tendencies, results presented here suggest that the azimuthally averaged tangential winds above the boundary layer satisfy the gradient wind balance within an error of 10%.

The analyses of instantaneous fields show pronounced asymmetries and well-organized wavenumber-2 structures of the agradient flows and forces in the form of azimuthally propagating vortex–Rossby waves in the eyewall. These waves propagate cyclonically downstream with a speed half the tangential winds near the top of the boundary layer and vertically upward. Agradient flows/forces and AAM transport in the eye are also discussed.

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Yong Liu, Huopo Chen, Huijun Wang, and Yubao Qiu

ABSTRACT

The changing characteristics of lake ice phenology over the Tibetan Plateau (TP) are investigated using historical satellite retrieved datasets during 2002–15 in this study. The results indicate that the freezing process mainly starts in December, and the ice melting process generally occurs in April for most lakes. However, the changes in lake ice phenology have varied depending on the location in recent years, with delayed break-up dates and prolonged ice durations in the southern TP, but no consistent changes have occurred in the lakes in the northern TP. Further analysis presents a close connection between the variation in the lake ice break-up date/ice duration over the southern TP and the winter North Atlantic Oscillation (NAO). The positive NAO generally excites an anomalous wave activity that propagates southward from the North Atlantic to North Africa and, in turn, strengthens the African–Asian jet stream at its entrance. Because of the blocking effect of the TP, the enhanced westerly jet can be divided into two branches and the south branch flow can deepen the India–Myanmar trough, which further strengthens the anomalous cyclonic circulation and water vapor transport. Therefore, the increased water vapor transport from the northern Indian Ocean to the southern region of the TP can increase the snowfall over this region. The increased snow cover over the lake acts as an insulating layer and lowers the lake surface temperature in the following spring by means of snow–ice feedback activity, resulting in a delayed ice break-up date and the increased ice duration of the lakes over the southern TP in recent years.

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Da-Lin Zhang, Yubao Liu, and M. K. Yau

Abstract

No abstract available.

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Yubao Liu, Da-Lin Zhang, and M. K. Yau

Abstract

Despite considerable research, understanding of the temporal evolution of the inner-core structures of hurricanes is very limited owing to the lack of continuous high-resolution observational data of a storm. In this study, the results of a 72-h explicit simulation of Hurricane Andrew (1992) with a grid size of 6 km are examined to explore the inner-core axisymmetric and asymmetric structures of the storm during its rapid deepening stage. Based on the simulation, a conceptual model of the axisymmetric structures of the storm is proposed. Most of the proposed structures confirm previous observations. The main ingredients include a main inflow (outflow) in the boundary layer (upper troposphere) with little radial flow in between, a divergent slantwise ascent in the eyewall, a penetrative dry downdraft at the inner edge of the eyewall, and a general weak subsiding motion in the eye with typical warming/drying above an inversion located near an altitude of about 2–3 km. The storm deepens as the axes of these features contract.

It is found that the inversion divides the eye of the hurricane vertically into two parts, with a deep layer of warm/dry air above and a shallow pool of warm/moist air below. The air aloft descends at an average rate of 5 cm s−1 and has a residency time of several days. In contrast, the warm/moist pool consists of air from the main inflow and penetrative downdrafts, offset somewhat by the air streaming in a returning outflow into the eyewall in the lowest 2 km; it is subject to the influence of the upward heat and moisture fluxes over the underlying warm ocean. The warm/moist pool appears to play an important role in supplying high-θe air for deep convective development in the eyewall. The penetrative downdraft is dry and originates from the return inflow in the upper troposphere, and it is driven by sublimative/evaporative cooling under the influence of the (asymmetric) radial inflow of dry/cold air in the midtroposphere. It penetrates to the bottom of the eye (azimuthally downshear with a width often greater than 100 km) in a radially narrow zone along the slantwise inner edge of the eyewall.

