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Nedjeljka Žagar

Abstract

This paper investigates the potential of line-of-sight (LOS) wind information from a spaceborne Doppler wind lidar to reduce uncertainties in the analysis fields of equatorial waves. The benefit of LOS winds is assessed by comparing their impact to that of a single wind component, full wind field information, and mass field data in three- and four-dimensional variational data assimilation.

The dynamical framework consists of nonlinear shallow-water equations solved in spectral space and a background error term based on eigenmodes derived from linear equatorial wave theory. Based on observational evidence, simulated wave motion fields contain equatorial Kelvin, Rossby, mixed Rossby–gravity, and the lowest two modes of the westward-propagating inertio–gravity waves. The same dynamical structures are included, entirely or partially, into the background error covariance matrix for the multivariate analysis. The relative usefulness of LOS data is evaluated by carrying out “identical twin” observing system simulation experiments and assuming a perfect model.

Results from the experiments involving a single observation or an imperfect background error covariance matrix illustrate that the assimilation increments due to LOS wind information rely more on the background error term specification than the full wind field information. This sensitivity is furthermore transferred to the balanced height field increments.

However, all assimilation experiments suggest that LOS wind observations have a capability of being valuable and need supplemental information to the existing satellite mass field measurements in the Tropics. Although the new wind information is incomplete, it has a potential to provide reliable analysis of tropical wave motions when it is used together with the height data.

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Vlado Malačič and Nedjeljka Žagar

Abstract

The marine icicles that form on coastal constructions (e.g., piers and railings) are very common in polar and subpolar areas. The occasional formation of icicles in the coastal zones of the Mediterranean Sea occurs in relation to cold-air outbreaks from the polar region, such as the one presented in this paper in February 2018. The air temperatures over the northern Adriatic Sea, the northernmost part of the Mediterranean, dropped below –2.1°C, a necessary condition for seawater to freeze, with salinity between 38.0 and 38.5 PSU. The formation of icicles on the coastal structures was further enabled by the bora wind and related high seas along the coast. Measurements presented in this paper confirm that the icicles in the Bay of Piran in the Gulf of Trieste (45.55°N) were formed from the seawater. The measured salinity level of the melted icicles, around 9 PSU, is a typical value reported for marine icicles in polar regions.

Open access
Nedjeljka Žagar and Istvan Szunyogh
Open access
Nedjeljka Žagar, Roberto Buizza, and Joseph Tribbia

Abstract

A new methodology for the analysis of ensemble prediction systems (ENSs) is presented and applied to 1 month (December 2014) of ECMWF operational ensemble forecasts. The method relies on the decomposition of the global three-dimensional wind and geopotential fields onto the normal-mode functions. The ensemble properties are quantified in terms of the 50-member ensemble spread associated with the balanced and inertio-gravity (IG) modes for forecast ranges every 12 h up to 7 days. Ensemble reliability is defined for the balanced and IG modes comparing the ensemble spread with the control analysis in each scale.

Modal analysis shows that initial uncertainties in the ECMWF ENS are largest in the tropical large-scale modes and their spatial distribution is similar to the distribution of the short-range forecast errors. Initially the ensemble spread grows most in the smallest scales and in the synoptic range of the IG modes but the overall growth is dominated by the increase of spread in balanced modes in synoptic and planetary scales in the midlatitudes. During the forecasts, the distribution of spread in the balanced and IG modes grows toward the climatological spread distribution characteristic of the analyses. In the 2-day forecast range, the global IG spread reaches 60% of its asymptotic value while the same percentage of the global balanced spread is reached after 5 days of forecasts. An underdispersiveness of the system is suggested to be associated with the lack of tropical variability, primarily the Kelvin waves.

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Nedjeljka Žagar, Žiga Zaplotnik, and Khalil Karami

Abstract

The globally integrated subseasonal variability associated with the two main atmospheric circulation regimes, the balanced (or Rossby) and unbalanced (or inertia–gravity) regimes, is evaluated for the four reanalysis datasets: ERA-Interim, JRA-55, MERRA, and ERA5. The results quantify amplitudes and trends in midlatitude traveling and quasi-stationary Rossby wave patterns as well as in the equatorial wave activity across scales. A statistically significant reduction of subseasonal variability is found in Rossby waves with zonal wavenumber k = 6 along with an increase in variability in wavenumbers k = 3–5 in the summer seasons of both hemispheres. The four reanalyses also agree regarding increased variability in the large-scale Kelvin waves, mixed Rossby–gravity waves, and westward-propagating inertio-gravity waves with the lowest meridional mode. The amplitude and sign of trends in inertia–gravity modes with smaller zonal scales and greater meridional modes differ between the ERA-Interim and JRA-55 datasets on the one hand and the ERA5 and MERRA data on the other. An increased variability in the ERA-Interim and JRA-55 accounts for positive trends in their total subseasonal variability.

