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Xin Xu, Ming Xue, and Yuan Wang

Abstract

The genesis of two mesovortices (MVs) within a real-data, convection-resolving simulation of the 8 May 2009 central U.S. bow echo system is studied. Both MVs form near the bow apex but differ distinctively in intensity, lifetime, and damage potential. The stronger and longer-lived mesovortex, MVa, stays near the bow apex where the system-scale rear-inflow jet (RIJ) is present. The descending RIJ produces strong downdrafts and surface convergence, which in turn induce strong vertical stretching and intensification of MVa into an intense mesovortex. In contrast, the weaker and shorter-lived mesovortex, MVb, gradually moves away from the bow apex, accompanied by localized convective-scale downdrafts.

Lagrangian circulation and vorticity budget analyses reveal that the vertical vorticity of MVs in general originate from the tilting of near-surface horizontal vorticity, which is mainly created via surface friction. The circulation of the material circuit that ends up to be a horizontal circuit at the foot of the MVs increases as the frictionally generated horizontal vortex tubes pass through the tilted material circuit (tilted following backward trajectories defining the material circuit) surface, especially in the final few minutes prior to mesovortex genesis. The tilted material circuit becomes horizontal at the MV foot, turning associated horizontal vorticity into vertical. The results show at least qualitatively that, in addition to baroclinicity, surface friction can also have significant contributions to the generation of low-level MVs, which was not considered in previous MV studies.

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Xin Xu, Ming Xue, and Yuan Wang

Abstract

A derecho-producing bow-echo event over the central United States on 8 May 2009 is analyzed based on radar observations and a successful real-data WRF simulation at 0.8-km grid spacing. Emphasis is placed on documenting the existence, evolution, and characteristics of low-level mesovortices (MVs) that form along the leading edge of the bowing system. The genesis of near-surface high winds within the system is also investigated.

Significant MVs are detected from the radar radial velocity using a linear least squares derivatives (LLSD) method, and from the model simulation based on calculated vorticity. Both the observed and simulated bow-echo MVs predominantly form north of the bow apex. MVs that develop on the southern bow tend to be weaker and shorter-lived than their northern counterparts. Vortex mergers occur between MVs during their forward movement, which causes redevelopment of some MVs in the decaying stage of the bow echo. MVs located at (or near) the bow apex are found to persist for a notably longer lifetime than the other MVs. Moreover, the model results show that these bow-apex MVs are accompanied with damaging straight-line winds near the surface. These high winds are mainly caused by the descent of the rear-inflow jet at the bow apex, but the MV-induced vortical flow also has a considerable contribution. The locally enhanced descent of the rear-inflow jet near the mesovortex is forced primarily by the dynamically induced downward vertical pressure gradient force while the buoyancy force only plays a minor role there.

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Xin Xu, Yuan Wang, and Ming Xue

Abstract

Linear mountain wave theory is used to derive the general formulas of the gravity wave momentum flux (WMF) and its vertical divergence that develop in directionally sheared flows with constant vertical shear. Height variations of the WMF and its vertical divergence are studied for a circular bell-shaped mountain. The results show that the magnitude of the WMF decreases with height owing to variable critical-level height for different wave components. This leads to continuous—rather than abrupt—absorption of surface-forced gravity waves, and the rate of absorption is largely determined by the maximum turning angle of the wind with height. For flows turning substantially with height, the wave momentum is primarily trapped in the lower atmosphere. Otherwise, it can be transported to the upper levels. The vertical divergence of WMF is oriented perpendicularly to the right (left) of the mean flow that veers (backs) with height except at the surface, where it vanishes. First, the magnitude of the WMF divergence increases with height until reaching its peak value. Then, it decreases toward zero above that height. The altitude of peak WMF divergence is proportional to the surface wind speed and inversely proportional to the vertical wind shear magnitude, increasing as the maximum wind turning angle increases. The magnitude of the peak WMF divergence also increases with the maximum wind turning angle, but it in general decreases as the ambient flow Richardson number increases. Implications of the findings for treating mountain gravity waves in numerical models are discussed.

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Shushi Zhang, David B. Parsons, and Yuan Wang

Abstract

This study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy.

