• Issue

    Quarterly Journal of the Royal Meteorological Society: Volume 146, Issue 732

    i-iv, 2981-3621
    October 2020 Part A

ISSUE INFORMATION

Free Access

Issue Information

  • Pages: i-iv
  • First Published: 09 November 2020

RESEARCH ARTICLES

On the Holton–Lindzen–Plumb model for mean flow reversals in stratified fluids

  • Pages: 2981-2997
  • First Published: 15 May 2020
Description unavailable

Bifurcation diagram of the Holten-Lindzen-Plumb model.

Open Access

Dynamics of concurrent and sequential Central European and Scandinavian heatwaves

  • Pages: 2998-3013
  • First Published: 16 May 2020
Description unavailable

We analyse the dynamics of concurrent and sequential heatwaves in Central Europe and Scandinavia. We show that (a) some circulation patterns are conducive to heatwaves in both regions, (b) the sequential occurrence of heatwaves in these regions is merely a random coincidence, and (c) that Central European and concurrent heatwaves are associated with weak pressure gradient situations over Central Europe.

Texture-based classification of high-resolution precipitation forecasts with machine-learning methods

  • Pages: 3014-3028
  • First Published: 16 May 2020
Description unavailable

Rainfall texture classification result obtained with a random forest model trained on raw image pixels. The two characterized rainfall textures, continuous and intermittent rainfall, are overlaid in black and light grey, respectively, on top of the analysed field (1-hr precipitation forecast from the French Arome model). This texture labelling matches perfectly the one given by an expert.

An application of the localized weighted ensemble Kalman filter for ocean data assimilation

  • Pages: 3029-3047
  • First Published: 17 May 2020
Description unavailable

Schematic diagram of the local block and local domain. The red dot indicates the model variable urn:x-wiley:qj:media:qj3824:qj3824-math-0001 corresponding to the proposal weight urn:x-wiley:qj:media:qj3824:qj3824-math-0002. The blue dots are model variables and the yellow squares are observations. The part enclosed by the solid blue line is the local block, and the area enclosed by the yellow solid line is the local domain.

Dynamics and oceanic response of the Madeira tip-jets

  • Pages: 3048-3063
  • First Published: 14 May 2020
Description unavailable

Isolated islands with a steep orography exert a significant impact on the regional circulation. Madeira island is one of such islands often within a flow regime characterized by a strong perturbation of the low-level wind (as shown in the figure) associated with the generation of atmospheric and oceanic vortices. It is suggested that such variability occurs within a quasi-permanent double tip-jet, and that the east jet has a more relevant impact on the ocean.

Increasing heavy rainfall events in south India due to changing land use and land cover

  • Pages: 3064-3085
  • First Published: 18 May 2020
Description unavailable

We find that heavy rainfall events are increasing in three major south Indian states: Telangana, Tamil Nadu and Kerala. Increasing urbanization is contributing by 20–25% to extremes of rainfall events during the monsoon, as demonstrated through our simulation and Land Use Land Cover change sensitivity experiments for twelve events with a convection-resolving high-resolution regional weather model. The following figure shows, as an example, the Chennai event from three observations (top panel), and the simulations with different LULCs.

Impacts of wind profile shear and curvature on the parameterized orographic gravity wave stress in the Weather Research and Forecasting model

  • Pages: 3086-3100
  • First Published: 17 May 2020
Description unavailable

Using a second-order Wentzel–Kramers–Brillouin approximation, this study extends the parameterization of orographic gravity wave drag in the Weather Research and Forecasting model by including the corrections of wind profile shear (WSHR) and curvature (WCUR) to the surface wave momentum flux (SWMF). In January, the parameterized SWMF at the Northern Hemisphere midlatitudes is weakened by the WSHR term, while the WCUR acts to enhance the SWMF over Antarctica. In July, the WSHR corrections are similar to those in January but the WCUR term produces corrections of opposite sign at the high latitudes of each hemisphere.

Contribution of mean and eddy momentum processes to tropical cyclone intensification

  • Pages: 3101-3117
  • First Published: 28 May 2020
Description unavailable

A high-resolution, three-dimensional, numerical simulation of a rapidly intensifying tropical cyclone is used to extend basic knowledge on the role of mean and eddy momentum transfer on the dynamics of the intensification process. Examination of terms in both the tangential and radial velocity tendency equations provides an improved quantitative understanding of the dynamics of the spin-up process within the inner-core boundary layer and eyewall regions of the system-scale vortex. The analysis provides a novel explanation for inflow jets sandwiching the upper-tropospheric outflow layer that are frequently found in numerical model simulations.

