bluuebunny/crossref_metadata_2025_split huggingface dataset:
Simulation of Flow Fields in Complex Terrain with WRF-LES: Sensitivity Assessment of Different PBL Treatments
Authors: Liu Yujue , Liu Yubao , Muñoz-Esparza Domingo , Hu Fei , Yan Chao , Miao Shiguang 
Published: August 2020

AbstractA multiscale modeling study of a real case has been conducted to explore the capability of the large-eddy simulation version of the Weather Research and Forecasting Model (WRF-LES) over Xiaohaituo Mountain (a game zone for the Beijing, China, 2022 Winter Olympic Games). In comparing WRF-LES results with observations collected during the Mountain Terrain Atmospheric Observations and Modeling (MOUNTAOM) field campaign, it is found that at 37-m resolution with LES settings, the model can reasonably capture both large-scale events and microscale atmospheric circulation characteristics. Employing the Shuttle Radar Topography Mission 1 arc s dataset (SRTM1; ~30 m) high-resolution topographic dataset instead of the traditional USGS_30s (~900 m) dataset effectively improves the model capability for reproducing fluctuations and turbulent features of surface winds. Five sensitivity experiments are conducted to investigate the impact of different PBL treatments, including YSU/Shin and Hong (SH) PBL schemes and LES with 1.5-order turbulence kinetic energy closure model (1.5TKE), Smagorinsky (SMAG), and nonlinear backscatter and anisotropy (NBA) subgrid-scale (SGS) stress models. In this case, at gray-zone scales, differences between YSU and SH are negligible. LES outperform two PBL schemes that generate smaller turbulence kinetic energy and increase the model errors for mean wind speed, energy spectra, and probability density functions of velocity. Another key finding is that wind field features in the boundary layer over complex terrain are more sensitive to the choice of SGS models than above the boundary layer. With the increase of model resolution, the effects of the SGS model become more significant, especially for the statistical characteristics of turbulence. Among these three SGS models, NBA has the best performance. Overall, this study demonstrates that WRF-LES is a promising tool for simulating real weather flows over complex terrain.
Diurnal Black-Sky Surface Albedo Parameterization of Snow
Authors: Manninen Terhikki , Jääskeläinen Emmihenna , Riihelä Aku 
Published: August 2020

AbstractSurface albedo, the fraction of incoming solar radiation reflected hemispherically by the surface, is an essential climate variable (ECV) directly related to the energy budget of Earth. The presence and properties of snow cover alter surface albedo significantly, with variability in temporal scales reaching from seasonal to diurnal. The diurnal variation of snow albedo is typically parameterized with the solar zenith angle, but it cannot take into account asymmetry with respect to midday. Using the solar azimuth angle instead is suggested, since especially in the melting season the snow albedo varies highly asymmetrically during the day. To derive a general time- and latitude-independent formula, the azimuth angle values are normalized. Baseline Surface Radiation Network data are used to derive an empirical formula for the diurnal variation of snow black-sky surface albedo. The overall accuracy is on the order of 0.02, and the relative accuracy is about 3%.
Ensemble Sensitivity Analysis of Tropical Cyclone Intensification Rate during the Development Stage
Authors: Hu Chih-Chi , Wu Chun-Chieh 
Published: August 2020

AbstractEnsemble sensitivity analysis based on convective-permitting ensemble simulations is used to understand the processes associated with tropical cyclone (TC) intensification under idealized conditions. Partial correlations between different variables and the future TC intensification rate, with the effect of intensity removed, are used to identify the sensitive factors. It is found that the equivalent potential temperature (θe) in the region from the radius of maximum wind (RMW) to 3 times the RMW below 2 km (hereafter, the sensitive region) has the largest correlation (over 0.7) with 2.5-h intensity change. It is found that higherθein the sensitive region is associated with not only a stronger updraft but also an inward shift of vertical motion in the mid- to upper eyewall. This suggests that higherθejust outside the RMW is favorable to TC intensification not only because of the larger amount of the heating, but also due to the heating location that is closer to the center. Trajectory analysis shows that the parcels in the sensitive region are mainly from the boundary layer inflow and the midlevel inflow. It is found that when the outer rainband is active, the midlevel inflow becomes stronger and is able to bring more low-θeair into the boundary layer, and theθeradially inward to the rainband decreases. Verification experiments justify that higherθearound the RMW to 3 times the RMW is favorable to TC intensification, while higherθeaway from 5 times the RMW is shown to be unfavorable for TC intensification.
Cloud-Top Entrainment in Mixed-Phase Stratocumulus and Its Process-Level Representation in Large-Eddy Simulation
Authors: Rauterkus Robert , Ansorge Cedrick 
Published: August 2020

