Climatology of High-frequency Gravity Waves Observed by an Airglow Imager at Andes Lidar Observatory

The long-term climatology of high-frequency quasi-monochromatic gravity waves is presented using multi-year airglow images observed at Andes Lidar Observatory (ALO, 30.3ºS, 70.7ºW) in northern Chile. A large number of high-frequency gravity waves were retrieved from OH airglow images. The distribution of primary wave parameters including horizontal wavelength, vertical wavelength, intrinsic wave speed, and intrinsic wave period are obtained and are in the ranges of 20–30 km, 15–25 km, 50–100 ms-1, and 5–10 min, respectively. The waves tend to propagate against the local background winds and show clear seasonal variations. In austral winter (Ma–Aug), the observed wave occurrence frequency is higher and preferential wave propagation is equator-ward. In austral summer (Nov–Feb), the wave occurrence frequency is lower and the waves mostly propagate pole-ward. Critical-layer filtering plays an important role in determining the preferential propagation direction in certain months, especially for waves with a small observed phase speed (less than typical background winds). The wave occurrence and preferential propagation direction are shown to be related to the locations of convection activities nearby and their relative distance to ALO. However, other possible wave sources such as secondary wave generation and possible ducted propagation cannot be ruled out. The estimated momentum fluxes have typical values of a few to 10 m2s-2 and show seasonal variations with a clear anti-correlation with local background wind directions

Statistical Characteristics of High-frequency Gravity Waves Observed by an Airglow Imager at Andes Lidar Observatory

The long-term statistical characteristics of high-frequency quasi-monochromatic gravity waves are presented using multi-year airglow images observed at Andes Lidar Observatory (ALO, 30.3° S, 70.7° W) in northern Chile. The distribution of primary gravity wave parameters including horizontal wavelength, vertical wavelength, intrinsic wave speed, and intrinsic wave period are obtained and are in the ranges of 20–30 km, 15–25 km, 50–100 m s−1, and 5–10 min, respectively. The duration of persistent gravity wave events captured by the imager approximately follows an exponential distribution with an average duration of 7–9 min. The waves tend to propagate against the local background winds and show evidence of seasonal variations. In austral winter (May–Aug), the observed wave occurrence frequency is higher, and preferential wave propagation is equator-ward. In austral summer (Nov–Feb), the wave occurrence frequency is lower, and the waves mostly propagate pole-ward. Critical-layer filtering plays a moderate role in determining the preferential propagation direction in certain months, especially for waves with a smaller observed phase speed (less than typical background winds). The observed wave occurrence and preferential propagation direction are related to the locations of convection activities nearby and their relative distance to ALO. However, direct wave generations are less likely due to the large distance between the ALO and convective sources. Other mechanisms such as secondary wave generation and possible ducted propagation should be considered. The estimated mean momentum fluxes have typical values of a few m2 s−2

Intermittency of Gravity Wave Momentum Flux in the Mesopause Region Observed with an All-Sky Airglow Imager

The intermittency of gravity wave momentum flux (MF) near the OH airglow layer (∼87 km) in the mesopause region is investigated for the first time using observation of all-sky airglow imager over Maui, Hawaii (20.7∘N, 156.3∘W), and Cerro Pachón, Chile (30.3∘S, 70.7∘W). At both sites, the probability density function (pdf) of gravity wave MF shows two distinct distributions depending on the magnitude of the MF. For MF smaller (larger) than ∼16 m2 s−2 (0.091 mPa), the pdf follows a lognormal (power law) distribution. The intermittency represented by the Bernoulli proxy and the percentile ratio shows that gravity waves have higher intermittency at Maui than at Cerro Pachón, suggesting more intermittent background variation above Maui. It is found that most of the MF is contributed by waves that occur very infrequently. But waves that individually contribute little MF are also important because of their higher occurrence frequencies. The peak contribution is from waves with MF around ∼2.2 m2 s−2 at Cerro Pachón and ∼5.5 m2 s−2 at Maui. Seasonal variations of the pdf and intermittency imply that the background atmosphere has larger influence on the observed intermittency in the mesopause region

A Modeling Study of O2 and OH Airglow Perturbations Induced by Atmospheric Gravity Waves

A one-dimensional model is used to investigate the relations between gravity waves and O2 and OH airglows perturbations. The amplitude and phase of the airglow perturbations induced by gravity waves (with period \u3e 20 min) are calculated for different vertical wavelength (10–50 km) and damping rate. The model shows that for vertically propagating gravity waves, the amplitude of airglow perturbations observed from ground is larger for longer vertical wavelength, because of the smaller cancellation effect within each layer. The ratio of the amplitudes between O2 and OH is smaller for larger wave damping. For upward propagating (downward phase progression) waves, the intensity perturbation in O2 leads OH, and their phase difference (O2 minus OH) is larger for smaller vertical length and/or stronger damping. The rotational temperature perturbation leads intensity perturbation in both layers. Their phase difference is also larger for smaller vertical length but is smaller for stronger damping. Based on these relations, the vertical wavelength and damping rate of gravity waves can be derived from simultaneous measurements of airglow perturbations in O2 and OH layers

Vertical Heat and Constituent Transport in the Mesopause Region by Dissipating Gravity Waves at Maui, Hawaii (20.7ºN), and Starfire Optical Range, New Mexico (35ºN)

Vertical heat flux profiles induced by dissipating gravity waves in the mesopause region (85–100 km altitude) are derived from Na lidar measurements of winds and temperatures at Maui (20.7ºN, 156.3ºW), Hawaii, and compared with earlier results from Starfire Optical Range (SOR, 35.0ºN, 106.5ºW), New Mexico. The heat flux profile at SOR has a single downward maximum of 2.25 ± 0.3 K m/s at 88 km, while the profile at Maui has two downward maxima of 1.25 ± 0.5 K m/s and 1.40 ± 0.5 K m/s at 87 and 95 km, respectively. The common maximum below 90 km can be attributed to high probability of convective instability. Comparison of the horizontal wind shear suggests that the second maximum at 95 km at Maui may be associated with dynamic instability. The measured Na flux and predicted Na flux based on measured heat flux at Maui agree well, further confirming earlier findings using SOR data. The dynamical flux of atomic oxygen estimated from the heat flux is smaller at Maui compared with that at SOR, but both are comparable to or larger than the eddy flux. The results also suggest that weaker gravity wave dissipation at Maui may cause two opposite effects on the energy balance in the mesopause region, a reduced cooling from heat transport and reduced chemical heating from atomic oxygen transport

Seasonal Variations of the Vertical Fluxes of Heat and Horizontal Momentum in the Mesopause Region at Starfire Optical Range, New Mexico

Lidar observations of wind and temperature profiles between 85 and 100 km, conducted at the Starfire Optical Range (SOR), New Mexico, are used to characterize the seasonal variations of the vertical fluxes of heat and horizontal momentum and their relationships to gravity wave activity in this region. The wind and temperature variances exhibit strong 6-month oscillations with maxima during the summer and winter that are about 3 times larger than the spring and fall minima. The vertical heat flux also exhibits strong 6-month oscillations with maximum downward flux during winter and summer. The downward heat flux peaks near 88 km where it exceeds -3 K m s-1 in mid-winter and is nearly zero during the spring and fall equinoxes. The heat flux is significantly different from zero only when the local instability probability exceeds 8%, i.e., the annual mean for the mesopause region. The momentum fluxes also exhibit strong seasonal variations, which are related to the horizontal winds. Two-thirds of the time the horizontal momentum flux is directed against the mean wind field

Maintenance of Circulation Anomalies during the 1988 Drought and 1993 Floods over the United States

The large-scale circulation anomalies associated with the 1988 drought and the 1993 floods are investigated with the National Centers for Environmental Prediction Reanalysis data and a linear stationary wave model. The transient vorticity and thermal forcings are explicitly calculated and the diabatic heating is derived as a residual in the thermodynamic energy equation. Using the April–June (AMJ) data for 1988, and June–August (JJA) data for 1993, the linear stationary wave model is able to reproduce the main features of the geopotential height anomaly for the two seasons when all forcings are included. This provides a basis for further investigation of stationary wave response to different forcing mechanisms using the linear model. Within the linear model framework, the linear model responses to different forcings are examined separately. The results indicate that the 1988 anomaly over the United States is a result of both the diabatic heating and the transient vorticity and thermal forcings. The large anticyclonic anomalies over the North Pacific and Canada are forced mainly by the diabatic heating. The 1993 anomaly, however, is dominated by the response to transient vorticity forcing. By further separating the linear model responses to regional diabatic heating anomalies in 1988, the results indicate that the western North Pacific heating is entirely responsible for the anticyclonic center over the North Pacific, which causes the northward shift and intensification of the Pacific jet stream. The eastern North Pacific heating/cooling couplet is the most important for maintaining the North American circulation anomaly. The tropical eastern Pacific cooling/heating anomalies associated with the La Nina condition have negligible influence on the North American circulation. In 1993, the strong diabatic heating over the North American continent largely compensates the effect of the cooling over the North Pacific. The dynamics of the AMJ and JJA climate is further explored by calculating its Green’s function for both diabatic heating and vorticity forcing. The results again show negligible influence from the equatorial Pacific. The most effective location for diabatic heating to generate a North American circulation anomaly is along the west coast of North America, where the zonal wind is relatively weak. There is little sensitivity in the Green’s function solution to the different basic states

First Na Lidar Measurements of Turbulence Heat Flux, Thermal Diffusivity, and Energy Dissipation Rate in the Mesopause Region

Turbulence is ubiquitous in the mesopause region, where the atmospheric stability is low and wave breaking is frequent. Measuring turbulence is challenging in this region and is traditionally done by rocket soundings and radars. In this work, we show for the first time that the modern Na wind/temperature lidar located at Andes Lidar Observatory in Cerro Pachón, Chile, is able to directly measure the turbulence perturbations in temperature and vertical wind between 85 and 100 km. Using 150 h of lidar observations, we derived the frequency (ω) and vertical wave number (m) spectra for both gravity wave and turbulence, which follow the power law with slopes consistent with theoretical models. The eddy heat flux generally decreases with altitude from about −0.5 Km s−1 at 85 km to −0.1 Km s−1 at 100 km, with a local maximum of −0.6 Km s−1 at 93 km. The derived mean turbulence thermal diffusivity and energy dissipation rate are 43 m2 s−1 and 37 mW kg−1, respectively. The mean net cooling resulted from the heat transport and energy dissipation is −4.9 ± 1.5 K d−1, comparable to that due to gravity wave transport at −7.9 ± 1.9 K d−1. Turbulence key parameters show consistency with turbulence theories