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Robert E. Dickinson and E. C. Ridley

Abstract

The structure, composition and winds of the mesosphere and thermosphere of Venus are investigated using a nonlinear time-dependent hydrodynamic model. The assumption that all variables depend only on altitude and distance from subsolar point allows a two-dimensional formulation of the problem. Within this framework the model provides an entirely self-consistent treatment of the multi-component fluid. The system solved consists of four time-dependent equations for motion, temperature, and the distributions of O and CO, and two diagnostic equations representing continuity and hydrostatic balance for the total fluid. The model is forced by absorption of solar radiation which provides heating of molecules and dissociation of CO2 into CO and O. A large-scale circulation is calculated, the gross features of which-resemble those derived in an earlier simplified model, consisting of a single cell with rising motion on the dayside, sinking motion on the nightside, and a day-to-night horizontal flow. This circulation in the global mean acts to remove the light gases to balance the photodissociation. Relatively large concentrations of light gases build up on the nightside. The consequent increase of the pressure at a given level acts to block the nightward circulation. Hence little motion occurs in a large region centered around the antisolar point. Instead, most of the downward vertical motion occurs within an internal boundary layer just to the night-side of the terminator. Exospheric temperatures predicted by the model, using an EUV heating efficiency of 0.30, range from greater than 600 K at the subsolar point to less than 300 K at the antisolar point, whereas there are typically 20 40 K horizontal variations of temperature in the mesosphere. Maximum horizontal velocities are order of 300 m s−1 and occur on the dayside near the terminator at the level of the exobase. The model predicts that CO and O will have relative number densities of 4% on the dayside at the level of the F-1 ionospheric peak, provided vertical eddy mixing is negligible.

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Robert E. Dickinson and E. C. Ridley

Abstract

We consider the thermosphere of a nonrotating planet with a large-scale circulation from dayside to nightside driven by differential solar heating. Return flow is assumed to occur only below the region considered. The motion is moixotonically upward on the dayside and downward on the nightside. A numerical model is developed and a method of solution derived for the distribution of N components in the presence of sources and sinks due to photodissociation. These components, any of which way be major species, are transported vertically by molecular diffusion and carried horizontally and vertically by the large-scale circulation. The model is integrated for parameters appropriate to the Venusian upper atmosphere, assuming only CO2 is carried upward on the dayside through the bottom boundary at 0.1 mb. This boundary condition requires that photodissociation fragments carried out at the bottom on the nightside essentially recombine into CO2 before they are carried back upward into the dayside integration region. Under these conditions, the calculations show that large-scale circulation alone can keep the relative number concentrations of O and CO at the Fl peak to ∼2%. Atomic oxygen concentrations of this magnitude have interesting consequences for the topside Venusian ionosphere.

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Robert E. Dickinson, E. C. Ridley, and R. G. Roble

Abstract

The mean meridional circulation and latitudinal variation of temperature in the thermosphere are calculated for solstice conditions. The heat and momentum sources that drive the thermospheric circulation are solar EUV and UV heating, high-latitude heating due to auroral processes, and a momentum source due to the correlation of diurnal variations of wind and ion drag. The results show a solar-driven, summer-to-winter circulation that is modified by the high-latitude heat source. The high-latitude heat source reinforces the summer-to-winter circulation in the summer hemisphere, but reverses the circulation in the mid-latitude winter hemisphere at F-layer heights with transition from one cell to another in the midlatitude winter hemisphere. Below about 150 km, however, the summer-to-winter circulation is maintained at all latitudes. The zonal winds at midlatitudes are generally eastward in the winter hemisphere and westward in the summer hemisphere. At F-layer heights, there is a significant temperature decrease from the summer pole to winter pole. Good agreement between the calculated and observed circulations and latitudinal temperature distributions is obtained for a total high-latitude heat source of about 2 × 1018 ergs s−1, but with 2½ times as much heating in the summer hemisphere as in the winter hemisphere.

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Robert E. Dickinson, E. C. Ridley, and R. G. Roble

Abstract

The mean meridional circulation and latitudinal variation of temperature in the thermosphere are considered for equinox conditions. With regard to these parameters there have been serious discrepancies between observational indications and theoretical expectations. A numerical model of the zonally symmetric thermospheric circulation is formulated and solved using a finite-difference initial value approach to steady-state solutions. Solutions are obtained for three different prescriptions of forcing terms: solar heating alone, solar heating plus an effective momentum source due to diurnal variations, and inclusion of a high-latitude heat source representing Joule dissipation of electric current systems. It is concluded that the Joule heating is essential for bringing theoretical predictions into agreement with observations but that the global mean of the required heating during geomagnetically quiet periods is necessarily small compared to global mean solar heating at the same levels.