AbstractCloud-top entrainment is a crucial process for the evolution of stratocumulus and is driven by interactions of radiation, microphysics, and turbulence on scales reaching down to less than one meter. Regardless of this fact, most large-eddy simulation studies still apply a horizontal resolution of tens of meters, not resolving these interactions sufficiently. Here, based on an extensive observational campaign, we define a weak-shear benchmark scenario for large-eddy simulation over Arctic ice and for the first time perform large-eddy simulation of mixed-phase stratocumulus with horizontal resolutions of 35, 10, and 3.5 m. Thereby, we investigate the processes contributing to cloud-top entrainment and their role for the evolution of stratocumulus with a particular focus on resolution sensitivity. First, we find that a horizontal grid spacing larger than 10 m insufficiently represents the effects of small-scale microphysical cooling and turbulent engulfment on cloud-top entrainment. Indeed, the small size of energy-containing eddies—a consequence of the intense stratification in the vicinity of the cloud-top region—violates the underlying assumptions of subgridscale models by buoyant suppression of eddies at the large-eddy simulation filter scale. Second, the decrease in cloud-top entrainment due to these insufficiently represented processes results in 15% less cloud water after 6 h of simulation and a corresponding optical thinning of the cloud. Third, we show that the applied nonequilibrium microphysics cause microphysical heating beneath the cloud top, which partly counteracts the evaporative cooling.
Evaluations of the Thermodynamic Phases of Clouds in a Cloud-System-Resolving Model Using CALIPSO and a Satellite Simulator over the Southern Ocean
Authors: Roh Woosub , Satoh Masaki , Hashino Tempei , Okamoto Hajime , Seiki Tatsuya 
Published: August 2020

AbstractA new evaluation method for the thermodynamic phases of clouds in cloud-system-resolving models is presented using CALIPSO observations and a satellite simulator. This method determines the thermodynamic phases using the depolarization ratio and a cloud extinction proxy. For the evaluation, we introduced empirical parameterization of the depolarization ratio of ice and water clouds using temperatures of a reanalysis dataset and total attenuated backscatters of CALIPSO. We evaluated the mixed-phase clouds simulated in a cloud-system-resolving model over the Southern Ocean using single-moment and double-moment bulk cloud microphysics schemes, referred to as NSW6 and NDW6, respectively. The NDW6 simulations reproduce supercooled water clouds near the boundary layer that are consistent with the observations. Conversely, the NSW6 simulations failed to reproduce such supercooled water clouds. Consistencies between the cloud classes diagnosed by the evaluation method and the simulated hydrometeor categories were examined. NDW6 shows diagnosed water and ice classes that are consistent with the simulated categories, whereas the ice category simulated with NSW6 is diagnosed as liquid water by the present method due to the large extinction from the ice cloud layers. Additional analyses indicated that ice clouds with a small effective radius and large ice water content in NSW6 lead to erroneous values for the fraction of the diagnosed liquid water. It is shown that the uncertainty in the cloud classification method depends on the details of the cloud microphysics schemes. It is important to understand the causes of inconsistencies in order to properly understand the cloud classification applied to model evaluations as well as retrievals.
A Businger Mechanism for Intermittent Bursting in the Stable Boundary Layer
Authors: van der Linden Steven J. A. , van de Wiel Bas J. H. , Petenko Igor , van Heerwaarden Chiel C. , Baas Peter , Jonker Harmen J. J. 
Published: August 2020

