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Abstract
Coastally trapped disturbances, which occur over the Gulf of California and are commonly referred to as gulf surges, are an important mechanism in the transport of low-level moisture into the southwestern United States. To determine whether or not gulf surges develop in association with the passage of tropical easterly waves, as hypothesized in previous studies, standard surface observations and European Centre for Medium-Range Weather Forecasts reanalysis data during the months of July and August over a 14-yr period are examined. Results indicate that tropical easterly wave troughs often cross western Mexico 1–3 days prior to surge onset at Yuma, Arizona, indicating a strong correlation between these two events. This analysis documents yet another mechanism by which the Tropics influences convection in the midlatitudes.
Abstract
Coastally trapped disturbances, which occur over the Gulf of California and are commonly referred to as gulf surges, are an important mechanism in the transport of low-level moisture into the southwestern United States. To determine whether or not gulf surges develop in association with the passage of tropical easterly waves, as hypothesized in previous studies, standard surface observations and European Centre for Medium-Range Weather Forecasts reanalysis data during the months of July and August over a 14-yr period are examined. Results indicate that tropical easterly wave troughs often cross western Mexico 1–3 days prior to surge onset at Yuma, Arizona, indicating a strong correlation between these two events. This analysis documents yet another mechanism by which the Tropics influences convection in the midlatitudes.
Abstract
A global, three-dimensional, time-dependent numerical model of the thermosphere has been created to simulate the dynamical behavior of the earth's thermosphere under a wide variety of geophysical conditions. Comparison of the model's predictions with the available data from ground-based, rocket and satellite techniques has shown that thermospheric dynamics can be realistically simulated by considering only three processes which deposit energy, or energy and momentum, in the thermosphere. Comparisons between the simulations and available data allow assessment of the magnitudes of the various processes as functions, particularly, of solar and geomagnetic activity.
The model is fully self-consistent in solving the neutral gas equations of momentum, energy and continuity, including all the Coriolis, inertial, viscosity and nonlinear terms, but assumes that the thermosphere contains a single species whose mean molecular weight varies only with the pressure. At times when the mean meridional wind is large (>20 m s−1), such as at the solstices and during strong and long geomagnetic storms, this assumption will cause the meridional temperature gradient, in particular, to be underestimated. The ion continuity equation is not handled in a self-consistent manner and, although the dynamo (V ∧ B) electric field term is universally computed, the present simulations do not include the “external” component of the electric field, either as a result of an empirical model or by computation of the polarization field.
With the above provisos, the dynamics of the thermosphere can be simulated under a wide variety of solar and geomagnetic conditions, using one major and ubiquitous energy source, due to solar UV and EUV radiation, and an additional geomagnetic source of both energy and momentum (Joule heating and ion drag) due to a polar electric field. Both the mean temperature and the mean meridional winds need further fine tuning, which may be realistically related to an additional high-latitude energy source for which relatively low-energy particle precipitation (suprathermal to a few hundred eV) is the most obvious candidate, rather than auroral (keV) electrons.
Recent values of the solar EUV flux are adequate to explain observed thermospheric temperatures at very low geomagnetic activity if a small residual polar source remains (geomagnetic) and if the heating efficiency is significantly increased, from about 0.3–0.35 to about 0.45. Polar electric fields, following the empirical models of Heppner (1977), as given analytically by Volland (1978), are adequate to explain wind structures at middle, auroral and polar latitudes under quiet geomagnetic conditions (Kp 0–1 = Volland model 1) and “average” geomagnetic conditions (Kp 3–4 = Volland model 2) at the equinox, and at summer and winter solstices, for low solar activity (F10.7 ∼ 75 × 10−22 W cm−2 s−1) and for moderately high solar activity (F10.7 ∼ 165 × 10−22 W cm−2 s−1), when a small amount of additional high-altitude high-latitude energy is included—values range between 109 and 1010 W per hemisphere depending on season, and solar and geomagnetic activity.