It is further shown that all the meteorological fields are highly asymmetric. Whereas the storm-scale flow features a source–sink couplet in the boundary layer and dual gyres aloft, the inner-core structures exhibit alternative radial inflow and outflow and a series of inhomogeneous updrafts and downdrafts. All the fields tilt more or less with height radially outward and azimuthally downshear. Furthermore, pronounced fluctuations of air motion are found in both the eye and the eyewall. Sometimes, a deep layer of upward motion appears at the center of the eye. All these features contribute to the trochoidal oscillation of the storm track and movement. The main steering appears to be located at the midtroposphere (∼4.5 km) and the deep-layer mean winds represent well the movement of the hurricane.

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Yubao Liu, Da-Lin Zhang, and M. K. Yau

Abstract

In this study, the inner-core structures of Hurricane Andrew (1992) are explicitly simulated using an improved version of the Penn State–NCAR nonhydrostatic, two-way interactive, movable, triply nested grid mesoscale model (MM5). A modified Betts–Miller cumulus parameterization scheme and an explicit microphysics scheme were used simultaneously to simulate the evolution of the larger-scale flows over the coarser-mesh domains. The intense storm itself is explicitly resolved over the finest-mesh domain using a grid size of 6 km and an explicit microphysics package containing prognostic equations for cloud water, ice, rainwater, snow, and graupel. The model is initialized with the National Centers for Environmental Prediction analysis enhanced by a modified moisture field. A model-generated tropical-storm-like vortex was also incorporated. A 72-h integration was made, which covers the stages from the storm’s initial deepening to a near–category 5 hurricane intensity and the landfall over Florida.

As verified against various observations and the best analysis, the model captures reasonably well the evolution and inner-core structures of the storm. In particular, the model reproduces the track, the explosive deepening rate (>1.5 hPa h−1), the minimum surface pressure of 919 hPa preceding landfall, the strong surface wind (>65 m s−1) near the shoreline, as well as the ring of maximum winds, the eye, the eyewall, the spiral rainbands, and other cloud features. Of particular significance is that many simulated kinematics, thermodynamics, and precipitation structures in the core regions compare favorably to previous observations of hurricanes.

The results suggest that it may be possible to predict reasonably the track, intensity, and inner-core structures of hurricanes from the tropical synoptic conditions if high grid resolution, realistic model physics, and proper initial vortices (depth, size, and intensity) in relation to their larger-scale conditions (e.g., SST, moisture content, and vertical shear in the lower troposphere) are incorporated.

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Da-Lin Zhang, Yubao Liu, and M. K. Yau

Abstract

In this study, the vertical force balance in the inner-core region is examined, through the analysis of vertical momentum budgets, using a high-resolution, explicit simulation of Hurricane Andrew (1992). Three-dimensional buoyancy- and dynamically induced perturbation pressures are then obtained to gain insight into the processes leading to the subsidence warming in the eye and the vertical lifting in the eyewall in the absence of positive buoyancy.

It is found from the force balance budgets that vertical acceleration in the eyewall is a small difference among the perturbation pressure gradient force (PGF), buoyancy, and water loading. The azimuthally averaged eyewall convection is found to be conditionally stable but slantwise unstable with little positive buoyancy. It is the PGF that is responsible for the upward acceleration of high-θ e air in the eyewall. It is found that the vertical motion and acceleration in the eyewall are highly asymmetric and closely related to the azimuthal distribution of radial flows in conjunction with large thermal and moisture contrasts across the eyewall. For example, the radially incoming air aloft is cool and dry and tends to suppress updrafts or induce downdrafts. On the other hand, the outgoing flows are positively buoyant and tend to ascend in the eyewall unless evaporative cooling dominates. It is also found that the water loading effect has to be included into the hydrostatic equation in estimating the pressure or height field in the eyewall.

