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In 2015, the International Association of Geodesy defined the International Height Reference System (IHRS) as the conventional gravity field-related global height system. The IHRS is a geopotential reference system co-rotating with the Earth. Coordinates of points or objects close to or on the Earth’s surface are given by geopotential numbers C ( P ) referring to an equipotential surface defined by the conventional value W 0  = 62,636,853.4 m 2  s −2 , and geocentric Cartesian coordinates X referring to the International Terrestrial Reference System (ITRS). Current efforts concentrate on an accurate, consistent, and well-defined realisation of the IHRS to provide an international standard for the precise determination of physical coordinates worldwide. Accordingly, this study focuses on the strategy for the realisation of the IHRS; i.e. the establishment of the International Height Reference Frame (IHRF). Four main aspects are considered: (1) methods for the determination of IHRF physical coordinates; (2) standards and conventions needed to ensure consistency between the definition and the realisation of the reference system; (3) criteria for the IHRF reference network design and station selection; and (4) operational infrastructure to guarantee a reliable and long-term sustainability of the IHRF. A highlight of this work is the evaluation of different approaches for the determination and accuracy assessment of IHRF coordinates based on the existing resources, namely (1) global gravity models of high resolution, (2) precise regional gravity field modelling, and (3) vertical datum unification of the local height systems into the IHRF. After a detailed discussion of the advantages, current limitations, and possibilities of improvement in the coordinate determination using these options, we define a strategy for the establishment of the IHRF including data requirements, a set of minimum standards/conventions for the determination of potential coordinates, a first IHRF reference network configuration, and a proposal to create a component of the International Gravity Field Service (IGFS) dedicated to the maintenance and servicing of the IHRS/IHRF.

Using ERA5 reanalysis data from March 2021 to February 2022 and the China Meteorological Administration Global Forecasting System (CMA‐GFS) operational forecast dataset of 500 hPa geopotential height in the Northern Hemisphere in the same period, the multiscale features of forecast errors are analyzed. The results indicate that the anomaly correlation coefficient (ACC) of 500 hPa geopotential height and its multiscale components in the Northern Hemisphere keep decreasing with the extension of forecast lead time, and there are no seasonal differences in the evolution of the ACC. The effective forecast skills by season for the CMA‐GFS model are above 6 days at multiscale, with the highest skills in winter and the planetary‐scale components. In space, significant seasonal differences are observed in the locations of the extreme values of multiscale forecast errors for 500 hPa geopotential height, and the spatial distribution of forecast errors reflects the inadequate prediction of the intensity of large‐scale trough and ridge systems at middle and high latitudes and the phase‐shift prediction of small troughs and ridges at middle latitudes. Generally, the forecast errors of the original field and planetary‐scale component show wavelike or banded distribution, and the synoptic‐scale forecast errors are always distributed in latitudinal wavelike patterns alternating between positive and negative, without significant differences in the distribution of land, sea, and terrain. The first empirical orthogonal function modes of multiscale forecast errors almost retain their respective feature. In temporal, the spring, summer, and autumn time series all have quasi‐biweekly positive and negative phase transitions within the monthly scale, and the significant phase transition in winter only occurs around January 1st. These results deepen the understanding of the distribution and possible causes of forecast errors of the CMA‐GFS model and provide ideas for the improvement and revision of the model.

The variation of synoptic systems impacting Iran's precipitation climatology can have significant climatic consequences. Among the cyclones contributing to the occasionally widespread precipitation in Iran are the coincident Mediterranean-Red Sea cyclones (CMRSC). This research aims to elucidate the long-term associated with the frequency and intensity of CMRSC by examining geopotential height (GH) and geopotential height gradient (GHG), along with CMRSC patterns, as influential factors on Iran's precipitation climatology. To achieve this, 4-daily GH data at 1000 hPa from the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-Interim, spanning from 1979 through 2018, were utilized. Throughout this period, a total of 97 CMRSC events accompanied precipitation in Iran. Methodologically, the non-parametric Chi-square test, alongside the Standard Normal Homogeneity Test (SNHT), were employed to assess variations in the cyclones' frequency and strength. The Chi-square non-parametric statistic was harnessed to discern trends in GH and GHG, while linear regression was applied to ascertain long-term trends. The findings indicate that the number of CMRSC did not display statistically significant changes over the study period when comparing successive decades and two successive halves of the time series. Notwithstanding, a more detailed examination of shorter timescales, particularly towards the end of the study period, disclosed statistically significant changes in two- and four-year averages. Additionally, the increase in GH at the formation site of Mediterranean cyclones and the decrease in GHG likely contributed to reduced atmospheric instability and precipitation in the affected regions. A pronounced GHG jump in the Mediterranean Sea in 1996 divided the time series into two distinguishable periods. The results demonstrate an upward trend in both periods; however, the second period exhibited a more gradual increase compared to the preceding period.

