Richard L. Collins

Gravity waves in the upper mesosphere over Antarctica : Lidar observations at the South Pole and Syowa

Lidar observations of the mesospheric Na layer were made at the south pole(90°S) and Syowa (69°S) during the winters of 1990 and 1985, respectively. These observations are used to characterize the gravity wave activity in the upper mesosphere at both sites. Strong wave activity is observed throughout the winter at both the south pole and Syowa and shows remarkable similarity with observations from several midlatitude and low-latitude sites. The quasi-monochromatic gravity waves exhibit the same general relationships between their wavelengths, observed periods, and amplitudes as observed at lower latitudes. The average growth length of these waves is approximately 26 km, indicating that the wave field at both Antarctic sites is strongly influenced by dissipation and saturation processes. The spectra and variances of the density perturbations associated with quasi-random wave field at the south pole are reported. The vertical wavenumber and temporal frequency spectra follow power-law shapes. The mean index of the vertical wavenumber spectrum is −2.4, and the mean characteristic wavelength is 14 km. The mean index of the temporal frequency spectrum is −1.7. The mean density variance at the south pole is (5.7%)2 and is similar in magnitude to that observed at a variety of lower-latitude sites. With no tropospheric convection during the polar night and little orographic forcing over the relatively featureless Antarctic plateau, these observations suggest that nonlinear processes, rather than the source characteristics, primarily determine the characteristics of the gravity wave field in the upper mesosphere. These observations show two other distinct features. The mean Na layer over Antarctica is significantly lower and broader (centroid height ≈ 90 km and rms width ≈ 4.8 km) than at lower latitudes, reflecting the stronger downwelling and warmer winter temperatures in the mesopause region at high latitudes. Strong coherent oscillations were observed in the bottomside density contours of the Na layer with periods close to the inertial period. These oscillations were also observed in OH airglow measurements and appear to be associate with planetary scale waves.

Gravity wave activity in the upper mesosphere over Urbana, Illinois: lidar observations and analysis of gravity wave propagation models

Abstract We analyze 375 h of Na Wind/Temperature lidar measurements of the mesopause region (≈ 80–105 km) Na density and temperature profiles on 57 nights distributed over 2 yr at Urbana, Illinois. These observations yield a high-resolution seasonal data set of gravity wave activity in the upper mesosphere. From this data, we present measurements of the Brunt-Vaisala period, the relative atmospheric density perturbations and their spectra, and the parameters of 143 quasi-monochromatic gravity waves. The direct measurement of the Brunt-Vaisala period allows accurate calculation of the horizontal velocity perturbations and vertical displacement perturbations from the density measurements. The horizontal velocity and vertical displacement vertical wave number spectrum magnitudes and indices show considerable seasonal and nightly variability. The gravity wave amplitudes, wavelengths, and observed periods exhibit systematic relationships similar to those found in previous studies, and are consistent with the MU radar measurements of intrinsic gravity wave parameters. Here, we present a detailed analysis of the observations in terms of Diffusive-Filtering Theory models of gravity wave propagation. The magnitudes of the vertical wave number spectrum, the form of the joint vertical wave number and frequency spectrum, and the systematic relationships between the monochromatic gravity wave parameters are consistent with the Diffusive-Filtering model. We compare these results with a variety of radar, lidar, and airglow observations from other sites. This observational study suggests that the complex nonlinear interactions of the gravity wave field may be modeled successfully as a diffusive damping process, where the effective diffusivity is a function of the total wave variance.

Signature of an overturning gravity wave in the mesospheric sodium layer: Comparison of a nonlinear photochemical-dynamical model and lidar observations

In this paper we study the evolution of the sodium layer in the presence of an overturning ( or convectively unstable) gravity wave using model simulations and lidar observations. The simulations employ a time-dependent, nonlinear, photochemical-dynamical model. The observations are a 9-day (210-hour) set of sodium density and temperature lidar measurements from Fort Collins, Colorado ( 41 degrees N, 105 degrees W). We model the evolution of large-scale ( vertical wavelength of 30 km) and small-scale ( vertical wavelength of 10 km) waves and the associated evolution of the sodium layer. We use filtering methods to identify waves of similar scales in the lidar measurements. The semidiurnal tide is the dominant large-scale wave in the lidar observations. We present the observed evolution of sodium density, mixing ratio, temperature, potential temperature, and buoyancy period over 24-hour periods. We find that the model and observations show similar behavior in the evolution of the sodium densities, mixing ratios, and potential temperature in response to large- and small-scale waves. The model and observations indicate that the sodium density perturbation has a more pronounced overturning behavior in the bottomside of the layer than the topside of the layer. The sodium density also has a more pronounced overturning behavior than the mixing ratio and potential temperature. The overturning signatures in the sodium density due to small-scale waves occur periodically at the wave period even before the wave itself becomes completely unstable. The study suggests that observations of single overturning events in sodium densities should be interpreted with caution and may not indicate complete overturning of a wave.

