576 research outputs found
Exploring hail and lightning diagnostics over the Alpine-Adriatic region in a km-scale climate model
The north and south of the Alps, as well as the eastern shores of the Adriatic Sea, are hot spots of severe convective storms, including hail and lightning associated with deep convection. With advancements in computing power, it has become feasible to simulate deep convection explicitly in climate models by decreasing the horizontal grid spacing to less than 4 km. These kilometer-scale models improve the representation of orography and reduce uncertainties associated with the use of deep convection parameterizations.
In this study, we perform km-scale simulations for eight observed cases of severe convective storms (seven with and one without observed hail) over the Alpine-Adriatic region. The simulations are performed with the climate version of the regional model Consortium for Small-scale Modeling (COSMO) that runs on graphics processing units (GPUs) at a horizontal grid spacing of 2.2 km. To analyze hail and lightning we have explored the hail growth model (HAILCAST) and lightning potential index (LPI) diagnostics integrated with the COSMO-crCLIM model.
Comparison with available high-resolution observations reveals good performance of the model in simulating total precipitation, hail, and lightning. By performing a detailed analysis of three of the case studies, we identified the importance of significant meteorological factors for heavy thunderstorms that were reproduced by the model. Among these are the moist unstable boundary layer and dry mid-level air, the topographic barrier, as well as an approaching upper-level trough and cold front. Although COSMO HAILCAST tends to underestimate the hail size on the ground, the results indicate that both HAILCAST and LPI are promising candidates for future climate research.</p
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Mean and extreme precipitation over European river basins better simulated in a 25km AGCM
Limited spatial resolution is one of the factors that may hamper applications of global climate models (GCMs), in particular over Europe with its complex coastline and orography. In this study, the representation of European mean and extreme precipitation is evaluated in simulations with an atmospheric GCM (AGCM) at different resolutions between about 135 and 25km grid spacing in the mid-latitudes. The continent-wide root-mean-square error in mean precipitation in the 25km model is about 25% smaller than in the 135km model in winter. Clear improvements are also seen in autumn and spring, whereas the model's sensitivity to resolution is very small in summer. Extreme precipitation is evaluated by estimating generalised extreme value distributions (GEVs) of daily precipitation aggregated over river basins whose surface area is greater than 50000km2. GEV location and scale parameters are measures of the typical magnitude and of the interannual variability of extremes, respectively. Median model biases in both these parameters are around 10% in summer and around 20% in the other seasons. For some river basins, however, these biases can be much larger and take values between 50% and 100%. Extreme precipitation is better simulated in the 25km model, especially during autumn when the median GEV parameter biases are more than halved, and in the North European Plains, from the Loire in the west to the Vistula in the east. A sensitivity experiment is conducted showing that these resolution sensitivities in both mean and extreme precipitation are in many areas primarily due to the increase in resolution of the model orography. The findings of this study illustrate the improved capability of a global high-resolution model in simulating European mean and extreme precipitation
Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared
The TRAPPIST-1 planetary system is a favorable target for the atmospheric
characterization of temperate earth-sized exoplanets by means of transmission
spectroscopy with the forthcoming James Webb Space Telescope (JWST). A possible
obstacle to this technique could come from the photospheric heterogeneity of
the host star that could affect planetary signatures in the transit
transmission spectra. To constrain further this possibility, we gathered an
extensive photometric data set of 25 TRAPPIST-1 transits observed in the
near-IR J band (1.2 m) with the UKIRT and the AAT, and in the NB2090 band
(2.1 m) with the VLT during the period 2015-2018. In our analysis of these
data, we used a special strategy aiming to ensure uniformity in our
measurements and robustness in our conclusions. We reach a photometric
precision of (RMS of the residuals), and we detect no significant
temporal variations of transit depths of TRAPPIST-1 b, c, e, and g over the
period of three years. The few transit depths measured for planets d and f hint
towards some level of variability, but more measurements will be required for
confirmation. Our depth measurements for planets b and c disagree with the
stellar contamination spectra originating from the possible existence of bright
spots of temperature 4500 K. We report updated transmission spectra for the six
inner planets of the system which are globally flat for planets b and g and
