A mobile, scanning eye-safe lidar for the study of atmospheric aerosol particles and transport processes in the lower troposphere [Elektronische Ressource] / vorgelegt von Sandip Pal

Sandip Dhomse <[e-mail]>

Informations

• Publié par: universitat_hohenheim
• Publié le: 01 janvier 2009
• Nombre de lectures: 27
• Langue: English
• Poids de l'ouvrage: 22 Mo

Extrait

A mobile, scanning eye-safe lidar for the study
of atmospheric aerosol particles and
transport processes in the lower troposphere




Dissertation zur Erlangung des Doktorgrades
der Naturwissenschaften (Dr. rer. nat.)

Fakultät Naturwissenschaften
Universität Hohenheim



Institut für Physik und Meteorologie


Vorgelegt von
Sandip Pal

aus Cooch Behar, Indien
2009









This thesis was accepted as a doctoral dissertation in fulfillment of the requirements for the degree
“Doktor der Naturwissenschaften” by the Faculty of Natural Sciences at the University of Hohenheim,
Stuttgart, Germany on 4 December 2008.

Date of oral examination: 23 January 2009



Examination committee:

Supervisor and reviewer: Prof. Dr. V. Wulfmeyer
Institute for Physics and Meteorology,
University of Hohenheim

Co-reviewer: Prof. Dr. Th. Foken
Institute for Micrometeorology,
University of Bayreuth

Additional examiner: Prof. Dr. K. Jetter
Institute for Applied Mathematics and Statistics,
University of Hohenheim

Dean: Prof. Dr. H. Breer
Faculty of Natural Sciences,
University of Hohenheim


Abstract

A high-power eye-safe scanning aerosol lidar system in the ultraviolet wavelength region is introduced for the
study of the optical properties of aerosol particles and transport processes in the atmosphere, especially in the
atmospheric boundary layer (ABL). This system operates with an average power of 9 W in combination with a
-140-cm scanner with a speed of up to 10° s . A modified version of the lidar inversion algorithm is developed for
the retrieval of optical properties of aerosols from scanning lidar measurements. The lidar data can be analyzed
with previously unachieved temporal and spatial resolution of 0.03 s and 3 m, respectively.
New methods are developed for both scanning and non-scanning lidar systems to study the evolution and
structure of the convective boundary layer (CBL). Three advanced techniques namely: logarithm gradient method,
inflection point method, and the Haar wavelet transform method are demonstrated for determining the
instantaneous CBL height from the high-resolution lidar measurements. The Haar wavelet-based approach is
found to be a robust technique for the automated detection of the ABL height. Vertical lidar measurements over an
urban valley-like location provided the ABL top to be within 0.7–2.3 km above ground level (AGL). The mean
entrainment zone thickness for the quasi-steady CBL is of about 75 m. The aerosol load showed high variability
both for a quasi-stationary well-mixed CBL and a CBL during its rapid growth in the morning. The fast Fourier
transform based spectral analysis of the instantaneous CBL height time series has yielded a spectral exponent
value of 1.502 ± 0.08314, confirming non-stationary behavior of CBL in the morning.
Higher-order moments are calculated with respect to fluctuations of the particle backscatter coefficients
for well-mixed CBL conditions under which hygroscopic growth of the aerosol particles can be neglected. The
- -5/3variance spectra show an f roll-off (inertial subrange) inside the quasi-steady CBL. It is demonstrated that the
major part of the inertial subrange is detected and that the integral scale (60-70 s) is significantly larger than the
temporal resolution of the lidar system. Consequently, the major part of turbulent fluctuations is resolved. Vertical
distribution of the variance, skewness, and kurtosis have reflected the turbulence features with an accuracy that is
mainly limited by sampling errors due to turbulence statistics. Negative values of skewness are found inside the
CBL while positive values are found in the entrainment zone near the top of the CBL for the quasi-stationary
regime. But for the case of a rapidly growing CBL, skewness profile has shown a high variability even inside the
CBL, most probably due to the presence of a rapid growth rate of 4-5 m/minute, non-stationarity, and strong
residual layer above the CBL.
The optical properties of aerosol particles emitted from a faint source (a livestock farm, located in flat
terrain) are determined by means of spatially and temporally high-resolved scanning lidar measurements in
combination with a high-resolved atmosphere-microphysics-chemistry model and in-situ aerosol measurements at
ground. Both model and lidar results have yielded the particle backscatter coefficient of the aerosol plume to be of
about 30 % higher than that of the background aerosol load near the facility. Results show a high spatial and
temporal variability of the plume. The lifting height of the plume is found to be of about 20 m AGL near the
source and of about 115 m AGL over long distances up to 3 km due to transport in downwind. The aerosol plume
has caused an increase of the aerosol number density downwind of up to 5 % in the lowermost 50 m of the ABL.
Combined high-resolution measurements of aerosol optical properties and temperature field over a
complex mountainous region with the rotational Raman lidar revealed undulating aerosol-rich layers in the
preconvective environment and a gradual warming trend of the lower troposphere as the nearby storm system
evolved. Simultaneous measurements of particle backscatter and extinction coefficients, and respective lidar ratios
confirm the presence of different particle types in different altitudes. Lidar ratios inside the CBL are found to vary
around 35 sr. High-resolution scanning and vertical measurements has shown the CBL height to vary between 0.8
and 1.2 km over the mountain peak. Measurements of aerosol distributions and collocated Doppler lidar derived
wind field confirm mountain induced flow modifications.
RHI scan measurements of aerosol optical properties and temperatures showed some more features that
were consistent with ascending air motion due to overflow over the ridge. The aerosol layers showed the terrain
following flow features. The particle backscatter coefficient is found to be slightly increased within a potential
temperature tongue. Small-scale wave structures are determined from the wavelet spectra of the time series of the
lidar signal intensity at cirrus layer. Lidar ratio values between the altitudes of cirrus cloud base (5.2 km) and top
(8 km) show high vertical variability with a minimum and maximum values of 3 sr and 25 sr, respectively.




