The atmospheric boundary layer – parameterizations, observations and applications

Bulgarian Journal of Meteorology and HydrologyBulgarianAcademyof Sciences

National Instituteof Meteorologyand Hydrology

Bul. J. Meteo & Hydro 16/1 (2011) 41-53

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The atmospheric boundary layer – parameterizations, observations and applications

Ekaterina Batchvarova*, Maria Kolarova, Blagorodka Veleva, Neyko Neykov, Plamen Neitchev, Plamen Videnov, Alexander Gamanov, Damyan Barantiev

National Institute of Meteorology and Hydrology-BAS, 1784 Sofia, Bulgaria

(received: December, 2010; accepted: December, 2010)

Abstract. The theoretical and experimental studies of the Atmospheric Boundary Layer (ABL) are traditional topic of research at the National Institute of Meteorology and Hydrology (NIMH). Moreover, it can be stated that the Bulgarian School of boundary-layer meteorology is worldwide recognized. The traditions in ABL meteorology were set by Akad. L. Krastanov in the 1960-ies and later by Akad. S. Panchev, Corr. Member D. Yourdanov, Corr. Member V. Andreev, Prof. D. Syrakov, Prof. E. Donev and Prof. E. Syrakov in the 1970–1980-ies. During these years, fruitful collaboration has been carried out with the Russian and German boundary-layer meteorology schools. In the 1990-ies a wider international collaboration has started through international projects under European, NATO and bilateral funding. The Department of Atmosphere and Hydrosphere Composition at NIMH has been very successful in the field, involving also wider group of scientists from NIMH, NIGGG (The National Institute of Geophysics, Geography and Geodesy), the Institute of electronics and Cathedra Meteorology at The Faculty of Physics in Sofia University. The ABL height (ABLH) studies have been a key issue in the air pollution modeling since 1970-ies, firstly as parameter provided by the meteorological preprocessors to the dispersion models. Lately, the ABLH is internally calculated in the complex systems of “meteorological drivers/emissions models/chemistry transport and transformation models”, such as MM5(WRF)/ SMOKE/CMAQ, WRF-CHEM and others. The ABL height; turbulent fluxes of momentum, heat, moisture and other substances from/to the surface; dispersion parameters; vertical profiles; etc are all examples of key parameters in models that are not yet sufficiently well described over the immense variety of surfaces existing on earth, thus constituting large part of the uncertainties in forecast, mesoscale and climate models at all scales. The complexity of physical and chemical processes taking place in the ABL requires further extended observation programs based on modern technology in order to achieve advances in meteorological, air pollution and climate modeling. Such advances are required for improved assessments of renewable energy potential; impacts of air pollution and climate on human health, economy and biodiversity; effectiveness of any mitigation

* corresponding author: [email protected]

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actions carried out by governments; and many other applications. The ABL is a thin air of the atmosphere and a small part of the models, but it is essential for all applications of meteorological sciences to social life, economy and environment. Therefore, the need of ABL observations and theoretical studies is recognized by WMO and action is taken worldwide. Especially in US and Europe comprehensive observation programs are funded both by governments and industry. At NIMH, the theoretical work has been carried out mainly based on international experimental data. In parallel, continuous effort has been going on to enhance and modernize the monitoring programs and the equipment for field studies through research projects.

Keywords: atmospheric boundary layer (ABL), ABL height, turbulence, energy exchange between surface and atmosphere, vertical profiles, radio soundings, SODAR, eddy correlation technique, field meteorological and air pollution experiments

1. INTRODUCTION

This paper contains information on the development and results of ABL studies carried out at NIMH for several decades. The studies can be grouped thematically according the modeled parameters and applications.

The studies on the ABL height (Yordanov and Batchvarova, 1988; Batchvarova and Gryning, 1989, 1991 and 1994; Gryning and Batchvarova 1990 and 1994; Valkov et al, 2006, Yordanov et al, 2003 and 2004, Kolarova et al, 2006 and 2007) and entrainment zone (Batchvarova and Gryning, 1994 and Gryning Batchvarova, 1990) were used in the development of meteorological preprocessors in Bulgaria, in many European countries and the US (Calmet-Calpuff system, Scire et al, 2007).

The studies on the vertical profiles of the meteorological parameters and the ABLH (Gryning et al, 2007a and b, Yordanov et al, 2003 and 2004) were used in air pollution and wind power assessments.

