Boundary-layer height detection with a ceilometer at a coastal ... - Orbit

More documents

Recommendations

Info

SPIE Newsroom function of height, P (z) provides information on the backscatter profile of the atmosphere. It is measured by the instrument and is related to the emitted signal and the characteristics of the propagating medium through the lidar equation: 10.1117/2.1200612.0512 oundary layer and air quality onitoring with a commercial idar ceilometer P (z) = P 0 ∆t c ∫ z 2z 2 Aβ(z)e−2 0 σ(z′ )dz ′ (15) where P 0 is the average power of the pulse [W], ∆t is the duration of the emitted laser pulse [s], c is the speed of light [m/s], A is the receiver area [m 2 ], β(z) is the volume backscatter coefficient at distance z [1/(srad m)], σ(z’) is the extinction (attenuation) coefficient at various distances z’ [1/m] and e −2 ∫ z 0 σ(z′ )dz ′ is the atmospheric transmittance. When there is no attenuation, in a clear atmosphere, the last term equals 1. hristoph Münkel roviding small, low-cost lidar systems with novel optical design and exible electronics sharpens the view of particles in the lower tropophere, and enables new environmental monitoring applications. idar (light detection and ranging) remote sensing has enorous potential in atmospheric research and air quality onitoring, 1 made possible by the portion of emitted laser light at is scattered back to the instrument by particles in the atmophere. The recorded backscatter profiles could be used, for exmple, to estimate the mixing layer’s aerosol density and thickess, or mixing height (MH). Substances introduced into the ixing layer become completely mixed by turbulence given sufcient time. These estimates would be useful in helping comply with Euroean air quality framework directive 96/62/EC, which requires recise monitoring of pollution loading. The loading can be estiated using aerosol optical thicknesses retrieved from satelliteased observations, 2 together with the MH. High costs, scarcity, and eye-safety considerations exclude reearch lidar systems as a candidate for a dense MH retrieval netork. I show here that eye-safe lidar ceilometers of the sort used report cloud base heights at most airfields can, with modst modifications introduced by Vaisala GmbH, provide a costffective alternative. The traditional ceilometer concentrates on cloud base detecon using either a two-lens or a one-lens approach. The two-lens esign provides little or no laser beam and receiver field-of-view verlap for collecting aerosol backscatter in the near range up to 00m, a crucial range for air quality applications. The single-lens design, which uses the same lens for transmitng and receiving, provides full overlap over the whole meauring range, but requires compensating mechanisms to prevent eceiver saturation from direct lens reflections that lower data uality in the near range. Poor range resolution, low pulse rep- Figure 1. Shown is the Vaisala Ceilometer CL31 and a schematic drawing of its optical concept. A hole in the mirror divides the lens into an inner part used by the transmitter and an outer ring visible to the receiver. Figure 6: The single lensed optics of the ceilometer CL31 (Münkel, 2006) The laser pulse signal is reduced by absorption (extinction coefficient) and scattering (backscatter coefficient), so when solving the lidar equation, there are two unknown quantities: β(z) and σ(z ′ ). The simplifying assumption is made that there is a linear relationship between extinction and backscatter. etition frequency, and long minimum report interval further decrease the performance of standard ceilometers. The shortcomings of both designs are overcome by the novel optical design of the Vaisala Ceilometer CL31, shown in Figure 1. This instrument incorporates the advantages of both optical concepts, and the disadvantages of neither. Its fast electronics make it an all-purpose backscatter lidar, fit for standard ceilometer tasks and air quality applications. 3 The right part of Figure 1 shows the enhanced single-lens design for the CL31. We use the center of the lens for collimating the outgoing laser beam, and the outer part to focus the backscattered light onto the receiver. This division between transmitting and receiving areas is provided by an inclined mirror with a hole β(z) = κσ(z) (16) where κ is called the lidar ratio and is assumed constant for a given height (Vaisala User’s Guide, 2004). It is then possible to obtain the vertical profiles of backscatter and extinction coefficients with the use of an inversion algorithm. It is not possible to know the exact ratio β(z)/σ(z) from the received signal, but the precise functional description is not needed to obtain useful results as stated by Klett (1985). Most lidars have the transmitter and receiver placed beside each other, which means that there is a zenith angle between emitted and received light Continued in the on next near page range. This produces an error in the signals that may be corrected with an overlap function (Leosphere User Manual, 2009). The ceilometer on the other hand is a single lens optical devise. Figure 6 shows the Vaisala Ceilometer CL31 and a schematic drawing of its optical concept. An inclined mirror with a hole in the center divides the lens into two areas. The inner area transmits the laser beam, while the outer area receives and focuses the backscattered light. This optical arrangement provides a full overlap of the whole measuring range, but has the disadvantage that emitted light is partially reflected by the lens directly to the receiver, when measuring signals from the near range (Münkel, 2006). 3.3 The wind LIDAR The main difference between the ceilometer and the wind lidar is that the wind lidar relies on measurements of the Doppler shift of the backscattered laser pulse (Leosphere, 2008; Cariou DTU Wind Energy Master Thesis M-0039 17
