Quarterly Journal of the Royal Meteorological Society

Journal: Quarterly Journal of the Royal Meteorological Society

Abbreviation

Q J R Meteorol Soc.

Publisher

Wiley

ISSN

0035-9009
1477-870X

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Publications 1 - 10 of 30

Multi-model assessment of sub-seasonal predictive skill for year-round Atlantic-European weather regimes
Osman, Marisol; Beerli, Remo; Büeler, Dominik; et al. (2023)
Quarterly Journal of the Royal Meteorological Society
The prediction skill of sub-seasonal forecast models is evaluated for seven year-round weather regimes in the Atlantic–European region. Reforecasts based on models from three prediction centers are considered and verified against weather regimes obtained from ERA-Interim reanalysis. Results show that predicting weather regimes as a proxy for the large-scale circulation outperforms the prediction of raw geopotential height. Greenland blocking tends to have the longest year-round skill horizon for all three models, especially in winter. On the other hand, the skill is lowest for the European blocking regime for all three models, followed by the Scandinavian blocking regime. Furthermore, all models struggle to forecast flow situations that cannot be assigned to a weather regime (so-called no regime), in comparison with weather regimes. Related to this, variability in the occurrence of no regime, which is most frequent in the transition seasons, partly explains the predictability gap between transition seasons and winter and summer. We also show that models have difficulties in discriminating between related regimes. This can lead to misassignments in the predicted regime during flow situations in which related regimes manifest. Finally, we document the changes in skill between model versions, showing important improvements for the ECMWF and NCEP models. This study is the first multi-model assessment of year-round weather regimes in the Atlantic–European domain. It advances our understanding of the predictive skill for weather regimes, reveals strengths and weaknesses of each model, and thus increases our confidence in the forecasts and their usefulness for decision-making.
The role of boundary‐layer friction in modulating midlatitude heatwaves
Russo, Emmanuele; Wicker, Wolfgang; Folini, Doris; et al. (2025)
Quarterly Journal of the Royal Meteorological Society
Heatwaves are among the most harmful extreme events, impacting human health, ecosystems, and infrastructure. Heatwaves are driven and modulated by a range of dynamical and thermodynamic factors, but their relative importance for the characteristics of these extremes, such as intensity, frequency, and location, for example, is not fully resolved. Dynamical models allow for a better understanding of the physical drivers of heatwaves, and idealised models in particular allow us to discriminate between the effects of different components of the climate system on heatwaves. One such factor is the boundary-layer friction associated with different surface types, which is known to influence the dynamics of the atmosphere. The geography and land cover of continental surfaces, as well as local characteristics in surface roughness, can influence the atmospheric circulation. In this study, we use an idealised general circulation model to conduct sensitivity experiments testing the role of changes in local and global boundary-layer friction for midlatitude heatwaves. The number of heatwave days and events is found to be larger for areas where the jet stream is strongly decelerated as a consequence of a local increase in boundary-layer friction. This effect strengthens when increasing the areas of modified friction. Changes in boundary-layer friction applied over areas with a geographical location and extent similar to those of continental areas of the Northern Hemisphere result in heatwave day differences of up to 20% between different locations at the same latitudes. These findings highlight how the extent, surface properties, and geographic position of continental and smaller land masses influence heatwave occurrence through their frictional effects on atmospheric dynamics.
Solar signals in CMIP‐5 simulations: the stratospheric pathway
Mitchell, Daniel M.; Misios, Stergios; Gray, Lesley; et al. (2015)
Quarterly Journal of the Royal Meteorological Society
The 11 year solar‐cycle component of climate variability is assessed in historical simulations of models taken from the Coupled Model Intercomparison Project, phase 5 (CMIP‐5). Multiple linear regression is applied to estimate the zonal temperature, wind and annular mode responses to a typical solar cycle, with a focus on both the stratosphere and the stratospheric influence on the surface over the period ∼1850–2005. The analysis is performed on all CMIP‐5 models but focuses on the 13 CMIP‐5 models that resolve the stratosphere (high‐top models) and compares the simulated solar cycle signature with reanalysis data. The 11 year solar cycle component of climate variability is found to be weaker in terms of magnitude and latitudinal gradient around the stratopause in the models than in the reanalysis. The peak in temperature in the lower equatorial stratosphere (∼70 