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Nambath K. Balachandran

Abstract

Gravity waves generated by severe thunderstorms in the eastern Ohio-Pennsylvania area were recorded by an array of microbarovariographs at Palisades, New York and by standard microbarographs across northeastern United States. The waves were associated with the cold mesohigh from the outflow of the thunderstorms. Along their path the waves apparently triggered new thunderstorms. The waves were observed to propagate with the velocity of the wind just below the tropopause. The long-distance propagation of the waves is explained by the presence of a duct associated with the critical level (steering level), in agreement with the derivation given by Lindzen and Tung (1976). The duct was directional and waves were absent to the west of the generating area. In the generating area wave-CISK might have been operating. Sharp vertical temperature gradients associated with the passage of the waves were observed by temperature sensors on a tower.

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Nambath K. Balachandran and David Rind

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 model's tropical response in the upper stratosphere is due to gravity wave drag.

Increased UV forcing warms tropical latitudes in the middle atmosphere, resulting in stronger extratropical wen winds, an effect which peaks in the upper stratosphere/lower mesosphere with the more extreme UV forcing but at lower altitudes and smaller wind variations with the more realistic forcing. The increased vertical gradient of the zonal wind leads to increased vertical propagation of planetary warm altering energy convergences and temperatures. The exact altitudes affected depend upon the UV forcing applied.

Results with combined QBO and UV forcing show that in the Northern Hemisphere, polar warming for the east QBO is stronger when the UV input is reduced by 25% and 5% as increased wave propagation to high latitudes(east QB0 effect) is prevented from then propagating vertically (reduced UV effect). The model results are thus in general agreement with observations associated with solar UV/QBO variations, although the west phase is not absolutely warmer with increased UV. Questions remain concerning the actual variation of stratospheric winds with the solar cycle as the magnitude of the variations reported in some observations cannot be associated with UV variations in this model (but do arise in the model without any external forcing). The model results actually come closer to reproducing observations with the reduced magnitude of UV forcing due to the lower altitude of west wind response, despite the smaller wind variations involved. An evaluation of the reality of the reported effects of combined QBO and solar UV variations on the middle atmosphere requires the use of proper UV solar cycle forcing and should include possible ozone variations.

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David Rind and Nambath K. Balachandran

Abstract

Results Of experiments with a GCM involving changes in UV input (±25%, ±5% at wavelengths below 0.3 µ) and simulated equatorial QBO are presented, with emphasis on the tropospheric response. The QBO and UV changes alter the temperature in the lower stratosphere/upper troposphere, altecting tropospheric/stratospheric vertical stability. When the extratropical lower stratosphere/upper troposphere warms, tropospheric eddy energy is reduced, leading to extratropical tropospheric cooling of some 0.5°C on the zonal average, and surface temperature changes up to ±5°C locally. Opposite effects occur when the extratropical lower stratosphere/upper troposphere cools. Cooling or warming of the comparable region in the Tropics decreases/increase static stability, accelerating/decelerating the Hadley circulation. Tropospheric dynamical changes are on the order of 5%.

The combined UV/QBO effect in the troposphere results from its impact on the middle atmosphere. in the QBO east phase, more energy is refracted to higher latitudes, due to the increased horizontal shear of the zonal wind, but with increased UV, this energy propagates preferentially out of the polar lower stratosphere, in response to the increased vertical shear of the zonal winds; therefore, it is less effective in warming the polar lower stratosphere. Due to their impacts on planetary wave generation and propagation, all combinations of UV and QBO phase affect the longitudinal patterns of tropospheric temperatures and potential heights. The modeled perturbations often agree qualitatively with observations and are of generally similar orders of magnitude.

The results are sensitive to the forcing employed. In particular, the nature of the tropospheric response depends upon the magnitude (and presumably wavelength) of the solar irradiance perturbation. The results of the smaller UV variations (±5%) are more in agreement with observations, showing clear differences between the UV impact in the cast and west QBO phase. However, since the UV magnitudes have been exaggerated relative to observed solar UV variations during the last solar cycle, the results cannot be used to prove an actual solar forcing of the troposphere. The results will also likely be sensitive to the model, particularly its planetary longwave energy, and may be influenced by other processes that have not been included, such as changes in stratospheric ozone.

The dynamical changes are accompanied by changes in cloud cover and snow cover that differ between maximum and minimum UV, and affect the radiative balance of the planet. As these influences do not cancel in the extreme phases of the UV variations, a net radiative forcing may result from solar cycling in conjunction with the QBO. An assessment of the solar impact on climate change must include these dynamically driven forcings.

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