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Rudi Xia, Da-Lin Zhang, Cuihong Zhang, and Yongqing Wang

Abstract

This study examines whether environmental conditions can control convective rainfall rates and cloud-to-ground (CG) lightning frequencies in mesoscale convective systems (MCSs) over north China (NC). A total of 60 identified MCSs over NC during June–August of 2008–13 were classified into 4 categories based on their high/low convective rainfall rates (HR/LR) and high/low CG lightning frequencies (HL/LL) (i.e., HRHL, HRLL, LRHL, and LRLL MCSs). MCSs with HR (HL) occurred most frequently in July (August), while those with LR or LL occurred most frequently in June; they followed closely seasonal changes. All MCSs were apt to form during afternoon hours. HRLL MCSs also formed during evening hours while HRHL MCSs could occur at any time of a day. A composite analysis of environmental conditions shows obvious differences and similarities among the HRHL, HRLL, and LRLL categories, while the LRHL MCSs exhibited little differences from the climatological mean because of its small sample size. Both the HRHL and HRLL MCSs occurred in the presence of upper-level anomalous divergence, a midlevel trough, and the lower-tropospheric southwesterly transport of tropical moist air. In contrast, LRLL MCSs took place as a result of daytime heating over mountainous regions, with little midlevel forcing over NC. The HRHL, HRLL, LRHL, and LRLL categories exhibited orders of the highest-to-smallest convective available potential energy and precipitable water but the smallest-to-largest convective inhibition and lifted indices. It is concluded that environmental conditions determine to some extent convective rainfall rates and CG lightning activity, although some other processes (e.g., cloud microphysics) also play certain roles, especially in CG lightning production.

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Stefan F. Cecelski, Da-Lin Zhang, and Takemasa Miyoshi

Abstract

In this study, the predictability of and parametric differences in the genesis of Hurricane Julia (2010) are investigated using 20 mesoscale ensemble forecasts with the finest resolution of 1 km. Results show that the genesis of Julia is highly predictable, with all but two members undergoing genesis. Despite the high predictability, substantial parametric differences exist between the stronger and weaker members. Notably, the strongest-developing member exhibits large upper-tropospheric warming within a storm-scale outflow during genesis. In contrast, the nondeveloping member has weak and more localized warming due to inhibited convective development and a lack of a storm-scale outflow. A reduction in the Rossby radius of deformation in the strongest member aids in the accumulation of the warmth, while little contraction takes place in the nondeveloping member. The warming in the upper troposphere is responsible for the development of meso-α-scale surface pressure falls and a meso-β surface low in the strongest-developing member. Such features fail to form in the nondeveloping member as weak upper-tropospheric warming is unable to induce meaningful surface pressure falls. Cloud ice content is nearly doubled in the strongest member as compared to its nondeveloping counterparts, suggesting the importance of depositional heating of the upper troposphere. It is found that the stronger member undergoes genesis faster due to the lack of convective inhibition near the African easterly wave (AEW) pouch center prior to genesis. This allows for the faster development of a mesoscale convective system and storm-scale outflow, given the already favorable larger-scale conditions.

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Da-Lin Zhang, Zuohao Cao, Jianmin Ma, and Aiming Wu

Abstract

The summer nonconvective severe surface wind (NCSSW) frequency over Ontario, Canada, in relation to regional climate conditions and tropical Pacific Ocean sea surface temperatures (SSTs) during the period of 1979–2006 is examined using surface wind reports and large-scale analysis data. A statistically robust positive trend in Ontario summer NCSSW frequency is identified using three independent statistical approaches, which include the conventional linear regression that has little disturbance to the original time series, the Mann–Kendall test without a lag-1 autoregressive process, and the Monte Carlo simulation. A composite analysis of the large-scale monthly mean data reveals that the high- (low-) NCSSW occurrence years are linked to stronger (weaker) large-scale horizontal pressure gradients and more (less) intensive vector wind anomalies in the upper troposphere. Unlike the low-event years, anomalous anticyclonic circulations are found at 500 and 250 hPa in the high-event years, which are conducive to downward momentum transport and favorable for severe surface wind development. It is also found that the summer NCSSW occurs more frequently under the conditions of warmer surface air temperature over Ontario. Further analyses indicate that an increase in the summer NCSSW frequency is well correlated with an increase in the previous winter SSTs over the eastern equatorial Pacific, namely, in the Niño-1+2 and Niño-3 areas, through a decrease in sea level pressure over northern Ontario and an increase in surface air temperature over central and southern Ontario.

