Geostrophic Wind Research Articles

Wind Turning in the Planetary Boundary Layer in CMIP6 Models

Abstract A set of CMIP6 models is evaluated for the turning of the wind over the planetary boundary layer (PBL) and the corresponding cross-isobaric mass flux. The bulk Richardson number method is used to calculate the height of the PBL to allow for comparisons with a climatology of observed wind-turning angles documented by Lindvall and Svensson based on more than 800 stations in the Integrated Global Radiosonde Archive. Wind-turning angles are found to be underestimated in all models, with the GFDL CM4 model having the closest distribution to the observations. Large, negative cross-isobaric mass fluxes (flow toward higher pressure) are found over high-terrain areas and the North Atlantic storm-track region in all models and the ERA-Interim reanalysis. Bulk Richardson number–derived PBLs are particularly shallow in the Norwegian Earth System Model, medium atmosphere-medium ocean resolution (NorESM2-MM), likely caused by a change in the turbulence and cloud scheme as compared to the CESM2 model that uses the same atmospheric model, leading to small wind-turning angles and cross-isobaric mass fluxes. Using the 850-hPa level as the upper boundary broadens the wind-turning angle distribution and increases the amount of cross-isobaric mass flux for all models. This makes the models closer to the observations, although substantial differences are still present. The assumption of a constant geostrophic wind throughout the PBL possibly affects the calculated mass fluxes.

Estimating the Volume Transport of Kuroshio Extension Based on Satellite Altimetry and Hydrographic Data

Abstract In this study, an effective method of estimating the volume transport of the Kuroshio Extension (KE) is proposed using surface geostrophic flow inferred from satellite altimetry and vertical stratification derived from climatological temperature/salinity (T/S) profiles. Based on velocity measurements by a subsurface mooring array across the KE, we found that the vertical structure of horizontal flow in this region is dominated by the barotropic and first baroclinic normal modes, which is commendably described by the leading mode of empirical orthogonal functions (EOFs) of the observed velocity profiles as well. Further analysis demonstrates that the projection coefficient of moored velocity onto the superimposed vertical normal mode can be represented by the surface geostrophic velocity as derived from satellite altimetry. Given this relationship, we proposed a dynamical method to estimate the volume transport across the KE jet, which is well verified with both ocean reanalysis and repeated hydrographic data. This finding implicates that, in the regions where the currents render quasi-barotropic structure, it takes only satellite altimetry observation and climatological T/S to estimate the volume transport across any section. Significance Statement The Kuroshio Extension (KE) plays an important role in the midlatitude North Pacific climate system. To better understand the KE dynamic and its influences, it is very important to estimate the KE transport. However, direct observation is very difficult in this area. Combining a subsurface mooring array and climatological temperature/salinity data, the vertical structure of the KE is explored in this study using mode decomposition methods. The relationship between the vertical structure of the zonal velocity and surface geostrophic flow observed by satellite altimetry in the KE region is further investigated. Based on this relationship, the KE transport can be well estimated by using satellite altimetry observation and historical hydrographic observation.

Theoretical characterization of gradient wind models of a translating tropical cyclone

To elucidate the underlying assumption behind four existing gradient wind (GW) models in relation to a translating tropical cyclone, we theoretically derived all the GW models solely from the storm-relative (SR) momentum equations in a unified manner. Then, we characterized the five GW models, including a new model, by deriving the mathematical form of the residuals of each GW model from the SR momentum equations, and evaluated the quantitative characteristics of the residuals with a numerical example. The numerical results indicated that the GW model used by Georgiou et al. (1983) exhibited the largest residuals among the five models, whereas the GW model used by Kepert (2001) exhibited virtually zero residuals as theoretically expected if the geostrophic balance and the synoptic-scale environmental pressure superposition can be assumed. The performance of each GW model was evaluated by root mean square errors (RMSEs) with reference to the horizontal wind velocities observed by wind profilers at high altitudes during the passage of Typhoon Jebi 2018 toward and across Japan. The RMSE results are found consistent with the residual characteristics theoretically deduced and numerically evaluated, indicating the excellence of the Kepert model in performance and adequacy of the related assumptions.

