COMPARISION OF VELOCITY SPECTRA DURING THE PASSAGE OF A COLD FRONT
OVER PANTANAL
Gannabathula S.S.D.Prasad 1*, K.P.R.Vittal Murty2*, Leonardo D.A. Sá1,Bart Kruijt3, Amaury
de Souza4
1
DCM/Instituto Nacional de Pesquisas Espaciais, São José dos Campos, SP
Andhra University, Waltair, INDIA,3Institute of Ecology and Resource Mangement, University of
Edingurth, Edinburgh UK.,4Dept. de Física/Universidade Federal do Mato Grosso do Sul
*
CNPq fellow
2
ABSTRACT: In this article the velocity spectra was examined over Pantanal region on two
contrasting days. One is a clear day and the other is a cloudy day with passage of a cold front. It is
observed that there is a shift in the spectral energy maximum on a clear day indicating more shear
production.
KEY WORDS: Velocity Spectra, Cold front, Surface Layer, Pantanal
INTRODUCTION
The gradient observations data measured by sensitive instruments provide an opportunity to
study the structure of the surface layer and its variation during passage of large synoptic scale weather
phenomena like passage of a front etc(Garret 1988). Van der Hoven (1957) analysed the horizontal
velocity spectrum which showed two eddy energy peaks. The first bigger one corresponds to synoptic
scale (a period of approximately 4 days) and the other small peak ( a period of approximately one
minute) corresponds to microscale motions. Between these two peaks the spectral energy is less (0.5
to 10 cycles per hour) which is known as the Spectral Gap. However, Kaimal (1973) showed that
atmospheric boundary layer can have eddy motions with periods ranging from several minutes to three
hours within the spectral gap.
In this paper the horizontal and vertical velocity spectra obtained from the fast response sonic
anemometer data at 24 meters measured at pantanal during the Interdiciplinary Pantanal
Experiment#1(IPE1) was compared for two Julian days 136 and 143. The former is a cloudy day with
a passage of cold front and the latter is a clear sunny day. The idea is to obtain changes in the structure
of the surface layer for different synpotic situations.
MATERIALS AND METHODS
The spectra of the u, v and w were obtained using a three dimensional sonic anemometers and
was sampled at 20.8samples per second. The spectra were computed using the procedure suggested
by Kaimal(1973). The data records are first checked for consisteny and any spurious spikes if any
were deleted. The Welsh periodogram was computed using an fft of size 4096 samples. The record
size was selected as 90minutes and care was taken to see that the actual data length is an exact multiple
of 4096 samples. This ensures that there is no zero padding. A low frequency spectrum was also
computed and the two spectra were then added. The trend present in the data was removed before
computing the spectra. The results are shown in figures1 and 2. All the figures were drawn on a loglog
scale. The lines in the figures have a slope of –2/3.
RESULTS
The spectra depending on the availability of the data is estimated at 00hours, 06:00hours and 18:00
hours on day 136 and at 00, 06:00 , 12:00 and 18:00hours on day 143. The shear production zone of
turbulance is upto 0.01 cycles and the spectral maximum also conicide within this frequency . Then
the spectral energy is dissipated following the –2/3 power law of the inertial range. The cluster is
more in the lower frequencies and it is less in the shear production zone.
Comparing the day 143 the maximum shift toward the positive side and in the night there is
more spectral gustiness and it is much smoother at 18:00hours. This may be due to the strong
instability at 12:00hours and the presence of the Brunt-Vaisala oscillatons and the presence of gravity
waves in the night.
So the main difference observed between the day 136 and 143 is the shift in the maximum, indicating
more shear production on the day 143 which is a clear day compared to the day 136 which is cloudy.
CONCLUSIONS:
The important difference in the velocity spectra is the shift in the spectra maximum. It shifts
towards the positive frequency during a clear day compared to a cloudy day with the passage of the
cold front.
The spectral gustiness is more in the night for both the clear and cloudy days. This may be
attributed to the presence of the gravity waves.
ACKNOWLEDGMENTS
The authors would like to thank, FAPESP (proc. #98/00105-5) and the CNPq (procs.#
381.699/97-8, 381.690/97-0and 139.164/96-0) for the finanacial support.
Thanks are also due to all the people involved in the IPE-1 Project organization and dat
collection: Drs Antonio O Manzi, Regina C.Santos Alvalá, Clovis A Sansigolo, Ralf Gielow, Plinio
C.A.Alvalá, Clovis M. do Espirito Santo and Messrs. Paulo R.A.Arlino, Luis E. Rosa, Celso Randow,
Jorge Martins de Melo, Sabrina B.M.Sambatti, Elizabete C.Moraes all of INPE. Acknowldgements
are also due to Drs. Edson Kassar, Lhamilton G. Pavão, Masao Uetanabaro and Mrs. Carla Muller,
Jorge Gonçalves and Waldeir M.Dias of Universidade Federal do Mato Grosso do Sul, Yadvinder
Malhi of University of Edinburgh, Dr. Maria Lucia Meirelles of EMBRAPA/CPC, Dr. Romisio
G.G.André of UNESP/Jaboticaba
Special Thanks are also due to Dr. Nelson L. Dias of SIMEPAR and Dr. Paulo H.Caramori of
IAPAR who provided a sonic anemometer and hygrometer for use duing the campaign.
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