pinecone/core-2020-05-10-deduplication huggingface dataset:
Relationships between total lightning activity, microphysics and kinematics during the 24 September 2012 HyMeX bow-echo system

International audienceThe structure of a bow-echo system that crossed southeast France on 24 September 2012 during the first Special Observation Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX) is investigated from a three-dimensional lightning mapping array (LMA), an operational lightning network, and operational polarimetric radar observations. Lightning observations reveal that 97% of flash initiation and 93% of flash propagation occurred within the convective region. Through combining LMA and radar-derived microphysical data it is also shown that lightning initiation events and propagation mostly occurred in regions containing graupel (initiation = 70%, propagation = 58%), ice (initiation = 22%, propagation = 33%), and to a lesser extent hail (initiation = 6%, propagation = 5%). The detailed analysis of the lightning activity during two periods of contrasted electrical activity suggests that wet hail growth processes and/or charge redistribution within the cloud may have had a negative impact on lightning initiation occurrences and propagation. The most active periods in terms of lightning activity occurred in conjunction with a vigorous, terrain driven, updraught that transported graupel particles above a height of 12 km
A new class of vacillations of the stratospheric polar vortex

A new class of persistent vacillations of the winter polar vortex, under the action of topographic wave forcing and radiative cooling, is identified in numerical integrations of the rotating shallow water equations. The vacillations are obtained provided only that care is taken to prevent the unconstrained growth of tropical easterlies that otherwise develop as the result of persistent angular momentum deposition at low latitudes. The vacillation cycle involves purely barotropic dynamics and is characterized by a dynamically controlled rapid splitting and rapid reformation of the vortex followed by a more gradual period of vortex intensification under the influence of radiative relaxation. The onset of the splitting occurs when the frequency of the free mode of the vortex approaches that of the forcing and resembles a resonant excitation. Experiments with an alternative basic state suggest that the vacillations are a robust feature of the topographically forced and radiatively relaxed vortex. In contrast to the behavior found in models with vertical structure, the period of the vacillation cycles here increases with increasing forcing amplitude. A wide range of forcing amplitude exists over which the vortex exhibits distinct regime transitions between a strong, vacillating state and a state in which the vortex is weak and the zonal mean polar flow nearly zero. Comparison with observational reanalysis suggest that the vacillation cycles obtained here may be relevant to the dynamics of some sudden warming events and that the onset of a radiatively dominated regime may be usefully linked to the loss of vortex area following such an event.PostprintPeer reviewe
Arctic System Reanalysis improvements in topographically forced winds near Greenland

Southern Greenland is home to a number of weather systems characterized by high speed low-level winds that are the result of topographic flow distortion. These systems include tip jets, barrier winds and katabatic flows. Global atmospheric reanalyses have proven to be important tools in furthering our understanding of these systems and their role in the climate system. However, there is evidence that their mesoscale structure may be poorly resolved in these global products. Here output from the regional Arctic System Reanalysis (ASRv1–30 km and ASRv2–15 km grid resolutions) are compared to the global ERA-Interim Reanalysis (ERA-I–80 km grid resolution), focusing on their ability to represent winds in the vicinity of southern Greenland. Comparisons are made to observations from surface and upper-air stations, as well as from research aircraft flights during the Greenland Flow Distortion Experiment (GFDex). The ERA-I reanalysis has a tendency to underestimate high wind speeds and overestimate low wind speeds, which is reduced in ASRv1 and nearly eliminated in ASRv2. In addition, there is generally a systematic reduction in the root mean square error between the observed and the reanalysis wind speeds from ERA-I to ASRv1 to ASRv2, the exception being low-level marine winds where the correspondence is similar in all reanalyses. Case studies reveal that mesoscale spatial features of the wind field are better captured in ASRv2 as compared to the ERA-I or ASRv1. These results confirm that a horizontal grid size on the order of 15 km is needed to characterize the impact that Greenland’s topography has on the regional wind field and climate. However even at this resolution, there are still features of the wind field that are under-resolved
Dynamical Analysis of Blocking Events: Spatial and Temporal Fluctuations
  of Covariant Lyapunov Vectors

