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Stefan F. Cecelski and Da-Lin Zhang

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While much attention has been given to investigating the dynamics of tropical cyclogenesis (TCG), little work explores the thermodynamical evolution and related cloud microphysical processes occurring during TCG. This study elaborates on previous research by examining the impact of ice microphysics on the genesis of Hurricane Julia during the 2010 North Atlantic Ocean hurricane season. As compared with a control simulation, two sensitivity experiments are conducted in which the latent heat of fusion owing to depositional growth is removed in one experiment and homogeneous freezing is not allowed to occur in the other. Results show that removing the latent heat of fusion substantially reduces the warming of the upper troposphere during TCG. This results in a lack of meso-α-scale hydrostatic surface pressure falls and no tropical depression (TD)-scale mean sea level pressure (MSLP) disturbance. In contrast, removing homogeneous freezing has little impact on the structure and magnitude of the upper-tropospheric thermodynamic changes and MSLP disturbance. Fundamental changes to the strength and spatial extent of deep convection and related updrafts are found when removing the latent heat of fusion from depositional processes. That is, deep convection and related updrafts are weaker because of the lack of heating in the upper troposphere. These changes to convective development impact the creation of a storm-scale outflow and thus the accumulation of upper-tropospheric warming and the development of the TD-scale MSLP disturbance.

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Huiqi Li, Xiaopeng Cui, and Da-Lin Zhang

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An isolated heavy-rain-producing thunderstorm was unexpectedly initiated in the afternoon of 9 August 2011 near the central urban area of the Beijing metropolitan region (BMR), which occurred at some distance from BMR’s northwestern mountains and two preexisting mesoscale convective systems (MCSs) to the west and north, respectively. An observational analysis shows the presence of unfavorable quasigeostrophic conditions but a favorable regional environment for the convective initiation (CI) of thunderstorms. A nested-grid cloud-resolving model simulation of the case with the finest 1.333-km resolution is performed to examine the CI of the thunderstorm and its subsequent growth. Results reveal that the growth of the mixed boundary layer, enhanced by the urban heat island (UHI) effects, accounts for the formation of a thin layer of clouds at the boundary layer top at the CI site and nearby locations as well as on the upslope sides of the mountains. It takes about 36 min for the latent-heating-driven updraft to penetrate through a 1-km “lid” layer above before the formation of the thunderstorm. However, this storm may not take place without sustained low-level convergence of a prevailing southerly flow with a northerly flow ahead of a cold outflow boundary associated with the northern MCS. The latter is driven by the latent heating of the shallow layer of clouds during the earlier CI stage and then a cold mesohigh underneath the northern MCS. This study indicates the important roles of the urban effects, mountain morphology, and convectively generated pressure perturbations in determining the CI location and timing of isolated thunderstorms during the summer months.

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Gaili Wang, Da-Lin Zhang, and Jisong Sun

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A multiscale observational analysis of a nocturnal extreme rainfall event that occurred at Changtu in Northeast China on 14 July 2017 is performed using global analysis, automated surface observations, Doppler radar, rawinsonde, and disdrometer data. Results show that the large-scale environment was characterized by high convective available potential energy and precipitable water, moderate convective inhibition, and a southwesterly low-level jet (LLJ) capped by an inversion layer. The first and subsequent convective cells developed along a quasi-stationary surface convergence zone in a convection-void region of a previously dissipated meso-α-scale convective line. Continuous convective initiation through backbuilding at the western end and the subsequent merging of eastward-moving convective cells led to the formation of a near-zonally oriented meso-β-scale rainband, with reflectivity exceeding 45 dBZ (i.e., convective core intensity). This quasi-stationary rainband was maintained along the convergence zone by the LLJ of warm moist air, aided by local topographical lifting and convectively generated outflows. A maximum hourly rainfall amount of 96 mm occurred during 0200–0300 Beijing standard time as individual convective cores with a melting layer of >55 dBZ reflectivity moved across Changtu with little intermittency. The extreme-rain-producing stage was characterized with near-saturated vertical columns, and rapid number concentration increases of all raindrop sizes. It is concluded that the formation of the meso-β-scale rainband with continuous convective backbuilding, and the subsequent echo-training of convective cores with growing intensity and width as well as significant fallouts of frozen particles accounted for the generation of this extreme rainfall event. This extreme event was enhanced by local topography and the formation of a mesovortex of 20–30 km in diameter.

