123 research outputs found
Scanning rotational Raman lidar at 355 nm for the measurement of tropospheric temperature fields
International audienceFor high-resolution measurements of temperature fields in the atmospheric boundary layer and the lower free troposphere a scanning eye-safe lidar which deploys the rotational Raman technique at 355 nm was developed. To optimize the filters of the receiver for both high nighttime and daytime performance, detailed simulation studies have been performed. The receiver is fiber-coupled to a sequential setup of multicavity interference filters used under small angles of incidence. Examples of nighttime and daytime measurements with the system which has a total power-aperture-efficiency product of 0.006 W m2 are presented. Noontime temperature measurements with a temporal resolution of 60 s result in 1-sigma statistical temperature uncertainty of <1 K up to 1 km height and <2 K up to 2 km height. With an integration time of 60 min and a gliding average of 750 m a 1-sigma statistical temperature uncertainty of <1 K up to 14 km height is achieved during night
Validating the Water Vapor Variance Similarity Relationship in the Interfacial Layer Using Observations and Large-Eddy Simulations
In previous work, the similarity relationship for the water vapor variance in the interfacial layer (IL) at the top of the convective boundary layer (CBL) was proposed to be proportional to the convective velocity scale and the gradients of the water vapor mixing ratio and the BruntâVaisala frequency in the entrainment zone. In the presence of wind shear in the IL, the similarity relationship was hypothesized to also include a dependence on the gradient Richardson number. Simultaneous measurements of the surface buoyancy flux, windâshear profiles from a radar wind profiler, water vapor mixing ratio and temperature measurements and their gradients from a Raman lidar provide a unique opportunity to thoroughly examine the function used in defining the variance and validate it. These observations were made over the Atmospheric Radiation Measurement Southern Great Plains site. We identified 19 cases from 2016 during which the CBL was quasiâstationary and well mixed for at least 2 hr in the afternoon. Furthermore, we simulated the CBL using a largeâeddy simulation (LES) model for these cases and derived the water vapor variance and other profiles to test the similarity function. Utilizing this unique combination of observations and LES, we demonstrate that the water vapor variance in the IL has littleâtoâno dependence on wind shear. Furthermore, we demonstrate that the predicted variance using the original similarity function matches the observed and LESâmodeled variance very well, with linear correlations between the two variances of 0.82 and 0.95, respectively
Validating the Water Vapor Variance Similarity Relationship in the Interfacial Layer Using Observations and Large-Eddy Simulations
In previous work, the similarity relationship for the water vapor variance in the interfacial layer (IL) at the top of the convective boundary layer (CBL) was proposed to be proportional to the convective velocity scale and the gradients of the water vapor mixing ratio and the BruntâVaisala frequency in the entrainment zone. In the presence of wind shear in the IL, the similarity relationship was hypothesized to also include a dependence on the gradient Richardson number. Simultaneous measurements of the surface buoyancy flux, windâshear profiles from a radar wind profiler, water vapor mixing ratio and temperature measurements and their gradients from a Raman lidar provide a unique opportunity to thoroughly examine the function used in defining the variance and validate it. These observations were made over the Atmospheric Radiation Measurement Southern Great Plains site. We identified 19 cases from 2016 during which the CBL was quasiâstationary and well mixed for at least 2 hr in the afternoon. Furthermore, we simulated the CBL using a largeâeddy simulation (LES) model for these cases and derived the water vapor variance and other profiles to test the similarity function. Utilizing this unique combination of observations and LES, we demonstrate that the water vapor variance in the IL has littleâtoâno dependence on wind shear. Furthermore, we demonstrate that the predicted variance using the original similarity function matches the observed and LESâmodeled variance very well, with linear correlations between the two variances of 0.82 and 0.95, respectively
HESS Opinions "Should we apply bias correction to global and regional climate model data?"
