191 research outputs found
Kelvin Waves and Internal Bores in the Marine Boundary Layer Inversion and Their Relationship to Coastally Trapped Wind Reversals
Detailed observations of a coastally trapped disturbance, or wind reversal, on 10–11 June 1994 along the
California coast provide comprehensive documentation of its structure, based on aircraft, wind profiler, radio
acoustic sounding system, and buoy measurements. Unlike the expectations from earlier studies based on limited
data, which concluded that the deepening of the marine boundary layer (MBL) was a key factor, the 1994 data
show that the perturbation was better characterized as an upward thickening of the inversion capping the MBL.
As the event propagated over a site, the reversal in the alongshore wind direction occurred first within the
inversion and then 3–4 h later at the surface. A node in the vertical structure (defined here as the altitude of
zero vertical displacement) is found just above the inversion base, with up to 200-m upward displacements of
isentropic surfaces above the node, and 70-m downward displacements below.
Although this is a single event, it is shown that the vertical structure observed is representative of most other
coastally trapped wind reversals. This is determined by comparing a composite of the 10–11 June 1994 event,
based on measurements at seven buoys, with surface pressure perturbations calculated from aircraft data. These
results are compared to the composite of many events. In each case a weak pressure trough occurred between
2.4 and 4.0 h ahead of the surface wind reversal, and the pressure rose by 0.32–0.48 mb between the trough
and the wind reversal. The pressure rise results from the cooling caused by the inversion’s upward expansion.
The propagation and structure of the event are shown to be best characterized as a mixed Kelvin wave–bore
propagating within the inversion above the MBL, with the MBL acting as a quasi-rigid lower boundary. If the
MBL is instead assumed to respond in unison with the inversion, then the theoretically predicted intrinsic phase
speeds significantly exceed the observed intrinsic phase speed. The hybrid nature of the event is indicated by
two primary characteristics: 1) the disturbance had a much shallower slope than expected for an internal bore,
while at the same time the upward perturbation within the inversion was quasi-permanent rather than sinusoidal,
which more closely resembles a bore; and 2) the predicted phase speeds for the ‘‘solitary’’ form of nonlinear
Kelvin wave and for an internal bore are both close to the observed intrinsic phase speed
Lagrangian Reachabililty
We introduce LRT, a new Lagrangian-based ReachTube computation algorithm that
conservatively approximates the set of reachable states of a nonlinear
dynamical system. LRT makes use of the Cauchy-Green stretching factor (SF),
which is derived from an over-approximation of the gradient of the solution
flows. The SF measures the discrepancy between two states propagated by the
system solution from two initial states lying in a well-defined region, thereby
allowing LRT to compute a reachtube with a ball-overestimate in a metric where
the computed enclosure is as tight as possible. To evaluate its performance, we
implemented a prototype of LRT in C++/Matlab, and ran it on a set of
well-established benchmarks. Our results show that LRT compares very favorably
with respect to the CAPD and Flow* tools.Comment: Accepted to CAV 201
Kelvin waves and internal bores in the marine boundary layer inversion and their relationship to coastally trapped wind reversals
Detailed observations of a coastally trapped disturbance, or wind reversal, on 10-11 June 1994 along the California coast provide comprehensive documentation of its structure, based on aircraft, wind profiler, radio acoustic sounding system, and buoy measurements. Unlike the expectations from earlier studies based on limited data, which concluded that the deepening of the marine boundary layer (MBL) was a key factor, the 1994 data show that the perturbation was better characterized as an upward thickening of the inversion capping the MBL. As the event propagated over a site, the reversal in the alongshore wind direction occurred first within the inversion and then 3-4 h later at the surface. A node in the vertical structure (defined here as the altitude of zero vertical displacement) is found just above the inversion base, with up to 200-m upward displacements of isentropic surfaces above the node, and 70-m downward displacements below. Although this is a single event, it is shown that the vertical structure observed is representative of most other coastally trapped wind reversals. This is determined by comparing a composite of the 10-11 June 1994 event, based on measurements at seven buoys, with surface pressure perturbations calculated from aircraft data. These results are compared to the composite of many events. In each case a weak pressure trough