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Nedjeljka Žagar, Koji Terasaki, and Hiroshi L. Tanaka

Abstract

This paper deals with the large-scale inertio-gravity (IG) wave energy in the operational ECMWF analyses in July 2007. Energy percentages of the IG waves obtained from the standard-pressure-level data are compared to those derived from various discretizations of the model-level data. The results show a small albeit systematic increase of the IG energy percentage as the vertical level density increases from the standard-pressure levels toward the model-level density; the small relative change is explained by the sufficient vertical resolution to resolve the large-scale IG waves in the tropics that make the majority of the global IG energy on large scales. A relatively larger increase of the IG energy is obtained when the mesospheric model levels are included; however, the analyses at these levels in July 2007 are less reliable. Furthermore, two numerical methods for the normal-mode function (NMF) decomposition are shown to provide similar results. The decomposition of atmospheric analyses into the NMF series is proposed as a tool to analyze the spatial and temporal variations of the large-scale equatorial waves and their role in global energetics.

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Vassili Kitsios, Terence J. O’Kane, and Nedjeljka Žagar

Abstract

The Madden–Julian oscillation (MJO) is presented as a series of interacting Rossby and inertial gravity waves of varying vertical scales and meridional extents. These components are isolated by decomposing reanalysis fields into a set of normal mode functions (NMF), which are orthogonal eigenvectors of the linearized primitive equations on a sphere. The NMFs that demonstrate spatial properties compatible with the MJO are inertial gravity waves of zonal wavenumber k = 1 and the lowest meridional index n = 0, and Rossby waves with (k, n) = (1, 1). For these horizontal scales, there are multiple small vertical-scale baroclinic modes that have temporal properties indicative of the MJO. On the basis of one such eastward-propagating inertial gravity wave (i.e., a Kelvin wave), composite averages of the Japanese 55-year Reanalysis demonstrate an eastward propagation of the velocity potential, and oscillation of outgoing longwave radiation and precipitation fields over the Maritime Continent, with an MJO-appropriate temporal period. A cross-spectral analysis indicates that only the MJO time scale is coherent between this Kelvin wave and the more energetic modes. Four mode clusters are identified: Kelvin waves of correct phase period and direction, Rossby waves of correct phase period, energetic Kelvin waves of larger vertical scales and meridional extents extending into the extratropics, and energetic Rossby waves of spatial scales similar to that of the energetic Kelvin waves. We propose that within this normal mode framework, nonlinear interactions between the aforementioned mode groups are required to produce an energetic MJO propagating eastward with an intraseasonal phase period. By virtue of the selected mode groups, this theory encompasses both multiscale and tropical–extratropical interactions.

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Katarina Kosovelj, Fred Kucharski, Franco Molteni, and Nedjeljka Žagar

Abstract

The paper presents four ensembles of numerical experiments that compare the response to monopole and dipole heating perturbations resembling different phases of the Madden–Julian oscillation (MJO). The results quantify the Rossby and inertio-gravity (IG) wave response using the normal-mode function decomposition. The day 3 response is characterized by about 60% variance in the IG modes, with about 85% of it belonging to the Kelvin waves. On day 14, only 10% of the response variance is due to the Kelvin waves. Although the n = 1 Rossby mode is the main contributor to the Rossby variance at all time scales, the n > 1 Rossby modes contribute over 50% of the balanced response to the MJO heating. In the short range, dipole perturbations produce a response with the maximal variance in zonal wavenumbers k = 2–3 whereas in the medium range the response maximizes at k = 1 in all experiments. Furthermore, the medium-range response to the heating perturbation mimicking MJO phase 6 is found also over Europe.

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Nedjeljka Žagar, Damjan Jelić, Marten Blaauw, and Peter Bechtold

Abstract

Several decades after E. Dewan predicted that the shallowing of the atmospheric energy spectrum in mesoscale is produced by the inertia–gravity (IG) waves, global analyses have reached the resolution at which the IG waves across many scales are resolved. The authors discuss the spatial filtering method based on the Hough harmonics that provides the temperature and wind perturbations associated with the IG waves in global analysis data. The method is applied to the ECMWF interim reanalysis and the operational 2014–16 analysis fields. The derived spectrum of IG wave energy is divided into three regimes: a part associated with the large-scale unbalanced circulations that has a slope close to −1 for zonal wavenumbers 1 ≤ k ≤ 6, a synoptic-scale range between 3000 and around 500 km (7 ≤ k ≲ 35) that is characterized by a nearly −5/3 slope, and a mesoscale range below 500 km where the slope of the IG energy spectrum in the 2015/16 analyses is steeper. In contrast, the energy spectrum of the Rossby waves has a −3 slope for all zonal wavenumbers k > 6. Presented results suggest that energy associated with the IG modes exceeds the level of energy associated with the Rossby waves around zonal wavenumber 35. The exact wavenumber depends on the season and considered atmospheric depth and it is suggested as a cutoff scale for studies of gravity waves.

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Nedjeljka Žagar, John Boyd, Akira Kasahara, Joseph Tribbia, Erland Källén, Hiroshi Tanaka, and Jun-ichi Yano
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