The influence of synoptic wind on land–sea breezes

AbstractParticularly challenging classes of heterogeneous surfaces are ones where strong secondary circulations are generated, potentially dominating the flow dynamics. In this study, we focus on land–sea breeze (LSB) circulations resulting from surface thermal contrasts, in the presence of increasing synoptic pressure forcing. The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a heterogeneity Richardson number (measuring the relative strength of geostrophic wind and convection induced by buoyancy), and the angle α between the shore and geostrophic wind. Large‐eddy simulations reveal the emergence of various regimes where the dynamics are asymmetric with respect to α. Along‐shore cases result in deep LSBs similar to the scenario with no synoptic background, irrespective of the geostrophic wind strength. Across‐shore simulations exhibit a circulation cell that decreases in height with increasing synoptic forcing. However, at the highest synoptic winds simulated, the circulation cell is advected away with sea‐to‐land winds, while a shallow circulation persists for land‐to‐sea cases. Scaling analysis that relates the internal parameters Qshore (net shore volumetric flux) and qshore (net shore advected kinematic heat flux) to the external input parameters results in a succinct model of the shore fluxes that also helps explain the physical implications of the identified LSBs. Finally, the vertical profiles of the shore‐normal velocity and shore‐advected heat flux are used, with the aid of k‐means clustering, to independently classify the LSBs into four regimes (canonical, sea‐driven, land‐driven, and advected), corroborating our visual categorization.

Theoretical and Computational Analysis of the Thermal Quasi-Geostrophic Model

This work involves theoretical and numerical analysis of the thermal quasi-geostrophic (TQG) model of submesoscale geophysical fluid dynamics (GFD). Physically, the TQG model involves thermal geostrophic balance, in which the Rossby number, the Froude number and the stratification parameter are all of the same asymptotic order. The main analytical contribution of this paper is to construct local-in-time unique strong solutions for the TQG model. For this, we show that solutions of its regularised version α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document}-TQG converge to solutions of TQG as its smoothing parameter α→0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha \\rightarrow 0$$\\end{document} and we obtain blow-up criteria for the α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document}-TQG model. The main contribution of the computational analysis is to verify the rate of convergence of α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document}-TQG solutions to TQG solutions as α→0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha \\rightarrow 0$$\\end{document}, for example, simulations in appropriate GFD regimes.

Wind Resource Assessment over the Hellenic Seas Using Dynamical Downscaling Techniques and Meteorological Station Observations

The current work focuses on establishing the parameters that influence the wind’s behavior over the Aegean and Ionian Seas and estimating the wind potential in the region based on long-term historic climate data. Combining a downscaling technique performed with the well-founded WRF-ARW computational algorithm and a number of simultaneous meteorological station time series, an attempt is made to investigate how regional changes may affect low-altitude wind speed distribution at hub height (100 m a.s.l.). The provided time-series coastal data span the entire region of interest from north to south. WRF-ARW v.3.9 is utilized to associate the geostrophic wind distribution obtained from long-term Copernicus ERA5 wind data with the localized wind potential over lower altitudes. Evaluation and correlation of the observational data to the predicted wind climate are performed, and the statistical differences that arise are investigated. High-accuracy wind resource potential maps are thus obtained in the region. Also, a few distinctive flow patterns are identified, such as wind speed cut-off regions and very high wind speed distributions, which are presented in specific southern regions of the Aegean Sea.

Analysis of a coastal-trapped wave generated by the 2016 extra-tropical cyclonic system in the Southern Brazilian continental shelf with COAWST modeling system

This study used the coupled ocean-atmosphere regional model (COAWST) to evaluate the effect of the trapped wave formed by a frontal system over the Southern Brazilian Continental Shelf (SBCS) during September 2016. The oceanic model was configured for the domain of 20–40°S/40–60°W, with a horizontal resolution of 1/9°, 32 vertical levels, and forced with Global Ocean Sea Physical Analysis and Forecasting Products. The model simulations were compared to remote sensing and tide gauges data, with a correlation higher than 0.78 between the wind and sea level rise data and higher than 0.82 for the wave amplitude and phase. Our results showed the presence of a sea level anomaly propagating northward along the continental shelf at 480 km.day−1, probably associated with the presence of a coastal-trapped wave. The analyzed period consisted of the rapid formation of an extra-tropical cyclone on the model grid, intensifying the south-quadrant winds and the associated Ekman transport. The zonal acceleration and Coriolis terms were strongly associated with the wave crossing, with an amplitude greater than 2.10−5 m.s−2. The Coriolis term follows the geostrophic flow induced by the wave crest, with values greater than 4.2 m.s−2, and a correlation coefficient of 0.82 with the pressure gradient and friction terms. The trapped wave reversed the pressure gradient and the over-shelf flow with a negative geostrophic flow (average westward shelf flow). Our results reinforce the fact that these processes change the physical and chemical seawater properties over the SBCS, changing the mixing layer and fertilizing its shelf waters.

