scholarly journals Venting of Heat and Carbon Dioxide from Urban Canyons at Night

2005 ◽  
Vol 44 (8) ◽  
pp. 1180-1194 ◽  
Author(s):  
J. A. Salmond ◽  
T. R. Oke ◽  
C. S. B. Grimmond ◽  
S. Roberts ◽  
B. Offerle
Keyword(s):  
Carbon Dioxide ◽  
Boundary Layer ◽  
Time Series ◽  
Wavelet Analysis ◽  
Air Temperature ◽  
Urban Boundary Layer ◽  
Heat Fluxes ◽  
Sensible Heat ◽  
Canopy Layer ◽  
Street Canyons

Abstract Turbulent fluxes of carbon dioxide and sensible heat were observed in the surface layer of the weakly convective nocturnal boundary layer over the center of the city of Marseille, France, during the Expérience sur Sites pour Contraindre les Modèles de Pollution Atmosphérique et de Transport d’Emission (ESCOMPTE) field experiment in the summer of 2001. The data reveal intermittent events or bursts in the time series of carbon dioxide (CO2) concentration and air temperature that are superimposed upon the background values. These features relate to intermittent structures in the fluxes of CO2 and sensible heat. In Marseille, CO2 is primarily emitted into the atmosphere at street level from vehicle exhausts. In a similar way, nocturnal sensible heat fluxes are most likely to originate in the deep street canyons that are warmer than adjacent roof surfaces. Wavelet analysis is used to examine the hypothesis that CO2 concentrations can be used as a tracer to identify characteristics of the venting of pollutants and heat from street canyons into the above-roof nocturnal urban boundary layer. Wavelet analysis is shown to be effective in the identification and analysis of significant events and coherent structures within the turbulent time series. Late in the evening, there is a strong correlation between the burst structures observed in the air temperature and CO2 time series. Evidence suggests that the localized increases of temperature and CO2 observed above roof level in the urban boundary layer (UBL) are related to intermittent venting of sensible heat from the warmer urban canopy layer (UCL). However, later in the night, local advection of CO2 in the UBL, combined with reduced traffic emissions in the UCL, limit the value of CO2 as a tracer of convective plumes in the UBL.

scholarly journals Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer

2014 ◽  
Vol 11 (16) ◽  
pp. 4507-4519 ◽  
Author(s):  
T. S. El-Madany ◽  
H. F. Duarte ◽  
D. J. Durden ◽  
B. Paas ◽  
M. J. Deventer ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Wind Speed ◽  
Eddy Covariance ◽  
Latent Heat ◽  
Heat Fluxes ◽  
Vertical Wind ◽  
Sensible Heat ◽  
Intermittent Turbulence ◽  
Low Level Jets ◽  
Vertical Wind Velocity

Abstract. Sodar (SOund Detection And Ranging), eddy-covariance, and tower profile measurements of wind speed and carbon dioxide were performed during 17 consecutive nights in complex terrain in northern Taiwan. The scope of the study was to identify the causes for intermittent turbulence events and to analyze their importance in nocturnal atmosphere–biosphere exchange as quantified with eddy-covariance measurements. If intermittency occurs frequently at a measurement site, then this process needs to be quantified in order to achieve reliable values for ecosystem characteristics such as net ecosystem exchange or net primary production. Fourteen events of intermittent turbulence were identified and classified into above-canopy drainage flows (ACDFs) and low-level jets (LLJs) according to the height of the wind speed maximum. Intermittent turbulence periods lasted between 30 and 110 min. Towards the end of LLJ or ACDF events, positive vertical wind velocities and, in some cases, upslope flows occurred, counteracting the general flow regime at nighttime. The observations suggest that the LLJs and ACDFs penetrate deep into the cold air pool in the valley, where they experience strong buoyancy due to density differences, resulting in either upslope flows or upward vertical winds. Turbulence was found to be stronger and better developed during LLJs and ACDFs, with eddy-covariance data presenting higher quality. This was particularly indicated by spectral analysis of the vertical wind velocity and the steady-state test for the time series of the vertical wind velocity in combination with the horizontal wind component, the temperature, and carbon dioxide. Significantly higher fluxes of sensible heat, latent heat, and shear stress occurred during these periods. During LLJs and ACDFs, fluxes of sensible heat, latent heat, and CO2 were mostly one-directional. For example, exclusively negative sensible heat fluxes occurred while intermittent turbulence was present. Latent heat fluxes were mostly positive during LLJs and ACDFs, with a median value of 34 W m−2, while outside these periods the median was 2 W m−2. In conclusion, intermittent turbulence periods exhibit a strong impact on nocturnal energy and mass fluxes.