The perturbation pressure inversions show that a large portion of surface perturbation pressures is caused by the moist-adiabatic warming in the eyewall and the subsidence warming in the eye. However, the associated buoyancy-induced PGF is mostly offset by the buoyancy force, and their net effect is similar in magnitude but opposite in sign to the dynamically induced PGF. Of importance is that the dynamically induced PGF points downward in the eye to account for the maintenance of the general descent. But it points upward in the outer portion of the eyewall, particularly in the north semicircle, to facilitate the lifting of high-θ e air in the lower troposphere. Furthermore, this dynamic force is dominated by the radial shear of tangential winds. Based on this finding, a new theoretical explanation, different from previously reported, is advanced for the relationship among the subsidence warming in the eye, and the rotation and vertical wind shear in the eyewall.

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Da-Lin Zhang, Yubao Liu, and M. K. Yau

Abstract

Although considerable progress has been made in understanding the development of hurricanes, our knowledge of their three-dimensional structures of latent heat release and inner-core thermodynamics remains limited. In this study, the inner-core budgets of potential temperature (θ), moisture (q), and equivalent potential temperature (θe) are examined using a high-resolution (Δx = 6 km), nonhydrostatic, fully explicit simulation of Hurricane Andrew (1992) during its mature or intensifying stage.

It is found that the heat energy is dominated by latent heat release in the eyewall, sublimative–evaporative cooling near the eye–eyewall interface, and the upward surface fluxes of sensible and latent heat from the underlying warm ocean. The latent heating (θ) rates in the eyewall range from less than 10°C h–1 to greater than 100°C h–1, depending upon whether latent heat is released in radial inflow or outflow regions. The latent heating rates decrease inward in the inflow regions and become negative near the eye–eyewall interface. It is shown that the radial θ advective cooling in the inflow regions accounts for the initiation and maintenance of the penetrative downdrafts at the eye–eyewall interface that are enhanced by the sublimative-evaporative cooling. It is also shown that the vertical θ advection overcompensates the horizontal θ advection for the generation of the warm-cored eye, and the sum of latent heating and radial advective warming for the development of intense cooling in the eyewall. The moisture budgets show the dominant upward transport of moisture in the eyewall updrafts (and spiral rainbands), partly by the low-level outflow jet from the bottom eye regions, so that the eyewall remains nearly saturated.

The θe budgets reveal that θe could be considered as an approximately conserved variable in the eyewall above the boundary layer even in the presence of deposition–sublimation and freezing–melting. The development of higher-θe surfaces at the eye–eyewall interface is discussed in the context of deep convection, the θe gradient and the mass recycling across the eyewall. It is concluded that the simulated hurricane is thermodynamically maintained by the upward surface flux of higher-θe air from the underlying warm ocean, the descent of higher-θe air in the upper troposphere along the eye–eyewall interface, and the recycling of some warmed-eye air at the eye–eyewall interface.

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M. K. Yau, Yubao Liu, Da-Lin Zhang, and Yongsheng Chen

Abstract

The objectives of Part VI of this series of papers are to (a) simulate the finescale features of Hurricane Andrew (1992) using a cloud-resolving grid length of 2 km, (b) diagnose the formation of small-scale wind streaks, and (c) perform sensitivity experiments of varying surface fluxes on changes in storm inner-core structures and intensity.

As compared to observations and a previous 6-km model run, the results show that a higher-resolution explicit simulation could produce significant improvements in the structures and evolution of the inner-core eyewall and spiral rainbands, and in the organization of convection. The eyewall becomes much more compact and symmetric with its width decreased by half, and the radius of maximum wind is reduced by ∼10 to 20 km. A zone of deep and intense potential vorticity (PV) is formed at the edge of the eye. A ring of maximum PV is collocated in regions of maximum upward motion in the eyewall and interacts strongly with the eyewall convection. The convective cores in the eyewall are associated with small-scale wind streaks.

The formation of the wind streaks is diagnosed from an azimuthal momentum budget. The results reveal small-scale Lagrangian acceleration of the azimuthal flow. It is found that at the lowest model level of 40 m, the main contributor to the Lagrangian azimuthal wind tendency is the radial advection of angular momentum per unit radius. At an altitude of 1.24 km, vertical advection of the azimuthal wind, in addition to the radial advection of angular momentum per unit radius, plays important roles.