Annual Cycle in Upper-Ocean Heat Content and the Global Energy Budget

Abstract As a major component of Earth’s energy budget, ocean heat content (OHC) plays a vital role in buffering climate change. The annual cycle is the most prominent change in OHC but has always been removed to study variations and changes in Earth’s energy budget. Here, we investigate the annual cycle of the upper-2000-m OHC at regional to global scales and assess the robustness of the signals using the spread of multiple observational products. The potential drivers are also investigated by comparing the annual OHC signal with the corresponding change in top-of-atmosphere radiation, surface heat flux, ocean heat divergence, and meridional heat transport. Results show that the robust signal of annual OHC change is significant down to a 1000-m depth globally and can reach down to 1500 m in some areas such as the tropical ocean. The global OHC (0–1500 m) changes from positive anomalies within September–February to negative anomalies within March–August, mainly because of the larger ocean area in the Southern Hemisphere and the seasonal migration of solar irradiance. Owing to the huge ocean heat capacity, the annual cycle of OHC dominates that of the global energy budget. The difference among the OHC annual cycles in the three major ocean basins is mainly attributed to ocean heat transport, especially in the tropics. In the upper 1500 m at mid- and high latitudes and in the upper 50 m of the tropics, the net sea surface heat flux dominates the OHC annual cycle, while in the tropics below 50 m, wind-driven Ekman heat transport associated with the geostrophic flow is the main driver.

Numerical investigation of neutral atmospheric boundary layer flows over flat terrain and three-dimensional hills considering the effects of Coriolis force

The characteristics of neutral atmospheric boundary layer (ABL) flows can be strongly affected by terrain and earth rotation-induced Coriolis force. The purpose of this study is to ascertain the effects of Coriolis force on neutral ABL flows over flat terrain and isolated three-dimensional hills. Based on large-eddy simulations (LES), a new formula for the maximum turbulence length scale lmax in the limited-length-scale (LLS) k-ε model is proposed to reproduce neutral ABL flows over flat terrain with different exposure conditions. It is demonstrated that lmax can be reasonably expressed as a function of the geostrophic wind speed Ug, the Coriolis parameter fc and the roughness length z0. The effects of Coriolis force on atmospheric flows over single isolated hills are subsequently elucidated from the perspective of flow patterns and turbulent statistics. The wake trajectory deflection and asymmetric vortex structures are identified on the lee side of hills owing to the wind veer. Additionally, lateral wind shear associated with the Coriolis force contributes to a faster wake recovery and larger turbulence fluctuations downstream of steep hills. Moreover, the speed-up factor at the hilltop is significantly enhanced with increasing hill slopes and height ratios, while it is not sensitive to the Coriolis effects.

Synoptic meteorological conditions and contributing factors to air quality during the SIJAQ campaign

The Satellite Integrated Joint Monitoring of Air Quality (SIJAQ) field campaign was conducted from October 10 to November 25, 2021, to assess air quality across the Korean Peninsula and validate the results of the Geostationary Environment Monitoring Spectrometer (GEMS) employing remote sensing, modeling, and satellite, aircraft, and ground measurements. To integrate all measurements collected during the campaign, synoptic features across the Korean Peninsula were analyzed. We identified cyclones and anticyclones by tracing weather charts and divided the campaign into five periods (P1 to P5): P1 - Sustained marine stagnation (October 10–28, 2021), P2 - Transitional stagnant high (October 29–November 7, 2021), P3 - Continental migratory low (November 8–14, 2021), P4 - Continental migratory high (November 15–21, 2021), and P5 - Continental migratory low (November 22–25, 2021). The synoptic features, such as the locations of stagnant or migratory cyclones and anticyclones were analyzed during the periods, in association with levels of particulate matter with a diameter of less than 2.5 μm (PM2.5 levels) over the Seoul metropolitan area (SMA). The synoptic characteristics showed that the presence of a high pressure system was well associated with the occurrence of high PM2.5 level over the SMA, and a pattern of near-surface divergence and descending motions originating from the mid-troposphere. In addition, we identified and compared two PM2.5 level peaks caused by local accumulation and transboundary transport process. We also investigated the underlying synoptic meteorological mechanisms of these peaks in terms of large-scale dynamic variables, such as geostrophic wind speed, transient synoptic scale features, vorticity, and vorticity advection at 850 hPa geopotential height, which are responsible for transboundary transport and local accumulation-driven process of PM2.5 in the SMA.

Controls of Topographic Rossby Wave Properties and Downslope Transport in Dense Overflows

Abstract Antarctic Bottom Water is primarily formed via overflows of dense shelf water (DSW) around the Antarctic continental margins. The dynamics of these overflows therefore influence the global abyssal stratification and circulation. Previous studies indicate that dense overflows can be unstable, energizing topographic Rossby waves (TRW) over the continental slope. However, it remains unclear how the wavelength and frequency of the TRWs are related to the properties of the overflowing DSW and other environmental conditions, and how the TRW properties influence the downslope transport of DSW. This study uses idealized high-resolution numerical simulations to investigate the dynamics of overflow-forced TRWs and the associated downslope transport of DSW. It is shown that the propagation of TRWs is constrained by the geostrophic along-slope flow speed of the DSW and by the dynamics of linear plane waves, allowing the wavelength and frequency of the waves to be predicted a priori. The rate of downslope DSW transport depends nonmonotonically on the slope steepness: steep slopes approximately suppress TRW formation, resulting in steady, frictionally dominated DSW descent. For slopes of intermediate steepness, the overflow becomes unstable and generates TRWs, accompanied by interfacial form stresses that drive DSW downslope relatively rapidly. For gentle slopes, the TRWs lead to the formation of coherent eddies that inhibit downslope DSW transport. These findings may explain the variable properties of TRWs observed in oceanic overflows, and they imply that the rate at which DSW descends to the abyssal ocean depends sensitively on the manifestation of TRWs and/or nonlinear eddies over the continental slope.

Two-Way Nesting in a Split-Implicit Ocean Model: NCOM

Abstract Presented is the approach, implementation, and evaluation of two-way nesting in a split-implicit ocean model, the Navy Coastal Ocean Model (NCOM). Emphasis is on the strategies applied to feed back fields from the fine-mesh nest (child grid) to the coarse mesh (parent grid). On an appropriate separation of dynamic and feedback interfaces, attention is especially needed for the feedback interface of surface elevation. One particular issue addressed is the inconsistency between the 3D baroclinic velocities and 2D barotropic transports in the feedback. The discrepancy is inherently associated with bathymetry, depth integration, and the need to average over spatial grid points. A simple remedy is proposed and proven to be effective and necessary in realistic coastal applications. In addition to the full two-way nesting, a simplified two-way nesting approach is provided in which only the temperature and salinity are fed back from the nest, and the velocity fields are assumed to self-adjust according to the geostrophic balance. The performance of both approaches is evaluated using the idealized benchmark, propagation of a baroclinic vortex, and an application to the Mississippi River outflow in the northeast Gulf of Mexico, including a comparison with available observations. Discussions are also made on the computational efficiency of the two-way nesting and its sensitivity to the open boundary conditions in regard to noise suppression. Significance Statement The two-way nesting approach reported here can be adapted to other structured-grid ocean models, in particular those using the split-implicit technique. The treatment of the feedback interface for surface elevation is especially important for suppressing the noise production and improving the feedback consistency. An effective procedure is given to amend the inconsistency in the velocity field feedback that is inherently due to bathymetry.

