A Multi-wavelength Mini Llidar for Measurements of

Marine Boundary Layer Aerosol and Water Vapor Fields

 

Shiv K. Sharma

Hawaii Institute of Geophysics & Planetology

2525 Correa Rd., Honolulu HI 96822

phone: (808) 956-8476     fax: (808) 956-3188  email: sksharma@soest.hawaii.edu

 

 Barry R. Lienert

Hawaii Institute of Geophysics & Planetology

phone: (808) 956-7815 fax: (808) 956-3188  email: lienert@soest.hawaii.edu

 

John N. Porter

Hawaii Institute of Geophysics & Planetology

phone: (808) 956-6483 fax: (808) 956-3188  email: jporter@soest.hawaii.edu

 

Antony D. Clarke

Dept. Oceanography, University of Hawaii at Manoa

1000 Pope Rd., Honolulu HI 96822

phone: (808) 956-6215 fax: (808) 956-7112  email: tclarke@soest.hawaii.edu

 

Award #: N00014-96-1-0317

http://www.soest.hawaii.edu/lidar

 

 

LONG-TERM GOALS

 

Our long-term goal is to improve our understanding of dynamics of marine aerosols and water vapor fields in the coastal marine boundary layer using a scanning lidar and meteorological parameters.

 

OBJECTIVES

 

Our scientific objectives are to collect well-calibrated lidar data sets that can be used to improve and develop models of the aerosol optical properties in the coastal marine boundary layer (MBL). Various aerosol models exist (e.g., Fitzgerald, 1989), but few are appropriate for coastal regions. We are studying the vertical aerosol structure in the 15-m of the atmosphere directly above the ocean surface.

 

APPROACH

 

We are using a scanning multi-wavelength lidar to measure the 4-D (space and time) aerosol optical fields in order to characterize the aerosol properties in a marine setting (Sharma et al., 1999; Lienert et al., 1999). These measurements have been carried out at Bellows Air Force  (AFS) next to the University of Hawaii's Meteorological Tower (21o 21.848' N, 157o 42.584' W). We have also made good progress towards obtaining calibrated lidar extinction values. Our efforts are now focussed on obtaining carefully calibrated multi-wavelength data from which we will recover aerosol properties. We are also linking our ongoing measurements to atmospheric stability and synoptic conditions. Dr. Shiv Sharma is the project director involved in all aspects of the efforts. Dr. Barry Lienert has developed the software and supervises the data collection. Dr. John Porter is involved in calibration and modeling efforts. Dr. Clarke is involved in comparisons with in situ measurements and data interpretation.

 

WORK COMPLETED

 

(1) We installed a 12” telescope and detector assembly incorporating 1064, 532 and 355 nm channels.

(2) We regularly measured horizontal and vertical distributions of aerosol at Bellows Beach.

(3) We have constructed a portable mini-lidar weighing <10 kg for future use at other coastal sites.

(4) We developed an approach to obtain calibrated aerosol extinction values in the marine atmosphere.

(5) We developed a method of deriving aerosol size distributions from multi-wavelength extinctions.

(6) We carried out preliminary aerosol phase function measurements at Bellows Beach.

 

RESULTS

 

Modeling of the optical extinction in the coastal regions is complicated by numerous possibilities, which include cloud processes, breaking waves, wind speeds and directions, atmospheric stability, and topographic and aerosol source effects. Based on our lidar measurements at Bellows Beach (SE Oahu) we have observed cases where each of these effects dominated. The atmospheric stability is an obvious controller of vertical mixing and therefore the mixing processed in the coastal marine boundary layer. To investigate the vertical boundary layer structure during the SEAS (Shoreline Environmental Aerosol Study) campaign (April 20-30, 2000), we released two radiosondes to measure the vertical

 

   Figure 1: Vertical lidar scan of extinction          Figure 2: Measured RH and virtual potential

   (in m-1) performed at Bellows Beach                   temperature calculated from radiosonde data   

   on 4/24/00 at 1600 hrs HST                                 obtained at Bellows  at 1600 hrs HST,  4/24/00.

