Žagar, N., V. Neduhal, S. Vasylkevych, Ž. Zaplotnik, and H. L. Tanaka, 2023: Decomposition of Vertical Velocity and Its Zonal Wavenumber Kinetic Energy Spectra in the Hydrostatic Atmosphere. J. Atmos. Sci., 80, 2747–2767, https://doi.org/10.1175/JAS-D-23-0090.1.
The spectrum of kinetic energy of vertical motions (VKE) is less well understood compared to the kinetic energy spectrum of horizontal motions (HKE). One challenge that has limited progress in describing the VKE spectrum is a lack of a unified approach to the decomposition of vertical velocities associated with the Rossby motions and inertia–gravity (IG) wave flows. This paper presents such a unified approach using a linear Rossby–IG vertical velocity normal-mode decomposition appropriate for a spherical, hydrostatic atmosphere. New theoretical developments show that for every zonal wavenumber k, the limit VKE is proportional to the total mechanical energy and to the square of the frequency of the normal mode. The theory predicts a VKE ∝ k −5 and a VKE ∝ k 1/3 power law for the Rossby and IG waves, assuming a k −3 and a k −5/3 power law for the Rossby and IG HKE spectra, respectively. The Kelvin and mixed Rossby–gravity wave VKE spectra are predicted to follow k −1 and k −5 power laws, respectively. The VKE spectra for ERA5 data from August 2018 show that the Rossby VKE spectra approximately follow the predicted a k −5 power law. The expected k 1/3 power law for the gravity wave VKE spectrum is found only in the SH midlatitude stratosphere for k ≈ 10–60. The inertial range IG VKE spectra in the tropical and midlatitude troposphere reflect a mixture of ageostrophic and convection-coupled dynamics and have slopes between −1 and −1/3, likely associated with too steep IG HKE spectra. The forcing by quasigeostrophic ageostrophic motions is seen as an IG VKE peak at synoptic scales in the SH upper troposphere, which gradually moves to planetary scales in the stratosphere.
Zhao, Y.-B., Žagar, N., Lunkeit, F., and Blender, R., 2023: Atmospheric bias teleconnections associated with systematic SST errors in the tropical Indian Ocean, EGUsphere [preprint], Weather Clim. Dynam., https://doi.org/10.5194/egusphere-2023-917
Li, L., N. Žagar, K. Raeder and J.L. Anderson, 2023: Comparison of temperature and wind observations in the tropics in a perfect-model, global EnKF data assimilation system, Q. J. R. Meteorol. Soc., 149(755), 2367–2385, https://doi.org/10.1002/qj.4511
Flow-dependent errors in tropical analyses and short-range forecasts are analysed using global observing-system simulation experiments assimilating only temperature, only winds, and both data types using the ensemble Kalman filter (EnKF) Data Assimilation Research Testbed (DART) and a perfect model framework. The idealised, homogeneous observation network provides profiles of wind and temperature data from the nature run for January 2018 using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) forced by the observed sea-surface temperature. The results show that the assimilation of abundant wind observations in a perfect model makes the temperature data in the Tropics largely uninformative. Furthermore, the assimilation of wind data reduces the background errors in specific humidity twice as much as the assimilation of temperature observations. In all experiments, the largest analysis uncertainties and the largest short-term forecast errors are found in regions of strong vertical and longitudinal gradients in the background wind, especially in the upper troposphere and lower stratosphere over the Indian Ocean and Maritime Continent. The horizontal error correlation scales are on average short throughout the troposphere, just several hundred km. The correlation scales of the wind variables in precipitating regions are half of those in nonprecipitating regions. In precipitating regions, the correlations are elongated vertically, especially for the wind variables. Strong positive cross-correlations between temperature and specific humidity in the precipitating regions are explained using the Clausius–Clapeyron equation.
Zaplotnik, Ž , N. Žagar and N. Semane, 2023: Flow-dependent wind extraction in strong-constraint 4D-Var. Q. J. R. Meteorol. Soc., 149(755), 2107–2124, https://doi.org/10.1002/qj.4497
Strigunova, I., Blender, R., Lunkeit, F., and Žagar, N., 2022: Signatures of midlatitude heat waves in global Rossby wave spectra, Weather Clim. Dynam., 3, 1399–1414, https://doi.org/10.5194/wcd-3-1399-2022
This paper investigates systematic changes in the global atmospheric circulation statistics during Eurasian heat waves (HWs). The investigation of Rossby wave energy anomalies during HWs is based on the time series of Hough expansion coefficients representing Rossby waves with the troposphere–barotropic structures through the extended boreal summer in the European ERA5, ERA-Interim, Japanese 55-year Reanalysis (JRA-55) and Modern-Era Retrospective analysis for Research and Applications (MERRA). The climatological Rossby wave energy distribution is shown to follow a χ2 distribution with skewness dependent on the zonal scale.
The applied multivariate decomposition reveals signatures of the Eurasian HWs in the probability density functions (PDFs) of the Rossby wave energy across scales. Changes in the PDFs are consistent with changes in the intramonthly variance during HWs. For the zonal-mean state (the zonal wavenumber k=0), a decrease in skewness is found, although it is not statistically significant. A reduction in skewness hints to an increase in the number of active degrees of freedom, indicating more independent modes involved in the circulation. A shift in the spectral distribution of the k=0 intramonthly variance is shown to describe a weakening of the mean westerlies near their core at 45∘ N and their strengthening at high latitudes. At planetary scales (k= 1–3), the skewness in the troposphere–barotropic Rossby wave energy significantly increases during HWs. This coincides with a reduction of intramonthly variance, in particular at k=3, and persistent large-scale circulation anomalies. Based on the χ2 skewness, we estimate a reduction of the active degrees of freedom for the planetary-scale Rossby waves of about 25 % compared to climatology. At synoptic scales (k= 4–10), no change in skewness is detected for the Eurasian HWs. However, synoptic waves k= 7–8 are characterised by a statistically significant increase in intramonthly variance of about 5 % with respect to the climatology. In addition, a shift of the entire Rossby wave energy distribution at synoptic scales, along with amplification, is observed during HWs.
Karwat, A., Franzke, C. L. E., and Blender, R., 2022: Long-Term Trends of Northern Hemispheric Winter Cyclones in the Extended ERA5 Reanalysis. Journal of Geophysical Research: Atmospheres, 127, e2022JD036952. https://doi.org/10.1029/2022JD036952
Fix, F., Buehler, S. A., & Lunkeit, F., 2022: How certain are El Niño–Southern Oscillation frequency changes in Coupled Model Intercomparison Project Phase 6 models? International Journal of Climatology, 1– 12. https://doi.org/10.1002/joc.7901
Denamiel, C., S. Vasylkevych, N. Žagar, P. Zemunik, I. Vilibić, 2022: Destructive potential of planetary meteotsunami waves beyond the Hunga Tonga–Hunga Ha’apai volcano eruption. Bull. Amer. Meteor. Soc., in press, https://doi.org/10.1175/BAMS-D-22-0164.1