Downwind case 10 October 23:30 – 24:00Upwind case 8 October 02:30 – 03:00 Motivation Our previous studies of turbulent structure of canopy airflow measured at the site situated near the top of a forested mountain ridge reveal the existence of low-frequency coherent structures detected in the high-frequency records of the wind velocity components and temperature. The spatial and time scale, as well as the persistence of these structures, is significantly different in the cases when the site is on the upwind, resp. downwind side of the ridge. Aims to analyse high-frequency records of CO 2 concentration measured in the upper crown layer at the Experimental Ecological Study Site Bílý Kříž by means of statistical and wavelet methods to assess the variability of CO 2 concentration for three different regimes of the airflow over the ridge to compare oscillations detected in the CO 2 concentration records with the fluctuations in the high- frequency temperature data K. Potužníková, P. Sedlák, P. Šauli, D. Hanslian Institute of Atmospheric Physics AS CR, Prague 4, Czech Republic Name1 st criterion2 st criterion Downwind caseσ w /U sc > ° < wind direction at 18 m < 60° Reversed flow caseσ w /U sc > ° < wind direction at 18 m < 235° Upwind caseσ w /U sc < 0.2 Data selection criteria Acknowledgements The study is supported by the grant IAA from the Grant Agency of Academy of Sciences of the Czech Republic (AS CR). Preparation and maintaining of the experiment as well as data acquisition by our colleagues from the collaborating Centre for Global Climate Change Impacts Studies AS CR is greatly acknowledged. NIGHT-TIME VARIABILITY OF CO 2 CONCENTRATION MEASURED IN THE FOREST CANOPY NEAR A MOUNTAIN RIDGE TOP: INFLUENCE OF INTERMITTENT VERTICAL MIXING Kateřina Potužníková, Pavel Sedlák, Petra Koucká Knížová, David Hanslian Institute of Atmospheric Physics AS CR, Prague, Czech Republic S ITE DESCRIPTION NameBily Kriz CountryCzech Republic Position N 49°30´17´´, E 18°32´28´´ Location Moravian-Silesian Beskydy Mts. Elevation m a.s.l. Topography 25% slope, SSW exposed Vegetation type Coniferous forest Tree speciesPicea abies, /L./, Karst Height13.5 m (in 2010) Period analyzed: September – November 2010 Sensors : 3D sonic anemometer (R.M.Young 81000, sampling frequency 20 Hz) and CO 2 analyzer Licor 840 (1 Hz)– both instaled at 10 m. Reducing the original data – the sonic temperature from 20 Hz to 2 Hz and the CO 2 concentrations from 1 Hz to 0.05 Hz, 30-min block averaging. Sonic anemometer Gill R3 at 18 m, 20 Hz data rotated into a planar fit coordinate frame, 30- min averages Excluding the 30-min time series with the occurrence of rain and/or the relative humidity higher than 99.5 %. D ATA R ESULTS Standard deviation of CO 2 concentration in the crown layer vs. mean wind direction above the canopy The “downwind“ and “reversed” cases are characterised by much larger variability of the CO 2 concentrations than the “upwind” case. This feature is in accordance with much larger flow variability in the lee of the ridge. Time series (top row in each panel), their wavelet power spectra generated by the Morlet wavelet (left columns) and the wavelet variance normalized by their maximum value (right columns). Colour contours are for 75%, 50%, 25% and 5% of the wavelet power. The green ellipses enclose the structures that are described below the figures. The “downwind case” is characterised by large amplitudes of the quantities studied here (see the time series plots). In agreement with this feature, the plots of wavelet power spectra indicate large number of turbulent oscillations with the period about 1 minute. On the contrary, in the “upwind case”, low-frequency turbulent structures are detected only in the minor part of the analysed time series. Moreover, the temperature amplitudes are several orders of magnitude smaller than in the downwind case. Three different types of oscillations are marked in the plots. The sharp temperature increase accompanied by a rapid drop of CO 2 concentration („Type I“) is probably caused by vertical mixing – a warmer air from above, poor in CO 2, penetrates the stable layer and „dilutes“ the air in the upper crown layer. The broad local maximum, scarcely visible in the temperature, is labelled „Type II“. And finally, the maxima visible in the CO 2 time series but not in the temperature time series, are labelled „Type III“. We speculate that this increase in CO 2 concentration appears when the air rich in CO 2 produced by the soil respiration is lifted up to the upper crown layer. F LOW TYPES AT B ILY K RIZ The research site Bílý Kříž is situated on a steep (25%) SSW-faced slope. North of the site, there is a W-E oriented mountain crest with a shallow saddle. The south (upslope) and the north (downslope) directions of flow above the canopy dominate at Bílý Kříž. In the most recent study of the large-scale flow direction above the mountain ridge, we have found a criterion for separating the cases when the site is on the upwind, resp. downwind side of the ridge, based on the ratio σ w /U sc above the canopy (at 18 m, see the table above). Here σ w is standard deviation of the wind component normal to the slope and U sc is the scalar mean wind speed in the plane parallel to the slope (calculated from the 20 Hz data of u and v wind components). However, in some downwind cases, with large value of the σ w /U sc ratio, the mean (30-min averaged) wind vector is oriented upslope, indicating a weak upslope flow. We explain this feature by occurrence of a flow reversal layer, which is formed on the downwind side of the ridge and reaches above the canopy layer, and we call such cases the Reversed flow cases. CO 2 average [ppm] Mean of CO 2 standard deviation [ppm] Median of CO 2 standard deviation [ppm] Downwind case Reversed flow case Upwind case CaseAverage time scale (period) of structures [s]Average horizontal spatial scale of structures [m] Main periodsAll periodsMain periodsAll periods Downwind case Reversed flow case Upwind case Data selection: We concentrate only on the nighttime periods, when the solar radiation effect and CO 2 uptake is eliminated. For the “downwind analysis” we selected the 30-min time series with mean CO 2 standard deviation ranging between 1.5 – 1.8 ppm, for the “reversed flow analysis” between 2.0 – 2.6 ppm, and for “upwind analysis” between 0.5 – 0.8. Method of analysis: The periods of oscillations or turbulent structures correspond to local maxima in wavelet variance spectra. The “Main period” (associated with the most intensive coherent structure) is computed as a major peak in the wavelet variance spectrum. The average horizontal spatial scale of structures is given by the product of mean wind speed and average period of structures. Summary of the CO 2 wavelet analysis Summary and outlook Our statistical and wavelet analysis of the measured variables under the three different regimes of the airflow over the mountain ridge reveals great variability of the CO 2 concentration in the lee of the ridge associated with large number of turbulent structures having relatively shorter time scale but relatively larger spatial scale. We prepare linking the CO 2 concentration variability to the records of vertical and horizontal wind velocity components with the purpose of specifying the genesis of different types of oscillations. We are working on the verification of the results presented here by using the data sets extended to the growing season © Aďa 6 Year Statistical analysis I III II