Rayleigh lidar observations of mesospheric inversion layers at Poker Flat, Alaska (65 °N, 147°W)

Rayleigh lidar measurements of the stratosphere and mesosphere have been made on an ongoing basis over a three-year period at Poker Flat, Alaska (65°N, 147°W). These observations have yielded 27 nightly measurements of the middle atmosphere temperature profile (∼ 40–80 km). These nighttime measurements are distributed between August and April. Mesospheric inversion layers have been observed on five occasions. The average altitude of the inversion layer peak is 60 km, with average amplitude of 18 K. The temperature gradients on the topside of the inversion layers approach the adiabatic lapse rate. The inversion layers do not exhibit the apparent downward phase velocities that are commonly observed at lower latitudes. Furthermore, the inversion layers appear significantly less frequently than at lower latitudes. The observations are discussed in terms of current models and observations at other sites.

Lidar observations of a large high‐altitude sporadic Na layer during active aurora

A large sporadic Na layer was observed near 109 km during active aurora at Poker Flat, Alaska, on the night of 12–13 January 1996. This sporadic layer is significantly denser and broader than previously reported thermospheric sporadic layers; having a peak concentration equal to the peak concentration of the background Na layer, a FWHM of 4.7 km, and a column abundance of 40% of the background Na layer. The appearence of the Nas layer coincided with a significant OH temperature gradient across the sky but did not coincide with the presence of a discrete arc overhead nor a distinct signature in the local magnetometer data. The presence of such a large amount of neutral Na at these altitudes contradicts the current models of Nas layer formation. The observations are discussed in the light of other observations, current models, layered auroral phenomena, and meteoric ablation.

Multi-year temperature measurements of the middle atmosphere at Chatanika, Alaska (65°N, 147°W)

Over an eight-year period (1997–2005) Rayleigh lidar temperature measurements of the stratosphere and mesosphere (40–80 km) have been made at Poker Flat Research Range, Chatanika, Alaska (65°N, 147°W). The Rayleigh lidar measurements have been made between mid-August and mid-May. These measurements have yielded a total of approximately 904 hours of temperature measurements of the middle atmosphere over 116 nights. The seasonal evolution of the middle atmosphere shows an annual cycle with maximum in summer below 60 km and a reversal of the cycle with minimum in summer above 60 km. The monthly mean stratopause has a highest temperature of 273 K at an altitude of 47.5 km in May and a lowest temperature of 243 K at an altitude of 54.7 km in January. However, nightly stratopause temperatures in January and December are sometimes warmer than those in May and August. An elevated stratopause (>65 km) is observed on 5 occasions in 41 observations in January and February. The Chatanika measurements are compared with five other Arctic data sets and models. The upper stratosphere at this site is slightly colder than the zonal mean as well as sites in Greenland and Scandinavia with the largest differences found in January. We discuss the wintertime temperatures in the upper stratosphere and lower mesosphere in terms of the position of the polar vortex and the increased occurrence of stratospheric warming events during the 1997–2005 observation period.

Mesospheric Inversion Layers at Chatanika, Alaska (65°N, 147°W): Rayleigh lidar observations and analysis

