1,798 research outputs found
On The Determination of MDI High-Degree Mode Frequencies
The characteristic of the solar acoustic spectrum is such that mode lifetimes
get shorter and spatial leaks get closer in frequency as the degree of a mode
increases for a given order. A direct consequence of this property is that
individual p-modes are only resolved at low and intermediate degrees, and that
at high degrees, individual modes blend into ridges. Once modes have blended
into ridges, the power distribution of the ridge defines the ridge central
frequency and it will mask the true underlying mode frequency. An accurate
model of the amplitude of the peaks that contribute to the ridge power
distribution is needed to recover the underlying mode frequency from fitting
the ridge.
We present the results of fitting high degree power ridges (up to l = 900)
computed from several two to three-month-long time-series of full-disk
observations taken with the Michelson Doppler Imager (MDI) on-board the Solar
and Heliospheric Observatory between 1996 and 1999.
We also present a detailed discussion of the modeling of the ridge power
distribution, and the contribution of the various observational and
instrumental effects on the spatial leakage, in the context of the MDI
instrument. We have constructed a physically motivated model (rather than some
ad hoc correction scheme) resulting in a methodology that can produce an
unbiased determination of high-degree modes, once the instrumental
characteristics are well understood.
Finally, we present changes in high degree mode parameters with epoch and
thus solar activity level and discuss their significance.Comment: 59 pages, 38 figures -- High-resolution version at
http://www-sgk.harvard.edu:1080/~sylvain/preprints/ -- Manuscript submitted
to Ap
Estimating the effect of rainfall on the surface temperature of a tropical lake
We make use of a unique high-quality, long-term observational
dataset on a tropical lake to assess the effect of rainfall on lake
surface temperature. The lake in question is Lake Kivu, one of the
African Great Lakes, and was selected for its remarkably uniform
climate and availability of multi-year over-lake meteorological
observations. Rain may have a cooling effect on the lake surface by
lowering the near-surface air temperature, by the direct rain heat
flux into the lake, by mixing the lake surface layer through the
flux of kinetic energy and by convective mixing of the lake surface
layer. The potential importance of the rainfall effect is discussed
in terms of both heat flux and kinetic energy flux. To estimate the
rainfall effect on the mean diurnal cycle of lake surface
temperature, the data are binned into categories of daily
rainfall amount. They are further filtered based on comparable
values of daily mean net radiation, which reduces the influence of
radiative-flux differences. Our results indicate that days with
heavy rainfall may experience a reduction in lake surface
temperature of approximately 0.3 K by the end of the day compared to
days with light to moderate rainfall. Overall this study highlights
a new potential control on lake surface temperature and suggests
that further efforts are needed to quantify this effect in other
regions and to include this process in land surface models used
for atmospheric prediction.</p
Future projections of temperature and mixing regime of European temperate lakes
The physical response of lakes to climate warming is regionally
variable and highly dependent on individual lake characteristics, making
generalizations about their development difficult. To qualify the role of
individual lake characteristics in their response to regionally homogeneous
warming, we simulated temperature, ice cover, and mixing in four intensively
studied German lakes of varying morphology and mixing regime with a
one-dimensional lake model. We forced the model with an ensemble of 12
climate projections (RCP4.5) up to 2100. The lakes were projected to warm at
0.10–0.11 ∘C decade−1, which is 75 %–90 % of the
projected air temperature trend. In simulations, surface temperatures
increased strongly in winter and spring, but little or not at all in summer
and autumn. Mean bottom temperatures were projected to increase in all lakes,
with steeper trends in winter and in shallower lakes. Modelled ice thaw and
summer stratification advanced by 1.5–2.2 and 1.4–1.8 days decade−1 respectively, whereas
autumn turnover and winter freeze timing was less sensitive. The projected
summer mixed-layer depth was unaffected by warming but sensitive to changes
in water transparency. By mid-century, the frequency of ice and
stratification-free winters was projected to increase by about 20 %,
making ice cover rare and shifting the two deeper dimictic lakes to a
predominantly monomictic regime. The polymictic lake was unlikely to become
dimictic by the end of the century. A sensitivity analysis predicted that
decreasing transparency would dampen the effect of warming on mean
temperature but amplify its effect on stratification. However, this
interaction was only predicted to occur in clear lakes, and not in the study
lakes at their historical transparency. Not only lake morphology, but also
mixing regime determines how heat is stored and ultimately how lakes respond
to climate warming. Seasonal differences in climate warming rates are thus
important and require more attention.</p
Evaluating and improving the Community Land Model's sensitivity to land cover