The impact of LNG offshore terminal on sea temperature and sea currents in the northern Adriatic Sea

The aim of this paper is to simulate the impact of a potential offshore LNG terminal on sea temperature (in autumn and spring/ summer) and sea currents (in autumn/winter) at three different depths (at the sea surface, at 25 m depth and at the seabed) in the northern Adriatic Sea from 14 November 2015 to 06 August 2016 using the Regional Ocean Modelling System (ROMS) model. The location of the potential offshore LNG terminal Istria (in the northern Adriatic Sea) was selected using the visual PROMETHEE method. The potential LNG terminal uses seawater for LNG heating and the seawater cooled to a temperature of 9°C returns to the marine environment. Although the differences in sea temperature with and without the discharge fit within normal temperature ranges, the simulations show that the discharge changed the speed and direction of sea currents at the sea surface not only in the wider northern Adriatic, but in the entire Adriatic. This is probably due to the specific circulation in the Adriatic, where cold water affects the geostrophic balance, an important part of the circulation field that depends on density (a function of salinity and temperature). Atmospheric conditions in the broader vicinity of the LNG terminal would also be affected by redistribution of air-sea fluxes due to changes in surface temperature. Changes in circulation would alter environmental conditions by redistributing nutrients, oxygen, etc. Further multi-year simulations of changes in the circulation system are needed, but other physical parameters (density, salinity, river inflow...) should also be included in the simulations to determine the cumulative impact of a potential LNG terminal on the marine environment.

Decadal changes in the relationship between Arctic stratospheric ozone and sea surface temperatures in the North Pacific

Using multi-source observations and ERA-Interim reanalysis data, this study found that the relationship between Arctic stratospheric ozone in March and North Pacific sea surface temperature (SST) in April shows dramatic decadal changes, and it significantly weakens since the 2000s. By enhancing the stratospheric circulations, a decreased Arctic stratospheric ozone favors tropospheric positive Arctic Oscillation (+AO)-like anomaly during PRE eras (1979–1999), with positive geopotential height (GH) anomalies over high-latitude Asia. The stratospheric ozone anomaly also favors the Asian positive GH anomalies via modulating high cloud and its related longwave radiation. According to geostrophic wind relationships, the Asian positive GH anomalies are accompanied by easterly anomalies over midlatitude Asia in late March, which further extend eastward and thereby induce easterly anomalies over the midlatitude North Pacific. The North Pacific easterly anomalies result in negative North Pacific Oscillation (–NPO)-like anomaly via vorticity forcing of zonal wind and the interactions between synoptic-scale eddies and mean flow, forcing significant SST anomalies. However, during POST eras (2000–2019), firstly, the +AO-related positive GH anomalies over high-latitude Asia are relatively weak, accompanied by weak easterly anomalies over midlatitude Asia according to geostrophic wind relations, which induce weak easterly anomalies over the North Pacific. Secondly, a westerly anomaly occurs over the midlatitude North Pacific, which partly offsets the easterly anomalies from Asia, weakening the North Pacific easterly anomalies. The weak easterly anomalies further induce weak –NPO and SST anomalies during the POST eras. Our analysis further indicates that the North Pacific westerly anomalies and weakening of the positive GH anomalies over high-latitude Asia associated with ozone are related to the interference of the westerly phase of quasi-biennial oscillation and the weakened stratospheric ozone anomaly over high-latitude Asia, respectively.

Homogenization of Observed Surface Wind Speed Based on Geostrophic Wind Theory over China from 1970 to 2017

Abstract The observed surface wind speed (SWS) over China has declined in the past four decades, and recently, the trend has reversed, which is known as SWS stilling and recovery. The observed SWS is vulnerable to changes in nonclimatic factors, i.e., inhomogeneity. Unfortunately, most of the existing studies on the long-term trend of SWS were based on raw datasets without homogenization. In this study, by means of geostrophic wind speed and penalized maximal t test, we conduct a systematic homogeneity test and exploration of the homogenization impact for SWS at over 2000 stations in China from 1970 to 2017. The results show that the inhomogeneity in the observed SWS over China is detectable at 59% of national weather stations. The breakpoint years are mainly concentrated in the late 1970s, mid-1990s, and early 2000s. Overall, 18% of breakpoints are caused by station relocations, and the remaining breakpoints are likely related to anemometer replacement and measurement environment changes that occurred during the mid-1990s and early 2000s. After homogenization, the decreasing trend in SWS during 1970–2017 decreased from −0.15 to −0.05 m s−1 decade−1. The homogenized SWS recovery period advanced from the early twenty-first century to the early 1990s, which is consistent with the SWS variations, excluding the impact of urbanization around weather stations. The phase change in the Western Hemisphere warm pool (WHWP) might be one of the causes of homogenized SWS recovery.