A method for joint statistical processing of the forecast fields of geopotential height and temperature over the Northern Hemisphere with a lead time up to 72 hours is described. These forecast fields were obtained from several operational hydrodynamic models. The approach reduces essentially the forecast errors for both meteorological fields for all lead times at standard pressure levels in the troposphere and lower stratosphere as compared to the best hydrodynamic schemes. Along with statistical methods, the variational adjustment of the vertical profiles of geopotential height and temperature is applied and reduces the forecast error when using the approximation of the profiles by cubic splines. The temperature forecast error increases approximately linearly with lead time, and the geopotential height error grows quadratically.

This study analyzes the predictability of the persistent maxima of 500-hPa geopotential height (Z500; PMZ) zonal eddies over the Northern Hemisphere in the long-term forecast datasets of the Global Ensemble Forecast System (GEFS) version 10. PMZ patterns, which potentially extending the predictability of severe weather events, include not only closed blocking anticyclones that occur more frequently in the Euro-Atlantic-Asia sector (EAAS) but also persistent open ridges and omega-shape blockings that prevail more often over the Pacific-North America sector (PNAS). The predicted PMZ occurrence frequencies in both the EAAS and the PNAS generally decrease with the lead time, which is consistent with classical blockings in early studies but different from the nearly invariant frequencies of blockings in a recent relevant diagnosis by Hamill and Kiladis. The Brier skill score associated with PMZ frequencies is generally higher in the PNAS than in the EAAS, indicating better predictions in the former. The forecast reliability decreases with the lead time in both sectors, particularly at the tails of probability distributions, suggesting some limitations of the GEFS. PMZ events longer than 1 week with anomaly correlation coefficients (ACCs) exceeding 0.6 in the Northern Hemisphere have a mean useful skill of nearly 10 lead days, which is approximately 0.5–1 day more than the average skill of all cases. Among these events, 50% extend useful ACC skills up to 12 days, and 25% extend the useful skill even further. A discussion is provided regarding how the better PMZ prediction skill in the PNAS can help improve 2 to 3-week predictions over North America.

This paper examines the three-dimensional geopotential height structure of the Madden–Julian oscillation (MJO) and proposes that the MJO convection signals can be well reflected by upper-tropospheric zonal anomalous height gradient (ZAHG, ∇ x z ′ ). A case study in 2011 and composite analyses have been done by applying a wavenumber-frequency filtering method on daily variable anomalies derived from different reanalysis datasets. Results show that the upper-level eastward-moving intraseasonal geopotential height anomaly does not exactly display a Rossby–Kelvin wave structure of the Gill model. Instead, it shows a height quadrupole pattern and is well associated with temperature and zonal wind anomalies. The notable tilted upper-tropospheric temperature structure can be derived from the height anomalies by the hydrostatic balance relationship. The first baroclinic mode vertical structure of zonal wind anomalies can be approximated from the height anomalies by the geostrophic equation. A highlight of the paper is the proposal of tracing the MJO convection center by the upper-tropospheric ZAHG ( ∇ x z ′ ). Composite analyses of all MJO events during extended winter (NDJFM) from 1979 to 2013 suggest that positive upper-tropospheric ZAHG is exactly in phase with negative outgoing longwave radiation (OLR) anomalies in the eastern Indian Ocean and Maritime Continent during a MJO event. On the other hand, positive upper-tropospheric ZAHG leads (lags) negative OLR anomalies by 1 (2) days in the western Indian Ocean (western Pacific Ocean), where MJO events generally initiate (dissipate). ZAHG can also be applied to modeling data and observation data before the satellite era. This study provides a new diagnostic parameter option to monitor MJO events, and an alternative for studying MJOs before the satellite era and model evaluation.