AbstractHigh-resolution large-eddy simulations of the Antarctic very stable boundary layer reveal a mechanism for systematic and periodic intermittent bursting. A nonbursting state with a boundary layer height of just 3 m is alternated by a bursting state with a height of ≈5 m. The bursts result from unstable wave growth triggered by a shear-generated Kelvin–Helmholtz instability, as confirmed by linear stability analysis. The shear at the top of the boundary layer is built up by two processes. The upper, quasi-laminar layer accelerates due to the combined effect of the pressure force and rotation by the Coriolis force, while the lower layer decelerates by turbulent friction. During the burst, this shear is eroded and the initial cause of the instability is removed. Subsequently, the interfacial shear builds up again, causing the entire sequence to repeat itself with a time scale of ≈10 min. Despite the clear intermittent bursting, the overall change of the mean wind profile is remarkably small during the cycle. This enables such a fast erosion and recovery of the shear. This mechanism for cyclic bursting is remarkably similar to the mechanism hypothesized by Businger in 1973, with one key difference. Whereas Businger proposes that the flow acceleration in the upper layer results from downward turbulent transfer of high-momentum flow, the current results indicate no turbulent activity in the upper layer, hence requiring another source of momentum. Finally, it would be interesting to construct a climatology of shear-generated intermittency in relation to large-scale conditions to assess the generality of this Businger mechanism.
Understanding How Complex Terrain Impacts Tornado Dynamics Using a Suite of High-Resolution Numerical Simulations
Authors: Satrio Martin A. , Bodine David J. , Reinhart Anthony E. , Maruyama Takashi , Lombardo Franklin T. 
Published: August 2020

AbstractA simulated vortex within a large-eddy simulation is subjected to various surface terrain, implemented through the immersed boundary method, to analyze the effects of complex topography on vortex behavior. Thirty simulations, including a control with zero-height terrain, are grouped into four categories—2D sinusoidal hills, 3D hills, valleys, and ridges—with slight modifications within each category. A medium-swirl-ratio vortex is translated over shallow terrain, which is modest in size relative to the vortex core diameter and with no explicitly defined surface roughness. While domain size restricts results to the very near-field effects of terrain, vortex–terrain interaction yields notable results. Terrain influences act to increase the variability of the near-surface vortex, including a notable leftward (rightward) deflection, acceleration (deceleration), and an expansion (a contraction) of the vortex as it ascends (descends) the terrain owing to changes in the corner flow swirl ratio. Additionally, 10-m track analyses show stronger horizontal wind speeds are found 1) on upslope terrain, resulting from transient subvortices that are more intense compared to the control simulation, and 2) in between adjacent hills simultaneous with strong pressure perturbations that descend from aloft. Composite statistics confirm that the region in between adjacent hills has the strongest horizontal wind speeds, while upward motions are more intense during ascent. Overall, valley (ridge) simulations have the largest horizontal (vertically upward) wind speeds. Last, horizontal and vertical wind speeds are shown to be affected by other terrain properties such as slope steepness and two-dimensionality of the terrain.
The Strong Impact of Weak Horizontal Convergence on Continental Shallow Convection
Authors: Kurowski Marcin J. , Grabowski Wojciech W. , Suselj Kay , Teixeira João 
Published: August 2020