Observations of a Filamentous Intrusion and Vigorous Submesoscale Turbulence within a Cyclonic Mesoscale Eddy

Abstract Oceanic submesoscale flows are considered to be a crucial conduit for the downscale transfer of oceanic mesoscale kinetic energy and upper-ocean material exchange, both laterally and vertically, but defining observations revealing submesoscale dynamics and/or transport properties remain sparse. Here, we report on an elaborate observation of a warm and fresh filament intruding into a cyclonic mesoscale eddy. By integrating cruise measurements, satellite observations, particle-tracking simulations, and the trajectory of a surface drifter, we show that the filament originated from an anticyclonic eddy immediately to the west of the cyclonic eddy, and the evolution of the filament was mainly due to the geostrophic flows associated with the eddy pair. Our observations reveal the mass exchange of the eddy pair and suggest that submesoscale flows can degrade the coherence of mesoscale eddies, providing important implications for the transport properties of mesoscale eddies. Vigorous submesoscale turbulence was found within the eddy core region, due to filamentous intrusion and frontogenesis. Our findings have thus offered novel insights into the dynamics and transport properties of oceanic submesoscale flows, which should be taken into account in their simulation and parameterization in ocean and climate models. Significance Statement Mesoscale eddies, with a spatial scale from tens to hundreds of kilometers, are ubiquitous in the global ocean. Carrying the largest proportion of oceanic kinetic energy, mesoscale eddies play a key role in ocean dynamics and have important applications for marine biology. Although mesoscale eddies have been studied extensively over the past decades, there are two major issues that remain inconclusive: (i) How do mesoscale eddies dissipate? (ii) Can eddies coherently trap waters when moving over a long distance? Recent studies, mostly through computer simulations, suggest that oceanic submesoscale processes, with a typical scale of a few kilometers, are highly relevant to the above two issues. This study presents a rare observation of a filament intruding into a cyclonic eddy. Because of this filament intrusion, submesoscale activities are enhanced near the eddy core area, in contrast to previous observations that normally suggest weaker submesoscale activities in the eddy core area than at eddy peripheries. Such dedicated process-oriented observations provide unique opportunities for better understanding the dynamics and transport properties of mesoscale eddies and submesoscale processes.

Sensitivity of the pseudo-global warming method under flood conditions: a case study from the northeastern US

Abstract. Intensified extreme precipitation and corresponding floods are the most relevant consequences of climate change over the northeastern US (NEUS). To evaluate the impacts of climate change or certain climate perturbations on future extreme weather events which are dynamically similar to historic analogs, the pseudo-global warming (PGW) method has been frequently employed; however, this method lacks precise definition and guidelines, thus limiting its application. More specifically, three key questions related to the application of the PGW method remain unanswered: at what spatial scale should climate perturbations be applied? Among the different meteorological variables available, which ones should be perturbed? And will PGW projections vary significantly when different perturbations are applied? To address these questions, we examine the sensitivity and robustness of conclusions drawn from the PGW method over the NEUS by conducting multiple PGW experiments with varied perturbation spatial scales and choice of perturbed meteorological variables. The results show that the projections of precipitation and other essential variables at the regional mean scale are consistent across the PGW simulations, with a relative difference of much less than 10 %; however, different perturbation modifications can cause significant displacements of the storm events being simulated. Several previously assumed advantages of modifying only the temperature at regional mean scale, such as the preservation of geostrophic balance, do not appear to hold. Also, for these experiments, we find the regional mean perturbation produces a positive precipitation bias because it ignores the land–ocean warming contrast, which is a robust regional response to global warming. Overall, PGW experiments with perturbations from temperature or the combination of temperature and wind at the grid point scale are both recommended, depending on the research questions. The first approach can isolate the spatially dependent thermodynamic impact, and the latter incorporates both the thermodynamic and dynamic impacts.