Modeling studies have shown the importance of biogeophysical effects of deforestation on local climate conditions but have also highlighted the lack of agreement across different models. Recently, remote-sensing observations have been used to assess the contrast in albedo, evapotranspiration (ET), and land surface temperature (LST) between forest and nearby open land on a global scale. These observations provide an unprecedented opportunity to evaluate the ability of land surface models to simulate the biogeophysical effects of forests. Here, we evaluate the representation of the difference of forest minus open land (i.e., grassland and cropland) in albedo, ET, and LST in the Community Land Model version 4.5 (CLM4.5) using various remote-sensing and in situ data sources. To extract the local sensitivity to land cover, we analyze plant functional type level output from global CLM4.5 simulations, using a model configuration that attributes a separate soil column to each plant functional type. Using the separated soil column configuration, CLM4.5 is able to realistically reproduce the biogeophysical contrast between forest and open land in terms of albedo, daily mean LST, and daily maximum LST, while the effect on daily minimum LST is not well captured by the model. Furthermore, we identify that the ET contrast between forests and open land is underestimated in CLM4.5 compared to observation-based products and even reversed in sign for some regions, even when considering uncertainties in these products. We then show that these biases can be partly alleviated by modifying several model parameters, such as the root distribution, the formulation of plant water uptake, the light limitation of photosynthesis, and the maximum rate of carboxylation. Furthermore, the ET contrast between forest and open land needs to be better constrained by observations to foster convergence amongst different land surface models on the biogeophysical effects of forests. Overall, this study demonstrates the potential of comparing subgrid model output to local observations to improve current land surface models' ability to simulate land cover change effects, which is a promising approach to reduce uncertainties in future assessments of land use impacts on climate
Modelling the water balance of Lake Victoria (East Africa) – Part 1: Observational analysis
Lake Victoria is the largest lake in Africa and one of the two major sources
of the Nile river. The water level of Lake Victoria is determined by its
water balance, consisting of precipitation on the lake, evaporation from the
lake, inflow from tributary rivers and lake outflow, controlled by two
hydropower dams. Due to a scarcity of in situ observations, previous estimates
of individual water balance terms are characterized by substantial
uncertainties, which means that the water balance is often not closed
independently. In this first part of a two-paper series, we present a water
balance model for Lake Victoria, using state-of-the-art remote sensing
observations, high-resolution reanalysis downscaling and outflow values
recorded at the dam. The uncalibrated computation of the individual water
balance terms yields lake level fluctuations that closely match the levels
retrieved from satellite altimetry. Precipitation is the main cause of
seasonal and interannual lake level fluctuations, and on average causes the
lake level to rise from May to July and to fall from August to December.
Finally, our results indicate that the 2004–2005 drop in lake level can be
about half attributed to a drought in the Lake Victoria Basin and about half
to an enhanced outflow, highlighting the sensitivity of the lake level to
human operations at the outflow dam.</p
Modelling the water balance of Lake Victoria (East Africa) – Part 2: Future projections
Lake Victoria, the second largest freshwater lake in the world, is one of the
major sources of the Nile river. The outlet to the Nile is controlled by two
hydropower dams of which the allowed discharge is dictated by the Agreed
Curve, an equation relating outflow to lake level. Some regional climate
models project a decrease in precipitation and an increase in evaporation
over Lake Victoria, with potential important implications for its water
balance and resulting level. Yet, little is known about the potential
consequences of climate change for the water balance of Lake Victoria. In
this second part of a two-paper series, we feed a new water balance model for
Lake Victoria presented in the first part with climate simulations available
through the COordinated Regional Climate Downscaling
Experiment (CORDEX) Africa
framework. Our results reveal that most regional climate models are not
capable of giving a realistic representation of the water balance of Lake
Victoria and therefore require bias correction. For two emission scenarios
(RCPs 4.5 and 8.5), the decrease in precipitation over the lake and an
increase in evaporation are compensated by an increase in basin precipitation
leading to more inflow. The future lake level projections show that the dam
management scenario and not the emission scenario is the main controlling
factor of the future water level evolution. Moreover, inter-model
uncertainties are larger than emission scenario uncertainties. The comparison
of four idealized future management scenarios pursuing certain policy
objectives (electricity generation, navigation reliability and environmental
conservation) uncovers that the only sustainable management scenario is
mimicking natural lake level fluctuations by regulating outflow according to
the Agreed Curve. The associated outflow encompasses, however, ranges from
14 m3 day−1 (−85 %) to 200 m3 day−1 (+100 %)
within this ensemble, highlighting that future hydropower generation and
downstream water availability may strongly change in the next decades even if
dam management adheres to he Agreed Curve. Our results overall underline that
managing the dam according to the Agreed Curve is a key prerequisite for
sustainable future lake levels, but that under this management scenario,
climate change might potentially induce profound changes in lake level and
outflow volume.</p
Oscillation frequencies and mode lifetimes in alpha Centauri A
We analyse our recently-published velocity measurements of alpha Cen A
(Butler et al. 2004). After adjusting the weights on a night-by-night basis in
order to optimize the window function to minimize sidelobes, we extract 42
oscillation frequencies with l=0 to 3 and measure the large and small frequency
separations. We give fitted relations to these frequencies that can be compared
with theoretical models and conclude that the observed scatter about these fits
is due to the finite lifetimes of the oscillation modes. We estimate the mode
lifetimes to be 1-2 d, substantially shorter than in the Sun.Comment: Accepted by Ap
Can climate‐effective land management reduce regional warming?