One of the most relevant weather regimes in the mid-latitudes atmosphere is
the persistent deviation from the approximately zonally symmetric jet to the
emergence of blocking patterns. Such configurations are usually connected to
exceptional local stability properties of the flow which come along with an
improved local forecast skills during the phenomenon. It is instead extremely
hard to predict onset and decay of blockings. Covariant Lyapunov Vectors (CLVs)
offer a suitable characterization of the linear stability of a chaotic flow,
since they represent the full tangent linear dynamics by a covariant basis
which explores linear perturbations at all time scales. Therefore, we assess
whether CLVs feature a signature of the blockings. As a first step, we examine
the CLVs for a quasi-geostrophic beta-plane 2-layer model in a periodic channel
baroclinically driven by a meridional temperature gradient $\Delta T$. An
orographic forcing enhances the emergence of localized blocked regimes. We
detect the blocking events with a Tibaldi-Molteni scheme adapted to the
periodic channel. When blocking occurs, the global growth rates of the fastest
growing CLVs are significantly higher. Hence, against intuition, the
circulation is globally more unstable in blocked phases. Such an increase in
the finite time Lyapunov exponents with respect to the long term average is
attributed to stronger barotropic and baroclinic conversion in the case of high
temperature gradients, while for low values of \Delta T, the effect is only due
to stronger barotropic instability. In order to determine the localization of
the CLVs we compare the meridionally averaged variance of the CLVs during
blocked and unblocked phases. We find that on average the variance of the CLVs
is clustered around the center of blocking. These results show that the blocked
flow affects all time scales and processes described by the CLVs.Comment: 23 pages, 10 figure
The interaction of moist convection and mid-level dry air in the advance of the onset of the Indian monsoon

The advance of the onset of the Indian monsoon is here explained in terms of a balance between the low-level monsoon flow and an over-running intrusion of mid-tropospheric dry air. The monsoon advances, over a period of about 6 weeks, from the south of the country to the northwest. Given that the low-level monsoon winds are westerly or southwesterly, and the midlevel winds northwesterly, the monsoon onset propagates upwind relative to midlevel flow, and perpendicular to the low-level flow, and is not directly caused by moisture flux toward the northwest. Lacking a conceptual model for the advance means that it has been hard to understand and correct known biases in weather and climate prediction models. The mid-level northwesterlies form a wedge of dry air that is deep in the far northwest of India and over-runs the monsoon flow. The dry layer is moistened from below by shallow cumulus and congestus clouds, so that the profile becomes much closer to moist adiabatic, and the dry layer is much shallower in the vertical, toward the southeast of India. The profiles associated with this dry air show how the most favourable environment for deep convection occurs in the south, and onset occurs here first. As the onset advances across India, the advection of moisture from the Arabian Sea becomes stronger, and the mid-level dry air is increasingly moistened from below. This increased moistening makes the wedge of dry air shallower throughout its horizontal extent, and forces the northern limit of moist convection to move toward the northwest. Wetting of the land surface by rainfall will further reinforce the north-westward progression, by sustaining the supply of boundary layer moisture and shallow cumulus. The local advance of the monsoon onset is coincident with weakening of the mid-level northwesterlies, and therefore weakened mid-level dry advection
Parametrizing the Antarctic stable boundary layer: synthesizing models and observations

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.The accurate representation of the stable boundary layer (SBL) is a key issue for weather prediction and climate models. The SBL exerts an important influence in controlling heat, moisture and momentum fluxes between the surface and the rest of the atmosphere. Some of the world's most stably stratified boundary layers develop on the Antarctic continent. Previous work investigating SBLs has tended to take either a purely observational or purely modelling-based approach. Here, a novel three-way methodology has been developed which uses observations from an Antarctic site, alongside large-eddy simulation (LES) and single-column model (SCM) techniques to examine a case-study. Reasonable agreement was generally achieved between the LES and observations. The choice of stability function is an important decision for column-based parametrizations of the SBL. Four schemes were tested in the SCM, providing persuasive evidence for the use of shorter-tailed stability functions. The LES data were also used to extract implied stability functions. These experiments reinforced the conclusion that shorter-tailed stability functions offered improved performance for the Antarctic SBL. This approach represents a powerful framework for verifying SCM and LES results against a range of insitu observations
On the dynamical downscaling and bias correction of seasonal-scale winter precipitation predictions over North India

This is the peer reviewed version of the following article: Tiwari, P. R., Kar, S. C., Mohanty, U. C., Dey, S., Sinha, P., Raju, P. V. S. and Shekhar, M. S., ???On the dynamical downscaling and bias correction of seasonal-scale winter precipitation predictions over North India???, quarterly Journal of the Royal Meteorological Society, Vol. 142 (699):2398-2410, June 2016, which has been published in final form at DOI: https://doi.org/10.1002/qj.2832. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. ?? 2016 Royal Meteorological SocietyThis study presents the results of high-resolution (30 km) climate simulations over North India using an optimized configuration of the Regional Climate Model (RegCM), driven by a global spectral model (T80 model with horizontal resolution of ???1.4???) for a period of 28 years (1982???2009). The main aim of this work is to analyze the capabilities of the RegCM to simulate the wintertime precipitation over North India in the recent past.The RegCM validation revealed a good improvement in reproducing the precipitation compared to results obtained from the T80 model. This improvement comes due to better representation of vertical pressure velocity, moisture transport, convective heating rate and temperature gradient at two different latitudinal zones. Moreover, orography in the high-resolution RegCM improves the precipitation simulation in the region where sharp orography gradient plays an important role in wintertime precipitation processes. Two bias correction (BC) methods namely mean bias-remove (MBR) and quantile mapping (QM) have been applied on the T80 driven RegCM model simulations. It was found that the QM method is more skillful than the MBR in simulating the wintertime precipitation over North India. A comparison of model-simulated and bias corrected precipitation with observed precipitation at 17 station locations has also been carried out. Overall, the results suggest that when the BC is applied on dynamically downscaled model, it has better skill in simulating the precipitation over North India and this model is a useful tool for further regional downscaling studies
Combining low-dimensional ensemble postprocessing with reordering
  methods