occurred between 2.4 and 4.0 h ahead of the surface wind reversal, and the pressure rose by 0.32-0.48 mb between the trough and the wind reversal. The pressure rise results from the cooling caused by the inversion's upward expansion. The propagation and structure of the event are shown to be best characterized as a mixed Kelvin wave-bore propagating within the inversion above the MBL, with the MBL acting as a quasi-rigid lower boundary. If the MBL is instead assumed to respond in unison with the inversion, then the theoretically predicted intrinsic phase speeds significantly exceed the observed intrinsic phase speed. The hybrid nature of the event is indicated by two primary characteristics: 1) the disturbance had a much shallower slope than expected for an internal bore, while at the same time the upward perturbation within the inversion was quasi-permanent rather than sinusoidal, which more closely resembles a bore; and 2) the predicted phase speeds for the "solitary" form of nonlinear Kelvin wave and for an internal bore are both close to the observed intrinsic phase speed
Recommended from our members
Vertical profiles of the 3-D wind velocity retrieved from multiple wind lidars performing triple range-height-indicator scans
Vertical profiles of 3-D wind velocity are retrieved from triple
range-height-indicator (RHI) scans performed with multiple simultaneous
scanning Doppler wind lidars. This test is part of the eXperimental Planetary
boundary layer Instrumentation Assessment (XPIA) campaign carried out at the
Boulder Atmospheric Observatory. The three wind velocity components are
retrieved and then compared with the data acquired through various profiling
wind lidars and high-frequency wind data obtained from sonic anemometers
installed on a 300 m meteorological tower. The results show that the
magnitude of the horizontal wind velocity and the wind direction obtained
from the triple RHI scans are generally retrieved with good accuracy.
However, poor accuracy is obtained for the evaluation of the vertical
velocity, which is mainly due to its typically smaller magnitude and to the
error propagation connected with the data retrieval procedure and accuracy in
the experimental setup
Recommended from our members
Assessment of virtual towers performed with scanning wind lidars and Ka-band radars during the XPIA experiment
During the
eXperimental Planetary boundary layer Instrumentation Assessment (XPIA)
campaign, which was carried out at the Boulder Atmospheric Observatory (BAO)
in spring 2015, multiple-Doppler scanning strategies were carried out with
scanning wind lidars and Ka-band radars. Specifically, step–stare
measurements were collected simultaneously with three scanning Doppler
lidars, while two scanning Ka-band radars carried out simultaneous range
height indicator (RHI) scans. The XPIA experiment provided the unique
opportunity to compare directly virtual-tower measurements performed
simultaneously with Ka-band radars and Doppler wind lidars. Furthermore,
multiple-Doppler measurements were assessed against sonic anemometer data
acquired from the meteorological tower (met-tower) present at the BAO site and a lidar wind
profiler. This survey shows that – despite the different technologies,
measurement volumes and sampling periods used for the lidar and radar
measurements – a very good accuracy is achieved for both remote-sensing
techniques for probing horizontal wind speed and wind direction with the
virtual-tower scanning technique
Recommended from our members
Evaluation of single and multiple Doppler lidar techniques to measure complex flow during the XPIA field campaign
Accurate three-dimensional information of wind flow fields can be an
important tool in not only visualizing complex flow but also understanding
the underlying physical processes and improving flow modeling. However, a
thorough analysis of the measurement uncertainties is required to properly
interpret results. The XPIA (eXperimental Planetary boundary layer
Instrumentation Assessment) field campaign conducted at the Boulder
Atmospheric Observatory (BAO) in Erie, CO, from 2 March to 31 May 2015 brought
together a large suite of in situ and remote sensing measurement platforms to
evaluate complex flow measurement strategies.
In this paper, measurement uncertainties for different single and
multi-Doppler strategies using simple scan geometries (conical, vertical
plane and staring) are investigated. The tradeoffs (such as time–space
resolution vs. spatial coverage) among the different measurement techniques
are evaluated using co-located measurements made near the BAO tower.
Sensitivity of the single-/multi-Doppler measurement uncertainties to
averaging period are investigated using the sonic anemometers installed on
the BAO tower as the standard reference. Finally, the radiometer measurements
are used to partition the measurement periods as a function of atmospheric
stability to determine their effect on measurement uncertainty.
