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Journal of the Atmospheric Sciences
American Meteorological Society
Journal of the Atmospheric Sciences

American Meteorological Society

0022-4928

Journal of the Atmospheric Sciences/Journal Journal of the Atmospheric SciencesSCIISTPEI
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    The Temperature Anomaly Pattern of the Pacific-North American Teleconnection: Growth and Decay

    Clark, Joseph P.Feldstein, Steven B.
    16页
    查看更多>>摘要：Applying composite analysis to ERA-Interim data, the surface air temperature (SAT) anomaly pattern of the Pacific-North American (PNA) teleconnection is shown to include both symmetric and asymmetric SAT anomalies with respect to the PNA phase. The symmetric SAT anomalies, overlying the Russian Far East and western and eastern North America, grow through advection of the climatological temperature by the anomalous meridional wind and vertical mixing. The asymmetric SAT anomalies, overlying Siberia during the positive PNA and the subtropical North Pacific during the negative PNA, grow through vertical mixing only. For all SAT anomalies, vertical mixing relocates the temperature anomalies of the PNA teleconnection pattern from higher in the boundary layer downward to the level of the SAT. Above the level of the SAT, temperature anomaly growth is caused by horizontal temperature advection in all locations except for the subtropical North Pacific, where adiabatic cooling dominates. SAT anomaly decay is caused by longwave radiative heating/cooling, except over Siberia, where SAT anomaly decay is caused by vertical mixing. Additionally, temperature anomaly decay higher in the boundary layer due to nonlocal mixing contributes indirectly to SAT anomaly decay by weakening downgradient diffusion. These results highlight a diverse array of mechanisms by which individual anomalies within the PNA pattern grow and decay. Furthermore, with the exception of Siberia, throughout the growth and decay stages, horizontal temperature advection and/or vertical mixing is nearly balanced by longwave radiative heating/cooling, with the former being slightly stronger during the growth stage and the latter during the decay stage.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    Theory of Parcel Vorticity Evolution in Supercell-Like Flows

    Davies-Jones, Robert
    18页
    查看更多>>摘要：In a prior paper, insights into tornadogenesis in supercell storms were gained by discovering analytical formulas for vorticity variations along streamlines in idealized, steady, frictionless, isentropic inflows of dry air imported from a horizontally uniform environment. This work is simplified and extended to the evolution of parcel vorticity in unsteady, nonisentropic flows by integrating the vorticity equation using nonorthogonal Lagrangian coordinates. The covariant basis vectors (e) over bar( 1), (e) over bar (2), and (e) over bar( 3) are material line elements attached to each parcel. Initially they form an orthonormal set with (e) over bar( 1) in the direction of and (e) over bar( 2) left normal to the storm-relative wind at each level in the environment, and e over bar 3 upward. The surface containing all parcels with the same initial height constitutes a material surface, within which initially streamwise and transverse material lines are reoriented and stretched or shrunk. The basis vectors propagate a parcel's barotropic vorticity through time by factoring in the "frozen-field " effect. With a horizontally uniform environment, the barotropic vorticity of a parcel depends on its initial streamwise vorticity times its current (e) over bar (1) plus its initial crosswise vorticity times its current (e) over bar( 2). For baroclinic and frictional vorticity, each contravariant component is the integral from initial to current time of the corresponding contravariant component of the generation vector. The "river-bend " effect acting on all parts (baroclinic, frictional, and barotropic) of transverse vorticity produces streamwise vorticity (parallel to 3D wind). In left-turning steady flow, it arises from (e) over bar( 2) rotating toward (e) over bar( 1). For steady, frictionless, dry isentropic flow, previous vorticity formulas are recovered.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    Linking Equatorial African Precipitation to Kelvin Wave Processes in the CP4-Africa Convection-Permitting Regional Climate Simulation