The investigations of the relation between air pollution and boundary layer parameters (Teneva and Batchvarova, 1989, Veleva et al, 2010, Neykov et al, 2010, Valkov et al, 2006) were used in or started setting new methods for use in environmental quality assessments.

The turbulence parameterizations, including dispersion parameters, local and aggregated fluxes of momentum and heat, solar radiation, etc (Batchvarova et al, 2001, Gryning et al, 2001, Gryning and Batchvarova, 2001, Beyrich et al, 2002) were essential in the improvement of corresponding parameterizations in mesoscale models.

The research on urban boundary layer (Batchvarova and Gryning, 2007; Rotach et al, 2004, 2006) was a part of international collaboration towards understanding the atmospheric processes within the built environment (COST Action 715a and b).

The comparisons of mesoscale models results with applied models and observations (Batchvarova and Gryning, 2010, Gryning and Batchvarova, 2002a, 2002b and 2003; Batchvarova et al, 1999, 2007, 2010; Pisoni et al, 2008) are contributing towards improved parameterizations and higher reliability of models.

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2. HISTORY OF THE ABL STUDIES AT NIMH

2.1. Urban climatology, surface layer - temporal and horizontal variability

Bulgarian climatologists and meteorologists were analyzing periodically the climate of cities, and especially the climate of Sofia. Hristov and Tanev (1970) have presented a thorough historical survey and rather detailed analysis of the climate of Sofia. Based on data from 5 climatological stations in the period 1952 – 1963, they have shown clearly a reduction of the wind speed up to 50 % comparing peripheral to central meteorological stations, and twice more days with fogs in the center, compared to the outskirts of the city.

Further on, a number of analytical papers and monographies have been published, such as “Climate and Microclimate of Sofia” by Blaskova et al, 1983, where the influence of the city on a number of climatological parameters has been thoroughly discussed. Comparing data from urban and rural stations (13 stations in total) for the period 1954-1964, a clear difference has been found for sun shine duration, temperature, wind speed, calm weather and other parameters.

Later, in an interdisciplinary study on the ecology of Sofia, the climate of the city was discussed (Andreev, 2005 and Andreev et al, 2005). The analysis has shown increased number of the days with maximal temperature above 30 degrees C after 1975 and increase of the annual average minimal temperature both in urban and rural stations, noting that the difference urban – rural still exists.

2.2. Experimental studies, surface layer – horizontal variability

In order to investigate the temperature and flow field over the city in greater detail, field experiments were organized in Sofia in 1976 (Andreev, 1980) and in 1992-1993 (Batchvarova et al, 1996; Andreev et al, 2005). Such studies revealed situations in which the difference in temperature between center and suburbs during a winter night was 6.3 degrees C. In other situations, there was no difference or even reversed one. The field experiments are still important research tool in meteorology, but the new technologies (automatic measurements and easy data transmission) define a tendency to keep such observation setups for longer periods.

Unfortunately, in the last 15 years many climatological stations were closed in Sofia and are not yet substituted with automatic measurements. In parallel, the city size, structure, green areas, density of built up areas, population, etc have changed significantly. So, new information and studies are needed now in order to find ways for improvement of the rapidly deteriorated urban environment. Moreover, the new field experiments or 3-5 years long term campaigns should include 3D observation programs, as the urban boundary layer is characterized by complex structure both in horizontal and vertical direction (Batchvarova and Gryning, 2006 and 2009, Batchvarova et al, 2010).

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2.3. Vertical structure of the ABL over Sofia

The vertical structure of the temperature field in Sofia has been studied throughout the years in different ways. Until 2001 the operational radiosoundings performed at NIMH were with very course resolution and not very suitable for boundary layer studies, although covering morning, noon and midnight hours.

The climatic characteristics of the temperature inversions in Sofia valley were obtained by Bluskova et al. (1968) based on 10-years observations at 2 and 40m above the earth surface (40 m is the top level of the meteorological tower of NIMH) and on aerological observations in height. Within the year, the ground based inversions up to 100 m were with greatest frequency of around 45% of all inversions. The average duration of the inversions reaching 40m of height was found to be around 19-20 hours in winter and 10-11 hours in summer. Inversions in the ground-to-40m layer in winter lasting over 6 hours were present in 80% of all days. Inversions in the layer up to 40m during all months of the year were formed at Sun altitude of 100 till sunset. The destruction of the inversions in this layer occurred usually when the Sun rose above 150. Inversions have been observed at all types of the surface pressure field, but most frequently at anticyclonic fields (about 60% of all days). At cyclonic conditions inversions were present in around 30%, and at transitional pressure fields - in around 10% of all days.