scholarly journals Importance of Using Observations of Mixing Depths in order to Avoid Large Prediction Errors by a Transport and Dispersion Model

2009 ◽  
Vol 26 (1) ◽  
pp. 22-32 ◽  
Author(s):  
J. M. White ◽  
J. F. Bowers ◽  
S. R. Hanna ◽  
J. K. Lundquist
Keyword(s):  
Boundary Layer ◽  
Atmospheric Transport ◽  
Similarity Theory ◽  
Dispersion Model ◽  
Heat Fluxes ◽  
Synoptic Situation ◽  
Prediction Errors ◽  
Sensible Heat ◽  
Mixing Depth ◽  
Near Surface

Abstract The mixing depth of the boundary layer is an input to most atmospheric transport and dispersion (ATD) models, which obtain mixing depths in one of four ways: 1) observations by radiosondes, sodars, or other devices; 2) simulations by regional or mesoscale meteorological models; 3) parameterizations based on boundary layer similarity theory; or 4) climatological averages. This paper describes a situation during a field experiment when exceptionally low mixing depths persisted in the morning and led to relatively high observed tracer concentrations. The low mixing depths were caused by synoptic effects associated with a nearby stationary front and the outflow from a mesoscale thunderstorm complex located 20–50 km away. For the same time period, the ATD model-parameterized mixing depth was a factor of 5–10 higher, leading to predicted concentrations that were less than the observations by a factor of 5–10. The synoptic situation is described and local radiosonde and radar observations of mixing depth are presented, including comparisons with other more typical days. Time series of local observations of near-surface sensible heat fluxes are also plotted to demonstrate the suppression of turbulence by negative sensible heat fluxes during the period in question.

The role of soil characteristics on measured and modelled carbon dioxide and energy fluxes for Arctic dwarf shrub tundra sites

2020 ◽  
Author(s):  
Gesa Meyer ◽  
Elyn Humphreys ◽  
Joe Melton ◽  
Peter Lafleur ◽  
Philip Marsh ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Growing Season ◽  
Cold Season ◽  
Soil Characteristics ◽  
Heat Fluxes ◽  
Dwarf Shrub ◽  
The Arctic ◽  
Sensible Heat ◽  
Shrub Tundra ◽  
Tundra Ecosystems

<p>Four years of growing season eddy covariance measurements of net carbon dioxide (CO<sub>2</sub>) and energy fluxes were used to examine the similarities/differences in surface-atmosphere interactions at two dwarf shrub tundra sites within Canada’s Southern Arctic ecozone, separated by approximately 1000 km. Both sites, Trail Valley Creek (TVC) and Daring Lake (DL1), are characterised by similar climate (with some differences in radiation due to latitudinal differences), vegetation composition and structure, and are underlain by continuous permafrost, but differ in their soil characteristics. Total atmospheric heating (the sum of latent and sensible heat fluxes) was similar at the two sites. However, at DL1, where the surface organic layer was thinner and mineral soil coarser in texture, latent heat fluxes were greater, sensible heat fluxes were lower, soils were warmer and the active layer thicker. At TVC, cooler soils likely kept ecosystem respiration relatively low despite similar total growing season productivity. As a result, the 4-year mean net growing season ecosystem CO<sub>2 </sub>uptake (May 1 - September 30) was almost twice as large at TVC (64 ± 19 g C m<sup>-2</sup>) compared to DL1 (33 ± 11 g C m<sup>-2</sup>). These results highlight that soil and thaw characteristics are important to understand variability in surface-atmosphere interactions among tundra ecosystems.</p><p>As recent studies have shown, winter fluxes play an important role in the annual CO<sub>2</sub> balance of Arctic tundra ecosystems. However, flux measurements were not available at TVC and DL1 during the cold season. Thus, the process-based ecosystem model CLASSIC (the Canadian Land Surface Scheme including biogeochemical Cycles, formerly CLASS-CTEM) was used to simulate year-round fluxes. In order to represent the Arctic shrub tundra better, shrub and sedge plant functional types were included in CLASSIC and results were evaluated using measurements at DL1. Preliminary results indicate that cold season CO<sub>2</sub> losses are substantial and may exceed the growing season CO<sub>2</sub> uptake at DL1 during 2010-2017. The joint use of observations and models is valuable in order to better constrain the Arctic CO<sub>2</sub> balance.  </p>

scholarly journals Carbon dioxide, water vapor and sensible heat fluxes over a tallgrass prairie

