LASE Investigations of Water Vapor & Aerosols

During PEM-Tropics B

Edward V. Browell, Principal Investigator*

Syed Ismail, Co-Principal Investigator

Atmospheric Sciences Division

NASA Langley Research Center

Hampton, VA 23681-0001

*Phone: 757-864-1273; Fax: 757-8647790

E-Mail: e.v.browell@larc.nasa.gov

During the NASA GTE/PEM-Tropics B field experiment, we will operate the LASE (Lidar Atmospheric Sensing Experiment) system from the DC-8 aircraft to provide high resolution profiles of water vapor, aerosols, and clouds from the surface to tropopause level and columnar amounts of water vapor. These data will be used in the study of many processes of interest to the PEM-Tropics B Mission including: the production of OH; atmospheric dynamics and transport including the deep tropical convection that transfers chemically active species to the upper troposphere, the transport of plumes and air masses across the ITCZ and SPCZ, andstratospheric-tropospheric exchange; the growth and development of the marine boundary layer (MBL); and the growth and development of aerosols and clouds. LASE water vapor data can also be used with aerosol and ozone data from other lidar measurements in the remote identification of airmass types. We will also collaborate in the investigation of the role of water vapor in meteorological conditions that are of interest to the PEM-Tropics study.

In addition, LASE measurements of aerosols at 815 nm will be used to characterize the aerosol structure of the tropical troposphere from the measurements taken during the mission. These data would be valuable in identifying the origin and dispersal of pollutants and aerosol plumes. Simultaneous LASE water vapor and aerosol measurements along with chemical species measured during the PEM-Tropics B Mission will be used to study the nucleation and growth of aerosols and clouds. LASE aerosol measurements at 815 nm will be used in conjunction with aerosol measurements by the multiwavelength UV DIAL system to conduct an aerosol type and size distribution study for remotely monitoring of aerosol and plumes in the Pacific Basin.

The LASE team will provide real-time (preliminary) and post-flight color images depicting the distributions of water vapor and aerosol backscatter to the science investigators to assist in flight planning as needed. High resolution cross-sections of water vapor distributions (300 m vertical, 10 km horizontal) and aerosol backscattering and scattering ratios (30 m vertical and 200 m horizontal) will be derived after post-processing following the field experiment. These data will be used to address the scientific questions related to the PEM-Tropics Mission.

Characteristics of the LASE System

LASE is a differential absorption lidar (DIAL) system that operates in the 815 nm region. The LASE system was developed as an autonomous DIAL system and was originally designed and operated from the Q-bay of the high altitude NASA ER-2 aircraft. LASE demonstrated autonomous operating capability, reliability of operation, and accuracy for profiling water vapor over the entire troposphere during the 1995 LASE Validation Experiment. LASE has since participated in the Tropospheric Aerosol Radiative Forcing Experiment (TARFOX) held over the Atlantic Ocean east of Wallops Island, VA in July 1996 and in the 1997 Southern Great Plains Experiment (SGP97) conducted in Oklahoma during June-July 1997 as part of the NASA Hydrology Program. LASE is being reconfigured to fly on the NASA DC-8. On the DC-8 LASE will operate during the PEM Tropics B mission to acquire data simultaneously in the nadir and zenith pointing mode to permit coverage over most of the troposphere.

The LASE system uses a Ti:sapphire-based laser system as transmitter. This laser system consists of a double-pulsed Ti:sapphire laser that operates in the 815-nm absorption band of water vapor and is pumped by a frequency-doubled flashlamp-pumped Nd:YAG laser. The wavelength of the Ti:sapphire laser is controlled by injection seeding with a diode laser that is frequency locked to a water vapor line using an absorption cell. The LASE detector system consists of two silicon avalanche photodiodes (Si:APD) and three digitizers to cover a large signal dynamic range (10⁶), and the signal processor system is designed to be relatively insensitive to rapid changes in signal levels. The LASE data system on the DC-8 will enable real-time and post-flight analyses on-board the aircraft. A 275 MHz Alpine w/DEC Alpha CPU with 128 MB memory 12 GB data storage is used for data processing and analysis Adjustments to the instrument will be made on-board the aircraft using on-board test equipment to maintain optimum performance.

In the current mode of operation LASE operates locked to a strong water vapor line and electronically tunes to any spectral position on the absorption line to choose the suitable absorption cross-section for optimum measurements over a range of water vapor concentrations in the atmosphere. In addition, LASE can operate over two or three water vapor concentration regions to cover a large altitude region in the troposphere. This unique method of operation permits rapid and more flexible absorption cross-section selection capability for water vapor measurements over the entire troposphere in a single pass. Water vapor measurements using this new method have been implemented and tested during the LASE validation experiment in September 1995.

For deployment on the DC-8 during PEM-Tropics B, LASE will be configured to acquire data simultaneously in both the nadir and zenith pointing modes to permit coverage over most of the troposphere. Figure 1 shows a schematic diagram of LASE. Since the DC-8 is expected to fly at a nominal altitude of 35,000 ft (10 km) altitude, the LASE system will operate, generally, beneath the cirrus clouds in the tropical Pacific. The addition of the simultaneous zenith data acquisition mode for LASE on the DC-8 will permit measurements of the water vapor, aerosols, and clouds above the aircraft in the upper troposphere as well. In cloud-free regions or in regions with optically thin cirrus clouds, LASE will measure the water vapor and aerosol distributions throughout the troposphere.

LASE Measurement Capabilities

During the September 1995 LASE Validation Field Experiment, LASE measurements were compared with a number of in-situ and remote sensors from the ground and other aircraft (Browell et al., 1997). During the Validation Experiment LASE demonstrated the capability to measure water vapor distributions over the entire troposphere with values ranging from about 15 g/kg near the ocean surface to about 0.01 g/kg near the tropopause in the tropical region and at mid-latitudes. The LASE water vapor measurements were found to have an accuracy of better than 6% or 0.01 g/kg, whichever is grater, across the entire troposphere. The projected water vapor and aerosol measurement capabilities of LASE during the PEM-Tropics B mission are listed in Table 1.

Figure 1. Schematic diagram of LASE as configured on DC-8 for PEM Tropics B.

Table 1. LASE Water Vapor and Aerosol Profiling Capability on NASA DC-8

Reference:

Browell, E. V., S. Ismail, W. M. Hall, A. S. Moore, S. A. Kooi et al., LASE Validation Experiment, in Advances in Atmospheric Remote Sensing with Lidar, A. Ansmann, R. Neuber, P. Rairoux, and U. Wandinger, eds. Springer-Verlag, Berlin, pp 289-295, 1997.