some structures are seen for planets c, d, e, and f.Comment: accepted for publication in MNRA
Adaptive Optics Images of Kepler Objects of Interest
All transiting planets are at risk of contamination by blends with nearby,
unresolved stars. Blends dilute the transit signal, causing the planet to
appear smaller than it really is, or produce a false positive detection when
the target star is blended with eclipsing binary stars. This paper reports on
high spatial-resolution adaptive optics images of 90 Kepler planetary
candidates. Companion stars are detected as close as 0.1 arcsec from the target
star. Images were taken in the near-infrared (J and Ks bands) with ARIES on the
MMT and PHARO on the Palomar Hale 200-inch. Most objects (60%) have at least
one star within 6 arcsec separation and a magnitude difference of 9. Eighteen
objects (20%) have at least one companion within 2 arcsec of the target star; 6
companions (7%) are closer than 0.5 arcsec. Most of these companions were
previously unknown, and the associated planetary candidates should receive
additional scrutiny. Limits are placed on the presence of additional companions
for every system observed, which can be used to validate planets statistically
using the BLENDER method. Validation is particularly critical for low-mass,
potentially Earth-like worlds, which are not detectable with current-generation
radial velocity techniques. High-resolution images are thus a crucial component
of any transit follow-up program.Comment: 9 pages, 4 figures, accepted to A
Weak evidence for variable occultation depth of 55 Cnc e with TESS
55 Cnc e is in a 0.73 day orbit transiting a Sun-like star. It has been
observed that the occultation depth of this Super-Earth, with a mass of
8 and radius of 2, changes significantly over
time at mid-infrared wavelengths. Observations with Spitzer measured a change
in its day-side brightness temperature of 1200 K, possibly driven by volcanic
activity, magnetic star-planet interaction, or the presence of a circumstellar
torus of dust. Previous evidence for the variability in occultation was in the
infrared range. Here we aim to explore if the variability exists also in the
optical. TESS observed 55 Cnc during sectors 21, 44 and 46. We carefully
detrend the data and fit a transit and occultation model for each sector in a
Markov Chain Monte Carlo routine. In a later stage we use the Leave-One-Out
Cross-Validation statistic to compare with a model of constant occultation for
the complete set and a model with no occultation. We report an occultation
depth of 82.5 ppm for the complete set of TESS observations. In
particular, we measured a depth of 154 ppm for sector 21, while for sector
44 we detect no occultation. In sector 46 we measure a weak occultation of
85 ppm. The occultation depth varies from one sector to the next between
1.6 and 3.4 significance. We derive the possible contribution on
reflected light and thermal emission, setting an upper limit on the geometric
albedo. Based on our model comparison the presence of an occultation is
favoured considerably over no occultation, where the model with varying
occultation across sectors takes most of the statistical weight. Our analysis
confirms a detection of the occultation in TESS. Moreover, our results weakly
lean towards a varying occultation depth between each sector, while the transit
depth is constant across visits.Comment: 9 pages, 9 figures, accepted for publication on A&
Rock magnetic investigation of possible sources of the Bangui magnetic anomaly
International audienceThe Bangui magnetic anomaly (BMA) is the largest lithospheric magnetic field anomaly on Earth at low latitudes. Previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements, as well as from surface magnetic susceptibility measurements on rocks from the Panafrican Mobile Belt Zone (PMBZ). Here we combine magnetic field data modelling and rock magnetic property measurements (susceptibility and natural remanent magnetization, NRM) on many samples from this PMBZ and the surrounding formations. It reveals that NRM is a significant component of the total magnetization (Mt) of the BMA source, which reaches 4.3 A/m with maximum thicknesses of 38 and 54 km beneath the western and eastern parts of the BMA. Only the isolated and relatively thin banded iron formations and some migmatites show such Mt values. Thus we suggest that the thick BMA source may be composed either by overlapped slices of such metamorphic rocks, or by an iron-rich mafic source, or by a combination of these two geological structures
The TRAPPIST survey of southern transiting planets. I. Thirty eclipses of the ultra-short period planet WASP-43 b
We present twenty-three transit light curves and seven occultation light
curves for the ultra-short period planet WASP-43 b, in addition to eight new
measurements of the radial velocity of the star. Thanks to this extensive data
set, we improve significantly the parameters of the system. Notably, the
largely improved precision on the stellar density (2.41+-0.08 rho_sun) combined
with constraining the age to be younger than a Hubble time allows us to break
the degeneracy of the stellar solution mentioned in the discovery paper. The
resulting stellar mass and size are 0.717+-0.025 M_sun and 0.667+-0.011 R_sun.
