R. J. Sica

The Untold Story of Pyrocumulonimbus

Wildfire is becoming the focus of increasing attention. It is now realized that changes in the occurrence frequency and intensity of wildfires has important significant consequences for a variety of important problems, including atmospheric change and safety in the urban–wildland interface. One important but poorly understood aspect of wildfire behavior—pyrocumulonimbus firestorm dynamics and atmospheric impact—has a curious history of theory and observation. The “pyroCb” is a fire-started or fire-augmented thunderstorm that in its most extreme manifestation injects huge abundances of smoke and other biomass-burning emissions into the lower stratosphere. The observed hemispheric spread of smoke and other biomass-burning emissions could have important climate consequences. PyroCbs have been spawned naturally and through anthropogenesis, and they are hypothesized as being part of the theoretical “Nuclear nuclear winter” work. However, direct attribution of the stratospheric aerosols to the pyroCb only occur...

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.

Measurements of superadiabatic lapse rates in the middle atmosphere

A large power-aperture product Rayleigh-scatter lidar system has been successfully built and over 175 nights of middle atmosphere temperature measurements have been obtained. The high signal-to-noise ratio of these measurements allows the stability of the air in the upper stratosphere and mesosphere to be determined. A detailed methodology has been developed to attempt to differentiate between lapse rate variations due to photon counting errors and actual geophysical variations. On nights when the geophysical variations are large compared to the photon counting errors, regions of convective stability and instability can be determined at a reasonably high confidence level. Both statistics of the layers and an “image” of the layers is presented for the night of May 31, 1996. The measured percentage of unstable layers is in agreement with the predictions of Hines (1991), as is the apparently sporadic formation and distribution of the unstable regions.

Modulation of upper mesospheric temperature inversions due to tidal-gravity Wave interactions

Two nights of coincident measurements with the University of Western Ontario’s MF radar and Purple Crow Lidar have been extensively analyzed to illustrate the possible e8ects due to tidal-gravity wave interactions on upper mesospheric inversion layers. On both nights an inversion layer in the upper mesosphere disappears as the westward component of the tide increases. Inversions similar in height and magnitude to these two nights, as seen in the nightly mean temperature pro:le, occur in 11% of the measurements made by the Purple Crow Lidar since 1994. Of these inversions, 76% show similar behavior to the two nights considered here in detail. The statistical results suggest the upper mesospheric inversion occur in two types. The :rst type, illustrated by the individual nights considered in this study, are associated with increased gravity wave variance as the magnitude of the westward tide decreases. These inversions are persistence over many hours. The second type of inversion has a similar nightly mean structure, but is due to shorter (on the order of i2 h) periods of heating with temperature changes much greater than that expected from tidal dissipation. c � 2002 Elsevier Science Ltd. All rights reserved.

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.

How many waves are in the gravity wave spectrum

Parametric modelling of density perturbation measurements obtained with the University of Western Ontarios Purple Crow Lidar on 5 nights are used to infer that the typical vertical wavenumber spectrum in the upper stratosphere is dominated by a few quasi-monochromatic waves. In general only 2 of these waves, with growth or decay rates on the order of 1/(14 km) or less, carry most of the spectral energy. These waves are present about half the time on the nights studied. When analyzed using classical spectral techniques these waves appear to form a continuous spectrum. These results may help explain why general circulation models obtain reasonable climatologies when gravity wave parameterization schemes based on a small number of propagating gravity waves are employed.

Measurements of the Effects of Gravity Waves in the Middle Atmosphere Using Parametric Models of Density Fluctuations. Part I: Vertical Wavenumber and Temporal Spectra

Abstract Parametric models of spectral analysis offer several distinct advantages over statistical methods such as the correlogram analysis. These advantages include higher spectral resolution and the ability, in principle, to separate correlated (i.e., wave) behavior from noise-driven (i.e., turbulent) behavior in the measurements. Here parametric models are used to highlight the spatial and temporal intermittency of the gravity wave spectrum. In Part II of this series the spatial and temporal spectrum are used to calculate energy dissipation and the eddy diffusion coefficient. The spectra are computed from measurements of density fluctuations obtained using a large power-aperture product lidar during a 6-h period on 30 August 1994. It is shown that parametric models provide an excellent representation of the temporal and spatial data series. One difficulty of parametric models is selecting the model order, an analogous situation to determining the proper lag in the correlogram procedure or the window le...

A Remotely Operated Lidar for Aerosol, Temperature, and Water Vapor Profiling in the High Arctic

AbstractA Rayleigh–Mie–Raman lidar has been installed and is operating in the Polar Environment Atmospheric Research Laboratory at Eureka in the High Arctic (79°59′N, 85°56′W) as part of the Canadian Network for the Detection of Atmospheric Change. The lidar operates in both the visible and ultraviolet and measures aerosol backscatter and extinction coefficients, depolarization ratio, tropospheric temperature, and water vapor mixing ratio. Variable field of view, aperture, and filtering allow fine-tuning of the instrument for different atmospheric conditions. Because of the remote location, operations are carried out via a satellite link. The instrument is introduced along with the measurement techniques utilized and interference filter specifications. The temperature dependence of the water vapor signal depends on the filter specifications, and this is discussed in terms of minimizing the uncertainty of the water vapor mixing ratio product. Finally, an example measurement is presented to illustrate the p...

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...