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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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Da-Lin Zhang, Menglin S. Jin, Yixuan Shou, and Chunqing Dong

Abstract

This paper examines the collective impacts of urban building complexes on the planetary boundary layer (PBL) winds using both observations and a mesoscale model. Horizontal winds measured on the rooftops of federal buildings over the regions of Washington, D.C., and a small city nearby (i.e., Reston, Virginia) show the blocking effects of urban building complexes on the downstream winds during the daytime of 9 July 2007. A modeling study of the case using a coupled version of the Weather Research and Forecasting (WRF)–multilayer urban canopy model in which the observed building height and density information is implemented to advance the calculations of momentum and heat, reproduces the rooftop-observed wind patterns and the related urban heat island effects, especially the wake flows on the downstream sides of the above-mentioned two cities. Results show that under daytime conditions the building complexes can collectively form a mesoscale wake on the downwind side of each city, about 2–10 km away, horizontally from the edge of the building complexes. The wake flow may extend to much higher levels than the building tops, depending on the incoming flow strength, the static stability in the PBL, the height of the building complexes, and the time of the day because of the strength of surface insolation.

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Da-Lin Zhang, Wei-Zhong Zheng, and Yong-Kang Xue

Abstract

The Pennsylvania State University–NCAR Mesoscale Model (MM5) and a simplified simple biosphere (SSiB) scheme are modified and then coupled to study various regional climate and weather problems. These modifications include correcting the moisture and cloud hydrometeor fields to ensure the mass conservation; incorporating the effects of dissipative heating to ensure total energy conservation; decoupling soil and vegetation types in specifying various surface parameters; and eliminating the shortwave radiation reaching the surface at points where deep convection occurs.

A 30-day integration of June 1998 over the Midwest states was used to examine the model's capability in capturing the observed wet regional climate and the passage of several mesoscale weather events. It is found that the coupled model reproduces the distribution and magnitude of monthly accumulated precipitation, the time series of area-integrated precipitation, surface pressures, and diurnal changes in surface temperatures, low-level winds and precipitation, as well as the evolution of precipitation systems across the central United States. In particular, the model reproduces well many daily weather events, including the distribution and intensity of low-level temperature and pressure perturbations and precipitation, even up to a month. The results suggest that the daily temperature, clouds, and precipitation events from the weekly to monthly scales, as well as their associated regional climate phenomena, could be reasonably simulated if the surface, boundary layer, radiation, and convective processes are realistically parameterized, and the large-scale forcing could be reasonably provided by general circulation models.

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Da-Lin Zhang, Kun Gao, and David B. Parsons

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A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-divergence flow structure; 6) the evolution of a traveling 700 mb shortwave; 7) the development of a rear-inflow jet; 8) the leading convective rainfall followed by a transition zone and trailing stratiform precipitation; 9) the observed configuration of front-to-rear relative flow at both upper and lower levels separated by the rear-to-front flow at midlevels; 10) the simulation of “onion-shaped” soundings; 11) the splitting of the wake low; 12) the maintenance and intensification of a mesovortex; 13) the distribution and magnitude of convective and stratiform rainfall; and 14) the diurnal cycle of the planetary boundary layer.

One of the encouraging results is that the model accurately simulates the rear-inflow jet as verified against Doppler windprofiler data after the 18-h integration from essentially synoptic-scale initial conditions. The results confirm the previously proposed hypothesis that the wake low develops hydrostatically as a consequence of adiabatic warming by descending flow entering the squall line within the rear-inflow jet The observed “onion-shaped” soundings are a manifestation of the warming and drying of air within the descending rear inflow jet. It is found that the present wake low is not a transient meso-β scale phenomenon, but has a time scale of more than 50% of the squall line lifetime. Another finding is that the present mesovortex is not produced by latent heat release associated with the squall line but was in existence prior to the model initialization time. The vortex appears to have a significant effect on the distribution of the rainfall associated with the squall line and on the intensity of the rear-inflow jet. Other mesoscale circulation features are also documented in this paper.

This study, along with previous investigations using the model, indicates that the meso-β scale structure and evolution of MCSs under certain synoptic-scale environmental conditions can be well simulated using the standard network observations if compatible grid resolution, reasonable model physics and initial conditions are utilized.