Despite considerable progress in recent years, output of both global and regional circulation models is still afflicted with biases to a degree that precludes its direct use, especially in climate change impact studies. This is well known, and to overcome this problem, bias correction (BC; i.e. the correction of model output towards observations in a post-processing step) has now become a standard procedure in climate change impact studies. In this paper we argue that BC is currently often used in an invalid way: it is added to the GCM/RCM model chain without sufficient proof that the consistency of the latter (i.e. the agreement between model dynamics/model output and our judgement) as well as the generality of its applicability increases. BC methods often impair the advantages of circulation models by altering spatiotemporal field consistency, relations among variables and by violating conservation principles. Currently used BC methods largely neglect feedback mechanisms, and it is unclear whether they are time-invariant under climate change conditions. Applying BC increases agreement of climate model output with observations in hindcasts and hence narrows the uncertainty range of simulations and predictions without, however, providing a satisfactory physical justification. This is in most cases not transparent to the end user.We argue that this hides rather than reduces uncertainty, which may lead to avoidable forejudging of end users and decision makers. We present here a brief overview of state-of-the-art bias correction methods, discuss the related assumptions and implications, draw conclusions on the validity of bias correction and propose ways to cope with biased output of circulation models in the short term and how to reduce the bias in the long term. The most promising strategy for improved future global and regional circulation model simulations is the increase in model resolution to the convection-permitting scale in combination with ensemble predictions based on sophisticated approaches for ensemble perturbation. With this article, we advocate communicating the entire uncertainty range associated with climate change predictions openly and hope to stimulate a lively discussion on bias correction among the atmospheric and hydrological community and end users of climate change impact studies
Continuous high-resolution midlatitude-belt simulations for JulyâAugust 2013 with WRF
Increasing computational resources and the demands of impact
modelers, stake holders, and society envision seasonal and climate simulations
with the convection-permitting resolution. So far such a resolution is only
achieved with a limited-area model whose results are impacted by zonal and
meridional boundaries. Here, we present the setup of a latitude-belt domain
that reduces disturbances originating from the western and eastern boundaries
and therefore allows for studying the impact of model resolution and physical
parameterization. The Weather Research and Forecasting (WRF) model coupled to
the NOAH landâsurface model was operated during July and August 2013 at two
different horizontal resolutions, namely 0.03 (HIRES) and
0.12° (LOWRES). Both simulations were forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) operational
analysis data at the northern and southern domain boundaries, and the
high-resolution Operational Sea Surface Temperature and Sea Ice Analysis
(OSTIA) data at the sea surface.The simulations are compared to the operational ECMWF analysis for the
representation of large-scale features. To analyze the simulated
precipitation, the operational ECMWF forecast, the CPC MORPHing (CMORPH), and
the ENSEMBLES gridded observation precipitation data set (E-OBS) were used as
references.Analyzing pressure, geopotential height, wind, and temperature fields as well
as precipitation revealed (1)Â a benefit from the higher resolution concerning
the reduction of monthly biases, root mean square error, and an improved
Pearson skill score, and (2)Â deficiencies in the physical parameterizations leading
to notable biases in distinct regions like the polar Atlantic for the LOWRES
simulation, the North Pacific, and Inner Mongolia for both resolutions.In summary, the application of a latitude belt on a convection-permitting
resolution shows promising results that are beneficial for future seasonal
forecasting
Simulation of semi-arid biomass plantations and irrigation using the WRF-NOAH model – a comparison with observations from Israel
A 10 Ă 10 km irrigated biomass plantation was simulated in an arid
region of Israel to simulate diurnal energy balances during the summer of
2012 (JJA). The goal is to examine daytime horizontal flux gradients between
plantation and desert. Simulations were carried out within the coupled
WRF-NOAH atmosphere/land surface model. MODIS land surface data was adjusted
by prescribing tailored land surface and soil/plant parameters, and by
adding a controllable sub-surface irrigation scheme to NOAH. Two model cases
studies were compared â Impact and Control. Impact simulates the irrigated plantation.
Control simulates the existing land surface, where the predominant land surface is
bare desert soil. Central to the study is parameter validation against land
surface observations from a desert site and from a 400 ha Simmondsia chinensis (jojoba)
plantation. Control was validated with desert observations, and Impact with Jojoba
observations. Model evapotranspiration was validated with two
PenmanâMonteith estimates based on the observations.
Control simulates daytime desert conditions with a maximum deviation for surface 2 m air temperatures (T2)
of 0.2 °C, vapour pressure deficit (VPD) of 0.25 hPa, wind
speed (U) of 0.5 m sâ1, surface radiation (Rn) of 25 W mâ2,
soil heat flux (G) of 30 W mâ2 and 5 cm soil temperatures (ST5) of
1.5 °C. Impact simulates irrigated vegetation conditions with a maximum deviation for T2 of 1â1.5 °C,
VPD of 0.5 hPa, U of 0.5 m sâ1, Rn of 50 W m−5,
G of 40 W mâ2 and ST5 of 2 °C.