Open Access

A conjugate BFGS method for accurate estimation of a posterior error covariance matrix in a linear inverse problem

  • Pages: 3118-3143
  • First Published: 27 May 2020
Description unavailable

This study proposes a new method for accurate and efficient estimation of a posterior error covariance matrix, in which conjugacy among the set of increment vector pairs is ensured. The accurate estimation of a posterior error covariance matrix resulting from the proposed method could provide valuable quantitative information regarding the uncertainties of estimated variables as well as the observational impacts. Furthermore, error correlations derived from the estimated off-diagonal elements could benefit the interpretation of optimised parameter variations.

Revisiting the relation between momentum and scalar roughness lengths of urban surfaces

  • Pages: 3144-3164
  • First Published: 07 June 2020
Description unavailable

Concepts of the micro-scale and macro-scale eddies viewed in the context of surface renewal theory with bluff bodies.

Open Access

Assimilating visible satellite images for convective-scale numerical weather prediction: A case-study

  • Pages: 3165-3186
  • First Published: 07 June 2020
Description unavailable

Generating synthetic satellite images for visible channels like the Meteosat image of Germany shown here was until recently computationally too expensive for operational data assimilation, but has now become feasible. Here we present first results of the assimilation of such images in a convective-scale data assimilation system based on a local ensemble transform Kalman filter in a near-operational set-up. The main results are that cloud cover is strongly improved and we also see a beneficial impact on precipitation forecasts.

Open Access

A compatible finite-element discretisation for the moist compressible Euler equations

  • Pages: 3187-3205
  • First Published: 07 June 2020
Description unavailable

We present a new discretisation of the moist compressible Euler equations, using a compatible finite-element framework. As well as detailing the discretisation using configurations with two different sets of function spaces, we demonstrate it through some test cases. A comparison of the two configurations is shown in the figure, which presents the urn:x-wiley:qj:media:qj3841:qj3841-math-0001 field from a rising thermal with (top) the lower order function spaces and (bottom) the higher order function spaces.

Towards a dry-mass conserving hydrostatic global spectral dynamical core in a general moist atmosphere

  • Pages: 3206-3224
  • First Published: 04 June 2020
Description unavailable

An improved hydrostatic global spectral dynamical core is presented. It conserves the mass of dry air inherently at the level of the continuous set of equations. In this new dynamical core, the diagnosed full pressure vertical velocity is decomposed into four components. The figure presents the evolution of the diagnosed full pressure vertical velocities at 850 hPa for an idealized tropical cyclone simulation. It highlights that the new dynamical core can maintain the eyewall well throughout the 10-day simulation.

Approaches toward improving the modelling of midlatitude cyclones entering at the lateral boundary corner in the limited area WRF model

  • Pages: 3225-3244
  • First Published: 07 June 2020
Description unavailable

Spatial and temporal adjustments are investigated to explore the challenge involved in introducing fast-moving large-scale meteorological information into the corner area of a limited area model. Adjustments include domain shift, nudging, relaxation layer adjustments and frequency of lateral boundary condition (LBC) updates. Spectral nudging reduces spatial distortion but smooths storm intensity, while increased LBC updates are beneficial in the case of fast-propagating systems. This reduces artificially created distortions and smoothing due to temporal interpolation.

Recent upgrades to the Met Office convective-scale ensemble: An hourly time-lagged 5-day ensemble

  • Pages: 3245-3265
  • First Published: 04 June 2020
Description unavailable

In this article, we introduce a new configuration of the Met Office convective-scale ensemble, based on hourly time-lagging and extending to 5 days, and compare the benefits of this new ensemble configuration against the previous operational non-lagged ensemble configuration.

User decisions, and how these could guide developments in probabilistic forecasting

  • Pages: 3266-3284
  • First Published: 07 June 2020
Description unavailable

Participants at a Live Science event were asked to make decisions based on weather forecast probabilities and their own subjective feelings. From this we were able to estimate the participants' distribution of cost–loss ratios and evaluate a ‘User Brier Score’. Being (asymptotically) proper, this user-relevant score would also be helpful for guiding forecast system development. With the potential for dangerous gales, some users were risk-prone; others would benefit from continued improvement in forecast refinement at low forecast probabilities.

Sensitivity of idealized mixed-phase stratocumulus to climate perturbations

  • Pages: 3285-3305
  • First Published: 09 June 2020
Description unavailable

Our study explores the sensitivities of an Arctic-like springtime stratocumulus to climate change through a series of idealized experiments. We examine in large-eddy simulations how cloud and boundary-layer properties respond to thermodynamic changes. We find that the condensed water path increases under warming and with increasing free-tropospheric relative humidity, but decreases when the inversion strengthens.