Limiting global warming to well below 2°C is an imminent challenge for humanity. However, even if this global target can be met, some regions are still likely to experience substantial warming relative to others. Using idealized global climate simulations, we examine the potential of land management options in affecting regional climate, with a focus on crop albedo enhancement and irrigation (climate-effective land management). The implementation is performed over all crop regions globally to provide an upper bound. We find that the implementation of both crop albedo enhancement and irrigation can reduce hot temperature extremes by more than 2°C in North America, Eurasia, and India over the 21st century relative to a scenario without management application. The efficacy of crop albedo enhancement scales with the magnitude, where a cooling response exceeding 0.5°C for hot temperature extremes was achieved with a large (i.e., ≥0.08) change in crop albedo. Regional differences were attributed to the surface energy balance response with temperature changes mostly explained by latent heat flux changes for irrigation and net shortwave radiation changes for crop albedo enhancement. However, limitations do exist, where we identify warming over the winter months when climate-effective land management is temporarily suspended. This was associated with persistent cloud cover that enhances longwave warming. It cannot be confirmed if the magnitude of this feedback is reproducible in other climate models. Our results overall demonstrate that regional warming of hot extremes in our climate model can be partially mitigated when using an idealized treatment of climate-effective land management
Can we use local climate zones for predicting malaria prevalence across sub-Saharan African cities?
Malaria burden is increasing in sub-Saharan cities because of rapid and uncontrolled urbanization. Yet very few studies have studied the interactions between urban environments and malaria. Additionally, no standardized urban land-use/land-cover has been defined for urban malaria studies. Here, we demonstrate the potential of local climate zones (LCZs) for modeling malaria prevalence rate (PfPR2-10) and studying malaria prevalence in urban settings across nine sub-Saharan African cities. Using a random forest classification algorithm over a set of 365 malaria surveys we: (i) identify a suitable set of covariates derived from open-source earth observations; and (ii) depict the best buffer size at which to aggregate them for modeling PfPR2-10. Our results demonstrate that geographical models can learn from LCZ over a set of cities and be transferred over a city of choice that has few or no malaria surveys. In particular, we find that urban areas systematically have lower PfPR2-10 (5%-30%) than rural areas (15%-40%). The PfPR2-10 urban-to-rural gradient is dependent on the climatic environment in which the city is located. Further, LCZs show that more open urban environments located close to wetlands have higher PfPR2-10. Informal settlements - represented by the LCZ 7 (lightweight lowrise) - have higher malaria prevalence than other densely built-up residential areas with a mean prevalence of 11.11%. Overall, we suggest the applicability of LCZs for more exploratory modeling in urban malaria studies. © 2020 The Author(s). Published by IOP Publishing Ltd.info:eu-repo/semantics/publishe
A semi-schematic model for the center of mass dynamics in supercooled molecular liquids
We introduce a semi-schematic mode-coupling model to describe the slow
dynamics in molecular liquids, retaining explicitly only the description of the
center of mass degrees of freedom. Angular degrees of freedom are condensed in
a q-vector independent coupling parameter. We compare the time and q-dependence
of the density fluctuation correlators with numerical data from a 250 ns long
molecular dynamics simulation. Notwithstanding the choice of a network-forming
liquid as a model for comparing theory and simulation, the model describes the
main static and dynamic features of the relaxation in a broad q-vector range.Comment: Revtex, 2 figure
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