occasionally use the term “radial velocity” v r or radial component of a velocity vector that is not parallel to the line of sight. v LOS and v r are fully synonymous, with the same sign convention. Now the collective movement of air masses which we call wind is superimposed by the individual, thermal, random movement of the and Boquet, 2011). Due to the wind field, the aerosols in the atmosphere are in motion. When the light emitted by a laser hits the moving aerosol this results in a frequency shift of the light, as illustrated in Figure 7. The moving particle scatters the pulse back and the received frequency, f, is calculated as: molecules. These normally move must faster than the wind speed and so much the faster the higher the temperature. The relative shift of their velocity distribution with the wind speed is therefore small. Aerosol particles, because of theirf higher = f 0 + ∆f mass, ≃ f 0 (1 move + 2V/c) more slowly at the same (17) temperature and have therefore a narrower velocity distribution. They are shifted by the same amount, but relative to its width this shift is much larger and amenable to measurement. The situation is schematically shown in Fig. 12.1. where f 0 is the emitted frequency, ∆f is the frequency shift, V the wind velocity and c the speed of light. The frequency shift of the returned light is proportional to the radial component of the wind, i.e the projection of the wind velocity vector on the line-of-sight. log β -7 -8 -9 -10 -11 λ0 λLOS -60 -30 0 +30 +60 frequency (MHz) f0 Fig. Figure 12.1. 7: Schematic Schematic representation representation of of the the original original (solid) (solid) and wind-shifted and wind-shifted (dotted) frequency (dotted) frequency distributions. distributions. If there are If there aerosols arepresent, aerosols a narrow present, spike a narrow is superimposed spike is superimposed onto the broad ontomolecular the broadpeak. molecular The return-signal peak. The return-signal frequency is shifted frequency here is toward shifted higher here toward values, indicating that indicating the wind that is the moving windtoward comes thetoward lidar. At the10.59 lidar. µm At 10.59 wavelength, µm wavelength, the 3-MHz shift the higher values, 3-MHz corresponds shift corresponds to a wind speed to roughly of ∼20 20 m/s m/s. (Werner, 2005). The return signal of the laser pulse is processed with heterodyne principle. In heterodyne detection the return signal is mixed with a reference signal of a local oscillator in the device, with a frequency f LO . The mixed signal contains two new frequencies, one is the sum f LO + (f 0 + ∆f), and the other the difference f LO − (f 0 + ∆f), known as the heterodyne- or beat frequency. If the frequencies of the two signals are close to each other the beat signal may be determined. The detector of the device measures the current, i, of the beat frequency signal only: i = ρ √ 2P LO P (z, λ) cos[2π(f LO − (f 0 + ∆f))] (18) where ρ is the detector sensitivity [A/W], P LO is the power of the local oscillator laser [W], f LO is the frequency of the local oscillator laser [Hz], P (z, λ) is the power of the backscattered signal [W] and f 0 + ∆f the frequency of the backscattered signal [Hz]. It is important to determine the difference between the transmitted frequency and the local oscillator frequency with great accuracy. This difference is kept as stable as possible (Werner, 2005). To provide information of all three wind vector components U, V and W , the lidar may scan in three or four directions. If the lines of sight are in the North, South, East and West directions, the measured radial velocities V r i are: 18 DTU Wind Energy Master Thesis M-0039
Page 1 and 2: DTU Wind Energy Master Thesis M-003
Page 3 and 4: Contents 1 Introduction 5 1.1 Thesi
Page 5 and 6: 1.1 Thesis contents The content of
Page 7 and 8: and Batchvarova, 1994; Stull, 1988)
Page 9 and 10: ∂ē ∂t = g w ′ θ v ′ − u
Page 11 and 12: 2.7 Boundary-layer clouds The prese
Page 13 and 14: the square root of the variance. Ro
Page 15: 3 Site and measurements In this sec
Page 19 and 20: Ultrasonic Wind Sensor uSonic-3 Sci
Page 21 and 22: 4 Boundary-layer height detection T
Page 23 and 24: 90/(10 8 m srad), respectively). Fo
Page 25 and 26: 1400 β profile 16/04/2010 11:40 14
Page 27 and 28: 24/04/2010 06:20 1000 σ w 1000 ē
Page 29 and 30: Decoupled Clouds Clouds that are no
Page 31 and 32: 1600 Zero filter 23/07/2010 22:00 N
Page 33 and 34: 5.2 Continuous and average data The
Page 35 and 36: of the error bars during the day. T
Page 37 and 38: also be explained by the shape of t
Page 39 and 40: increases from 19:00-20:00. A resid
Page 41 and 42: 1400 1200 BLH estimations 01/05/201
Page 43 and 44: 5.4 Cold front passage On April 8 2
Page 45 and 46: Wind vectors at 10m height. Metmast
Page 47 and 48: 0.3 The inverse Obukhov length 1/L
Page 49 and 50: Wind vectors at 10m height. Metmast
Page 51 and 52: 0.1 The inverse Obukhov length 1/L
Page 53 and 54: 1400 BLH from minimum TKE 16 June 2
Page 55 and 56: 70 60 Threshold night day Easterly
Page 57 and 58: Threshold Westerly winds Ideal prof
Page 59 and 60: 140 120 Threshold night day Easterl
Page 61 and 62: Threshold Easterly winds Ideal prof
Page 63 and 64: 400 300 Threshold night day Easterl
Page 65 and 66: 5.9 Intra-annual variation The BLH
Page 67 and 68:
wind lidar (Sathe et al., 2011). Ho
Page 69 and 70:
distributions. It is clear that the
Page 71 and 72:
A Appendix Abbreviations ABL Atmosp
Page 73 and 74:
References Batchvarova, E., X. Cai,
Page 75:
Sinclair, V. A., S. Niemelä, and M
show all

Boundary-layer height detection with a ceilometer at a coastal ... - Orbit

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?