The study of Teneva et al. (1989) has reported some supplementary characteristics of the temperature inversions in Sofia, separately for each of the time of three radiosoundings at 02, 08 and 14 hrs local time for the 1970-1979 period.

While for the study of 1968 the location of NIMH could be considered as rural, for the second study it was already a suburb. After the year 2000, the description of the station changed to urban.

In 2001, a Vaisala make aerological equipment was installed and the vertical resolution increased significantly reaching the order of 15-20 meters. Due to limited budget though, only one sounding per day is performed during the last 10 years. Thus, higher resolution in vertical direction was achieved, but the resolution in time decrease comparing to the 1970-ies when 3 sounding per day were operationally performed. This made the climatological studies on the vertical profiles only possible for early afternoon hours.

Studies on the data after 2001 were performed by Valkov et al, 2006, Batchvarova et al, 2007, Kolarova et al, 2006 and 2007. The connection between ABL parameters and air pollution was studied by Neykov et al, 2010 and Veleva et al, 2010.

2.4. Experimental studies, vertical structure of the ABL

The vertical structure of the urban ABL is important in the understanding of the meteorological processes over big cities, the dispersion of pollutants and the microclimate. Studying the structure and the mechanisms for destruction of the radiative

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temperature inversions over Sofia is a part of this knowledge. “Experimental study of the structure and dynamics of temperature inversions and the convective boundary layer for the Sofia region” (Branzov et al., 1992) was organized in the period 1988-1991. The measurements were perforemed with a tethered balloon and a meteorological sounding device developed by the researchers. The studies of Branzov and Panchev, 1991 showed that a ground radiative temperature inversion in Sofia is not observed at wind speed higher than 3 ms-1 at 10 m height. Some results on the mechanisms of long lasting inversions formation were also reported. It was found that the speed of formation of ground based temperature inversion in autumn is greater than in spring.

Another complex experiment in Sofia was performed in the summer of 1994 using a lidar system for determining the concentration of aerosol in the air together with standard meteorological measurements (meteorological ground station, a standard radiosounding, pilot balloons). The high resolution in space and time of the lidar data allowed studying the diurnal evolution of the atmospheric boundary layer in general, and also its features over different ground surfaces in the city. The lidar and meteorological equipment were based in the southeastern part of Sofia at the Institute of electronics and NIMH. The night time stably stratified boundary layer was typically destroyed after sunrise, when a convective boundary layer (also called mixed layer or mixing height) was formed. The two-dimensional horizontal lidar scans of the aerosol stratification showed that the height of the mixed layer over the park zones is lower than over the built up areas (respectively around 100 and 180 m 1 hour after sunrise and up to around 260 and 470m in noon hours (Donev et al, 1995; Parvanov et al, 2000).

Kolev et al. (2000) suggested a physical model for the local air flows and for the structure of the boundary layer over a city, situated in a complex topography region such as the Sofia valley, in line with valley inversion model of Whiteman, 1982 and 1986.

The latest experimental study in Sofia was organized in 2003 with the purpose of studying the turbulence in the urban boundary layer (Batchvarova et al, 2004 and 2006). Turbulence profile measurements using ultrasonic anemometers (METEK make) and fast hygrometer (Campbell make) were performed on the meteorological tower of NIMH at 20 and 40 m above ground. The equipment was purchased within a Swiss-Bulgarian institutional partnership (SCOPUS 7/IP65650). Along with turbulence, consecutive radio soundings were performed using slowly ascending balloons in order to obtain higher resolution data both in height and time. All measurements were concentrated at NIMH. Combining ABL (Batchvarova and Gryning, 1994; Batchvarova et al, 2001 and Gryning and Batchvarova, 1999) and turbulence modeling (Batchvarova and Gryning, 2005 and 2006 and Gryning and Batchvarova, 2005) suggested that the roughness sub-layer depth in the suburban type region of NIMH is more than 40 meters. Interesting observation was made that both the urban ABL height and the aggregated heat fluxes were lower at easterly winds (combined rural and urban footprint of the flow) and higher at westerly winds (urban footprint of the flow towards NIMH). During this “Sofia 1993” experiment the modern Vaisala aerological sounding equipment was employed