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Shanshan Wang, Jianping Huang, and Xing Yuan
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Xiaojun Yuan, Dake Chen, Cuihua Li, Lei Wang, and Wanqiu Wang

Abstract

A linear Markov model has been developed to predict sea ice concentration (SIC) in the pan-Arctic region at intraseasonal to seasonal time scales, which represents an original effort to use a reduced-dimension statistical model in forecasting Arctic sea ice year-round. The model was built to capture covariabilities in the atmosphere–ocean–sea ice system defined by SIC, sea surface temperature, and surface air temperature. Multivariate empirical orthogonal functions of these variables served as building blocks of the model. A series of model experiments were carried out to determine the model’s dimension. The predictive skill of the model was evaluated by anomaly correlation and root-mean-square errors in a cross-validated fashion . On average, the model is superior to the predictions by anomaly persistence, damped anomaly persistence, and climatology. The model shows good skill in predicting SIC anomalies within the Arctic basin during summer and fall. Long-term trends partially contribute to the model skill. However, the model still beats the anomaly persistence for all targeted seasons after linear trends are removed. In winter and spring, the predictability is found only in the seasonal ice zone. The model has higher anomaly correlation in the Atlantic sector than in the Pacific sector. The model predicts well the interannual variability of sea ice extent (SIE) but underestimates its accelerated long-term decline, resulting in a systematic model bias. This model bias can be reduced by the constant or linear regression bias corrections, leading to an improved correlation skill of 0.92 by the regression bias correction for the 2-month-lead September SIE prediction.

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Kai-Yuan Cheng, Pao K. Wang, and Chen-Kang Wang

Abstract

The ventilation coefficients that represent the enhancement of mass transfer rate due to the falling motion of spherical hailstones in an atmosphere of 460 hPa and 248 K are computed by numerically solving the unsteady Navier–Stokes equation for airflow past hailstones and the convective diffusion equation for water vapor diffusion around the falling hailstones. The diameters of the hailstones investigated are from 1 to 10 cm, corresponding to Reynolds number from 5935 to 177 148. The calculated ventilation coefficients vary approximately linearly with the hailstone diameter, from about 19 for a 1-cm hailstone to about 208 for a 10-cm hailstone. Empirical formulas for ventilation coefficient variation with hailstone diameter as well as Reynolds and Schmidt numbers are given. Implications of these ventilation coefficients are discussed.

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Tao Luo, Renmin Yuan, Zhien Wang, and Damao Zhang

Abstract

In this study, collocated satellite and buoy observations as well as satellite observations over an extended region during 2006–10 were used to quantify the humidity effects on marine boundary layer (MBL) aerosols. Although the near-surface aerosol size increases with increasing near-surface relative humidity (RH), the influence of RH decreases with increasing height and is mainly limited to the lower well-mixed layer. In addition, the size changes of MBL aerosols with RH are different for low and high surface wind () conditions as revealed by observations and Mie scattering calculations, which may be related to different dominant processes (i.e., the hygroscopic growth process during low wind and the evaporation process during sea salt production during high wind). These different hygroscopic processes under the different conditions, together with the MBL processes, control the behaviors of the MBL aerosol optical depth () with RH. In particular, under high conditions, the MBL stratifications effects can overwhelm the humidity effects, resulting in a weak relationship of MBL on RH. Under low conditions, the stronger hygroscopic growth can overwhelm the MBL stratification effects and enhance the MBL with increasing RH. These results are important to evaluate and to improve MBL aerosols simulations in climate models.

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Chungu Lu, Huiling Yuan, Edward I. Tollerud, and Ning Wang
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Yuan Wang, Lifeng Zhang, Jun Peng, and Jiping Guan

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High-resolution cloud-permitting simulations with the Weather Research and Forecasting (WRF) Model are performed to study the generation, structure, and characteristics of mesoscale gravity waves in an idealized mei-yu front system. Two classes of waves are generated successively during the control simulation. The first class of waves, which is typical of vertically propagating waves excited by the front itself, appears as the front develops before the generation of the prefrontal moist convection and has a coherent fanlike pattern from the troposphere to the lower stratosphere. The second class of waves, which is much stronger than the fanlike waves, appears accompanied by the generation of the moist convection. It is nearly vertically trapped in the troposphere, while it propagates vertically upstream and downstream in the lower stratosphere. The source function analysis is introduced to demonstrate that the mechanical oscillator mechanism plays a dominant role in the generation of convective gravity waves in the lower stratosphere. The vertical motion induced by the deep convection develops upward in the troposphere, overshoots the level of neutral buoyancy (LNB), and impinges on the tropopause. The net buoyancy forces the air parcels to oscillate about the LNB, thus initiating gravity waves in the lower stratosphere. Further spectral analysis shows that the upstream waves have more abundant wavenumber–frequency and phase speed space distributions than the downstream waves. And the former amplify with height while the latter weaken in general under the effect of background northerly wind. The power spectral densities of downstream waves concentrate on faster phase speed than those of upstream waves.

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