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Zonghui Huo, Da-Lin Zhang, John Gyakum, and Andrew Staniforth

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In this study, the synoptic evolution of the March 1993 superstorm is documented using conventional observations, and the predictability and deepening mechanisms of the storm are investigated using a newly implemented mesoscale version of the Canadian Regional Finite-Element (RFE) Mode. It is found that although the RFE model can predict well the large-scale background flow associated with the storm, it fails to predict various important mesoscale elements when it is initialized with fields that contain weak signals of the low-level circulations. However, when the storm is located close to the data-rich region, the model has considerable skill in predicting those mesoscale elements, such as a prefrontal squall line, upper- and low-level jets, as well as the quantitative aspects of their associated precipitation.

Observational analysis reveals that the storm developed in a convectively unstable prestorm environment with ample moisture content. It first experienced an antecedent surface vorticity growth over the anomalously warm water of the Gulf of Mexico mainly through condensation processes. Then the storm deepened rapidly as two midlevel short-wave troughs approached, merged, and interacted with a thermally direct ageostrophic circulation in the entrance region of an upper-level jet streak. A potential vorticity (PV) diagnosis reveals that the intensification of the midlevel troughs is related to the pronounced descent of stratospheric PV-rich air associated with tropopause depressions. It is found that the tropopause depression, latent heat release, weak static stability, the jet streak-induced ageostrophic circulation, and surface sensible and latent heat fluxes, act together to determine the amplification and evolution of the storm, although their relative importance differs at different stages of the storm's development. In particular, latent heat release accounts for roughly 40% of the total deepening, and the heating-induced ageostrophic circulations provide an important coupling of the surface cyclogenesis with the above-mentioned processes.

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Zonghui Huo, Da-Lin Zhang, and John R. Gyakum

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The relative importance of various potential vorticity (PV) perturbations and their mutual interactions associated with the superstorm of 12–14 March 1993 are investigated by applying a piecewise PV inversion diagnostic system to a 36-h simulation of the storm. It is shown that the contributions from all PV anomalies to the surface development increase with time, although their relative significance varies during the rapid deepening stage. In general, the upper-level dry PV anomalies contribute the most to the rapid deepening of the storm, followed, in order, by the lower-level thermal anomaly and latent heat release.

Comparing the PV anomalies and their inverted circulations reveals that there exists a favorable phase tilt between the upper- and lower-level anomalies that allows lower- and upper-level mutual interactions, in which the circulations associated with the upper-level PV anomalies enhance the lower-level anomalies and vice versa. In addition to the vertical interactions, lateral interactions are also present among the upper-level PV anomalies and the background flow. It is also found that the background flow advection dominates the vortex–vortex and vortex–background flow interactions in the deepening of the storm. The vortex–vortex interactions of the two upper-level positive PV anomalies cause the negative tilt of the main upper-level trough during the rapid deepening period.

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Marco L. Carrera, John R. Gyakum, and Da-Lin Zhang

Abstract

Secondary cyclogenesis has been identified as a difficult forecast challenge. In this paper, the authors examine the dominant physical processes associated with the predictability of a case of explosive secondary marine cyclogenesis and provide a better understanding of the large variability in the recent model-intercomparison simulations of the case. A series of sensitivity experiments, involving changes to the model initial conditions and physical parameterizations, is performed using the Canadian Mesoscale Compressible Community Model with a grid size of 50 km.

It is found that errors in the model initial conditions tend to decay with time, and more rapidly so in “dry” simulations. The model fails to produce the secondary cyclogenesis in the absence of latent heating. Water vapor budget calculations from the control experiment show that the surface moisture flux from 6 to 12 h is the largest contributor of water vapor to the budget area in the vicinity of the cyclone center, and remains an important moisture supply throughout the integration period. During the first 12 h, these fluxes are crucial in inducing grid-scale diabatic heating and destabilizing the lower troposphere, thereby facilitating the subsequent rapid deepening of the storm. A secondary maximum in surface latent heat flux to the north and east of the primary maximum acts to force the cyclogenesis event to the south and east of a coastal circulation center. When the surface evaporation is not allowed, much less precipitation is produced and the secondary cyclone fails to develop. Calculations of the potential temperature on the dynamic tropopause (i.e., 2-PVU surface) in the absence of surface evaporation indicate a significantly damped thermal wave when compared with the control integration.

This result for a case of secondary cyclogenesis differs from those generally found for large-scale extratropical cyclogenesis where upper-level baroclinic forcings tend to dominate, and motivates the need for better physical parameterizations, including the condensation and boundary layer processes, in operational models. The authors speculate that the different treatment of condensation and boundary layer processes may have been partly responsible for the enhanced variability in the simulation of this case in a recently completed international mesoscale model intercomparison experiment.

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