scholarly journals Mount Kenya Global Atmosphere Watch Station (MKN): Installation and Meteorological Characterization

2008 ◽  
Vol 47 (11) ◽  
pp. 2946-2962 ◽  
Author(s):  
Stephan Henne ◽  
Wolfgang Junkermann ◽  
Josiah M. Kariuki ◽  
John Aseyo ◽  
Jörg Klausen
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Convective Boundary Layer ◽  
Atmospheric Composition ◽  
Specific Humidity ◽  
Convective Boundary ◽  
Thermally Induced ◽  
Air Masses ◽  
Downslope Wind

Abstract The meteorological conditions at the Mount Kenya (station identifier MKN) tropical Global Atmosphere Watch Programme station are described. Like other stations in mountainous terrain, the site experiences thermally induced wind systems that disturb free tropospheric conditions. Therefore, the adequacy of the site for long-term background atmospheric composition measurements needs to be evaluated. Meteorological parameters for the period June 2002–June 2006 were analyzed, focusing on the development of thermally induced wind systems and boundary layer influence. Filters based on the local wind and day–night differences in specific humidity were developed for selection of times representative of undisturbed free tropospheric conditions. In addition, the convective boundary layer depth was evaluated. Throughout the whole year the station is influenced by thermally induced wind systems and the atmospheric boundary layer. The filters distinguished between thermally and synoptically influenced days. Thermally influenced days (86%) dominated. However, maxima in specific humidity were also reached in the afternoon on synoptically influenced days and were attributed to mixing in the convective boundary layer. During nighttime, downslope wind dominated that carries undisturbed free tropospheric air masses. Nevertheless, during 24% of all nights the specific humidity was also elevated, possibly indicating the presence of residual layers. It is recommended that nighttime data only (2100–0400 UTC) be used for analysis of long-term trends of the free tropospheric background while the remaining data can be used to characterize composition and trends of the regional atmospheric boundary layer. Further exclusion of apparent pollution events and residual layer influence should be considered. With these constraints, the Mount Kenya Global Atmosphere Watch site is adequate for the study of trends and budgets of background atmospheric composition.

scholarly journals Finescale Clusterization Intermittency of Turbulence in the Atmospheric Boundary Layer

2020 ◽  
Vol 77 (7) ◽  
pp. 2375-2392
Author(s):  
Lei Liu ◽  
Fei Hu
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Return Time ◽  
Statistical Simulation ◽  
Convective Boundary ◽  
Wind Speeds ◽  
Stable Conditions ◽  
Telegraphic Approximation ◽  
Better Than

AbstractThe intermittency of atmospheric turbulence plays an important role in the understanding of particle dispersal in the atmospheric boundary layer and in the statistical simulation of high-frequency wind speed in various applications. There are two kinds of intermittency, namely, the magnitude intermittency (MI) related to non-Gaussianity and the less studied clusterization intermittency (CI) related to long-term correlation. In this paper, we use a 20 Hz ultrasonic dataset lasting for 1 month to study CI of turbulent velocity fluctuations at different scales. Basing on the analysis of return-time distribution of telegraphic approximation series, we propose to use the shape parameter of the Weibull distribution to measure CI. Observations of this parameter show that contrary to MI, CI tends to weaken as the scale increases. Besides, significant diurnal variations, showing that CI tends to strengthen during the daytime (under unstable conditions) and weaken during the nighttime (under stable conditions), are found at different observation heights. In the convective boundary layer, the mixed-layer similarity is found to scale the CI exponent better than the Monin–Obukhov similarity. At night, CI is found to vary less with height in the regime with large mean wind speeds than in the regime with small mean wind speeds, according to the hockey-stick theory.

scholarly journals Convective boundary layer characterization in the Amazon rainforest before and after the passage of Mesoscale convective systems

2020 ◽  
Vol 42 ◽  
pp. e2
Author(s):  
Vanessa Monteiro
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Growth Rates ◽  
Mesoscale Convective Systems ◽  
Amazon Rainforest ◽  
Specific Humidity ◽  
Convective Boundary ◽  
Data Set ◽  
Convective Systems ◽  
Mesoscale Convective

This study describes thermodynamic variables (e.g., temperature and humidity) of the atmospheric convective boundary layer (CBL) and its growth rates preceding and following the passage of mesoscale convective systems (MCSs) in the Amazon rainforest. Using the data set provided by GoAmazon 2014/15, this study will address its objectives through the evaluation of case studies and an ensemble of days when there was the passage of MCSs. The results show that the convective boundary layer experiences reductions in the equivalent potential temperature within 2 to 8K and in the specific humidity up to 2 g kg-1 after the passage of a MCS, due to the cold and dry air brought to the surface by storms downdrafts. These two variables in addition to others (e.g., energy fluxes) are responsible for the low growth rates of the convective boundary layer, that were reduced by 100 m h-1 in the following two hours after the rainfall ceases, when compared to undisturbed conditions. Nonetheless, this work provides a quantitative evaluation of the thermodynamic features of the convective boundary layer under the passage of mesoscale convective systems in the Amazon rainforest.

scholarly journals The Turbulent Structure and Diurnal Growth of the Saharan Atmospheric Boundary Layer

2015 ◽  
Vol 72 (2) ◽  
pp. 693-713 ◽  
Author(s):  
L. Garcia-Carreras ◽  
D. J. Parker ◽  
J. H. Marsham ◽  
P. D. Rosenberg ◽  
I. M. Brooks ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Convective Boundary Layer ◽  
Large Scale ◽  
Temperature Inversion ◽  
Residual Layer ◽  
Turbulent Structure ◽  
Radiosonde Data ◽  
Convective Boundary ◽  
Boundary Layer Processes