Latent heat curves in Impact correspond closely with PenmanâMonteith estimates,
and magnitudes of 160 W mâ2 over the plantation are usual. Sensible heat
fluxes, are around 450 W mâ2 and are at least 100â110 W mâ2
higher than the surrounding desert. This surplus is driven by reduced albedo
and high surface resistance, and demonstrates that high evaporation rates
may not occur over Jojoba if irrigation is optimized. Furthermore, increased
daytime T2 over plantations highlight the need for hourly as well as daily
mean statistics. Daily mean statistics alone may imply an overall cooling
effect due to surplus nocturnal cooling, when in fact a daytime warming
effect is observed
Gaming with eutrophication: Contribution to integrating water quantity and quality management at catchment level
The Metropolitan Region of Sao Paulo (MRSP) hosts 18 million inhabitants. A complex system of 23 interconnected reservoirs was built to ensure its water supply. Half of the potable water produced for MRSP's population (35 m3/s) is imported from a neighbour catchment, the other half is produced within the Alto TietĂȘ catchment, where 99% of the population lives. Perimeters of land use restriction were defined to contain uncontrolled urbanization, as domestic effluents were causing increasing eutrophication of some of these reservoirs. In the 90's catchment committees and sub committees were created to promote discussion between stakeholders and develop catchment plans. The committees are very well structured "on paper". However, they are not very well organised and face a lack of experience. The objective of this work was to design tools that would strengthen their discussion capacities. The specific objective of the AguAloca process was to integrate the quality issue and its relation to catchment management as a whole in these discussions. The work was developed in the Alto TietĂȘ Cabeceiras sub-catchment, one of the 5 sub catchments of the Alto-TietĂȘ. It contains 5 interconnected dams, and presents competitive uses such as water supply, industry, effluent dilution and irrigated agriculture. A RPG was designed following a companion modelling approach (Etienne et al., 2003). It contains a friendly game-board, a set of individual and collective rules and a computerized biophysical model. The biophysical model is used to simulate water allocation and quality processes at catchment level. It articulates 3 modules. A simplified nutrient discharge model permits the estimation of land use nutrient exportation. An arc-node model simulates water flows and associated nutrient charges from one point of the hydrographical network to another. The Vollenweider model is used for simulating specific reservoir dynamics. The RPG allows players to make individual and collective decisions related to water allocation and the management of its quality. Impacts of these decisions are then simulated using the biophysical model. Specific indicators of the game are then updated and may influence player's behaviour (actions) in following rounds. To introduce discussions on the management of water quality at a catchment level, an issue that is rarely explicitly dealt with, four game sessions were implemented involving representatives of basin committees and water and sanitation engineers. During the game session, the participants took advantage of the water quality output of the biophysical model to test management alternatives such as rural sewage collection or effluent dilution. The biophysical model accelerated calculations of flows and eutrophication rates that were then returned to the game board with explicit indicators of quantity and quality. Players could easily test decisions impacting on qualitative water processes and visualize the simulation results directly on the game board that was representing a friendly, virtual and simplified catchment. The Agualoca game proved its ability to turn complex water processes understandable for a non totally initiated public. This experience contributed to a better understanding of multiple-use water management and also of joint management of water quality and quantity. (RĂ©sumĂ© d'auteur
HESS Opinions âshould we apply bias correction to global and regional climate model data?â
Despite considerable progress in recent years, output of both global and regional circulation models is still afflicted with biases to a degree that precludes its direct use, especially in climate change impact studies. This is well known, and to overcome this problem, bias correction (BC; i.e. the correction of model output towards observations in a post-processing step) has now become a standard procedure in climate change impact studies. In this paper we argue that BC is currently often used in an invalid way: it is added to the GCM/RCM model chain without sufficient proof that the consistency of the latter (i.e. the agreement between model dynamics/model output and our judgement) as well as the generality of its applicability increases. BC methods often impair the advantages of circulation models by altering spatiotemporal field consistency, relations among variables and by violating conservation principles. Currently used BC methods largely neglect feedback mechanisms, and it is unclear whether they are time-invariant under climate change conditions. Applying BC increases agreement of climate model output with observations in hindcasts and hence narrows the uncertainty range of simulations and predictions without, however, providing a satisfactory physical justification. This is in most cases not transparent to the end user.We argue that this hides rather than reduces uncertainty, which may lead to avoidable forejudging of end users and decision makers. We present here a brief overview of state-of-the-art bias correction methods, discuss the related assumptions and implications, draw conclusions on the validity of bias correction and propose ways to cope with biased output of circulation models in the short term and how to reduce the bias in the long term. The most promising strategy for improved future global and regional circulation model simulations is the increase in model resolution to the convection-permitting scale in combination with ensemble predictions based on sophisticated approaches for ensemble perturbation. With this article, we advocate communicating the entire uncertainty range associated with climate change predictions openly and hope to stimulate a lively discussion on bias correction among the atmospheric and hydrological community and end users of climate change impact studies
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