 

profiles of pressure, temperature and humidity. The radiosondes were released from a small boat 2 km upwind of the lidar site to obtain cases representative of the open ocean. Fig. 1 shows the lidar scan during one of the soundings. It can be seen that the sea salt is well mixed up to approximately 0.55 km with cleaner layers above. Between 0.5 and 2 km several layered cloud and aerosol fields exist which are consistent with aged trade wind clouds evaporating in a stable stratified atmosphere. Fig. 2 shows the relative humidity is 75% near the surface and increases with height through the mixed layer similar to the model proposed by Fitzgerald (1989). The virtual potential temperature is approximately constant in the lowest mixed layer (0-0.5 km) with a fixed rate of increase from 500 to ~2250m with a strong increase from 2250 to 2500 m corresponding to the trade wind inversion. These data indicate that the atmosphere below 0.5 km is well mixed, whereas it is stable above 0.5 km. These observations from the radiosonde data are consistent with the vertical lidar measurements (Fig. 1), which show a well-mixed layer below 0.5 km and increased scatter near the top of the mixed layer due to increased relative humidity (Fitzgerald, 1989). Soundings collected at Lihue, Kauai (by the National Weather Service) indicate that the trade wind inversion varied from 1.4-2.7 km during the SEAS experiment. Substantial variability in the low level stability (both dry adiabatic an moist adiabatic processes) existed in this short period, which means on some occasions the low level thermals were capped at the mixed layer while on other more convective occasions the vertical motions should be able to penetrate past the mixed layer and produce trade wind clouds.

 

Based on our lidar data and the variability seen in the Lihue soundings we find rapid transitions in the low level vertical stability and mixing of the marine boundary. When low-level instability occurs, it is typically associated with convective trade wind clouds and increased sea salt scatter even at the lowest

                           1                     2                      3                      4                     5                        6

 

Figure 3: Vertical scan (upper left), followed by a time sequence of horizontal scans at 90 second intervals performed at Bellows Beach on 4/27/00, 12:05 hrs HST.

 

levels. During the SEAS experiment, we were able to obtain horizontal scans during a disturbed cloudy period on 4/27/00. The wind speed was 9.1 m/s and the wind direction was 75OE (slightly east of typical trade wind conditions). Lidar measurements during this period are shown in Fig. 3. The first (left) panel shows a vertical lidar scan with the cloud base extending down to ~200 m above sea level which is lower than the typical value of  ~500 m. Panels 2-6 show horizontal scans taken at 5 m height over the ocean. They illustrate regions of enhanced extinction (red/yellow patches) moving from the open ocean towards the coast. Panels 2 and 3 capture an enhanced scattering region moving towards the shore. By panel 4 this feature has merged into a larger coastal region with enhanced scatter. By panel 5 a new region of enhanced scatter has appeared and by panel 6 has merged into the coastal pool of enhanced scatter/extinction. The fact that these enhanced surface scattering regions appear more than 4 km from the lidar means that they cannot be associated with the spray-generating reefs at Bellows (1.2-2 km upwind) and are open ocean features. Relative humidity’s measured at both 5 and 20 m altitudes 10 minutes after these scans were 92% and 90%, respectively, which was significantly above their daily mean values of 82% and 73%. This suggests cloud downdrafts with drizzle could be the cause of the increased surface optical scattering although surface gust fronts are also likely. 

 

As the trade winds encounter the blocking due to an island mountain they must either go over the island (like Oahu) or around if the island mountains are higher than the trade wind inversion (like Hawaii). This island blocking effect causes a pileup of the trade winds upstream of the islands, which causes air to rise even before it reaches the island. The manner in which the air rises is controlled by the atmospheric stability and wind speed and is related to the Froude number (Stull, 1988). We have frequently observed this blocking effect at Bellows Beach with trade wind clouds forming or growing some 200 m upwind of the coast. This blocking and raising of the air causes the hygroscopic marine aerosol to pick up water increasing the optical scattering effect. This is evident in the horizontal scans in Fig. 3 where it can be seen that the extinction increases near the lidar (at the coastline). Fig. 4 is a vertical scan pointed out to sea and illustrates the blocking effect with both the clouds and aerosol scatter increasing as they near the island. It can also be seen that the aerosol extinction coefficient increases with height consistent with the idea that air is raising as it nears the coastline. The enhanced scatter near the coastline is a feature we routinely see in our lidar measurements.