Water resources are crucial to the livelihood and sustainability of the general public across the western United States. This study covers the timespan of both the third driest drought in Californian history between 2012 and 2015 as well as the extreme atmospheric river year in 2016–2017. The evaluation of vertical moisture profiles using Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Radio Occultation (RO) data, National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis of 500 hPa geopotential heights, 1000–500 hPa thickness, Optimum Interpolation (OI) Sea Surface Temperature (SST), NOAA/NDBC buoy data, and NASA, MEaSUREs, Gridded Sea Surface Height Anomalies (SSHA) were performed. The daily COSMIC time evolution from 2006 through 2015 showed a flat to slightly upward trend of both temperature and water vapor profiles through the entirety of the western US drought. Subsequently, a significant increase of temperatures and water vapor were recorded in early 2016 before the extreme Atmospheric River (AR) season of 2016–2017. The quantitative analyses suggest that warmer SST and higher SSHA lead to an increase of heat fluxes from the ocean into the troposphere, which forces thickness changes and thus the position of troughs in the geopotential height field changes afterwards, consequently pushing the trough eastward over the Pacific Northwest and potentially leading to an active AR year in the western US. It appears that regional COSMIC RO moisture profiles, seasonal SST, and SLH anomalies may serve as a precursor for seasonal or sub-seasonal precipitation outlook along the western US.

Bayesian model averaging (BMA) and ensemble model output statistics (EMOS) were used to improve the prediction skill of the 500 hPa geopotential height field over the northern hemisphere with lead times of 1–7 days based on ensemble forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction (NCEP), and UK Met Office (UKMO) ensemble prediction systems. The performance of BMA and EMOS were compared with each other and with the raw ensembles and climatological forecasts from the perspective of both deterministic and probabilistic forecasting. The results show that the deterministic forecasts of the 500 hPa geopotential height distribution obtained from BMA and EMOS are more similar to the observed distribution than the raw ensembles, especially for the prediction of the western Pacific subtropical high. BMA and EMOS provide a better calibrated and sharper probability density function than the raw ensembles. They are also superior to the raw ensembles and climatological forecasts according to the Brier score and the Brier skill score. Comparisons between BMA and EMOS show that EMOS performs slightly better for lead times of 1–4 days, whereas BMA performs better for longer lead times. In general, BMA and EMOS both improve the prediction skill of the 500 hPa geopotential height field.

A detailed analysis of springtime ozone outbreaks in South/Southwest China is presented in this paper, providing an insight into a regional photochemical and climate problem. A major ozone episode in 2013 was the first ever in April and the worst in Hong Kong up to 2018, measuring a peak ozone concentration of 293 μg m−3. This multi-day, ozone pollution was evidenced by similar conditions in the Pearl River Delta (PRD), and an even more severe episode in Kunming (Yunnan) in Southwest China. Concurrently, widespread air temperature composite anomalies of up to about + 4°K were observed in the region, particularly during 6Z (14:00 local time). The global annual geopotential height anomaly implied increased atmospheric stability and inhibited dispersion—consistent with global warming impacts for the region. Backward trajectories, satellite observations, and transport model simulations characterized the biomass burning sources. Results indicated that activities in Indochina, South and Southwest China, and Africa were the main sources in South China while those in Burma dominated Southwest China. The close succession of outbreaks from west to east (Kunming, Guangzhou, and Hong Kong) suggests an eastward transport of ozone and precursors.

The paper is devoted to analysis of the cold half-year (October to March) frequency of cyclones in the Mediterranean-Black Sea region associated with the global processes in the ocean-atmosphere system - the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Firstly, climatic conditions in the North Atlantic-European region during positive and negative phases of these global oscillations were shown using NCEP/NCAR reanalysis data for 1948 - 2016 and data of the NASA GISS Atlas of Exratropical Strom Tracks in 1961 - 1998. Mean monthly values / anomalies (composites) for equal periods of the negative and positive AMO and PDO phases were calculated and compared. The results of the study support the idea that the Pacific and Atlantic influence on the climatic (multidecadal) scale is realized via change of the large-scale fields of the North Atlantic anomalies typical for the interannual scale. Then spatial distribution of the frequency of cyclones in the Mediterranean-Black Sea region was obtained using global NCEP/NCAR reanalysis data sets on 1000 hPa geopotential height in 1948 - 2013. It was shown that during the positive AMO phase, frequency of cyclones in the Mediterranean was higher by absolute values in November to March over the Tyrrhenian and Ionic Seas and lower over the Anatolian peninsula. During the negative PDO phase, frequency of cyclones over the Anatolian peninsula in January to March is significantly higher than in the positive PDO phase.