Abstract The turbulent structure and growth of the remote Saharan atmospheric boundary layer (ABL) is described with in situ radiosonde and aircraft measurements and a large-eddy simulation model. A month of radiosonde data from June 2011 provides a mean profile of the midday Saharan ABL, which is characterized by a well-mixed convective boundary layer, capped by a small temperature inversion (<1 K) and a deep, near-neutral residual layer. The boundary layer depth varies by up to 100% over horizontal distances of a few kilometers due to turbulent processes alone. The distinctive vertical structure also leads to unique boundary layer processes, such as detrainment of the warmest plumes across the weak temperature inversion, which slows down the warming and growth of the convective boundary layer. As the boundary layer grows, overshooting plumes can also entrain free-tropospheric air into the residual layer, forming a second entrainment zone that acts to maintain the inversion above the convective boundary layer, thus slowing down boundary layer growth further. A single-column model is unable to accurately reproduce the evolution of the Saharan boundary layer, highlighting the difficulty of representing such processes in large-scale models. These boundary layer processes are special to the Sahara, and possibly hot, dry, desert environments in general, and have implications for the large-scale structure of the Saharan heat low. The growth of the boundary layer influences the vertical redistribution of moisture and dust, and the spatial coverage and duration of clouds, with large-scale dynamical and radiative implications.

scholarly journals Long-Term Observations of the Convective Boundary Layer Using Insect Radar Returns at the SGP ARM Climate Research Facility

2010 ◽  
Vol 23 (21) ◽  
pp. 5699-5714 ◽  
Author(s):  
Arunchandra S. Chandra ◽  
Pavlos Kollias ◽  
Scott E. Giangrande ◽  
Stephen A. Klein
Keyword(s):  
Boundary Layer ◽  
Vertical Velocity ◽  
Convective Boundary Layer ◽  
Turbulent Transport ◽  
Doppler Velocity ◽  
Research Facility ◽  
Convective Boundary ◽  
Climate Research ◽  
Turbulent Statistics

Abstract A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004–08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes–thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.

scholarly journals ESTIMATIVA DA PARTIÇÃO DE ENERGIA NA SUPERFÍCIE

2016 ◽  
Vol 38 ◽  
pp. 412
Author(s):  
Daiane De Vargas Brondani ◽  
Otávio Costa Acevedo ◽  
Fabíola Valente
Keyword(s):  
Boundary Layer ◽  
Latent Heat ◽  
Convective Boundary Layer ◽  
Method Development ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Specific Humidity ◽  
Ratio Method ◽  
Convective Boundary ◽  
Monthly Scale

This paper is a complementary analysis to method development, which proposes to estimate the energy partition on the surface by Bowen ratio method and height of convective boundary layer on the monthly scale based on the average temporal evolution of the variables air temperature and specific humidity. The basic hypothesis is that the evolution of these quantities is controlled solely by the convergence of surface fluxes of sensible and latent heat. This assumption is valid for monthly scale and in regions of middle latitudes away from the coast. Thus, it is assumed that the advective terms of the balance equation of these quantities in the convective boundary layer, the prefrontal situations and post-frontal have opposite sign. Therefore, using for a longer time scale than the typical scale of the passage of synoptic systems, the cancellation terms of hypothesis can be tested. In this study, the method is applied to the region of Santa Maria, where it is assumed that the conditions allowing despise the advective terms in monthly scale are valid. When testing the method on different time scales: 5,10,15,20 and 30 days, smaller errors temperature and specific humidity were for 15 and 30 days, while the sensible heat and latent heat fluxes showed lower relative errors at 20 and 30 days, respectively.

scholarly journals A Combined Local and Nonlocal Closure Model for the Atmospheric Boundary Layer. Part I: Model Description and Testing

2007 ◽  
Vol 46 (9) ◽  
pp. 1383-1395 ◽  
Author(s):  
Jonathan E. Pleim
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Convective Boundary Layer ◽  
Large Scale ◽  
Eddy Diffusion ◽  
Small Scale ◽  
Convective Conditions ◽  
Convective Boundary ◽  
Scale Turbulence ◽  
Convective Model

Abstract The modeling of the atmospheric boundary layer during convective conditions has long been a major source of uncertainty in the numerical modeling of meteorological conditions and air quality. Much of the difficulty stems from the large range of turbulent scales that are effective in the convective boundary layer (CBL). Both small-scale turbulence that is subgrid in most mesoscale grid models and large-scale turbulence extending to the depth of the CBL are important for the vertical transport of atmospheric properties and chemical species. Eddy diffusion schemes assume that all of the turbulence is subgrid and therefore cannot realistically simulate convective conditions. Simple nonlocal closure PBL models, such as the Blackadar convective model that has been a mainstay PBL option in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) for many years and the original asymmetric convective model (ACM), also an option in MM5, represent large-scale transport driven by convective plumes but neglect small-scale, subgrid turbulent mixing. A new version of the ACM (ACM2) has been developed that includes the nonlocal scheme of the original ACM combined with an eddy diffusion scheme. Thus, the ACM2 is able to represent both the supergrid- and subgrid-scale components of turbulent transport in the convective boundary layer. Testing the ACM2 in one-dimensional form and comparing it with large-eddy simulations and field data from the 1999 Cooperative Atmosphere–Surface Exchange Study demonstrates that the new scheme accurately simulates PBL heights, profiles of fluxes and mean quantities, and surface-level values. The ACM2 performs equally well for both meteorological parameters (e.g., potential temperature, moisture variables, and winds) and trace chemical concentrations, which is an advantage over eddy diffusion models that include a nonlocal term in the form of a gradient adjustment.

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