Abstract
Idealized large-eddy simulation (LES) is a basic tool for studying three-dimensional turbulence in the planetary boundary layer. LES is capable of providing benchmark solutions for parameterization development efforts. However, real small-scale atmospheric flows develop in heterogeneous and transient environments with locally varying vertical motions inherent to open multiscale interactive dynamical systems. These variations are often too subtle to detect them by state-of-the-art remote and in situ measurements, and are typically excluded from idealized simulations. The present study addresses the impact of weak [i.e., O(10−6) s−1] short-lived low-level large-scale convergence/divergence perturbations on continental shallow convection. The results show a strong response of shallow nonprecipitating convection to the applied weak large-scale dynamical forcing. Evolutions of CAPE, mean liquid water path, and cloud-top heights are significantly affected by the imposed convergence/divergence. In contrast, evolving cloud-base properties, such as the area coverage and mass flux, are only weakly affected. To contrast those impacts with microphysical sensitivity, the baseline simulations are perturbed assuming different observationally based cloud droplet number concentrations and thus different rainfall. For the tested range of microphysical perturbations, the imposed convergence/divergence provides significantly larger impact than changes in the cloud microphysics. Simulation results presented here provide a stringent test for convection parameterizations, especially important for large-scale models progressing toward resolving some nonhydrostatic effects.
Role of Gravity Waves in a Vortex-Split Sudden Stratospheric Warming in January 2009
Authors: Song Byeong-Gwon , Chun Hye-Yeong , Song In-Sun 
Published: August 2020

AbstractThe role of gravity waves (GWs) in a sudden stratospheric warming (SSW) event that occurred in January 2009 (SSW09) is investigated using the MERRA-2 dataset. Nearly 2 weeks prior to the central date (lag = 0), at which the zonal-mean zonal wind at 10 hPa and 60°N first becomes negative, westward GW drag (GWD) is significantly enhanced in the lower mesosphere and stratosphere. At 5 days before lag = 0, planetary waves (PWs) of zonal wavenumber 2 (ZWN-2) in the stratosphere are enhanced, while PWs of ZWN-1 are weakened, which are evident from the amplitudes of the PWs and their Eliassen–Palm flux divergence (EPD). To examine the relationship between PWs and GWs, a nonconservative GWD (NCGWD) source term of the linearized quasigeostrophic potential vorticity equation is considered. A ZWN-2 pattern of the NCGWD forcing is developed around z = 55–60 km with a secondary peak around z = 40 km just before the PWs of ZWN-2 in the stratosphere began to enhance. A significant positive correlation between the NCGWD forcing in the upper stratosphere and lower mesosphere (USLM; 0.3–0.1 hPa in the present data) and the PWs of ZWN-2 in the stratosphere (5–1 hPa) exists. This result demonstrates that the amplification of the PWs of ZWN-2 in the stratosphere before the onset of SSW09 is likely related to the generation of PWs by GWD in the USLM, which is revealed by the enhanced downward-propagating PWs of ZWN-2 into the stratosphere from above.
Stochastic Variability of Tropical Cyclone Intensity at the Maximum Potential Intensity Equilibrium
Authors: Nguyen Phuong , Kieu Chanh , Fan Wai-Tong (Louis) 
Published: August 2020

Abstract
This study examines the variability of tropical cyclone (TC) intensity associated with stochastic forcings at the maximum potential intensity (PI) equilibrium. By representing TC intensity as an Itô diffusion process in the framework of TC-scale dynamics, we show from both theoretical and numerical analyses that there exists an invariant intensity distribution whose variance is proportional to the variances of stochastic forcings. This result provides further evidence that TC dynamics possess an intrinsic variability that prevents the TC absolute intensity errors in numerical models from being reduced below an arbitrarily small threshold. Analysis of the invariant intensity distribution at the PI limit reveals also that the stochastic forcing component associated with tangential wind and the warm-core anomaly in the TC central region have the largest contribution to TC intensity variability. These results suggest that future development of stochastic representation in TC models should focus on the tangential wind and thermodynamic structure to capture proper TC intensity random fluctuations.
Description of Atmospheric Aerosol Dynamics Using an Inverse Gaussian Distributed Method of Moments
Authors: Shen J. , Yu M. , Lin J. 
Published: August 2020