The results presented in this paper relate exclusively to the steady-state behavior of the thermosphere, and a series of further papers are in preparation describing some of the interesting results of the various time-dependent simulations we have performed, in particular to study the consequences of geomagnetic substorms of different magnitudes and at different seasons and different levels of solar activity. In later papers we will also consider the constraints arising from the major energy and momentum sources of the thermosphere in a quantitative way.
Abstract
A global, three-dimensional, time-dependent numerical model of the thermosphere has been created to simulate the dynamical behavior of the earth's thermosphere under a wide variety of geophysical conditions. Comparison of the model's predictions with the available data from ground-based, rocket and satellite techniques has shown that thermospheric dynamics can be realistically simulated by considering only three processes which deposit energy, or energy and momentum, in the thermosphere. Comparisons between the simulations and available data allow assessment of the magnitudes of the various processes as functions, particularly, of solar and geomagnetic activity.
The model is fully self-consistent in solving the neutral gas equations of momentum, energy and continuity, including all the Coriolis, inertial, viscosity and nonlinear terms, but assumes that the thermosphere contains a single species whose mean molecular weight varies only with the pressure. At times when the mean meridional wind is large (>20 m s−1), such as at the solstices and during strong and long geomagnetic storms, this assumption will cause the meridional temperature gradient, in particular, to be underestimated. The ion continuity equation is not handled in a self-consistent manner and, although the dynamo (V ∧ B) electric field term is universally computed, the present simulations do not include the “external” component of the electric field, either as a result of an empirical model or by computation of the polarization field.
With the above provisos, the dynamics of the thermosphere can be simulated under a wide variety of solar and geomagnetic conditions, using one major and ubiquitous energy source, due to solar UV and EUV radiation, and an additional geomagnetic source of both energy and momentum (Joule heating and ion drag) due to a polar electric field. Both the mean temperature and the mean meridional winds need further fine tuning, which may be realistically related to an additional high-latitude energy source for which relatively low-energy particle precipitation (suprathermal to a few hundred eV) is the most obvious candidate, rather than auroral (keV) electrons.
Recent values of the solar EUV flux are adequate to explain observed thermospheric temperatures at very low geomagnetic activity if a small residual polar source remains (geomagnetic) and if the heating efficiency is significantly increased, from about 0.3–0.35 to about 0.45. Polar electric fields, following the empirical models of Heppner (1977), as given analytically by Volland (1978), are adequate to explain wind structures at middle, auroral and polar latitudes under quiet geomagnetic conditions (Kp 0–1 = Volland model 1) and “average” geomagnetic conditions (Kp 3–4 = Volland model 2) at the equinox, and at summer and winter solstices, for low solar activity (F10.7 ∼ 75 × 10−22 W cm−2 s−1) and for moderately high solar activity (F10.7 ∼ 165 × 10−22 W cm−2 s−1), when a small amount of additional high-altitude high-latitude energy is included—values range between 109 and 1010 W per hemisphere depending on season, and solar and geomagnetic activity.
The results presented in this paper relate exclusively to the steady-state behavior of the thermosphere, and a series of further papers are in preparation describing some of the interesting results of the various time-dependent simulations we have performed, in particular to study the consequences of geomagnetic substorms of different magnitudes and at different seasons and different levels of solar activity. In later papers we will also consider the constraints arising from the major energy and momentum sources of the thermosphere in a quantitative way.
Abstract
We examine the generation of trapped edge waves on a continental shelf when a long wave from the deep ocean reaches an irregular coast, and alterations in the propagation characteristics of trapped edge waves due to the coastal irregularities. The continental shelf is modeled by a single flat-step model, and the coast is straight except for irregularities represented as a centered stationary random function of distance along the coast. The relevant boundary value problem are thus stochastic, with the randomness introduced through the boundary condition at the coast. We find the power flux into trapped edge waves and into a continuous spectrum of leaky modes, both generated by the scattering of an incident wave from the deep ocean. Numerical results, assuming a Gaussian spectrum for coastal irregularities, indicate that there is less power transferred to the forward traveling trapped wave than the backward one, and less power to the scattered leaky modes than to either the forward or backward traveling trapped modes. We obtain the attenuation coefficient of a trapped edge wave, the “tilting” of the wave toward the coast, and the correction to the dispersion relation due to the coastal irregularities. The results are valid for wave periods much shorter than the period associated with the Coriolis parameter f and for wavelengths much greater than the average size of the coastal irregularities.