hPa) reported in some studies is found in the models to depend on the length of the analysis period, with the last 30 years yielding the strongest response. A modification of the Polar Jet Oscillation (PJO) in response to the 11 year solar cycle is not robust across all models, but is more apparent in models with high spectral resolution in the short‐wave region. The PJO evolution is slower in these models, leading to a stronger response during February, whereas observations indicate it to be weaker. In early winter, the magnitude of the modelled response is more consistent with observations when only data from 1979–2005 are considered. The observed North Pacific high‐pressure surface response during the solar maximum is only simulated in some models, for which there are no distinguishing model characteristics. The lagged North Atlantic surface response is reproduced in both high‐ and low‐top models, but is more prevalent in the former. In both cases, the magnitude of the response is generally lower than in observations.
Resolution dependence of southern Atlantic Ocean stratocumulus decks
Canton, Jacopo; Dipankar, Anurag; Schär, Christoph (2024)
Quarterly Journal of the Royal Meteorological Society
Oceanic stratocumulus decks of clouds are among the largest contributors to the Earth’s radiation budget, covering around a fifth of the planet’s surface and reflecting a large part of the incoming solar radiation. Unfortunately, these clouds are not well represented in modern climate models, resulting in one of the leading causes of uncertainty in climate change projections. This con tribution analyses this issue from a novel perspective and sheds light on the mechanisms behind the misrepresentation and resolution dependence of these large clouds. The analysis is based on realistic week-long simulations performed over a 563 × 563 km2 oceanic domain. Four horizontal resolutions, between 4.4 and 0.55 km, are employed, resulting in a timely investigation, especially in light of the high resolutions employed by present and near-future climate projections. Results show that the liquid cloud water, the main contributor to the simulated grid-scale clouds, decreases with a power-law decay as the resolution increases, whereas the water vapour, responsible for subgrid-scale clouds, is much less affected by the grid spacing. The leading cause is identified as an imbalance between the rates of change of the advection and turbulence parametrisation terms. In order to verify this observation and provide a possible mitigation to the issue, a second set of simulations is performed where the turbulence parametri sation is tuned. The strategy proves to be successful, confirming the hypotheses and resulting not only in a resolution-independent radiation budget but also cloud coverage.
On the applicability of urban canopy parametrization in building grey zone
Chen, Song; Dipankar, Anurag (2022)
Quarterly Journal of the Royal Meteorological Society
With increasing interest in urban meteorology and related services, the need to appropriately represent the urban environment in climate/weather models is rising. These regional weather/climate models typically use a km-scale horizontal grid, which is insufficient to resolve the flow around buildings. Effects of the urban environment on the atmosphere above are represented through a bulk approach using the Urban Canopy Parametrization (UCP) schemes. Existing UCPs usually use the repeating canyon-roof representation that assumes homogeneous distribution of buildings within the grid box. It is commonly accepted that the assumption of homogeneity holds at km-scale grid resolution but whether it also holds at sub-km scale, where the regional models are increasingly approaching, is questionable. For this reason, among others, the resolution ranges from a few hundred metres to tens of metres (i.e. building-resolving scales) is termed the building grey zone in the existing literature. This work shows that the assumption of homogeneity indeed does not hold at the building grey zone for the city-state Singapore. To understand the possible influences of the use of UCPs at scales from the building grey zone to the conventional mesoscale, we use an urban-grid method that allows us to estimate the parametrized fluxes from a typical UCP at varying resolutions over the urban landcover while keeping the same atmospheric model grid. Numerical results show that different urban-grid resolutions yield variations in the near-surface temperature and wind to a maximum of 0.5 K and 1 m center dot s(-1). Their impact on the boundary-layer parameters is found to be limited. Although these near-surface variations are small, they are comparable to the near-surface scaling variables and are thus physically significant.
Assessing the calibration of multivariate probabilistic forecasts
Allen, Sam; Ziegel, Johanna; Ginsbourger, David (2024)
Quarterly Journal of the Royal Meteorological Society