Rayleigh lidar observations at Poker Flat Research Range, Chatanika, Alaska (65°N, 147°W), have yielded density and temperature measurements from 40 km to 80 km. These measurements have been made under clear skies at night between November 1997 and May 2009. We have identified and characterized 79 Mesospheric Inversion Layers (MILs) in the 40–70 km altitude region on 56 of 117 nights of observations between August and May. These 79 MILs have an average amplitude of 13 K, peak altitude of 61 km, topside lapse rate of 6 K/km, depth or thickness of 3 km, base altitude of 58 km, bottomside gradient of 5 K/km, and downward vertical velocity of 0.1 km/h based on the 2 h temperature profiles. MILs occur with a nightly occurrence rate of 48%, more frequently than previously reported at Arctic sites, but less frequently than at lower latitude sites. MILs are embedded in the larger planetary wave structure. MILs are found to occur most commonly in January as has been reported from lower latitude sites. We find that the planetary wave-1 amplitudes are a factor 1.3 larger at an altitude of 5.0–6.5 scale heights (~32–39 km) on days when MILs are observed than when they are not observed. The planetary wave-2 amplitudes are not significantly different when MILs are present and absent. We find that Eliassen-Palm flux divergences at an altitude of 7–8 scale height (~45–52 km) are a factor 2.0 larger on nights when MILs are observed than when they are not observed. We also find that gravity wave potential energy densities are a factor 1.3 larger on nights when MILs are observed than on nights when they are not observed.

Comparative MF radar and Na lidar measurements of fluctuating winds in the mesopause region

Na lidar and MF radar observations of the mesopause region (≈ 80–105 km) were made over a 2-year period at Urbana, Illinois (40°N, 88°W). The Na lidar data yielded both temperature profiles and Na density profiles which are used to derive independent estimates of the rms horizontal winds. Estimates of the rms horizontal winds from concurrent radar and lidar measurements are obtained on 37 nights, totaling 263 hours. These three sets of rms wind estimates are processed in a common fashion. This analysis yields vertical profiles of the rms winds that do not grow exponentially with altitude, indicating that the gravity wave field is saturated throughout this altitude region. All three techniques reveal an annual maximum in the summer rms horizontal winds. However, the Na density estimates also have a semiannual variation with a second maximum in winter, which was not produced by the other techniques. Estimates of the rms winds derived from lidar Na density and Na temperature data are comparable in magnitude. The radar estimates are systematically larger than both sets of lidar estimates at higher altitudes. The discrepancy between the radar and lidar estimates is found to be greatest at those altitudes where the saturation is most pronounced.

Statistical Analysis of Sodium Doppler Wind–Temperature Lidar Measurements of Vertical Heat Flux

Abstract A statistical study is presented of the errors in sodium Doppler lidar measurements of wind and temperature in the mesosphere that arise from the statistics of the photon-counting process that is inherent in the technique. The authors use data from the Colorado State University (CSU) sodium Doppler wind-temperature lidar, acquired at a midlatitude site, to define the statistics of the lidar measurements in different seasons under both daytime and nighttime conditions. The CSU lidar measurements are scaled, based on a 35-cm-diameter receiver telescope, to the use of large-aperture telescopes (i.e., 1-, 1.8-, and 3.5-m diameters). The expected biases in vertical heat flux measurements at a resolution of 480 m and 150 s are determined and compared to Gardner and Yang’s reported geophysical values of 2.3 K m s−1. A cross-correlation coefficient of 2%–7% between the lidar wind and temperature estimates is found. It is also found that the biases vary from −4 × 10−3 K m s−1 for wintertime measurements a...

Gravity Wave Dynamics in a Mesospheric Inversion Layer: 2. Instabilities, Turbulence, Fluxes, and Mixing

A companion paper by Fritts et al. [2017a] employed an anelastic numerical model to explore the dynamics of gravity waves (GWs) encountering a mesospheric inversion layer (MIL) having a moderate static stability enhancement and a layer of weaker static stability above. That study revealed that MIL responses, including GW transmission, reflection, and instabilities, are sensitive functions of GW parameters. This paper expands on two of the Fritts et al. [2017a] simulations to examine GW instability dynamics and turbulence in the MIL, forcing of the mean wind and stability environments by GW, instability, and turbulence fluxes, and associated heat and momentum transports. These direct numerical simulations resolve turbulence inertial-range scales and yield the following results: GW breaking and turbulence in the MIL occur below where they would otherwise due to enhancements of GW amplitudes and shears in the MIL, 2D GW and instability heat and momentum fluxes are ~20-30 times larger than 3D instability and turbulence fluxes, mean fields are driven largely by 2D GW and instability dynamics rather than 3D instabilities and turbulence, 2D and 3D heat fluxes in regions of strong turbulence yield small departures from initial T(z) and N2(z) profiles, hence do not yield nearly adiabatic “mixed” layers, and our MIL results are consistent with the relation between the turbulent vertical velocity variance and energy dissipation rate proposed by Weinstock [1981] for the limited intervals evaluated.