Downwind case 10 October 23:30 – 24:00Upwind case 8 October 02:30 – 03:00 Motivation Our...
If you can't read please download the document
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
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 Bl K 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. Potunkov, P. Sedlk, P. auli, D. Hanslian Institute of
Atmospheric Physics AS CR, Prague 4, Czech Republic
([email protected]) Name1 st criterion2 st criterion Downwind case w
/U sc > 0.35 300 < wind direction at 18 m < 60 Reversed
flow case w /U sc > 0.35145 < wind direction at 18 m < 235
Upwind case w /U sc < 0.2 Data selection criteria
Acknowledgements The study is supported by the grant IAA300420803
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 Kateina Potunkov, Pavel Sedlk, Petra
Kouck Knov, David Hanslian Institute of Atmospheric Physics AS CR,
Prague, Czech Republic S ITE DESCRIPTION NameBily Kriz CountryCzech
Republic Position N 493017, E 183228 Location Moravian-Silesian
Beskydy Mts. Elevation800-900 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 Bl K 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 Bl K. 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 w /U
sc above the canopy (at 18 m, see the table above). Here 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 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 case3771.761.55 Reversed flow case3752.292.23 Upwind
case3810.710.45 CaseAverage time scale (period) of structures
[s]Average horizontal spatial scale of structures [m] Main
periodsAll periodsMain periodsAll periods Downwind case13218562130
Reversed flow case141200104 Upwind case122194105184 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 2011. Aa 6 Year Statistical analysis I III II