i
Zusammenfassung

Zur Untersuchung optischer Eigenschaften von Aerosolpartikeln und Transportprozessen in der Atmosphäre,
speziell in der atmosphärischen Grenzschicht (atmospheric boundary layer, ABL), wird ein augensicheres
Hochleistungs-Scanning-Lidarsystem im ultravioletten Wellenlängenbereich vorgestellt. Das System arbeitet mit
einer durchschittlichen Leistung von 9 W in Kombination mit einem 40 cm Scanner mit einer Geschwindigkeit bis
-1zu 10° s . Eine modifizierte Version des Lidar-Inversionsalgorithmus zur Rekonstruktion der optischen
Eigenschaften von Aerosolpartikeln aus den Scanning-Lidar-Messungen wird entwickelt. Die Lidar-Daten können
mit einer bisher nicht erreichten zeitlichen und räumlichen Auflösung von 0,03 s bzw. 3 m analysiert werden.
Zur Untersuchung von Entwicklung und Struktur der konvektiven Grenzschicht (convective boundary
layer, CBL) werden neue Verfahren, sowohl für scannende als auch für nicht-scannende Lidar-Systeme,
entwickelt. Zur Bestimmung der Höhe der instantanen CBL aus den hochaufgelösten Lidar-Messungen werden
drei fortschrittliche Techniken, und zwar: die Methode des logarithmischen Gradienten, die Inflection-Point-
Methode und die Wavelet-Transformation nach Haar, angewendet. Die auf Wavelets basierende Methode nach
Haar zeigte bemerkenswert gute Resultate. Vertikale Lidar-Messungen über einem städtischen talähnlichen
Messplatz ergaben eine Obergrenze der ABL zwischen 0,7 km und 2,3 km über Grund. Die abgeschätzte mittlere
Dicke der Entrainmentzone für die quasistabile CBL war etwa 75 m. Die Aerosolpartikelbeladung zeigte eine
hohe Variabilität sowohl für eine quasistationäre gut durchmischte CBL wie auch für eine CBL während des
schnellen morgendlichen Anwachsens der Schicht. Die auf einer Fast-Fourier-Transformation basierende
spektrale Analyse der Höhe der instantanen CBL ergab einen spektralen Exponentenwert von 1,502±0,0314, was
das nicht-stationäre Verhalten der CBL am Morgen bestätigt.
Für gut durchmischte CBL-Bedingungen unter denen hygroskopisches Wachstum der Aerosolpartikel
vernachlässigt werden kann, Momente höherer Ordnung unter Berücksichtigung von Fluktuationen des
-5/3Partikelrückstreukoeffizienten berechnet. Die Varianz-Spektren zeigen ein von f Roll-Off (Inertialbereich) im
Innern der quasistabilen CBL. Es wird gezeigt, dass der Großteil des Inertialbereichs nachgewiesen wird und dass
die integrale Skala (60 s –70 s) signifikant größer ist als die zeitliche Auflösung des Lidar-Systems. Folglich wird
der Großteil der turbulenten Fluktuationen aufgelöst. Die vertikale Verteilung v