    Ayesiga, GodwinHolloway, Christopher E.Williams, Charles J. R.Yang, Gui-Ying...
    19页
    查看更多>>摘要：Observational studies have shown the link between convectively coupled Kelvin waves (CCKWs) and eastward-propagating rainfall anomalies. We explore the mechanisms in which CCKWs modulate the propagation of precipitation from west to east over equatorial Africa. We examine a multiyear state-of-the-art Africa-wide climate simulation from a convection-permitting model (CP4A) along with a parameterized global driving-model simulation (G25) and evaluate both against observations (TRMM) and ERA-Interim (ERA-I), with a focus on precipitation and Kelvin wave activity. We show that the two important related processes through which CCKWs influence the propagation of convection and precipitation from west to east across equatorial Africa are 1) low-level westerly wind anomalies that lead to increased low-level convergence, and 2) westerly moisture flux anomalies that amplify the lower- to midtropospheric specific humidity. We identify Kelvin wave activity using zonal wind and geopotential height. Using lagged composite analysis, we show that modeled precipitation over equatorial Africa can capture the eastward-propagating precipitation signal that is associated with CCKWs. Composite analysis on strong (high-amplitude) CCKWs shows that both CP4A and G25 capture the connection between the eastward-propagating precipitation anomalies and CCKWs. In comparison to TRMM, however, the precipitation signal is weaker in G25, while CP4A has a more realistic signal. Results show that both CP4A and G25 generally simulate the key horizontal structure of CCKWs, with anomalous low-level westerlies in phase with positive precipitation anomalies. These findings suggest that for operational forecasting, it is important to monitor the day-to-day Kelvin wave activity across equatorial Africa.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    Northern Hemisphere Winter Blocking: Differing Onset Mechanisms across Regions

    Miller, Douglas E.Wang, Zhuo
    19页
    查看更多>>摘要：Atmospheric blocking is a prolific producer of extreme weather with significant socioeconomic impacts. Different physical mechanisms for blocking onset have been proposed and are generally focused on two sectors: the Eurasian and the North Pacific. Here, we objectively separate blocking into four regions and investigate how the blocking onset mechanisms vary from one region to another, focusing on three factors: scale interactions between three frequency bands, Rossby wave breaking (RWB), and diabatic heating. Atlantic blocks are dominated by the low-frequency flow evolution that resembles the negative phase of the North Atlantic Oscillation and are influenced by cyclonic RWB toward the western edge of the anticyclone. Europe blocks are influenced by high-frequency, traveling waves across the Atlantic Ocean and develop rapidly, mainly attributed to strong anticyclonic RWB and interaction between high- and intermediate-frequency flow components. Asian blocks are fixated within a stationary wave train that spans upstream to the western Atlantic Ocean and do not have strong potential vorticity or RWB features. The Pacific blocks are mainly influenced by an intermediate-frequency retrograding wave train, while a low-frequency component resembling the Pacific-North American pattern is evident. The Pacific blocks also contain precursor signals in the stratosphere. Backward trajectory analysis revealed that 35%-45% of parcels initialized within the Atlantic, Europe, and Pacific blocking anticyclones experience heating and ascent, while adiabatic processes dominate Asian blocking. Overall, our analysis demonstrates the importance of decomposing the flow into three frequency bands and illustrates different blocking onset mechanisms over four sectors.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    The Sensitivity of Superrotation to the Latitude of Baroclinic Forcing in a Terrestrial Dry Dynamical Core

    Zurita-Gotor, PabloAnaya-Benlliure, AlvaroHeld, Isaac M.
    13页
    查看更多>>摘要：Previous studies have shown that Kelvin-Rossby instability is a viable mechanism for producing equatorial superrotation in small and/or slowly rotating planets. It is shown in this paper that this mechanism can also produce superrotation with terrestrial parameters when the baroclinic forcing moves to low latitudes, explaining previous results by Williams. The transition between superrotating and subrotating flow occurs abruptly as the baroclinic forcing moves poleward. Although Kelvin-Rossby instability weakens when the baroclinic zone moves away from the equator, the key factor explaining the abrupt transition is the change in the baroclinic eddies. When differential heating is contained within the tropics, baroclinic eddies do not decelerate the subtropical jet and the upper-tropospheric flow approximately conserves angular momentum, providing conditions favorable for Kelvin-Rossby instability. In contrast, when baroclinic eddies are generated in the extratropics, they decelerate the subtropical jet and prevent the Kelvin-Rossby coupling. Due to this sensitivity to baroclinic eddies, the system exhibits hysteresis: near the transition parameter, extratropical eddies can prevent superrotation when they are initially present.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    Representation of Lake-Atmosphere Interactions and Lake-Effect Snowfall in the Laurentian Great Lakes Basin among HighResMIP Global Climate Models

