Nambath K. Balachandran

The GISS Global Climate-Middle Atmosphere Model. Part I: Model Structure and Climatology

Abstract The GISS global climate model (Hansen et al.) has been extended to include the middle atmosphere up to an altitude of approximately 85 km. The model has the full array of processes used for climate research, i.e., numerical solutions of the primitive equations, calculation of radiative and surface fluxes, a complete hydrologic cycle with convective and cloud cover parameterizations, etc. In addition, a parameterized gravity wave drag formulation has been incorporated, in which gravity-wave momentum fluxes due to flow over topography, wind shear and convection are calculated at each grid box, using theoretical relationships between the grid-scale variables and expected source strengths. The parameterized waves then propagate vertically upward depending on the instantaneous wind and temperature profiles, with waves breaking at levels in which their momentum flux exceed the background saturation value. Radiative damping is also calculated, and the total momentum convergence in each layer is used to ...

Climate change and the middle atmosphere. I - The doubled CO2 climate

Abstract The impact of doubled atmospheric CO2 on the climate of the middle atmosphere is investigated using the GISS global climate/middle atmosphere model. In the standard experiment, the CO2 concentration is doubled both in the stratosphere and troposphere, and the sea surface temperatures are increased to match those of the doubled CO2 run of the GISS 9 level climate model. Additional experiments are run to determine how the middle atmospheric effects are influenced by tropospheric changes, and to separate the dynamic and radiative influences. These include the use of the greater high latitude/low latitude surface warming ratio generated by the Geophysical Fluid Dynamics Laboratory doubled CO2 experiments, doubling the CO2 only in either the troposphere or stratosphere, and allowing the middle atmosphere to react only radiatively. As expected, doubled CO2 produces warmer temperatures in the troposphere, and generally cooler temperatures in the stratosphere. The net result is a decrease of static stabi...

Modeling the Effects of UV Variability and the QBO on the Troposphere–Stratosphere System. Part I:. The Middle Atmosphere

Abstract Results of experiments with a GCM involving changes in UV input (±25%, ±10%, ±5% at wavelengths below 0.3 µm) and simulated equatorial QBO are presented, with emphasis on the middle atmosphere response. The UV forcing employed is larger than observed during the last solar cycle and does not vary with wavelength, hence the relationship of these results to those from actual solar UV forcing should be treated with caution. The QBO alters the location of the zero wind line and the horizontal shear of the zonal wind in the low to middle stratosphere, while the UV change alters the magnitude of the polar jet and the vertical shear of the zonal wind. Both mechanisms thus affect planetary wave propagation. The east phase of the QBO leads to tropical cooling and high-latitude warming in the lower stratosphere, with opposite effects in the upper stratosphere. This quadrupole pattern is also wen in the observations. The high-latitude responses are due to altered planetary wave effect, while the models trop...

Climate Change and the Middle Atmosphere. Part III: The Doubled CO2 Climate Revisited

Abstract The response of the troposphere–stratosphere system to doubled atmospheric CO2 is investigated in a series of experiments in which sea surface temperatures are allowed to adjust to radiation imbalances. The Goddard Institute for Space Studies (GISS) Global Climate Middle Atmosphere Model (GCMAM) warms by 5.1°C at the surface while the stratosphere cools by up to 10°C. When ozone is allowed to respond photochemically, the stratospheric cooling is reduced by 20%, with little effect in the troposphere. Planetary wave energy increases in the stratosphere, producing dynamical warming at high latitudes, in agreement with previous GCMAM doubled CO2 simulations; the effect is due to increased tropospheric generation and altered refraction, both strongly influenced by the magnitude of warming in the model’s tropical upper troposphere. This warming also results in stronger zonal winds in the lower stratosphere, which appears to reduce stratospheric planetary wave 2 energy and stratospheric warming events. ...

Climate change and the middle atmosphere. II: The impact of volcanic aerosols

Abstract The effects of volcanic aerosols on the middle atmosphere are investigated with the Goddard Institute for Space Studies (GISS) Global Climate/Middle Atmosphere model. Volcanic aerosols with a visible optical depth of 0.15 are put into the lower stratosphere, and their influence is explored for different time scales: instantaneous effect (sea surface temperatures not allowed to adjust); influence for the first few years, with small tropospheric cooling; and long-term effect (50 years) with significant tropospheric cooling. The aerosols induce a direct stratospheric response, with warming in the tropical lower stratosphere, and cooling at higher latitudes. On the shorter time scales, this radiative effect increases tropospheric static stability at low- to midlatitudes, which reduces the intensity of the Hadley cell and Ferrel cell. There is an associated increase in tropospheric standing wave energy and a decrease in midlatitude west winds, which result in additional wave energy propagation into th...