It was found that with an increase in spatial coverage and measurement
complexity, the uncertainty in the wind measurement also increased. For
multi-Doppler techniques, the increase in uncertainty for temporally
uncoordinated measurements is possibly due to requiring additional
assumptions of stationarity along with horizontal homogeneity and less
representative line-of-sight velocity statistics. It was also found that wind speed
measurement uncertainty was lower during stable conditions compared to
unstable conditions
Complexity in water and carbon dioxide fluxes following rain pulses in an African savanna
The idea that many processes in arid and semi-arid ecosystems are dormant until activated by a pulse of rainfall, and then decay from a maximum rate as the soil dries, is widely used as a conceptual and mathematical model, but has rarely been evaluated with data. This paper examines soil water, evapotranspiration (ET), and net ecosystem CO2 exchange measured for 5 years at an eddy covariance tower sited in an Acacia–Combretum savanna near Skukuza in the Kruger National Park, South Africa. The analysis characterizes ecosystem flux responses to discrete rain events and evaluates the skill of increasingly complex “pulse models”. Rainfall pulses exert strong control over ecosystem-scale water and CO2 fluxes at this site, but the simplest pulse models do a poor job of characterizing the dynamics of the response. Successful models need to include the time lag between the wetting event and the process peak, which differ for evaporation, photosynthesis and respiration. Adding further complexity, the time lag depends on the prior duration and degree of water stress. ET response is well characterized by a linear function of potential ET and a logistic function of profile-total soil water content, with remaining seasonal variation correlating with vegetation phenological dynamics (leaf area). A 1- to 3-day lag to maximal ET following wetting is a source of hysteresis in the ET response to soil water. Respiration responds to wetting within days, while photosynthesis takes a week or longer to reach its peak if the rainfall was preceded by a long dry spell. Both processes exhibit nonlinear functional responses that vary seasonally. We conclude that a more mechanistic approach than simple pulse modeling is needed to represent daily ecosystem C processes in semiarid savannas
The charcoal trap: Miombo forests and the energy needs of people
<p>Abstract</p> <p>Background</p> <p>This study evaluates the carbon dioxide and other greenhouse gas fluxes to the atmosphere resulting from charcoal production in Zambia. It combines new biomass and flux data from a study, that was conducted in a <it>miombo </it>woodland within the Kataba Forest Reserve in the Western Province of Zambia, with data from other studies.</p> <p>Results</p> <p>The measurements at Kataba compared protected area (3 plots) with a highly disturbed plot outside the forest reserve and showed considerably reduced biomass after logging for charcoal production. The average aboveground biomass content of the reserve (Plots 2-4) was around 150 t ha<sup>-1</sup>, while the disturbed plot only contained 24 t ha<sup>-1</sup>. Soil carbon was not reduced significantly in the disturbed plot. Two years of eddy covariance measurements resulted in net ecosystem exchange values of -17 ± 31 g C m<sup>-2 </sup>y<sup>-1</sup>, in the first and 90 ± 16 g C m<sup>-2 </sup>in the second year. Thus, on the basis of these two years of measurement, there is no evidence that the <it>miombo </it>woodland at Kataba represents a present-day carbon sink. At the country level, it is likely that deforestation for charcoal production currently leads to a per capita emission rate of 2 - 3 t CO<sub>2 </sub>y<sup>-1</sup>. This is due to poor forest regeneration, although the resilience of <it>miombo </it>woodlands is high. Better post-harvest management could change this situation.</p> <p>Conclusions</p> <p>We argue that protection of <it>miombo </it>woodlands has to account for the energy demands of the population. The production at national scale that we estimated converts into 10,000 - 15,000 GWh y<sup>-1 </sup>of energy in the charcoal. The term "Charcoal Trap" we introduce, describes the fact that this energy supply has to be substituted when woodlands are protected. One possible solution, a shift in energy supply from charcoal to electricity, would reduce the pressure of forests but requires high investments into grid and power generation. Since Zambia currently cannot generate this money by itself, the country will remain locked in the charcoal trap such as many other of its African neighbours. The question arises whether and how money and technology transfer to increase regenerative electrical power generation should become part of a post-Kyoto process. Furthermore, better inventory data are urgently required to improve knowledge about the current state of the woodland usage and recovery. Net greenhouse gas emissions could be reduced substantially by improving the post-harvest management, charcoal production technology and/or providing alternative energy supply.</p
- …