Our deduced physical parameters for the planet are 2.034+-0.052 M_jup and
1.036+-0.019 R_jup. Taking into account its level of irradiation, the high
density of the planet favors an old age and a massive core. Our deduced orbital
eccentricity, 0.0035(-0.0025,+0.0060), is consistent with a fully circularized
orbit. We detect the emission of the planet at 2.09 microns at better than
11-sigma, the deduced occultation depth being 1560+-140 ppm. Our detection of
the occultation at 1.19 microns is marginal (790+-320 ppm) and more
observations are needed to confirm it. We place a 3-sigma upper limit of 850
ppm on the depth of the occultation at ~0.9 microns. Together, these results
strongly favor a poor redistribution of the heat to the night-side of the
planet, and marginally favor a model with no day-side temperature inversion.Comment: 14 pages, 6 tables, 11 figures. Accepted for publication in A&
55 Cancri e’s occultation captured with CHEOPS
Past occultation and phase-curve observations of the ultra-short period super-Earth 55 Cnc e obtained at visible and infrared wavelengths have been challenging to reconcile with a planetary reflection and emission model. In this study, we analyse a set of 41 occultations obtained over a two-year timespan with the CHEOPS satellite. We report the detection of 55 Cnc e’s occultation with an average depth of 12 ± 3 ppm. We derive a corresponding 2σ upper limit on the geometric albedo of Ag < 0.55 once decontaminated from the thermal emission measured by Spitzer at 4.5 µm. CHEOPS’s photometric performance enables, for the first time, the detection of individual occultations of this super-Earth in the visible and identifies short-timescale photometric corrugations likely induced by stellar granulation. We also find a clear 47.3-day sinusoidal pattern in the time-dependent occultation depths that we are unable to relate to stellar noise, nor instrumental systematics, but whose planetary origin could be tested with upcoming JWST occultation observations of this iconic super-Earth
The stable climate of KELT-9b
Even among the most irradiated gas giants, so-called ultra-hot Jupiters, KELT-9b stands out as the hottest planet thus far discovered with a dayside temperature of over 4500 K. At these extreme irradiation levels, we expect an increase in heat redistribution efficiency and a low Bond albedo owed to an extended atmosphere with molecular hydrogen dissociation occurring on the planetary dayside. We present new photometric observations of the KELT-9 system throughout 4 full orbits and 9 separate occultations obtained by the 30 cm space telescope CHEOPS. The CHEOPS bandpass, located at optical wavelengths, captures the peak of the thermal emission spectrum of KELT-9b. In this work we simultaneously analyse CHEOPS phase curves along with public phase curves from TESS and Spitzer to infer joint constraints on the phase curve variation, gravity-darkened transits, and occultation depth in three bandpasses, as well as derive 2D temperature maps of the atmosphere at three different depths. We find a day-night heat redistribution efficiency of ~0.3 which confirms expectations of enhanced energy transfer to the planetary nightside due to dissociation and recombination of molecular hydrogen. We also calculate a Bond albedo consistent with zero. We find no evidence of variability of the brightness temperature of the planet, excluding variability greater than 1
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