Assessment of climate models in relation to the low-level clouds over the southern Indian Ocean

  • Pages: 3306-3325
  • First Published: 10 June 2020
Description unavailable

Differences between the simulated average seasonal mean (June–September) latitude–altitude cross-section of the frequency of occurrence of clouds (FALT, expressed as a percentage) during Asian Summer Monsoon (ASM) averaged over 80–100°E by the 26 CMIP5 models and three different versions of CFSv2 T126 (CTRL, RSAS and CFSCR), respectively, from CloudSat-CALIPSO observations.

Open Access

Observation and simulation of mountain wave turbulence above Iceland: Turbulence intensification due to wave interference

  • Pages: 3326-3346
  • First Published: 19 June 2020
Description unavailable

Breaking gravity waves can lead to strong turbulence for aircraft. We analyse such a case where a research aircraft experienced strong turbulence above Iceland (at the position of the black dot). It is found that the interference of horizontally propagating mountain waves, which are excited by the two nearby mountains, amplified the turbulence intensity and enlarged the vertical extent of the turbulent region.

Identifying the key challenges for fog and low stratus forecasting in complex terrain

  • Pages: 3347-3367
  • First Published: 16 June 2020
Description unavailable

Forecasting fog and low stratus (FLS) poses a challenge for numerical weather prediction models. We compare FLS forecasts with satellite observations and conduct a series of sensitivity experiments for a case study with overly rapid FLS dissipation in the model. Model changes, which had been reported to improve FLS simulations in previous studies, have little impact. We show that the advection scheme's implicit numerical diffusion is the primary cause for erroneous FLS dissipation in complex terrain.

A physically based raindrop–cloud droplet accretion parametrization for use in bulk microphysics schemes

  • Pages: 3368-3383
  • First Published: 11 June 2020
Description unavailable

The accretion of cloud droplets by raindrops is an important cloud microphysical process in warm clouds. This study develops a new accretion parametrization for use in bulk microphysics schemes derived analytically from the stochastic collection equation where the collection efficiency of each raindrop–cloud droplet pair is applied. The new accretion parametrization well describes the dependence of the accretion rates on cloud droplet and raindrop size distributions.

Spatial variability and possible cause analysis of regional precipitation complexity based on optimized sample entropy

  • Pages: 3384-3398
  • First Published: 16 June 2020
Description unavailable

The distinction degree theory was introduced to optimize sample entropy. The precipitation in the study area has unsteady complex fluctuation characteristics. The complexity of extreme daily precipitation is higher than that of monthly precipitation. Industrial economic development may affect the complexity of extreme daily precipitation.

Open Access

Weight structure of the Local Ensemble Transform Kalman Filter: A case with an intermediate atmospheric general circulation model

  • Pages: 3399-3415
  • First Published: 16 June 2020
Description unavailable

The Local Ensemble Transform Kalman Filter (LETKF) computes analysis using a weighted average of the first-guess ensemble with surrounding observations. This study explores the spatial structure of the weights with an intermediate atmospheric global circulation model, and aims to improve the weight interpolation (WI) method based on the characteristics of the weight structure. An advanced WI method with observation-density-dependent reference points was proposed, and resulted in better forecasts than those with uniformly distributed reference points owing to the spatially inhomogeneous localization function.

Open Access

Statistical post-processing of heat index ensemble forecasts: Is there a royal road?

  • Pages: 3416-3434
  • First Published: 17 June 2020
Description unavailable

We investigate the effect of statistical post-processing on the predictive performance of heat index ensemble forecasts calculated from the corresponding forecasts of temperature and dew point temperature from the European Centre for Medium-Range Weather Forecasts. Joint calibration of the input variables is compared with direct post-processing of the output heat indices. Result show that for short lead times calibration significantly improves the forecast skill, direct post-processing is generally more efficient; however, post-processing of the input variables is a competitive approach.

Open Access

The generalized Ekman model for the tropical cyclone boundary layer revisited: The myth of inertial stability as a restoring force

  • Pages: 3435-3449
  • First Published: 19 June 2020

Description unavailable

Tangential wind profiles as a function of radius. This profile has the form vg(r) = v1s/(1 + sx), where s = smr/rm, r is the radius, rm = 50 km and sm and v1 are constants chosen to make v = vgm, the maximum tangential wind speed, when r = rm. The red curve has x = 1.6, and blue curve has x = 2.3. The thin black reference curves are discussed in the text. These have the form v = vgm(rm/r)n, where the exponent n equals either 0.5 or 1

Locally forced convection in subkilometre-scale simulations with the Unified Model and WRF

  • Pages: 3450-3465
  • First Published: 01 July 2020
Description unavailable

Convection-resolving models are the basis of weather forecasting but are also used to study the properties of convection where observations are not available. Further applications include studies of otherwise under-resolved weather extremes, or to serve as “truth” for climate model validation and development of parametrizations. Here we show that these models can still be far from the truth, and much work still needs to be done to represent the involved physics accurately.