Rank and probability integral transform histograms are established tools to assess the calibration of probabilistic forecasts. They not only check whether a forecast is calibrated, but they also reveal what systematic biases (if any) are present in the forecasts. Several extensions of rank histograms have been proposed to evaluate the calibration of probabilistic forecasts for multivariate outcomes. These extensions introduce a so-called pre-rank function that condenses the multivariate forecasts and observations into univariate objects, from which a standard rank histogram can be produced. Existing pre-rank functions typically aim to preserve as much information as possible when condensing the multivariate forecasts and observations into univariate objects. Although this is sensible when conducting statistical tests for multivariate calibration, it can hinder the interpretation of the resulting histograms. In this article, we demonstrate that there are few restrictions on the choice of pre-rank function, meaning forecasters can choose a pre-rank function depending on what information they want to extract concerning forecast performance. We introduce the concept of simple pre-rank functions and provide examples that can be used to assess the mean, spread, and dependence structure of multivariate probabilistic forecasts, as well as pre-rank functions that could be useful when evaluating probabilistic spatial field forecasts. The simple pre-rank functions that we introduce are easy to interpret, easy to implement, and they deliberately provide complementary information, meaning several pre-rank functions can be employed to achieve a more complete understanding of multivariate forecast performance. We then discuss how e-values can be employed to formally test for multivariate calibration over time. This is demonstrated in an application to wind-speed forecasting using the EUPPBench post-processing benchmark dataset.
Stratus over rolling terrain: Large-eddy simulation reference and sensitivity to grid spacing and numerics
Weinkaemmerer, Jan; Ďurán, Ivan Bašták; Westerhuis, Stephanie; et al. (2022)
Quarterly Journal of the Royal Meteorological Society
The formation of low stratus cloud over idealized hills is investigated using numerical model simulations. The main driver for the cloud formation is radiative cooling due to outgoing longwave radiation. Despite a purely horizontal flow, the advection terms in the prognostic equations for heat and moisture produce vertical mixing across the upper cloud edge, leading to a loss of cloud water content. This behavior is depicted via a budget analysis. More precisely, this spurious mixing is caused by the diffusive error of the advection scheme in regions where the sloping surfaces of the terrain-following vertical coordinate intersect the cloud top. This study shows that the intensity of the (spurious) numerical diffusion depends strongly on the horizontal resolution, the order of the advection schemes, and the choice of scalar advection scheme. A large-eddy simulation with 4-m horizontal resolution serves as a reference. For horizontal resolutions of a few hundred meters and simulations carried out with a model setup as used in numerical weather prediction, a strong reduction of the simulated liquid-water path is observed. In order to keep the (spurious) numerical diffusion at coarser resolutions small, at least a fifth-order advection scheme should be used. In the present case, a weighted essentially nonoscillatory scalar advection scheme turns out to increase the numerical diffusion along a sharp cloud edge compared with an upwind scheme. Furthermore, the choice of vertical coordinate has a strong impact on the simulated liquid-water path over orography. With a modified definition of the sigma coordinate, it is possible to produce cloud water where the classical sigma coordinate does not allow any cloud formation.
Northern Hemisphere extratropical cyclone biases in ECMWF subseasonal forecasts
Büeler, Dominik; Sprenger, Michael; Wernli, Heini (2024)
Quarterly Journal of the Royal Meteorological Society
Extratropical cyclones influence midlatitude surface weather directly via precipitation and wind and indirectly via upscale feedbacks on the large-scale flow. Biases in cyclone frequency and characteristics in medium-range to subseasonal numerical weather prediction might therefore hinder exploitation of potential predictability on these timescales. We thus, for the first time, identify and track extratropical cyclones in 20 years (2000-2020) of subseasonal ensemble reforecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) in the Northern Hemisphere in all seasons. The reforecasts reproduce the climatology of cyclone frequency and life-cycle characteristics qualitatively well up to six weeks ahead. However, there are significant regional biases in cyclone frequency, which can result from a complex combination of biases in cyclone genesis, size, location, lifetime, and propagation speed. Their magnitude is largest in summer, with the strongest regional deficit of cyclones of more than 30% in the North Atlantic, relatively large in spring, and smallest in winter and autumn. Moreover, the reforecast cyclones reach too-high intensities during most seasons, although intensification rates are captured well. An overestimation of cyclone lifetime might partly but not exclusively explain this intensity bias. While the cyclone bias patterns often appear in lead-time weeks 1 and 2, their magnitudes typically grow further at subseasonal lead times, in some cases up to weeks 5 and 6. Most of the dynamical sources of these biases thus likely appear in the early medium range, but sources on longer timescales probably contribute to the further increase of biases with lead time. Our study provides a useful basis to identify, better understand, and ultimately reduce biases in the large-scale flow and in surface weather in subseasonal weather forecasts. Given the considerable biases during summer, when subseasonal predictions of precipitation and surface temperature will become increasingly important, this season deserves particular attention for future research.