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Da-Lin Zhang and Wei-Zhong Zheng

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Although most of the planetary boundary layer (PBL) parameterizations have demonstrated the capability to reproduce many meteorological phenomena in the lowest few kilometers, little attention has been paid to the prediction of the diurnal cycles of surface wind speed (V SFC) in relation to surface temperature (T SFC). In this study, the performance of five widely used PBL parameterizations in reproducing the diurnal cycles of V SFC and T SFC is evaluated using the 3-day mesoscale simulations of summertime weak-gradient flows over the central United States where little organized convection and topographical forcing were present. The time series of area-averaged V SFC and T SFC, as well as the vertical wind and thermal profiles from the five sensitivity simulations, are compared with hourly surface observations and other available data. The hourly surface observations reveal that the diurnal cycles of V SFC are in phase (but surface wind directions are 5–6 h out of phase) with those of T SFC. It is shown that both V SFC and T SFC are very sensitive to the PBL parameterizations, given the identical conditions for all of the other model parameters. It is found that all five of the PBL schemes can reproduce the diurnal phases of T SFC (and wind directions), albeit with different amplitudes. However, all of the schemes underestimate the strength of V SFC during the daytime, and most of them overestimate it at night. Moreover, some PBL schemes produce pronounced phase errors in V SFC or substantially weak V SFC all of the time, despite their well-simulated diurnal cycle of T SFC. The results indicate that a perfect simulation of the diurnal T SFC cycle (and the thermal structures above) does not guarantee the reproduction of the diurnal cycles of V SFC. The final outcome would depend on how various physical processes, such as the vertical turbulent exchanges of the mass and momentum under different stability conditions, are parameterized. Because the upper portion of the PBL flow is often nearly opposite in phase to V SFC under weak-gradient conditions, the results have significant implications for the predictability of diurnal precipitation and the studies of air quality, wind energy, and other environmental problems.

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Tong Zhu, Da-Lin Zhang, and Fuzhong Weng

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In this study, a 5-day explicit simulation of Hurricane Bonnie (1998) is performed using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) with the finest grid length of 4 km. The initial mass, wind, and moisture fields of the hurricane vortex are retrieved from the Advanced Microwave Sounding Unit-A (AMSU-A) satellite measurements, and the sea surface temperature (SST) is updated daily. It is shown that the simulated track is within 3° latitude–longitude of the best track at the end of the 5-day integration, but with the landfalling point close to the observed. The model also reproduces reasonably well the hurricane intensity and intensity changes, asymmetries in cloud and precipitation, as well as the vertical structures of dynamic and thermodynamic fields in the eye and eyewall.

It is shown that the storm deepens markedly in the first 2 days, during which period its environmental vertical shear increases substantially. It is found that this deepening could occur because of the dominant energy supply by a strong low-level southeasterly flow into the eastern eyewall plus the presence of underlying warm SST and favorable upper-level divergent outflow. However, the approaching of a strong upper-level northwesterly flow tends to generate mass convergence and subsidence warming and drying, thereby suppressing the development of deep convection in the western semicircle. This gives rise to wavenumber-1 asymmetries in clouds and precipitation (i.e., a partial eyewall) and the eastward tilt of the eyewall and storm center. Both the observed and simulated storms also appear to exhibit eyewall replacement scenarios in which the storms weaken as double eyewalls appear, and then reintensify as their inner eyewalls diminish and concentric eyewalls develop. The results indicate that the eyewall replacement process may be predictable because it appears to depend on the large-scale flow.

