3,429 research outputs found
Recommended from our members
Doppler W-band polarization diversity space-borne radar simulator for wind studies
CloudSat observations are used in combination with collocated European Centre for Medium-Range
Weather Forecasts (ECMWF) reanalysis to simulate spaceborne W-band Doppler observations from slant-looking
radars. The simulator also includes cross-polarization effects
which are relevant if the Doppler velocities are derived from
polarization diversity pulse pair correlation. A specific conically scanning radar configuration (WIVERN), recently proposed to the ESA-Earth Explorer 10 call that aims to provide
global in-cloud winds for data assimilation, is analysed in
detail in this study.
One hundred granules of CloudSat data are exploited to investigate the impact on Doppler velocity estimates from three
specific effects: (1) non-uniform beam filling, (2) wind shear
and (3) crosstalk between orthogonal polarization channels
induced by hydrometeors and surface targets. Errors associated with non-uniform beam filling constitute the most important source of error and can account for almost 1 m s−1
standard deviation, but this can be reduced effectively to less
than 0.5 m s−1 by adopting corrections based on estimates
of vertical reflectivity gradients. Wind-shear-induced errors
are generally much smaller (∼ 0.2 m s−1
). A methodology
for correcting these errors has been developed based on estimates of the vertical wind shear and the reflectivity gradient. Low signal-to-noise ratios lead to higher random errors
(especially in winds) and therefore the correction (particularly the one related to the wind-shear-induced error) is less
effective at low signal-to-noise ratio. Both errors can be underestimated in our model because the CloudSat data do not
fully sample the spatial variability of the reflectivity fields,
whereas the ECMWF reanalysis may have smoother velocity fields than in reality (e.g. they underestimate vertical wind
shear).
The simulator allows for quantification of the average
number of accurate measurements that could be gathered by
the Doppler radar for each polar orbit, which is strongly impacted by the selection of the polarization diversity H − V
pulse separation, Thv. For WIVERN a selection close to 20 µs
(with a corresponding folding velocity equal to 40 m s−1
)
seems to achieve the right balance between maximizing the
number of accurate wind measurements (exceeding 10 % of
the time at any particular level in the mid-troposphere) and
minimizing aliasing effects in the presence of high winds.
The study lays the foundation for future studies towards
a thorough assessment of the performance of polar orbiting
wide-swath W-band Doppler radars on a global scale. The
next generation of scanning cloud radar systems and reanalyses with improved resolution will enable a full capture of the
spatial variability of the cloud reflectivity and the in-cloud
wind fields, thus refining the results of this study
Comparison of lightning location data and polarisation radar observations of clouds
Simultaneous observations of both the precipitation and the lightning associated with thunderstorms show that the lightning is within 3 km of the maximum precipitation echo. The intensity and type of the precipitation is observed with 500 m spatial accuracy using an S-band polarization radar and the position of the lightning is inferred from a low frequency magnetic direction finding location system. Empirical adjustment to the angles using the redundancy of the lightning data reduce this error. Radar echoes above 45dBZ may be caused by soft hail or hailstones, but similarly intense echoes may result from melting snow. The data show that a new polarization radar parameter, the linear depolarization ratio, can distinguish between soft hail and melting snow, and that the intense radar echoes associated with melting snow pose no threat of lightning. A lightning risk only exists when the radar indicates that the clouds contain soft hail or hailstones
Recommended from our members
Implementation of polarization diversity pulse-pair technique using airborne W-band radar
This work describes the implementation of polarization diversity on the National Research Council Canada W-band Doppler radar and presents the first-ever airborne Doppler measurements derived via polarization diversity pulse-pair processing. The polarization diversity pulse-pair measurements are interleaved with standard pulse-pair measurements with staggered pulse repetition frequency, this allows a better understanding of the strengths and drawbacks of polarization diversity, a methodology that has been recently proposed for wind-focused Doppler radar space missions. Polarization diversity has the clear advantage of making possible Doppler observations of very fast decorrelating media (as expected when deploying Doppler radars on fast-moving satellites) and of widening the Nyquist interval, thus enabling the observation of very high Doppler velocities (up to more than 100 m s−1 in the present work). Crosstalk between the two polarizations, mainly caused by depolarization at backscattering, deteriorated the quality of the observations by introducing ghost echoes in the power signals and by increasing the noise level in the Doppler measurements. In the different cases analyzed during the field campaigns, the regions affected by crosstalk were generally associated with highly depolarized surface returns and depolarization of backscatter from hydrometeors located at short ranges from the aircraft. The variance of the Doppler velocity estimates can be well predicted from theory and were also estimated directly from the observed correlation between the H-polarized and V-polarized successive pulses. The study represents a key milestone towards the implementation of polarization diversity in Doppler space-borne radars
Recommended from our members
Estimating drizzle drop size and precipitation rate using two-colour lidar measurements
A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution.
The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions
The Dearth of z~10 Galaxies in all HST Legacy Fields -- The Rapid Evolution of the Galaxy Population in the First 500 Myr
We present an analysis of all prime HST legacy fields spanning >800 arcmin^2
for the search of z~10 galaxy candidates and the study of their UV luminosity
function (LF). In particular, we present new z~10 candidates selected from the
full Hubble Frontier Field (HFF) dataset. Despite the addition of these new
fields, we find a low abundance of z~10 candidates with only 9 reliable sources
identified in all prime HST datasets that include the HUDF09/12, the HUDF/XDF,
all the CANDELS fields, and now the HFF survey. Based on this comprehensive
search, we find that the UV luminosity function decreases by one order of
magnitude from z~8 to z~10 at all luminosities over a four magnitude range.