Aerosol–cloud–precipitation interactions during a Saharan dust event – A summertime case‐study from the Alps
Eirund, Kirstin Gesa; Drossaart van Dusseldorp, Saskia; Brem, Benjamin T.; et al. (2022)
Quarterly Journal of the Royal Meteorological Society
Changes in the ambient aerosol concentration are known to affect the microphysical properties of clouds. Especially regarding precipitation formation, increasing aerosol concentrations are assumed to delay the precipitation onset, but may increase precipitation rates via convective invigoration and orographic spillover further downstream. In this study, we analyse the effect of increased aerosol concentrations on a heavy precipitation event observed in summer 2017 over northeastern Switzerland, an event which was considerably underestimated by the operational weather forecast model. Preceding the precipitation event, Saharan dust was advected towards the Alps, which could have contributed to increased precipitation rates north of the Alpine ridge. To investigate the potential impact of the increased ambient aerosol concentrations on surface precipitation, we perform a series of sensitivity simulations using the Consortium for Small-scale Modeling (COSMO) model with different microphysical parametrizations and prognostic aerosol perturbations. The results show that the choice of the microphysical parametrization scheme in terms of a one- or two-moment scheme has the relatively largest impact on surface precipitation rates. In the one-moment scheme, surface precipitation is strongly reduced over the Alpine ridge and increased further downstream. Simulated changes in surface precipitation in response to aerosol perturbations remain smaller in contrast to the impact of the microphysics scheme. Elevated cloud condensation nuclei (CCN) concentrations lead to increased cloud water and decreased cloud ice mass, especially in regions of high convective activity south of the Alps. These altered cloud properties indeed increase surface precipitation further downstream, but the simulated change is too small to explain the observed heavy precipitation event. Additional ice-nucleating particles (INPs) increase cloud ice mass, but only trigger local changes in downstream surface precipitation. Thus, increased aerosol number concentrations during the Saharan dust outbreak are unlikely to have caused the heavy precipitation event in summer 2017.
Identifying the key challenges for fog and low stratus forecasting in complex terrain
Westerhuis, Stephanie; Fuhrer, Oliver; Cermak, Jan; et al. (2020)
Quarterly Journal of the Royal Meteorological Society
Forecasting fog and low stratus (FLS) accurately poses a challenge to current numerical weather prediction models, despite many advancements in recent years. We present a novel method to quantify FLS extent bias by comparing forecasts with satellite observations. Evaluating a four-month period, we show that COSMO-1, the MeteoSwiss high-resolution operational model, exhibits a considerable negative FLS bias during wintertime. To study the cause, we conduct a series of sensitivity experiments for a representative case study, where COSMO-1 dissipated extensive FLS erroneously. Replacing the one-moment bulk microphysics parameterisation scheme by a two-moment scheme, as well as increasing the number of vertical levels, did not show any improvements. The FLS dissipation was delayed (but not prevented) by decreasing the lower bound imposed on the turbulent diffusion coefficients from 0.4 to 0.01 m(2)center dot s(-1), or by reducing horizontal grid spacing from 1.1 km to 550 m. Additionally, simulations at 1.1-km grid spacing with smoothed orography led to more extensive FLS than the same simulations without smoothed orography. An analysis of the cloud water budget revealed that the model's advection scheme is causing a loss of liquid water content near the cloud top. A simulation with an alternative terrain-following coordinate system, in which the vertical coordinates are quasihorizontal near the cloud top, reduced the loss of cloud water through advection and improved the evolution of FLS in the case study. In combination, our findings suggest that the advection scheme exhibits numerical diffusion, which promotes spurious mixing in the vertical of cloudy and adjacent cloud-free grid cells in terrain-following vertical coordinates; this process can become the root cause for too rapid dissipation of FLS during nighttime in complex terrain.

Publications 1 - 10 of 30

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