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specifically for ABL research. Seven soundings per day were performed with resolution time resolution of 2 hours to cover the diurnal cycle of atmospheric stability and with resolution in height less than 10 meters. Several studies of the thermal stratification and height of the urban boundary layer were performed based on these data (Valkov et al, 2006, Veleva et al, 2010, Kolarova et al, 2006 and 2007).

3. ONGOING STUDIES

3.1. Remote sensing and turbulence measurements at a coastal site

The observations at the meteorological observatory of Ahtopol on the southeast Black Sea coast of Bulgaria started in July 2008 under a joint research project between NIMH and the Research and Production Association “Typhoon” at Roshydromet. The site is located in flat grassland, about 500 m inland, starting from a steep 30 m high coast. As shown in Fig. 1, the ultra sonic anemometer is mounted on a meteorological mast at height of 4.5m. The SODAR is located on the roof of the administrative building.

These measurements are the start of high resolution in time and space climatological observations of the breeze circulation at the Black Sea coast.

The combination of remote sensing and eddy correlation turbulence measurements allows to analyze and to classify the sea breeze days according to a number of different parameters.

Fig. 1. The meteorological park at the meteorological observatory (MO) Ahtopol. The SODAR is located on the roof of the administration building. The ultrasonic anemometer is mounted at

4.5 m height

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Here we illustrate the data from a distinct sea breeze day at Ahtopol – 5th August 2008. On this day in the morning the transition from land to sea breeze was marked by a change in wind direction by 180 degrees within only 6-8 minutes. Moreover, this happened not only near the ground, but in the entire 500 m deep layer covered by the sodar (Figs. 2 and 3). Although it is known that the sea breeze days at the Black sea coast are common phenomenon between May and September, such clear cases are not often observed.

Fig. 2. Wind direction from the sodar, Ahtopol, 05.08.2008

The maximum wind speed in the morning was detected at 50 m height and was about 4 ms-1, while in the early afternoon it is observed at 180 m height and is 7 ms-1 (Fig. 4). The vertical wind profile data show clearly the structure of the marine boundary layer. The long term information from this observation system (sodar and sonic anemometer) evaluation of mesoscale models results for the Bulgarian Black Sea coast, which can be used with higher reliability in air pollution and wind power assessments.

Fig. 3. Wind direction from the sonic anemometer at 4.5 m, Ahtopol, 05.08.2008

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Fig. 4. Vertical profiles of wind speed at 8:40 (upper panel) and 14:50 (lower panel) from sodar, Ahtopol, 05.08.2008

4. CONCLUSIONS

The ABL studies at NIMH have started in the 1960-ies based on climatological data. Later a number of field studies were organized to cover the city spatially. Presently, the operational network over Sofia is very sparse.

Aerological soundings were performed 3 times per day until the 1990-ies, though with very coarse resolution. Since 2001, high resolution is achieved, but only a noon sounding performed. Thus, no climatology of the inversions is available after 1990-ies.

Turbulence measurements are performed only during field campaigns within different projects.

The first ground based ABL remote sensing instrument (a Scintec sodar) at NIMH is operating at Ahtopol at the Black Sea site since summer 2008.

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ACKNOWLEDGEMENTS

Currently, the ABL studies at NIMH are supported by a research project initiated between the National Institute of Meteorology and Hydrology - Bulgarian Academy of Sciences (NIMH-BAS) and the Research and Production Association (RPA) ‘Typhoon’ – Russian Federal Service on Hydrometeorology and Environmental Monitoring (Roshydromet) in the frame of intergovernmental collaboration and the EU FP7-People-IEF VSABLA (PIEF-GA-2009-237471). Personally, we are thankful to Prof. M. Novitzky for his devotion to our common work in Ahtopol. Finally, we would like to acknowledge the contribution of our late colleague Dr. Nedialko Valkov who worked with us on ABL research in the period 1989 – 2010.

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The atmospheric boundary layer – parameterizations, observations and applications