Abstract
We examine the generation of trapped edge waves on a continental shelf when a long wave from the deep ocean reaches an irregular coast, and alterations in the propagation characteristics of trapped edge waves due to the coastal irregularities. The continental shelf is modeled by a single flat-step model, and the coast is straight except for irregularities represented as a centered stationary random function of distance along the coast. The relevant boundary value problem are thus stochastic, with the randomness introduced through the boundary condition at the coast. We find the power flux into trapped edge waves and into a continuous spectrum of leaky modes, both generated by the scattering of an incident wave from the deep ocean. Numerical results, assuming a Gaussian spectrum for coastal irregularities, indicate that there is less power transferred to the forward traveling trapped wave than the backward one, and less power to the scattered leaky modes than to either the forward or backward traveling trapped modes. We obtain the attenuation coefficient of a trapped edge wave, the “tilting” of the wave toward the coast, and the correction to the dispersion relation due to the coastal irregularities. The results are valid for wave periods much shorter than the period associated with the Coriolis parameter f and for wavelengths much greater than the average size of the coastal irregularities.
Abstract
We show results from the first set of measurements conducted to validate extinction data from the satellite sensor SAM II. Dustsonde-measured number density profiles and lidar-measured backscattering profiles for two days are converted to extinction profiles using the optical modeling techniques described in the companion Paper I (Russell et al., 1981). At heights ∼2 km and more above the tropopause, the dustsonde data are used to restrict the range of model size distributions, thus reducing uncertainties in the conversion process. At all heights, measurement uncertainties for each sensor are evaluated, and these are combined with conversion uncertainties to yield the total uncertainty in derived data profiles.
The SAM II measured, dustsonde-inferred, and lidar-inferred extinction profiles for both days are shown to agree within their respective uncertainties at all heights above the tropopause. Near the tropopause, this agreement depends on the use of model size distributions with more relatively large particles (radius ≳0.6 μm) than are present in distributions used to model the main stratospheric aerosol peak. The presence of these relatively large particles is supported by measurements made elsewhere and is suggested by in situ size distribution measurements reported here. These relatively large particles near the tropopause are likely to have an important bearing on the radiative impact of the total stratospheric aerosol.
The agreement in this experiment supports the validity of the SAM II extinction data and the SAM II uncertainty estimates derived from an independent error analysis. Recommendations are given for reducing the uncertainties of future correlative experiments.
Abstract
We show results from the first set of measurements conducted to validate extinction data from the satellite sensor SAM II. Dustsonde-measured number density profiles and lidar-measured backscattering profiles for two days are converted to extinction profiles using the optical modeling techniques described in the companion Paper I (Russell et al., 1981). At heights ∼2 km and more above the tropopause, the dustsonde data are used to restrict the range of model size distributions, thus reducing uncertainties in the conversion process. At all heights, measurement uncertainties for each sensor are evaluated, and these are combined with conversion uncertainties to yield the total uncertainty in derived data profiles.
The SAM II measured, dustsonde-inferred, and lidar-inferred extinction profiles for both days are shown to agree within their respective uncertainties at all heights above the tropopause. Near the tropopause, this agreement depends on the use of model size distributions with more relatively large particles (radius ≳0.6 μm) than are present in distributions used to model the main stratospheric aerosol peak. The presence of these relatively large particles is supported by measurements made elsewhere and is suggested by in situ size distribution measurements reported here. These relatively large particles near the tropopause are likely to have an important bearing on the radiative impact of the total stratospheric aerosol.
The agreement in this experiment supports the validity of the SAM II extinction data and the SAM II uncertainty estimates derived from an independent error analysis. Recommendations are given for reducing the uncertainties of future correlative experiments.