    Notaro, MichaelJorns, JennaBriley, Laura
    23页
    查看更多>>摘要：Credible modeling, tools, and guidance, regarding the changing Laurentian Great Lakes and the climatic impacts, are needed by local decision-makers to inform their management and planning. The present study addresses this need through a model evaluation study of the representation of lake-atmosphere interactions and resulting lake-effect snowfall in the Great Lakes region. Analysis focuses on an extensive ensemble of 74 historical simulations generated by 23 high-resolution global climate models (GCMs) from the High-Resolution Model Intercomparison Project (HighResMIP). The model assessment addresses the modeling treatment of the Great Lakes, the spatial distribution and seasonality of climatological snowfall, the seasonal cycle of lake-surface temperatures and overlake turbulent fluxes, and the lake-effect ratio between upwind and downwind precipitation. A deeper understanding of model performance and biases is achieved by partitioning results between HighResMIP GCMs that are 1) coupled to 1D lake models versus GCMs that exclude lake models, 2) between prescribed-ocean model configurations versus fully coupled configurations, and 3) between deep Lake Superior versus relatively shallow Lake Erie. While the HighResMIP GCMs represent the Great Lakes by a spectrum of approaches that include land grid cells, ocean grid cells (with lake surface temperature and ice cover boundary conditions provided by the Met Office Hadley Center Sea Ice and Sea Surface Temperature Dataset), and 1D lake models, the current investigation demonstrates that none of these rudimentary approaches adequately represent the complex nature of seasonal lake temperature and ice cover evolution and its impact on lake-atmosphere interactions and lake-effect precipitation in the Great Lakes region. Significance StatementThe purpose of this study is to evaluate the capability of high-resolution global climate models to simulate lake-atmosphere interactions and lake-effect snowfall in the Great Lakes region, given the critical influence of the lakes on regional climate and vast societal and environmental impacts of lake-effect snowfall. It is determined that the models inadequately represent lake temperatures and ice cover, often leading to insufficient annual snowfall in the lake-effect zones. More advanced, three-dimensional lake models need to be coupled to climate models to support greater credibility in regional lake and climate simulations and future climate projections.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    Cross-Tropopause Transport of Surface Pollutants during the Beijing 21 July Deep Convection Event

    Wu, LuolinChen, XiaoyangChen, XiZhang, Yan...
    14页
    查看更多>>摘要：Air transport from the troposphere to the stratosphere plays an important role in altering the vertical distribution of pollutants in the upper troposphere and lower stratosphere (UTLS). On 21 July 2012, Beijing was hit by an unprecedented extreme rainfall event. In the present study, the Community Multiscale Air Quality Modeling System (CMAQ) is used to simulate the change in vertical profiles of pollutants during this event. The integrated process rate (IPR) method was applied to quantify the relative contributions from different atmospheric processes to the changes in the vertical profile of pollutants and to estimate the vertical transport flux across the tropopause. The results revealed that, in the tropopause layer, during the torrential rainfall event, the values of O-3 decreased by 35% and that of CO increased by 98%, while those of SO2, NO2, and PM2.5 increased slightly. Atmospheric transport was the main cause for the change in O-3 values, contributing 32% of the net increase and 99% of the net decrease of O-3. The calculations showed that the transport masses of CO, O-3, PM2.5, NO2, and SO2 to the stratosphere by this deep convection in 25 h were 6.0 x 10(7), 2.4 x 10(7), 7.9 x 10(5), 2.2 x 10(5), and 2.7 x 10(3) kg, respectively, within the similar to 300 km x 300 km domain. In the midlatitudes of the Northern Hemisphere, penetrating deep convective activities can transport boundary layer pollutants into the UTLS layer, which will have a significant impact on the climate of this layer.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    Emergence of a Nocturnal Low-Level Jet from a Broad Baroclinic Zone