Effects of solar cycle variability on the lower stratosphere and the troposphere

The effects of solar irradiance variability on the lower stratosphere and the troposphere are investigated using observed and general circulation model (GCM)-generated 30 and 100 mbar geopotential heights. The GCM includes changes in UV input (+ or −5% at wavelengths below 0.3 micron and no ozone photochemistry and transport) to roughly approximate the combined effects of UV and ozone changes associated with the solar variability. The annual and seasonal averages of the height differences between solar maximum and solar minimum conditions are evaluated. In the subtropics, observations indicate statistically highly significant increased geopotential heights during solar maximum, compared to solar minimum, in composite annual and seasonal averages. The model simulates this feature reasonably well, although the magnitude and statistical significance of the differences are often weaker than in observations, especially in summer. Both the observations and the model results show a strong dipole pattern of height differences when the data are partitioned according to the phase of the quasi-biennial oscillation (QBO), with the pattern reversing itself with the change in the phase of the QBO. The connection between solar variability and lower atmospheric changes are interpreted as follows: The solar changes directly affect the stratosphere by changing the vertical gradients of temperature and zonal wind. This leads to changes in propagation conditions for planetary waves resulting in changes of E-P flux divergence and then by the downward control principle, affecting the circulation in the lower stratosphere and the troposphere.

The GISS Global Climate-Middle Atmosphere Model. Part II. Model Variability Due to Interactions between Planetary Waves, the Mean Circulation and Gravity Wave Drag

Abstract The variability which arises in the GISS Global Climate-Middle Atmosphere Model on two time scales is reviewed: interannual standard deviations, derived from the five-year control run, and intraseasonal variability as exemplifited by stratospheric warmings. The models extratropical variability for both mean fields and eddy statistics appears reasonable when compared with observations, while the tropical wind variability near the stratopause may be excessive, possibly due to inertial oscillations. Both wave 1 and wave 2 warmings develop, with connections to tropospheric forcing. Variability on both time scales results from a complex set of interactions among planetary waves, the mean circulation, and gravity wave drag. Specific examples of these interactions are presented, which imply that variability in gravity wave forcing and drag may be an important component of the variability of the middle atmosphere.

Concorde sonic booms as an atmospheric probe.

Infrasound generated by the sonic boom from the inbound Concorde supersonic transport is recorded at Palisades, New York (Lamont-Doherty Geological Observatory), as a series of impulses from distances varying from 165 to about 1000 kilometers. Refraction effects determined by temperature and wind conditions return the signal to the surface from both stratospheric (40 to 50 kilometers) and thermospheric (100 to 130 kilometers) levels. The frequency of the recorded signal is a function of the level of reflection; the frequency decreases from impulse stretching as the atmosphere becomes more rarified relative to the sound pressure. The horizontal trace velocity of the signal across the array of instruments is equal to the acoustic velocity at the reflection level. The sonic boom can thus be used to provide temperature-wind parameters at reflection levels estimated from the signal frequency. Daily observed signal variations have indicated significant variations in these parameters.

On the Propagation of Infrasound from Rockets: Effects of Winds

Acoustic signals received in the northeast coastal regions of the United States from rockets launched at Cape Kennedy show strong seasonal effects. For the Saturn V rockets, strong signals are received in winter, very weak signals in summer, and weak signals in the transitional months of early fall and spring. These seasonal effects are attributed to the winds in the stratosphere (around an altitude of 50 km). In winter, when strong signals are received, the stratospheric winds have strong components in the direction of propagation of the signals. These components are weak during the transitional months, and during summer the stratospheric winds have components in the direction opposite to that of the signal propagation. It is shown that calculations of horizontal trace velocities provide an indirect method of estimating upper atmospheric winds.

Infrasound at Long Range from Saturn V, 1967

Two distinct groups of infrasonic waves from Saturn V, 1967, were recorded at Palisades, New York, 1485 kilometers from the launch site. The first group, of 10-minute duration, began about 70 minutes after launch time; the second, having more than twice the amplitude and a duration of 9 minutes, commenced 81 minutes after launch time. From information on the Saturn V trajectory and analysis of recorded data, it is established that the first group represents sound emitted either by the first stage reentry or by the second stage when its elevation was above 120 kilometers. The second, more intense wave group represents the sound from the powered first stage. A reversal of signal occurs because the rocket outran its own sound. Fourier analyses indicate that the energy extends to relatively long periods—10 seconds for the first stage and 7 seconds for the second. Trapping of sound in the upper atmospheric sound channel can be the cause of the separation of the signal into two distinct groups.