38 research outputs found
An improved algorithm for polar cloud-base detection by ceilometer over the ice sheets
Optically thin ice and mixed-phase clouds play an important role in polar
regions due to their effect on cloud radiative impact and precipitation.
Cloud-base heights can be detected by ceilometers, low-power backscatter
lidars that run continuously and therefore have the potential to provide
basic cloud statistics including cloud frequency, base height and vertical
structure. The standard cloud-base detection algorithms of ceilometers are
designed to detect optically thick liquid-containing clouds, while the
detection of thin ice clouds requires an alternative approach. This paper
presents the polar threshold (PT) algorithm that was developed to be
sensitive to optically thin hydrometeor layers (minimum optical depth
Ï„ ≥ 0.01). The PT algorithm detects the first hydrometeor layer
in a vertical attenuated backscatter profile exceeding a predefined threshold
in combination with noise reduction and averaging procedures. The optimal
backscatter threshold of 3 × 10<sup>−4</sup> km<sup>−1</sup> sr<sup>−1</sup> for
cloud-base detection near the surface was derived based on a sensitivity
analysis using data from Princess Elisabeth, Antarctica and Summit,
Greenland. At higher altitudes where the average noise level is higher than
the backscatter threshold, the PT algorithm becomes signal-to-noise ratio
driven. The algorithm defines cloudy conditions as any atmospheric profile
containing a hydrometeor layer at least 90 m thick. A comparison with
relative humidity measurements from radiosondes at Summit illustrates the
algorithm's ability to significantly discriminate between clear-sky and
cloudy conditions. Analysis of the cloud statistics derived from the PT
algorithm indicates a year-round monthly mean cloud cover fraction of 72%
(±10%) at Summit without a seasonal cycle. The occurrence of
optically thick layers, indicating the presence of supercooled liquid water
droplets, shows a seasonal cycle at Summit with a monthly mean summer peak of
40 % (±4%). The monthly mean cloud occurrence frequency in summer
at Princess Elisabeth is 46% (±5%), which reduces to 12%
(±2.5%) for supercooled liquid cloud layers. Our analyses
furthermore illustrate the importance of optically thin hydrometeor layers
located near the surface for both sites, with 87% of all detections below
500 m for Summit and 80% below 2 km for Princess Elisabeth. These
results have implications for using satellite-based remotely sensed cloud
observations, like CloudSat that may be insensitive for hydrometeors near
the surface. The decrease of sensitivity with height, which is an inherent
limitation of the ceilometer, does not have a significant impact on our
results. This study highlights the potential of the PT algorithm to extract
information in polar regions from various hydrometeor layers using
measurements by the robust and relatively low-cost ceilometer instrument
WorldCereal: a dynamic open-source system for global-scale, seasonal, and reproducible crop and irrigation mapping
The challenge of global food security in the face of population growth, conflict and climate change requires a comprehensive understanding of cropped areas, irrigation practices and the distribution of major commodity crops like maize and wheat. However, such understanding should preferably be updated at seasonal intervals for each agricultural system rather than relying on a single annual assessment. Here we present the European Space Agency funded WorldCereal system, a global, seasonal, and reproducible crop and irrigation mapping system that addresses existing limitations in current global-scale crop and irrigation mapping. WorldCereal generates a range of global products, including temporary crop extent, seasonal maize and cereals maps, seasonal irrigation maps, seasonal active cropland maps, and confidence layers providing insights into expected product quality. The WorldCereal product suite for the year 2021 presented here serves as a global demonstration of the dynamic open-source WorldCereal system. The presented products are fully validated, e.g., global user's and producer's accuracies for the annual temporary crop product are 88.5 % and 92.1 %, respectively. The WorldCereal system provides a vital tool for policymakers, international organizations, and researchers to better understand global crop and irrigation patterns and inform decision-making related to food security and sustainable agriculture. Our findings highlight the need for continued community efforts such as additional reference data collection to support further development and push the boundaries for global agricultural mapping from space. The global products are available at https://doi.org/10.5281/zenodo.7875104 (Van Tricht et al., 2023)
The Greenland and Antarctic ice sheets under 1.5â—¦C global warming
Even if anthropogenic warming were constrained to less than 2°C above pre-industrial, the Greenland and Antarctic ice sheets will continue to lose mass this century, with rates similar to those observed over the last decade. However, nonlinear responses cannot be excluded, which may lead to larger rates of mass loss. Furthermore, large uncertainties in future projections still remain, pertaining to knowledge gaps in atmospheric (Greenland) and oceanic (Antarctica) forcing. On millennial timescales, both ice sheets have tipping points at or slightly above the 1.5-2.0°C threshold; for Greenland, this may lead to irreversible mass loss due to the surface mass balance elevation feedback, while for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening
In vitro set-up of modified Blalock Taussig shunt: Vascular resistance-flow relationship
Background: A modified Blalock-Taussig (mBT) shunt is an anastomosis created between the systemic and pulmonary arterial tree in order to improve pulmonary blood flow in neonates and children with congenital heart disease. The aim of this study was to assess vascular resistance-flow relationship in an in vitro set-up of a modified Blalock Taussig shunt
Cloud and precipitation properties from ground-based remote-sensing instruments in East Antarctica
A new comprehensive cloud–precipitation–meteorological observatory has been
established at Princess Elisabeth base, located in the escarpment zone of
Dronning Maud Land (DML), East Antarctica. The observatory consists of a set of
ground-based remote-sensing instruments (ceilometer, infrared pyrometer and
vertically profiling precipitation radar) combined with automatic weather
station measurements of near-surface meteorology, radiative fluxes, and snow
height. In this paper, the observatory is presented and the potential for
studying the evolution of clouds and precipitating systems is illustrated by
case studies. It is shown that the synergetic use of the set of instruments
allows for distinguishing ice, liquid-containing clouds and precipitating clouds,
including some information on their vertical extent. In addition, wind-driven
blowing snow events can be distinguished from deeper precipitating systems.
Cloud properties largely affect the surface radiative fluxes, with
liquid-containing clouds dominating the radiative impact. A statistical
analysis of all measurements (in total 14 months mainly during summer–beginning of winter)
indicates that these liquid-containing clouds occur during as much as 20% of
the cloudy periods. The cloud occurrence shows a strong bimodal distribution
with clear-sky conditions 51% of the time and complete overcast conditions
35% of the time. Snowfall occurred during 17% of the cloudy periods with a
predominance of light precipitation and only rare events with snowfall
>1 mm h−1 water equivalent (w.e.). Three of such intense
snowfall events occurred during 2011 contributing to anomalously large annual
surface mass balance (SMB). Large accumulation events
(>10 mm w.e. day−1) during the radar-measurement period of 26
months were always associated with snowfall, but at the same time other
snowfall events did not always lead to accumulation. The multiyear deployment
of a precipitation radar in Antarctica allows for assessing the contribution of
the snowfall to the local SMB and comparing it to the other SMB components.
During 2012, snowfall rate was 110 ± 20 mm w.e. yr−1, from
which surface and drifting snow sublimation removed together 23%. Given the
measured yearly SMB of 52 ± 3 mm w.e., the residual term of
33 ± 21 mm w.e. yr−1 was attributed to the wind-driven snow
erosion. In general, this promising set of robust instrumentation allows for
improved insight into cloud and precipitation processes in Antarctica and can
be easily deployed at other Antarctic stations
Clouds enhance Greenland ice sheet meltwater runoff
The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m-2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise