Nikolai M. Gavrilov

The nonlinear mechanism of gravity wave generation by meteorological motions in the atmosphere

Abstract Using asymptotic expansions of the hydrodynamic equations in the Rossby number and the method of multiple time scales, we derive approximate expressions for the inhomogeneous “forcing” terms which describe the continuous generation of inertio-gravity waves by quasi-geostrophic motions. As a result of numerical modelling applied to the evolution of tropospheric meso- and macro-scale wave sources, the values of these forcing terms are estimated. A three-dimensional numerical simulation of wave propagation from a mesometeorological tropospheric eddy into the upper atmosphere was done to estimate the gravity wave response to the sources described. The results of the calculations show that the most part of the wave energy propagates quasi-horizontally carried by two-dimensional inertio-gravity waves. At the same time, a part of the energy is transported into the upper atmosphere by internal-gravity waves which can create regions of wave disturbance in the upper atmosphere at considerable distances from the source site. The amplitudes of these waves increase with increasing intensity and decreasing time scales of the wave sources and can reach the values observed in the upper atmosphere.

Comparative study of interannual changes of the mean winds and gravity wave activity in the middle atmosphere over Japan, Central Europe and Canada

Abstract Results are presented of measurements of the mean wind and its variances attributed to the intensity of internal gravity waves (IGWs) with the MU radar at Shigaraki, Japan, from 1986 to 1999, with the low-frequency ionospheric drift method Dl at Collm, Germany and with the medium frequency (MF) radar at Saskatoon, Canada, between 1983 and 1999 at altitudes 65–80, 80–110 and 60– 100 km , respectively. The amplitudes of eastward–westward seasonal variations of the mean zonal wind in the mesosphere and lower thermosphere are different in different years. The mean wind and IGW intensities show substantial seasonal, quasi-biennial and interannual variations, which may be different at different altitudes. The measurements show differences in the interannual changes, IGW intensities at different locations. Comparison of interannual changes of the wind and drift velocity variances reveal substantial differences between Saskatoon and Collm and Shigaraki. Such differences in seasonal and interannual changes in the wind perturbation variances may be attributed to the changes in the strengths of IGW sources in the atmosphere at different altitudes and locations and with the conditions of wave propagation in the lower, middle and upper atmosphere. Among the possible reasons for these changes could be solar activity, the eruption of the Pinatubo volcano in June 1991 and, probably, interannual changes of the temperature of oceans in tropics (El Nino events) on the circulation of the middle and upper atmosphere.

Parameterization of accelerations and heat flux divergences produced by internal gravity waves in the middle atmosphere

Abstract Simple formulae have been derived for calculating the accelerations and heat flux divergences produced by internal gravity waves (IGW) in the middle atmosphere. The parameterization does not use the assumption of IGW saturation; rather, it takes into account the influence of the atmospheric circulation and temperature inhomogeneity on IGW propagation and amplitude variation.

Multi-beam MU radar measurements of advective accelerations in the atmosphere

A method is developed to calculate nonlinear zonal, sx, and meridional, sy, longitudinal advective accelerations from MU radar observations with antenna beam scanning in eight directions. The results of measurements of sx and sy at altitudes of 2–18 km on April 16–18, 1997, are presented. The values of sx and sy are mainly between −0.01 m s−2 and 0.01 m s−2 with increases up to ±0.04 m s−2 during tens of minutes to several hours in regions with vertical scales up to several kilometers. Such extremes are more frequent at the altitudes of tropo-stratospheric jet at 10–15 km and may indicate regions of inclination from geostrophic balance and of increased generation of atmospheric waves.

Peculiarities of interannual changes in the mean wind and gravity wave characteristics in the mesosphere over Shigaraki, Japan

Results are presented of month by month measurements of the mean wind and intensity and momentum fluxes of internal gravity waves (IGWs) with the Middle and Upper atmosphere radar (MU radar) at the altitudes 65–80 km at Shigaraki, Japan (35°N, 136°E) in 1986–1997. The mean zonal wind is eastward in winter and westward in summer, but the amplitude of these annual variations is different in different years, having a minimum in 1992–1994. The mean meridional wind shows substantial quasi-bi-annual variations. IGW intensities and momentum fluxes had a maximum in 1991–1994. Some possible reasons for such interannual changes are discussed.

Hydrodynamic tropospheric wave sources and their role in gravity wave climatology of the upper atmosphere from the MU radar observations

Abstract The results of measurements of generation of internal gravity waves (IGWs) in the troposphere by mesoscale dynamical processes with the MU radar are presented. Zonal and meridional horizontal nonlinear advective accelerations are estimated using eight directions of the MU radar antenna beam. Seasonal variations of the intensity of tropospheric IGW sources are discussed using the results of the observations in all seasons. A contribution of IGWs generated in the troposphere into dynamics of the mesosphere and lower thermosphere is considered by comparing the MU radar data with a theoretical model. Possible contribution of IGWs generated in the troposphere at middle and equatorial latitudes to interannual variations of the wave intensity and general circulation of the middle and upper atmosphere over the MUradar is discussed.

Statistical analysis of gravity waves observed with the middle and upper atmosphere radar in the middle atmosphere: 2. Waves propagated in different directions

The statistical method developed in the first part of the paper is applied to obtain parameters of internal gravity wave (IGW) harmonics with periods from 5 min to 6 hours propagating upward and downward, and in the direction of the mean wind and opposite to it, from the observations of motions in the middle atmosphere with the middle and upper atmosphere radar. Usually, wave amplitudes and modulus of momentum fluxes are larger for IGWs having upward energy fluxes and propagated opposite to the mean wind (eastward propagation in summer and westward propagation in winter). Most of IGWs have momentum fluxes directed to the northeast in winter and to the east in summer.

Simulating influences of QBO phases and orographic gravity wave forcing on planetary waves in the middle atmosphere

Recently developed parameterization of stationary orographic gravity waves (OGWs) generated by the Earth’s topography was implemented into a general circulation model of the middle and upper atmosphere. We performed numerical simulations of the zonal mean wind and amplitudes of stationary planetary waves and normal atmospheric modes with periods of 4–16 days at altitudes from the troposphere to the lower thermosphere in January for easterly and westerly phases of the quasi-biennial oscillation (QBO) including and excluding the stationary OGW parameterization. Simulations show that accounting dynamical and thermal effects of stationary OGWs can lead to substantial changes (up to 50–90 %) in the amplitudes of stationary planetary waves. Amplitudes of westward travelling normal atmospheric modes change (up to 50–90 %) at different altitudes and latitudes of the northern hemisphere due to OGW effects. Transitions from the easterly to westerly QBO phases can change planetary wave amplitudes up to ±30–90 % at middle and high latitudes. These changes in PW amplitudes are consistent with distributions of EP-flux and refractive index under different QBO phases simulated including our parameterization of stationary OGWs.

Average statistical characteristics of long gravity waves observed with the middle and upper atmosphere radar in the mesosphere

Average statistical characteristics of internal gravity waves (IGWs) with horizontal wavelengths longer than 60 km are obtained at the altitudes 65–80 km from the observations with the middle and upper atmosphere radar at Shigaraki, Japan (35°N, 136°E) in 1986–1995. The numbers of signals selected with narrow frequency band filters attributed to IGW having periods 0.1–6 hours vary between 870 and 2300 for different seasons and altitudes. Long-period IGWs have larger amplitudes and larger vertical wavelengths than short-period IGWs. For waves with periods 0.1–6 hours the mean values of the horizontal and vertical wavelengths are 100–300 km and 6–10 km, respectively. IGW amplitudes and momentum fluxes have maxima in winter and summer and minima in spring and autumn. Distributions of the azimuths of IGW horizontal wave vectors and momentum fluxes are nearly isotropic with maxima in zonal directions in summer. The westward momentum flux is larger than the eastward flux in winter and is smaller than that in summer. The differences between eastward and westward wave momentum fluxes are larger for long-period IGWs. The net zonal IGW momentum flux is mainly directed opposite to the mean wind in the mesosphere.

Short-period variations of ionospheric drifts at Collm an their connection with the dynamics of the lower and middle atmosphere

Abstract Studies of the intensity of horizontal drift velocity perturbations with periods 0.7 – 3 hours at the altitudes 80 – 110 km are made from the data of regular low-frequency D1 ionospheric reflection observations at Collm, Germany (52° N, 15° E) in 1983 – 1999. Seasonal changes of variances of short-period perturbations of zonal velocity reveal the main maximum in summer and a smaller maximum in winter near the altitude 80 km and the two maxim at equinoxes near and above 100 km. Perturbation variances had the minima in 1985 -- 1987 and in 1996 -- 1998. Numerical modeling shows that IGWs generated in the strato-mesospheric jet stream may have substantial amplitudes at altitudes near and above 100 km and may modify seasonal variations of tropospheric IGWs.