    Shapiro, AlanGebauer, Joshua G.Parsons, David B.
    21页
    查看更多>>摘要：An analytical model is presented for the generation of a Blackadar-like nocturnal low-level jet in a broad baroclinic zone. The flow is forced from below (flat ground) by a surface buoyancy gradient and from above (free atmosphere) by a constant pressure gradient force. Diurnally varying mixing coefficients are specified to increase abruptly at sunrise and decrease abruptly at sunset. With attention restricted to a surface buoyancy that varies linearly with a horizontal coordinate, the Boussinesq-approximated equations of motion, thermal energy, and mass conservation reduce to a system of one-dimensional equations that can be solved analytically. Sensitivity tests with southerly jets suggest that (i) stronger jets are associated with larger decreases of the eddy viscosity at sunset (as in Blackadar theory); (ii) the nighttime surface buoyancy gradient has little impact on jet strength; and (iii) for pure baroclinic forcing (no free-atmosphere geostrophic wind), the nighttime eddy diffusivity has little impact on jet strength, but the daytime eddy diffusivity is very important and has a larger impact than the daytime eddy viscosity. The model was applied to a jet that developed in fair weather conditions over the Great Plains from southern Texas to northern South Dakota on 1 May 2020. The ECMWF Reanalysis v5 (ERA5) for the afternoon prior to jet formation showed that a broad north-south-oriented baroclinic zone covered much of the region. The peak model-predicted winds were in good agreement with ERA5 winds and lidar data from the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) central facility in north-central Oklahoma.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    Evidence of Aggregation Dependence of 5 degrees-Scale Tropical Convective Evolution Using a Gross Moist Stability Framework

    Tsai, Wei-MingMapes, Brian E.
    20页
    查看更多>>摘要：Spatial aggregation of deep convection and its possible role in larger-scale atmospheric behavior have received growing attention. Here we seek aggregation-correlated statistical properties of convective events in 5 degrees x 5 degrees boxes over the tropical Indian Ocean. Events are identified by box-averaged rainfall exceeding 5 mm day(-1) at the center of a 4-day time window, and aggregation is estimated by an index [simple convective aggregation index (SCAI)] based on contiguous cold cloud areas and their geometrical distances in infrared imagery. A physical framework using gross moist stability (GMS) helps to interpret relationships between aggregation, box-scale ascent profiles, moist static energy budgets, and time evolution both within composite events and on longer time scales. For a given precipitation rate, more-aggregated events (with fewer and larger cloud objects on average) exhibit a drier area mean, greater horizontal gradient of moisture, more bottom-heavy ascent profile, and a greater prevalence of low-altitude cloud tops, especially for lower rain rates. In the GMS budget, this bottom-heavy ascent implies net energy import into the atmospheric column during the 4-day event composite. Consistently, net energy variations filtered to reveal longer time scales do indeed exhibit more-aggregated rain events in their growth phase than in their flat and decaying phases. More-aggregated scenes also have more drying by analysis than less-aggregated scenes in MERRA-2's assimilation budgets. This suggests that parameterized convection (lacking any organization effect) is raining out less water than nature's real, aggregated convection in such scenes.
      原文链接:
    • NSTL
    • Amer Meteorological Soc

    On a Solution of the Closure Problem for Dry Convective Boundary Layer Turbulence and Beyond

    Gryanik, Vladimir M.Hartmann, Joerg
    24页
    查看更多>>摘要：We consider the closure problem of representing the higher-order moments (HOMs) in terms of lower-order moments, a central feature in turbulence modeling based on the Reynolds-averaged Navier-Stokes (RANS) approach. Our focus is on models suited for the description of asymmetric, nonlocal, and semiorganized turbulence in the dry atmospheric convective boundary layer (CBL). We establish a multivariate probability density function (PDF) describing populations of plumes that are embedded in a sea of weaker randomly spaced eddies, and apply an assumed delta-PDF approximation. The main content of this approach consists of capturing the bulk properties of the PDF. We solve the closure problem analytically for all relevant HOMs involving velocity components and temperature and establish a hierarchy of new non-Gaussian turbulence closure models of different content and complexity ranging from analytical to semianalytical. All HOMs in the hierarchy have a universal and simple functional form. They refine the widely used Millionshchikov closure hypothesis and generalize the famous quadratic skewness-kurtosis relationship to higher order. We examine the performance of the new closures by comparison with measurement, LES, and DNS data and derive empirical constants for semianalytical models, which are best for practical applications. We show that the new models have a good skill in predicting the HOMs for atmospheric CBL. Our closures can be implemented in second-, third-, and fourth-order RANS turbulence closure models of bi-, tri-, and four-variate levels of complexity. Finally, several possible generalizations of our approach are discussed.
      原文链接:
    • NSTL
    • Amer Meteorological Soc