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Da-Lin Zhang and J. Michael Fritsch

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A mesoscale warm-core vortex associated with the mesoscale convective complex (MCC) that produced the 1977 Johnstown flood is examined using a three-dimensional nested-grid model simulation of the flood episode. In the simulation, the vortex plays a key role in determining the evolution of the MCC, a squall line, and the distribution of heavy precipitation. The vortex has a space scale of 100–200 km in diameter and a time scale of more than 18 hours. Its low pressure center extends from the midtroposphere down to the surface, and its maximum vorticity occurs between 850 and 700 mb. A pool of cool moist downdraft air develops in the surface to the 850 mb layer beneath the warm core, while a cold dome forms in the vicinity of the tropopause above the warm core. Following forcing from repeated deep convection prior to model initial time, the vortex is initiated by mesoscale ascent associated with a traveling meso-α scale wave. Genesis takes place in a nearly saturated, slightly conditionally unstable environment with weak horizontal deformation and vertical shear. The vortex is then energetically supported primarily by latent heat release from stratiform (resolvable-scale) cloud condensation in the low- to midtroposphere. In the decaying stage, the vortex is maintained by inertial stability. The evolution of the warm-core mesovortex appears to depend upon the concurrent development of deep convection and the mesoscale flow structure. In particular, moist downdrafts play an important role in controlling the strength of the vortex and the amount of resolvable-scale rainfall. Associated with the mesovortex, an intense vertical circulation with strong low-level convergence and upper-level divergence develops. In addition, a strong cyclonic circulation extends to 300 mb where a changeover to anticyclonic circulation occurs. It is found that equivalent potential temperature and the horizontal momentum are nearly uniformly distributed in the immediate environment of the vortex. The resultant weak horizontal deformation provides an important energy-preserving mechanism for the maintenance of the warm-core structure while inertial stability of large tangential winds helps the longevity of the vortex circulation. At upper levels, a mesohigh with strong anticyclonic outflow develops above the vortex. The mesohigh behaves like an “obstacle,” forcing the horizontal environmental wind flow around it. To the northeast of the upper level mesohigh, a northwesterly jet streak develops between the strong anticyclonic outflow and a baroclinic zone farther north.

The results suggest that successful prediction of the evolution of mesoscale convective weather systems not only hinges upon the convective parameterization, but also depends upon the model's ability to reproduce the timing and location of resolvable-scale condensation. The resolvable-scale phase changes, and associated latent heat release, strongly affect the mesoscale circulation and contributes about 30% to 40% of the total precipitation from the mesoscale convective systems.

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Zuohao Cao, Qin Xu, and Da-Lin Zhang

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Unlike the classical point vortex model, a new method is developed to extract flows induced not only by vorticity but also by divergence in a well-defined vortex core area of a cyclone. This new method is applied to diagnosing the interactions of three midlatitude cyclones (called A, B, and C) that account for a missed summer severe rainfall forecast, in which the daily precipitation predicted by the Canadian operational model is an order of magnitude smaller than the rain gauge and radar measurements. In this event, cyclone B, responsible for the severe rainfall occurrence, was advected largely by flows induced by two neighboring cyclones: A and C to the west and east, respectively. This work attempts to assess whether and to what degree the vertical tilt of the observed cyclone versus that of the forecast cyclone B is caused by advections of the environmental flows (including A- and C-induced flows) at 500 and 1000 hPa. Results show that the observed cyclone B was advected mainly by the cyclone A–induced flow at 500 hPa into a vertically tilted structure that was northwestward against the vertical shear of the environmental flow and thus favorable for upward motion and cyclone intensification around the time of severe rainfall. However, the forecast cyclone B was advected largely by the cyclone A–induced flow at 500 hPa and the cyclone C–induced flow at 1000 hPa into an increasingly northward-tilted structure that was along the vertical shear of the environmental flow and thus unfavorable for upward motion and cyclone intensification, leading to the missed forecast of severe rainfall. Suggestions are made for future improvements of model forecasts.