Horizontal gravity disturbance vector in atmospheric dynamics

All meteorological and oceanographic textbooks unintentionally define the Earth gravitation outside the solid Earth through treating the Earth as a point-mass located at the Earth center containing the entire Earth mass or assuming the Earth with uniform mass density. Combination of such simplified gravitational and centrifugal accelerations leads to the effective gravity geff. The geopotential surface corresponding to geff (called the effective geopotential surface) is the ellipsoidal surface. The true Earth gravitation outside the solid Earth is the volume integration of gravitation of all point-masses inside the Earth. Combination of true gravitational and centrifugal accelerations leads to the true gravity g. The geopotential surface corresponding to g (called the true geopotential surface) and respect to the Earth ellipsoidal reference surface is the geoid, which varies approximately from -106 m to 86 m worldwide. The true gravity g minus the effective gravity geff is the gravity disturbance vector, δg = g – geff, which is a major variable in geodesy and solid Earth dynamics. Contrarily, δg is neglected completely in dynamics of atmosphere and ocean. In this study, the true gravity g replaces the effective gravity geff in atmospheric dynamic equations. Seven non-dimensional numbers are proposed to identify the magnitude of horizontal gravity disturbance vector (δg)⊥versus traditional horizontal forcing terms such as pressure gradient force and Coriolis force. These non-dimensional numbers are calculated from two publicly available and independent datasets with the geoid undulation (N) from the static gravity field model EIGEN-6C4 and long-term mean geopotential height (Z), wind velocity (u, v), and temperature (T) at 12 pressure levels in troposphere from the NCEP/NCAR reanalyzed climatology. The results demonstrate (δg)⊥ nonnegligible in hydrostatic equilibrium, geostrophic wind, geostrophic vorticity, Q vector, and Omega equation.

Geomagnetic activity dependence and dawn-dusk asymmetry of thermospheric winds from 9-year measurements with a Fabry–Perot interferometer in Tromsø, Norway

Ion drag associated with the ionospheric plasma convection plays an important role in the high-latitude thermospheric dynamics, yet changes in the thermospheric wind with geomagnetic activity are not fully understood. We performed a statistical analysis of the thermospheric wind measurements with a Fabry–Perot interferometer (FPI; 630 nm wavelength) in Tromsø, Norway, in the winter months for 9 years. The measurements were sorted by a SuperMAG (SME) index, and a quiet-time wind pattern was defined as an hourly mean under SME ≤ 40 nT. The quiet-time wind pattern can be expected to be represented by a pressure gradient associated with the solar radiation and a geostrophic force balance. With an increase in the geomagnetic activity level, the thermospheric wind turned over from eastward to westward at dusk and increased the equatorward magnitude from midnight to dawn. Deviations from the quiet-time wind presented similar patterns in the direction with the ionospheric plasma convection but were larger in magnitude at dusk than at dawn. This is the first study to report a dawn-dusk asymmetry of the thermospheric wind acceleration feature and signatures of the eastward wind acceleration at dawn by ion drag.Graphical

Investigation of low-level supergeostrophic wind and Ekman spiral as observed by a radar wind profiler in Beijing

The supergeostrophic flow remains seldomly reported from an observational perspective. Here, 1 year record of radar wind profiler measurements and ERA-5 reanalysis collected at Beijing observatory station are used to characterize the vertical structures of supergeostrophic wind and Ekman spirals in the lower troposphere. It is found that supergeostrophic flow shows significant diurnal variation, with lowest frequency for the supergeostrophic wind forming during daytime under clear-sky conditions, largely due to strong turbulent mixing and friction in the daytime. By comparison, the planetary boundary layer at night is stably stratified, the supergeostrophic wind occurs more frequently due to friction-induced decoupling from the ground surface. Furthermore, the presence of cloud makes the supergeostrophic wind occur more often in the daytime. Also, the geostrophic wind deviation within 1 km of atmosphere is found to be more negatively associated with the difference between surface temperature and 2-m air temperature compared with that in the altitude range of 1–3 km, indicating that the supergeostrophic wind near ground surface is more subject to the influence of heat flux. Intriguingly, most of the vertical wind profiles in the PBL are found not to follow Ekman spiral under neutral atmospheric conditions. The supergeostrophic winds contribute significantly to the magnitude of Ekman spirals in the upper mixed layer. Overall, the profiles and evolution features of the supergeostrophic wind and Ekman spirals observed in the lower troposphere in Beijing are much complicated than expected. The findings lay a solid foundation for better elucidating the low-level atmospheric dynamics in Beijing.