Results of a series of sensitivity tests, performed to examine the impact of several critical factors in the surface and boundary layer processes on the inner-core structures and the evolution of the hurricane intensity, are presented.

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Ying-Hwa Kuo, Richard J. Reed, and Yubao Liu

Abstract

Modeling studies have consistently shown the importance of latent heat release in explosive marine cyclogenesis. However, a systematic evaluation of precipitation parameterization in the simulation of marine cyclones has been rare in the literature. This paper is the third in a series of modeling studies on the ERICA IOP 5 storm. The objective is to assess the performance of various subgrid-scale cumulus parameterization and resolvable-scale microphysics schemes in the simulation of the storm using the Penn State–NCAR mesoscale model MM5 at grid resolutions of 20 and 60 km. Emphasis is placed on the intensity, distribution, and character of precipitation and on the mesoscale low pressure centers embedded within the synoptic-scale cyclone. Principal findings are as follows.

  • The distribution and intensity of precipitation, its partitioning into grid-resolvable and subgrid-scale portions, the atmospheric thermodynamic structure in the precipitation region, and the evolution of mesoscale low pressure centers were extremely sensitive to the choice of cumulus parameterization scheme. This is true for both the 20- and 60-km MM5.
  • The partitioning of precipitation into subgrid scale and resolvable scale for a given convective parameterization is nearly the same for both the 20- and 60-km models.
  • The detailed cold-cloud microphysics did not have a significant impact on cyclone deepening for tests carried out on the 20-km grid.
  • The CAPE-based scheme developed by Kain and Fritsch gave the best simulation of this explosive marine cyclone on both the 60- and 20-km grids. This scheme effectively consumed the convective instability and captured the evolution of two observed mesolows.
  • Analysis of the simulated mesolows showed that rapid intensification took place just in advance of a strong upper-level PV maximum. At low levels, the mesolows were characterized by large, diabatically produced PV maxima and by nearly coincident rainfall maxima.
  • A number of PV-rainfall maxima, less visible in the pressure field, with a spacing of 150 km along the cold front, were also simulated in the 20-km Kain–Fritsch experiment. The realism of these PV-rainfall maxima cannot be confirmed due to the lack of observations. Similar features with enhanced pressure signature were seen in the simulations with the Grell scheme and no-cumulus scheme.
  • The number, intensity, and evolution of mesolows were highly variable in the 20-km simulations of this case with different convective parameterization schemes. The amount of mesoscale perturbation of the pressure fields appears to be inversely proportional to the percentage of convective rainfall, being the least in the Kuo scheme (with 90% convective rainfall) and the most in the scheme without subgrid-scale convective parameterization (0%). No mesolows were simulated in an experiment that excluded latent heat release.
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Amir Givati, Barry Lynn, Yubao Liu, and Alon Rimmer

Abstract

The Weather Research and Forecasting (WRF) model was employed to provide precipitation forecasts during the 2008/09 and 2009/10 winters (wet season) for Israel and the surrounding region where complex terrain dominates. The WRF precipitation prediction has been coupled with the Hydrological Model for Karst Environment (HYMKE) to forecast the upper Jordan River streamflow. The daily WRF precipitation forecasts were verified against the measurements from a dense network of rain gauges in northern and central Israel, and the simulation results using the high-resolution WRF indicated good agreement with the actual measurements. The daily precipitation amount calculated by WRF at rain gauges located in the upper parts of the Jordan River basin showed good agreement with the actual measurements. Numerical experiments were carried out to test the impact of the WRF model resolution and WRF microphysical schemes, to determine an optimal model configuration for this application. Because of orographic forcing in the region, it is necessary to run WRF with a 4–1.3-km grid increment and with sophisticated microphysical schemes that consider liquid water, ice, snow, and graupel to produce quality precipitation predictions. The hydrological modeling system that ingests the high-resolution WRF forecast precipitation produced good results and improved upon the operational streamflow forecast method for the Jordan River that is now in use. The modeling tools presented in this study are used to support the water-resource-assessment process and studies of seasonal hydroclimatic forecasting in this region.

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