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Nannan Qin, Da-Lin Zhang, and Ying Li

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It is well known that hurricane intensification is often accompanied by continuous contraction of the radius of maximum wind (RMW) and eyewall size. However, a few recent studies have shown rapid and then slow contraction of the RMW/eyewall size prior to the onset and during the early stages of rapid intensification (RI) of hurricanes, respectively, but a steady state in the RMW (S-RMW) and eyewall size during the later stages of RI. In this study, a statistical analysis of S-RMWs associated with rapidly intensifying hurricanes is performed using the extended best-track dataset during 1990–2014 in order to examine how frequently, and at what intensity and size, the S-RMW structure tends to occur. Results show that about 53% of the 139 RI events of 24-h duration associated with 55 rapidly intensifying hurricanes exhibit S-RMWs, and that the percentage of the S-RMW events increases to 69% when RI events are evaluated at 12-h intervals, based on a new RI rate definition of 10 m s−1 (12 h)−1; both results satisfy the Student’s t tests with confidence levels of over 95%. In general, S-RMWs tend to appear more frequently in more intense storms and when their RMWs are contracted to less than 50 km. This work suggests a new fruitful research area in studying the RI of hurricanes with S-RMWs.

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Chanh Q. Kieu and Da-Lin Zhang

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In this study, a piecewise potential vorticity (PV) inversion algorithm for an arbitrary number of PV pieces is developed by extending Wang and Zhang’s PV inversion scheme, and then the nonlinear responses to various types and magnitudes of axisymmetric PV anomalies (PVAs) in hurricane vortices are investigated. Results show that the upper- and lower-level PVAs in the eye help enhance cyclonic flows in the eyewall, but with weak vertical interactions between them. The balanced flows corresponding to the PVAs in the eyewall appear to account for a substantial portion of the warm core and the minimum pressure in the eye. However, the lower-level PVA in the eye inversion layer is more effective in contributing to the hurricane intensity than that at the upper levels. Results also show that the radius of the balanced response of PVAs is more sensitive to the mean vortex intensity than the vertical penetration; the weaker the mean vortex intensity, the larger the radius of the influence will be. Similar behaviors are also observed for the quasi-balanced secondary circulations. That is, given a diabatic heating profile in the eyewall, the weaker the background vortex or the PVAs, the stronger the secondary circulations will be.

It is found that the development of an outer eyewall (or spiral rainbands) could be inimical to the inner eyewall in several ways, such as by 1) adding an anticyclonic flow inside to offset the cyclonic rotation of the inner eyewall, 2) enhancing a ring of a lower pressure zone underneath to broaden the inner-core lower pressure region, 3) inducing an inward (outward) radial flow outside (inside) in the PBL (upper level) to block the energy supply to (outflow of) the inner eyewall, and 4) generating subsidence between the two eyewalls to suppress the development of deep convection in the inner eyewall.

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Rudi Xia, Da-Lin Zhang, and Bailin Wang

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The cloud-to-ground (CG) lightning climatology and its relationship to rainfall over central and eastern China is examined, using data from 32 million CG lightning flashes and Tropical Rainfall Measuring Mission measurements during a 6-yr period covering 2008–13. Results show substantial spatial and temporal variations of flash density across China. Flash counts are the highest (lowest) in summer (winter) with the lowest (highest) proportion of positive flashes. CG lightning over northern China is more active only in summer, whereas in winter CG lightning is more active only in the Yangtze River basin. The highest CG lightning densities, exceeding 9 flashes per kilometer squared per year and more than 70 CG lightning days per year, are found in the northern Pearl River delta region, followed by the Sichuan basin, the Yangtze River delta, and the southeastern coast of China in that order. Lower-flash-density days occur over mountainous regions as a result of the development of short-lived afternoon storms, while higher-flash-density days, typically associated with nocturnal thunderstorms, appear over the north China plain and Sichuan basin. The highest number of CG lightning flashes is found in August whereas monthly convective rainfall peaks in May or July. Flash rates during the warm season are typically maximized in the afternoon hours in coincidence with a convective rainfall peak except for the Sichuan basin and its surrounding mountainous areas where a single late-night convective rainfall peak dominates. Much less lightning activity corresponds to a late-night to morning rainfall peak over the plains in eastern China because of the increased proportion of stratiform rainfall during that period.

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