State-of-the-art weather forecasts usually rely on ensemble prediction
systems, accounting for the different sources of uncertainty. As ensembles are
typically uncalibrated, they should get statistically postprocessed. Several
multivariate ensemble postprocessing techniques, which additionally consider
spatial, inter-variable and/or temporal dependencies, have been developed.
These can be roughly divided into two groups. The first group comprises
parametric, mainly low-dimensional approaches that are tailored to specific
settings. The second group involves non-parametric reordering methods that
impose a specific dependence template on univariately postprocesed forecasts
and are suitable in any dimension. In this paper, these different strategies
are combined, with the aim to exploit the benefits of both concepts.
Specifically, a high-dimensional postprocessing problem is divided into
multiple low-dimensional instances, each of which is postprocessed via a
suitable multivariate parametric method. From each postprocessed
low-dimensional distribution, a sample is drawn, which is then reordered
according to the corresponding multidimensional rank structure of an
appropriately chosen dependence template. In this context, different ranking
concepts for multivariate settings are discussed. Finally, all reordered
samples are aggregated to obtain the overall postprocessed ensemble. The new
approach is applied to ensemble forecasts for temperature and wind speed at
several locations from the European Centre for Medium-Range Weather Forecasts,
using a recent bivariate ensemble model output statistics postprocessing
technique and a reordering based on the raw ensemble forecasts similar to the
ensemble copula coupling method. It shows good predictive skill and outperforms
reference ensembles
Semigeostrophic time evolution of velocity gradient tensor invariants

The behaviour of quadratic invariants of the velocity gradient tensor is explored when the time evolution is governed by semigeostrophic forms of the shallow water equations. The evolution equation of a certain Jacobian involving the geostrophic flow is formally similar to its counterpart under the primitive shallow water equations. The resultant deformation and the Frobenius norm do not behave in this symmetrical way. A product of the study is a straightforward derivation of the semigeostrophic potential vorticity conservation property. Results are extended to 3D baroclinic flow by using isentropic coordinates
ENSO-related variation of equatorial MRG and Rossby waves and forcing from higher latitudes

The contrasting behaviour of westward-moving mixed Rossby–gravity (WMRG) and the first Rossby (R1) waves in El Niño (EN) and La Niña (LN) seasons is documented with a focus on the Northern Hemisphere winter. The eastward-moving variance in the upper troposphere is dominated by WMRG and R1 structures that appear to be Doppler-shifted by the flow and are referred to as WMRG-E and R1-E. In the east Pacific and Atlantic the years with stronger equatorial westerly winds, LN in the former and EN in the latter, have the stronger WMRG and WMRG-E. In the east Pacific, R1 is also a maximum in LN. However, R1-E exhibits an eastward shift between LN and EN. The changes with El Niño/Southern Oscillation (ENSO) phase provide a test bed for the understanding of these waves. In the east Pacific and Atlantic, the stronger WMRG-E and WMRG with stronger westerlies are in accord with the dispersion relation with simple Doppler-shifting by the zonal flow. The possible existence of free waves can also explain stronger R1 in EN in the Eastern Hemisphere. 1-D free-wave propagation theory based on wave activity conservation is also important for R1. However, this theory is unable to explain the amplitude maxima for other waves observed in the strong equatorial westerly regions in the Western Hemisphere, and certainly not their ENSO-related variation. The forcing of equatorial waves by higher-latitude wave activity and its variation with ENSO phase is therefore examined. Propagation of extratropical eastward-moving Rossby wave activity through the westerly ducts into the equatorial region where it triggers WMRG-E is favoured in the stronger westerlies, in LN in the east Pacific and EN in the Atlantic. It is also found that WMRG is forced by Southern Hemisphere westward-moving wave trains arching into the equatorial region where they are reflected. The most significant mechanism for both R1 and R1-E appears to be lateral forcing by subtropical wave trains
Zonal-mean circulation response to reduced air-sea momentum roughness

The impact of uncertainties in surface layer physics on the atmospheric
general circulation is comparatively unexplored. Here the sensitivity of the
zonal-mean circulation to reduced air-sea momentum roughness ($Z_{0m}$) at low
flow speed is investigated with the Community Atmosphere Model (CAM3). In an
aquaplanet framework with prescribed sea surface temperatures, the response to
reduced $Z_{0m}$ resembles the La Ni$\tilde{\text{n}}$a minus El
Ni$\tilde{\text{n}}$o response to El Ni$\tilde{\text{n}}$o Southern Oscillation
variability with: i) a poleward shift of the mid-latitude westerlies extending
all the way to the surface; ii) a weak poleward shift of the subtropical
descent region; and iii) a weakening of the Hadley circulation, which is
generally also accompanied by a poleward shift of the inter-tropical
convergence zone (ITCZ) and the tropical surface easterlies. Mechanism-denial
experiments show this response to be initiated by the reduction of tropical
latent and sensible heat fluxes, effected by reducing $Z_{0m}$. The circulation
response is elucidated by considering the effect of the tropical energy fluxes
on the Hadley circulation strength, the upper tropospheric critical layer
latitudes, and the lower-tropospheric baroclinic eddy forcing. The ITCZ shift
is understood via moist static energy budget analysis in the tropics. The
circulation response to reduced $Z_{0m}$ carries over to more complex setups
with seasonal cycle, full complexity of atmosphere-ice-land-ocean interaction,
and a slab ocean lower boundary condition. Hence, relatively small changes in
the surface parameterization parameters can lead to a significant circulation
response.Comment: 13 pages, 15 figures, accepted for publication in QJRM
Validating weather and climate models at small Rossby numbers: including a boundary layer

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.Ideally, the validation of weather and climate models requires that the predictions remain close to an exact solution of the governing equations. The complexity of weather and climate models means that it is not possible to compute exact solutions except in trivial cases. However, in the limit of small Rossby number, the exact solution of the Euler equations can be shown to be close to that of a semi-geostrophic model, which can be computed. Previous studies have used the small-Rossby-number limit to validate numerical methods for a baroclinic wave without sub-grid physics. However, the method of coupling to the sub-grid physics plays an important role in the performance of weather and climate models. The aim of this article is thus to extend the previous studies to include a boundary-layer parametrization. We use a balanced model that includes a known boundary-layer parametrization, the semi-geotriptic model. We then demonstrate that the semi-geotriptic model is the appropriate small-Rossby-number limit of the solution of the Euler equations with the same boundary-layer representation. The semi-geotriptic model is then used to expose weaknesses in the numerical methods for coupling the boundary layer to the rest of the model
Object-oriented processing of CRM precipitation forecasts by stochastic filtering

International audienceIn order to cope with small-scale unpredictable details of mesoscale structuresin cloud-resolving models, it is suggested in this paper to process the modeloutputs following a fuzzy object-oriented approach to extract and trackprecipitating features (associated with a higher predictability than the directmodel outputs). The present approach uses the particle filter method torecognize patterns based on predefined texture or spatial variability of themodel output. This provides an ensemble of precipitating objects, which arethen propagated in time using a stochastic advection-diffusion process. Thismethod is applied to both deterministic and ensemble forecasts provided bythe AROME-France convective-scale model. Specific case studies support theability of the approach to handle precipitation of different types
Estimation of the local diffusion tensor and normalization for heterogeneous correlation modelling using a diffusion equation

This is a preprint of an article accepted for publication in QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 134:1425?1438 (2008) see the website for final version http://www.interscience.wiley.comInternational audienceAs the background error covariance matrix is a key component of any assimilation system, its modelling is an important step. Usually, this matrix is decomposed into correlations and variance matrices. An interesting method for modelling the correlation matrix of the background error for complex geometry, like ocean grid, consists in computing correlation functions using a diffusion operator. The background error correlation functions can be estimated for example from an ensemble of perturbed forecasts. The diffusion operator is able to represent heterogeneous correlation functions at a reasonable numerical cost. But a first challenge resides in the determination of the local diffusion tensor corresponding to the local correlation function. Then the second challenge resides in the determination of the normalization to make sure that the matrix modelled through the diffusion operator is a correlation matrix. In this article, we propose to build a background error correlation matrix using a diffusion operator based on a local diffusion tensor. The estimation of this local tensor is performed using an ensemble of perturbed forecasts. A validation within a randomization method illustrates the feasibility and the accuracy of the proposed method. In particular, it is shown that the local geographical variations of diagnosed correlation functions (through an ensemble of perturbed forecast) are well represented