1989 ◽  
Vol 46 (1-2) ◽  
pp. 53-67 ◽  
Author(s):  
Shashi B. Verma ◽  
Joon Kim ◽  
Robert J. Clement
Keyword(s):  
Carbon Dioxide ◽  
Water Vapor ◽  
Tallgrass Prairie ◽  
Heat Fluxes ◽  
Sensible Heat

scholarly journals Diurnal Course of Carbon Dioxide Mixing Ratios in the Urban Boundary Layer in Response to Surface Emissions

2016 ◽  
Vol 55 (3) ◽  
pp. 507-529 ◽  
Author(s):  
B. Crawford ◽  
A. Christen ◽  
I. McKendry
Keyword(s):  
Carbon Dioxide ◽  
Boundary Layer ◽  
Eddy Covariance ◽  
Local Scale ◽  
Urban Boundary Layer ◽  
Model Calculations ◽  
Horizontal Advection ◽  
Mixing Ratios ◽  
Diurnal Course ◽  
Entrainment Fluxes

AbstractObservations of carbon dioxide (CO2) mixing ratios in the urban boundary layer (UBL) are rare, even though there is potential for such measurements to be used to monitor city-scale net CO2 emissions. This work presents a unique dataset of CO2 mixing ratios observed in the UBL above Vancouver, British Columbia, Canada, by means of a tethered balloon system over a continuous 24-h summertime period. Vertical profiles of CO2 mixing ratios are found to vary according to UBL thermal structure and mechanical dynamics (development of convective and nocturnal boundary layers, vertical mixing from mechanical turbulence, horizontal advection from land–sea thermal breezes, and vertical entrainment). A box model is applied to quantify net city-scale surface emissions to the UBL volume using the measured rate of change of UBL CO2 mixing ratios and estimated CO2 advection and entrainment fluxes. The diurnal course of city-scale net emissions predicted by the model is similar to simultaneous local-scale eddy-covariance CO2 flux measurements, although there are relatively large uncertainties in hourly model calculations of horizontal advection and vertical entrainment fluxes due to inputs of regional background CO2 mixing ratios. Daily city-scale emissions totals predicted by the model (20.2 gC m−2 day−1) are 35% larger than those measured simultaneously on an urban local-scale eddy-covariance flux tower and are within 32% of a spatially scaled municipal greenhouse gas inventory. However, these methods are not expected to agree exactly because they represent different spatial source areas and include different CO2 source and sink processes.

scholarly journals Footprint issues in scintillometry over heterogeneous landscapes

2009 ◽  
Vol 6 (2) ◽  
pp. 2099-2127 ◽  
Author(s):  
W. J. Timmermans ◽  
Z. Su ◽  
A. Olioso
Keyword(s):  
Heat Flux ◽  
Time Series ◽  
Sensible Heat Flux ◽  
Simulated Data ◽  
Heat Fluxes ◽  
Two Dimensional ◽  
Sensible Heat ◽  
Heterogeneous Landscapes ◽  
Heterogeneous Landscape ◽  
Potential Tool

Abstract. Scintillometry is widely recognized as a potential tool for obtaining spatially aggregated sensible heat fluxes. Although many investigations have been made over contrasting component surfaces, few aggregation schemes consider footprint contributions. In this paper an approach is presented to infer average sensible heat flux over a very heterogeneous landscape by using a large aperture scintillometer. The methodology is demonstrated on simulated data and tested on a time series of measurements obtained during the SPARC2004 experiment in Barrax, Spain. Results show that the two-dimensional footprint approach yields more accurate results of aggregated sensible heat flux than traditional methods.

scholarly journals Sensible Heat Fluxes in the Nearly Neutral Boundary Layer: The Impact of Frictional Heating within the Surface Layer