Open Access

Upper-tropospheric inflow layers in tropical cyclones

  • Pages: 3466-3487
  • First Published: 17 June 2020
Description unavailable

Three-dimensional numerical simulations of tropicalcyclone intensification with sufficient vertical resolution have shown the development of a layer of strong inflow just beneath the upper-tropospheric outflow layer as well as, in some cases, a shallower layer of weaker inflow above the outflow layer. An example is shown in the figure. Here we provide an explanation for these inflow layers in the context of the prototype problem for tropical cyclone intensification, which considers the evolution of a vortex on an f-plane in a quiescent environment, starting from an initially symmetric, moist, cloud-free vortex over a warm ocean. We attribute the inflow layers to a subgradient radial force that exists through much of the upper troposphere beyond a certain radius. Some effects of the inflow layers on the storm structure are discussed.

A simple immersed boundary forcing for flows over steep and complex orography

  • Pages: 3488-3502
  • First Published: 16 June 2020
Description unavailable

Gravity current encountering two steep hills using the Immersed Boundary Method.

Differences between the 2018 and 2019 stratospheric polar vortex split events

  • Pages: 3503-3521
  • First Published: 19 June 2020
Description unavailable

The stratospheric polar vortex experienced a dynamic weakening and split into two vortices in both February 2018 and January 2019. The planetary wave forcing before the events, and the coupling from the stratosphere to the surface after the events, were quite different, which influenced their predictability. We investigate the role of large-scale climate patterns in these differences.

Open Access

Following moist intrusions into the Arctic using SHEBA observations in a Lagrangian perspective

  • Pages: 3522-3533
  • First Published: 19 June 2020
Description unavailable

Direct observations of the air mass transformation into the cloudy state during the wintertime Arctic is lacking. Here, we compile such observations, showing cooling and drying of air masses over sea ice and moistening over the open ocean. Air masses that generate cloudy conditions over the observation site originate over open ocean, whereas air masses originating over sea ice or continents generate radiatively clear conditions.

Baroclinic instability and nonlinear oscillations in the truncated SQG model

  • Pages: 3534-3547
  • First Published: 27 June 2020
Description unavailable

Baroclinic instability of a flow with a constant vertical shear in a zonal channel has been studied. A dynamic system (obtained from maximally truncated SQG model) describing nonlinear interactions between counter-propagating Rossby waves and a neutral mode independent of zonal coordinate has been formulated. Both analytical and numerical solutions of the system show that exponentially increasing disturbances at the linear stage of instability development give way to nonlinear oscillations/vacillations shown on the figure for the model variables. According to numerical estimates, the period of oscillations (caused by the law of conservation of surface potential energy) is of the order of a month, which is in agreement with measurement data obtained for the winter atmosphere.

All-sky microwave humidity sounder assimilation in the Korean Integrated Model forecast system

  • Pages: 3570-3586
  • First Published: 16 July 2020
Description unavailable

The radiative transfer model for the TIROS Operational Vertical Sounder (RTTOV) works most effectively, in terms of accurate simulation of brightness temperature and reliable Jacobian sensitivity, when hydrometeor components in the convective area are taken into consideration. The availability of the radiance data over the convective region increases the persistence of weather forecasting with the aid of extended satellite observation coverage. Forecast error decreases globally with a statistical significance, and a case study related to heavy rainfall over Japan shows that the all-sky radiance data assimilation improves a longer-range weather forecast case study.

Observational analysis and simulations of a severe hailstorm in northeastern Italy

  • Pages: 3587-3611
  • First Published: 31 July 2020
Description unavailable

(a) Map showing the times assigned to each hailpad impacted during the July 4, 2007 event, after synchronizing them with the radar VMI. Square brackets toward the number means that the extreme is included, while if they point away from the number he extreme is not included into the interval. Topography is shown as the grey scale. (b) is as (a), but for classes of the maximum hailstone diameter found for each hailpad. Most of the largest hailstones (labelled for those larger than 22mm) are located on the western FVG plain. (c) is as (a), but for classes of the flux of kinetic energy (labelled for values larger than 62 J·m−2)

NOTES AND CORRESPONDENCE

Monthly and zonally averaged zonal wind information in the equatorial stratosphere provided by GNSS radio occultation

  • Pages: 3612-3621
  • First Published: 13 July 2020
Description unavailable

The ERA5 monthly mean, zonally averaged zonal winds on pressure levels, for the period January 2007 to December 2018. The zonal averaging is between ±5° latitude.