 

Over the past year we have added a new detector box, which collects simultaneous lidar data at wavelengths of 1064, 532 and 355 nm. The right panel in Fig 4 shows the ratio of aerosol extinctions at 1064 over 532 nm. The approach we are using is to calibrate one wavelength using the horizontal method (Porter et al., 2000a, b) and then transfer the calibration to other wavelengths by comparing the light scattered off surfaces of known reflectance such as hard targets, clouds or islands. We are currently working on this new approach and hope to have it working soon. Once we are able to obtain good wavelength dependent calibration we will employ several techniques to derive aerosol properties (Lienert et al., 2000, Porter et al., 1997).

 

  

 

Figure 4. Left panel shows aerosol  the extinction coefficient (532 nm) derived from lidar measurements collected on 9/12/2000 at 0809 hrs, HST. The right panel shows the ratio of derived aerosol extinction at 1064 divided by 532 nm.

 

IMPACT/APPLICATIONS

 

Through our efforts we have already developed new techniques to calibrate lidars to derive aerosol scattering and extinction values. We have also developed new methods to derive aerosol size distributions from multi-wavelength extinction measurements. We have also found extinction in the coastal boundary layer is frequently dominated by cloud drizzle effects and that future models must include small scale cloud processes.

 

TRANSITIONS

 

We are making our efforts known through publications and meetings. Our new lidar methods have been meet with interest by the Lidar community (IGARSS 2000). In particular Dr. John Reagan has expressed interest in applying our lidar method to his lidar measurements. Both Dr. Kaufman and Dr. M. J. Post have expressed keen interest in our new aerosol inversion. 

 

RELATED PROJECTS

 

Our lidar efforts are closely related to the work being carried out by Dr. Antony Clarke. We are comparing our lidar measurements with his in situ aerosol size distribution measurements.  We have also supplied lidar data to Dr Kuseil Shifrin, who is applying his own analysis techniques to it.

 

REFERENCES

 

Fitzgerald, J.W., 1989, Model of the aerosol extinction profile in a well-mixed marine boundary layer, Applied Optics, 28, 3534-3538.

Lienert, B.R., Porter, J.N., S.K. Sharma, 1999, Real Time Analysis and Display of Scanning Lidar Scattering Data, Marine Geodesy, 22, 259-265.

Lienert, B.R., Porter, J.N., S.K. Sharma,2000, Repetitive Genetic Inversion of Optical Scattering Data,  submitted to Applied Optics.

Porter, J., B. Lienert, S. Sharma, T. Cooney, A. Clarke, 1997: Deriving Aerosol Properties from Combined Passive and Active Measurements. Optical Society of America, Santa Fe, New Mexico.

Porter, J.N., Lienert, B., S.K. Sharma, 2000a, Using the Horizontal and Slant Lidar Calibration Methods To Obtain Aerosol Scattering Coefficients From A Coastal Lidar In Hawaii, Journal of Atmospheric and Oceanic Technology, (in press).

Porter, J. N.,  S. K. Sharma, and B. R. Lienert, 2000b, Obtaining Calibrated Marine Aerosol Extinction Measurements Using Horizontal Lidar Measurements, Differential Lidar-Target Measurements and A Polar Nephelometer, Proceedings of the SPIE Lidar Remote Sensing for Industry and Environment Monitoring, Sendai, Japan (in press).

Stull, R.B., 1988, An Introduction to Boundary Layer Meteorolgy, Kluer Academic Publishers, Dordrecht.

 

PUBLICATIONS

 

Lienert, B.R., Porter, J.N., S.K. Sharma, 2000, Repetitive Genetic Inversion of Optical Scattering Data,  Applied Optics (submitted).

Porter, J.N., Lienert, B., S.K. Sharma, 2000, Using the Horizontal and Slant Lidar Calibration Methods To Obtain Aerosol Scattering Coefficients From A Coastal Lidar In Hawaii, Journal of Atmospheric and Oceanic Technology (in press).

Porter, J. N.,  S. K. Sharma, and B. R. Lienert, 2000, Obtaining Calibrated Marine Aerosol Extinction Measurements Using Horizontal Lidar Measurements, Differential Lidar-Target Measurements and A Polar Nephelometer, Proceedings of the SPIE Lidar Remote Sensing for Industry and Environment Monitoring, Sendai, Japan (in press).

S. K. Sharma, B. R. Lienert and J.N. Porter, 2000, Scanning Lidar Measurements of Marine Aerosol Field at a Coastal  Lidar Site in Hawaii, Proceedings of the SPIE Lidar Remote Sensing for Industry and Environment Monitoring, Sendai, Japan.