This also implies a decrease of the cosmic star-formation rate density by an
order of magnitude within 170 Myr from z~8 to z~10. We show that this
accelerated evolution compared to lower redshift can entirely be explained by
the fast build-up of the dark matter halo mass function at z>8. Consequently,
the predicted UV LFs from several models of galaxy formation are in good
agreement with this observed trend, even though the measured UV LF lies at the
low end of model predictions. In particular, the number of only 9 observed
candidate galaxies is lower, by ~50%, than predicted by galaxy evolution
models. The difference is generally still consistent within the Poisson and
cosmic variance uncertainties. However, essentially all models predict larger
numbers than observed. We discuss the implications of these results in light of
the upcoming James Webb Space Telescope mission, which is poised to find much
larger samples of z~10 galaxies as well as their progenitors at less than 400
Myr after the Big Bang.Comment: 13 pages, 6 figures, minor updates to match accepted versio
Theory and observations of ice particle evolution in cirrus using Doppler radar: evidence for aggregation
Vertically pointing Doppler radar has been used to study the evolution of ice
particles as they sediment through a cirrus cloud. The measured Doppler fall
speeds, together with radar-derived estimates for the altitude of cloud top,
are used to estimate a characteristic fall time tc for the `average' ice
particle. The change in radar reflectivity Z is studied as a function of tc,
and is found to increase exponentially with fall time. We use the idea of
dynamically scaling particle size distributions to show that this behaviour
implies exponential growth of the average particle size, and argue that this
exponential growth is a signature of ice crystal aggregation.Comment: accepted to Geophysical Research Letter
Recommended from our members
The formation of ice in a long-lived supercooled layer cloud
This article focuses on the characteristics of persistent thin single-layer mixed-phase clouds. We seek to answer two important questions: (i) how does ice continually nucleate and precipitate from these clouds, without the available ice nuclei becoming depleted? (ii) how do the supercooled liquid droplets persist in spite of the net flux of water vapour to the growing ice crystals? These questions are answered quantitatively using in situ and radar observations of a long-lived mixed-phase cloud layer over the Chilbolton Observatory.
Doppler radar measurements show that the top 500 m of cloud (the top 250 m of which is mixed-phase, with ice virga beneath) is turbulent and well-mixed, and the liquid water content is adiabatic. This well-mixed layer is bounded above and below by stable layers. This inhibits entrainment of fresh ice nuclei into the cloud layer, yet our in situ and radar observations show that a steady flux of ≈100 m−2s−1 ice crystals fell from the cloud over the course of ∼1 day. Comparing this flux to the concentration of conventional ice nuclei expected to be present within the well-mixed layer, we find that these nuclei would be depleted within less than 1 h. We therefore argue that nucleation in these persistent supercooled clouds is strongly time-dependent in nature, with droplets freezing slowly over many hours, significantly longer than the few seconds residence time of an ice nucleus counter.
Once nucleated, the ice crystals are observed to grow primarily by vapour deposition, because of the low liquid water path (21 g m−2) yet vapour-rich environment. Evidence for this comes from high differential reflectivity in the radar observations, and in situ imaging of the crystals. The flux of vapour from liquid to ice is quantified from in situ measurements, and we show that this modest flux (3.3 g m−2h−1) can be readily offset by slow radiative cooling of the layer to space
Extremely Small Sizes for Faint z~2-8 Galaxies in the Hubble Frontier Fields: A Key Input For Establishing their Volume Density and UV Emissivity
We provide the first observational constraints on the sizes of the faintest
galaxies lensed by the Hubble Frontier Fields (HFF) clusters. Ionizing
radiation from faint galaxies likely drives cosmic reionization, and the HFF
initiative provides a key opportunity to find such galaxies. Yet, we cannot
really assess their ionizing emissivity without a robust measurement of their
sizes, since this is key to quantifying both their prevalence and the faint-end
slope to the UV luminosity function. Here we provide the first such size
constraints with 2 new techniques. The first utilizes the fact that the
detectability of highly-magnified galaxies as a function of shear is very
dependent on a galaxy's size. Only the most compact galaxies will remain
detectable in regions of high shear (vs. a larger detectable size range for low
shear), a phenomenon we carefully quantify using simulations. Remarkably,
however, no correlation is found between the surface density of faint galaxies
and the predicted shear, using 87 faint high-magnification mu>10 z~2-8 galaxies
seen behind the first 4 HFF clusters. This can only be the case if such faint
(~-15 mag) galaxies have significantly smaller sizes than luminous galaxies. We
constrain their half-light radii to be <~30 mas (<160-240 pc). As a 2nd size
probe, we rotate and stack 26 faint high-magnification sources along the major
shear axis. Less elongation is found than even for objects with an intrinsic
half-light radius of 10 mas. Together these results indicate that extremely
faint z~2-8 galaxies have near point-source profiles in the HFF dataset
(half-light radii conservatively <30 mas and likely 5-10 mas). These results
suggest smaller completeness corrections and hence much lower volume densities
for faint z~2-8 galaxies and shallower faint-end slopes than have been derived
in many recent studies (by factors of ~2-3 and by dalpha>~0.1-0.3).Comment: 19 pages, 15 figures, 3 tables, accepted for publication in Ap
- …