2019 ◽  
Vol 76 (4) ◽  
pp. 1039-1053
Author(s):  
J. M. Edwards
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Frictional Heating ◽  
Sensible Heat Flux ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Sensible Heat ◽  
Simple Modification ◽  
Neutral Boundary Layer ◽  
The Impact

Abstract The effect of frictional dissipative heating on the calculation of surface fluxes in the atmospheric boundary layer using bulk flux formulas is considered. Although the importance of frictional dissipation in intense storms has been widely recognized, it is suggested here that its impact is also to be seen at more moderate wind speeds in apparently enhanced heat transfer coefficients and countergradient fluxes in nearly neutral conditions. A simple modification to the bulk flux formula can be made to account for its impact within the surface layer. This modification is consistent with an interpretation of the surface layer as one across which the flux of total energy is constant. The effect of this modification on tropical cyclones is assessed in an idealized model, where it is shown to reduce the predicted maximum wind speed by about 4%. In numerical simulations of three individual storms, the impacts are more subtle but indicate a reduction of the sensible heat flux into the storm and a cooling of the surface layer.

scholarly journals Footprint issues in scintillometry over heterogeneous landscapes

2009 ◽  
Vol 13 (11) ◽  
pp. 2179-2190 ◽  
Author(s):  
W. J. Timmermans ◽  
Z. Su ◽  
A. Olioso
Keyword(s):  
Heat Flux ◽  
Time Series ◽  
Sensible Heat Flux ◽  
Simulated Data ◽  
Heat Fluxes ◽  
Two Dimensional ◽  
Sensible Heat ◽  
Heterogeneous Landscapes ◽  
Heterogeneous Landscape ◽  
Potential Tool

Abstract. Scintillometry is widely recognized as a potential tool for obtaining spatially aggregated sensible heat fluxes at regional scales. Although many investigations have been made over contrasting component surfaces, few aggregation schemes consider footprint contributions. In this paper, an approach is presented to infer average sensible heat flux over a very heterogeneous landscape by using a large aperture scintillometer. The methodology is demonstrated on simulated data and tested on a time series of measurements obtained during the SPARC2004 experiment in Barrax, Spain. Results show that the two-dimensional footprint approach yields more accurate results of aggregated sensible heat flux than traditional methods.

scholarly journals Neural Network Retrieval of Ocean Surface Parameters from SSM/I Data

2007 ◽  
Vol 135 (2) ◽  
pp. 586-597 ◽  
Author(s):  
Lei Meng ◽  
Yijun He ◽  
Jinnian Chen ◽  
Yumei Wu
Keyword(s):  
Neural Network ◽  
Heat Flux ◽  
Air Temperature ◽  
Surface Air Temperature ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Sensible Heat ◽  
The Neural Network ◽  
Global Oceans ◽  
Rms Errors

Abstract A new algorithm based on the multiparameter neural network is proposed to retrieve wind speed (WS), sea surface temperature (SST), sea surface air temperature, and relative humidity (RH) simultaneously over the global oceans from Special Sensor Microwave Imager (SSM/I) observations. The retrieved geophysical parameters are used to estimate the surface latent heat flux and sensible heat flux using a bulk method over the global oceans. The neural network is trained and validated with the matchups of SSM/I overpasses and National Data Buoy Center buoys under both clear and cloudy weather conditions. In addition, the data acquired by the 85.5-GHz channels of SSM/I are used as the input variables of the neural network to improve its performance. The root-mean-square (rms) errors between the estimated WS, SST, sea surface air temperature, and RH from SSM/I observations and the buoy measurements are 1.48 m s−1, 1.54°C, 1.47°C, and 7.85, respectively. The rms errors between the estimated latent and sensible heat fluxes from SSM/I observations and the Xisha Island (in the South China Sea) measurements are 3.21 and 30.54 W m−2, whereas those between the SSM/I estimates and the buoy data are 4.9 and 37.85 W m−2, respectively. Both of these errors (those for WS, SST, and sea surface air temperature, in particular) are smaller than those by previous retrieval algorithms of SSM/I observations over the global oceans. Unlike previous methods, the present algorithm is capable of producing near-real-time estimates of surface latent and sensible heat fluxes for the global oceans from SSM/I data.

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