P. S. Argall

Lidar measurements taken with a large-aperture liquid mirror. 1. Rayleigh-scatter system

A lidar system has been built to measure atmospheric-density fluctuations and the temperature in the upper stratosphere, the mesosphere, and the lower thermosphere, measurements that are important for an understanding of climate and weather phenomena. This lidar system, the Purple Crow Lidar, uses two transmitter beams to obtain atmospheric returns resulting from Rayleigh scattering and sodium-resonance fluorescence. The Rayleigh-scatter transmitter is a Nd:YAG laser that generates 600 mJ/pulse at the second-harmonic frequency, with a 20-Hz pulse-repetition rate. The sodium-resonance-fluorescence transmitter is a Nd:YAG-pumped ring dye laser with a sufficiently narrow bandwidth to measure the line shape of the sodium D(2) line. The receiver is a 2.65-m-diameter liquid-mercury mirror. A container holding the mercury is spun at 10 rpm to produce a parabolic surface of high quality and reflectivity. Test results are presented which demonstrate that the mirror behaves like a conventional glass mirror of the same size. With this mirror, the lidar systems performance is within 10% of theoretical expectations. Furthermore, the liquid mirror has proved itself reliable over a wide range of environmental conditions. The use of such a large mirror presented several engineering challenges involving the passage of light through the system and detector linearity, both of which are critical for accurate retrieval of atmospheric temperatures. These issues and their associated uncertainties are documented in detail. It is shown that the Rayleigh-scatter lidar system can reliably and routinely measure atmospheric-density fluctuations and temperatures at high temporal and spatial resolutions.

Mesospheric temperature from UARS MLS: retrieval and validation

A research algorithm is developed to retrieve temperature at 20 –90 km using 63 GHz O2 emission measurements from Microwave Limb Sounder (MLS) on Upper Atmosphere Research Satellite (UARS). The algorithm is based on a previous MLS radiative transfer model but improved to produce more accurate radiance calculations in the cases where the geomagnetic Zeeman splitting is important. A fast version of the model is developed and implemented for practical uses of the temperature retrieval, which uses a single temperature and O2 density pro=le as the linearization basis. The calculated radiances and linearization coe>cients are =t to a set of explicit functions of the geomagnetic =eld and its direction at tangent heights of 0 –120 km, which are pre-stored in order to speed up the computation. The new algorithm has been used to process all the data available during 1991–1997 before MLS 63 GHz radiometer was powered oA. The estimated precision of MLS temperature varies from 2 K at ∼20 km to 8 K at ∼80 km and increases sharply above ∼90 km. The retrieved MLS temperature are compared against CIRA’86, satellite, lidar, and rocket observations. Comparisons to CIRA’86 seasonal climatology show that the diAerences are latitude-and-season dependent and generallyi 5 K below 50 km and 10 K in the mesosphere. Comparisons with other satellite observations (ISAMS, HRDI, CRISTA1) show diAerent patterns but a cold bias at 85 –90 km seems common in all these comparisons. Comparisons to ground-based lidar measurements suggest that MLS temperatures are warmer by 2–4 K in the stratosphere and colder by 5 –15 K at 85 –90 km. The MLS-minus-lidar diAerence shows a 3–10 K cold bias near 70 km for most of the sites selected. The comparisons with rocket measurements are similar to those with lidars at these altitudes, giving cold biases in the MLS temperatures at 85 –90 km. Most of these biases are understandable in terms of sampling and resolution diAerences, and some biases can be reduced with further improvements in the MLS retrieval algorithm. Despite the existing biases, the MLS temperature have been found useful in studying large-scale mesospheric phenomena such as the temperature inversion layer. c

Lidar measurements taken with a large-aperture liquid mirror. 2. Sodium resonance-fluorescence system

Sodium resonance-fluorescence lidar is an established technique for measuring atmospheric composition and dynamics in the mesopause region. A large-power-aperture product (6.6-W m(2)) sodium resonance-fluorescence lidar has been built as a part of the Purple Crow Lidar (PCL) at The University of Western Ontario. This sodium resonance-fluorescence lidar measures, with high optical efficiency, both sodium density and temperature profiles in the 83-100-km region. The sodium lidar operates simultaneously with a powerful Rayleigh- and Raman-scatter lidar (66 W m(2)). The PCL is thus capable of simultaneous measurement of temperature from the tropopause to the lower thermosphere. The sodium resonance-fluorescence lidar is shown to be able to measure temperature to an absolute precision of 1.5 K and a statistical accuracy of 1 K with a spatial-temporal resolution of 72 (km s) at an altitude of 92 km. We present results from three nights of measurements taken with the sodium lidar and compare these with coincident Rayleigh-scatter lidar measurements. These measurements show significant differences between the temperature profiles derived by the two techniques, which we attribute to variations in the ratio of molecular nitrogen to molecular oxygen that are not accounted for in the standard Rayleigh-scatter temperature analysis.

Ozone Corrections for Rayleigh-Scatter Temperature Determinations in the Middle Atmosphere

Abstract A well-established technique for the determination of temperature in the middle atmosphere is the retrieval of temperature profiles from density profiles of air. The measurement of air density profiles from the ground and from space are typically determined from measurements of Rayleigh-scattered light. Most researchers using the Rayleigh-scatter temperature technique do not state whether they correct their measurements for the absorption of light due to ozone in the upper stratosphere. Such corrections may have been less significant for initial studies of temperature, but with the current need for temperature measurements of sufficient quality to access atmospheric change, these corrections take on an added importance. Significant improvement to the temperature measurements in the stratosphere are shown to result by including this effect for any reasonable choice of ozone profile. Simple correction functions are presented for temperature measurements, appropriate for low, middle, and high latitu...

Retrieval of molecular nitrogen and molecular oxygen densities in the upper mesosphere and lower thermosphere using ground‐based lidar measurements

Two new techniques have been developed to estimate the densities of N2, O2, and O from Rayleigh lidar backscatter photocount profiles and from independent temperature determinations. Both techniques involve solving initial value problems for the N2, O2, and O densities. These initial value problems are derived from the lidar equation, the ideal gas law, and the assumption of hydrostatic equilibrium. Their solutions are valid in regions where the Rayleigh lidar backscatter can be fully isolated from other scattering sources, e.g., aerosol scattering and where independent temperature determinations can been made. Estimates of the N2 and O2 density profiles in the upper mesosphere and lower thermosphere have been performed on three nights using simultaneous sodium-resonance-fluorescence lidar measurements to independently determine the temperature profile. The available measurements are of insufficient quality to retrieve O density profiles. The resulting N2 and O2 density retrievals appear reasonable when compared with previous determinations from sounding rockets and give hope that this method will allow routine ground-based measurements of major constituent density in the middle atmosphere. A detailed error analysis is described which shows that the primary source of random errors is the Rayleigh photocounts, while the most important systematic errors are uncertainties in the N2 and O2 Rayleigh backscatter cross sections and in the normalization of the lidar density profiles in the upper stratosphere and lower mesosphere.