AbstractFor nearly 60 years, the lognormal distribution has been the most widely used function in the field of atmospheric science for characterizing atmospheric aerosol size distribution. We verify whether the three-parameter inverse Gaussian distribution (IGD) is a more suitable function than the lognormal distribution for characterizing aerosol size distribution. An attractive feature of IGD is that with it a new method of moments (MOM) can be established for resolving atmospheric aerosol dynamics which is described by a kinetic aerosol dynamics equation, i.e., inverse Gaussian distributed MOM (IGDMOM). The advantage of IGDMOM is that all of its moments can be analytically calculated using a closure moment function inherited from IGD. The precision and efficiency of IGDMOM are verified by comparing it with other recognizable methods in test cases of four representative atmospheric aerosol dynamics. Several key statistical quantities determining aerosol size distributions, including kth moments (k = 0, 1/3, 2/3, and 2), geometric standard deviation, skewness, and kurtosis, are evaluated. IGDMOM has higher precision than the lognormal MOM with nearly identical efficiency. The article provides a novel alternative to atmospheric scientists for solving kinetic aerosol dynamics equations.
An Assessment of MJO Circulation Influence on Air–Sea Interactions for Improved Seasonal Rainfall Predictions over East Africa
Authors: Kimani Margaret , Hoedjes Joost C. B. , Su Zhongbo 
Published: August 2020

AbstractRainfall variability affects agriculture planning and water resource management. In extreme flood and drought events, lives and property are destroyed. This study aims to improve East Africa’s seasonal rainfall prediction by determining the impact of the standard eight Real-time Multivariate Madden–Julian Oscillation (MJO) (RMM) phases on rainfall and using sea surface temperature (SST) response to test the predictability of the March–May (MAM) and October–December (OND) main rainfall seasons over a period of 33 years (1981–2013). Pearson correlation patterns, composite maps, and regression analyses were applied, and the Brier skill score (BSS) and correlation coefficients (CC) were utilized as validation metrics. Low correspondence of rainfall to MJO 1 and MJO 2 was observed except for the months of November and December. Seasonally, MAM and OND correlation patterns with MJO 2 revealed enhanced rainfall over the highlands and insignificant correspondence over coastal areas. Conversely, enhanced MJO 8 corresponded to suppressed rainfall during the June–August season over the coast and the eastern highlands. MAM rainfall was shown to be predictable using Maritime Continent SST indices, with a BSS of 0.41, while OND rainfall was shown to be predictable using Atlantic and Maritime Continent SSTs with a BSS of 0.62. Positive and negative MJO 2 corresponded, respectively, to enhanced and suppressed rainfall during the OND season and was confirmed to be related to, respectively, a positive and negative Indian Ocean dipole (IOD). An IOD year could possibly be triggered by changes in MJO 2 amplitudes observed as early peaks between February and June.
The Effect of Explicit Convection on Couplings between Rainfall, Humidity, and Ascent over Africa under Climate Change
Authors: Jackson Lawrence S. , Finney Declan L. , Kendon Elizabeth J. , Marsham John H. , Parker Douglas J. , Stratton Rachel A. , Tomassini Lorenzo , Tucker Simon 
Published: August 2020

AbstractThe Hadley circulation and tropical rain belt are dominant features of African climate. Moist convection provides ascent within the rain belt, but must be parameterized in climate models, limiting predictions. Here, we use a pan-African convection-permitting model (CPM), alongside a parameterized convection model (PCM), to analyze how explicit convection affects the rain belt under climate change. Regarding changes in mean climate, both models project an increase in total column water (TCW), a widespread increase in rainfall, and slowdown of subtropical descent. Regional climate changes are similar for annual mean rainfall but regional changes of ascent typically strengthen less or weaken more in the CPM. Over a land-only meridional transect of the rain belt, the CPM mean rainfall increases less than in the PCM (5% vs 14%) but mean vertical velocity at 500 hPa weakens more (17% vs 10%). These changes mask more fundamental changes in underlying distributions. The decrease in 3-hourly rain frequency and shift from lighter to heavier rainfall are more pronounced in the CPM and accompanied by a shift from weak to strong updrafts with the enhancement of heavy rainfall largely due to these dynamic changes. The CPM has stronger coupling between intense rainfall and higher TCW. This yields a greater increase in rainfall contribution from events with greater TCW, with more rainfall for a given large-scale ascent, and so favors slowing of that ascent. These findings highlight connections between the convective-scale and larger-scale flows and emphasize that limitations of parameterized convection have major implications for planning adaptation to climate change.