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Robert E. Dickinson, E. C. Ridley, and R. G. Roble

Abstract

A general circulation model has been developed for the atmosphere above 97 km. It uses a 5° latitude × 5° longitude grid and 24 vertical levels in increments of 0.5 scale height. The prognostic variables are horizontal winds, temperature, and the mass mixing ratios of atomic and molecular oxygen, which are obtained using hydrodynamic equations and which include vertical transport by realistic models of molecular diffusion. All the prognostic variables are in near diffusive equilibrium in the vertical as the top of the model is approached. Realistic ion drag is included in the model equations for horizontal winds, including the rapid polar drifts of magnetic field fines due to magnetospheric convection. Excellent agreement is achieved between the calculated and observed global averaged composition, provided a reasonable amount of vertical eddy mixing is included in the compositional equations over the lowest model scale height. Calculations are carried out for solar minimum equinox conditions. The calculated variation of composition with latitude is opposite to that observed for the model forced only by solar heating but is brought into reasonable agreement with observations with the inclusion of auroral heating. Generally speaking, auroral heating changes significantly the global patterns of wind, temperature, and composition, and brings the model composition in reasonable agreement with that given by the MSIS empirical model. The calculated diurnal variations of composition with auroral heating are in acceptable agreement with observation. Calculated temperature variations in the upper thermosphere are consistent with a tendency for the coupled model to minimize the ratio of temperature to mean molecular mass.

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Robert E. Dickinson, R. G. Roble, and E. C. Ridley

Abstract

Departures from a mean global-scale ionization distribution are commonly found in the ionospheric F region. Global-scale winds experience an acceleration where the ion drag is locally less than its global-scale smooth value and, they likewise, experience a deceleration where the ion drag is locally greater. Thus, a perturbation in the horizontal flow is set up in response to this ion-drag momentum source.

A two-dimensional, steady-state dynamic model of the neutral thermosphere, incorporating thermal conduction, viscosity and ion drag, is used to calculate the temperature perturbation and circulation pattern caused by these ion-drag anomalies. The forcing is given by a momentum source which depends on the interaction of a basic-state neutral wind with the anomaly. For horizontal-scale anomalies of a few hundred kilometers, such as the electron density depression within the stable auroral red arc, the momentum source due to perturbation ion drag is almost completely balanced by a perturbation pressure force. The perturbation temperature and circulation responses are, therefore, negligibly small. For horizontal-scale anomalies of the order of a few thousand kilometers, such as the day-night electron density variation at sunset, the force exerted by the perturbation pressure is not able to cancel the addition of momentum by the ion-drag anomaly. Thus, such a momentum source produces a significant perturbation in the horizontal velocity, vertical motion, and temperature field.

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T. C. Johns, E. W. Blockley, and J. K. Ridley

Abstract

We present a coupled retrospective forecast (hindcast) study using the Met Office Global Coupled Model, version 2 (GC2), in which we identify and mitigate causes of initialization shock that lead to rapid error growth in sea ice forecasts. Sea ice state variables and volume budget terms as a function of forecast lead time are evaluated relative to analyses from an uncoupled Met Office ocean–sea ice analysis system [Forecast Ocean Assimilation Model, version 13 (FOAMv13)]. Two sources of initialization shock are highlighted and addressed, both of which are related to effective differences in physics between the analysis system and coupled forecast model. The primary shock to sea ice state variables arises from the use of a salinity-independent freezing temperature for seawater in GC2 as opposed to a salinity-dependent formulation in FOAMv13. A secondary effect arises from differences in the sea ice roughness and hence air–ice drag in the GC2 forecast model compared to the FOAMv13 analysis system. Generalizing from the findings of this study, we suggest that using nonnative analyses as initial conditions for coupled numerical weather prediction (NWP) studies will likely make them prone to initialization shock in some model components, to the detriment of forecast skill. To reduce the undesirable impacts of initialization shock on short-range forecast skill noted in this study we would therefore recommend the use of initial conditions (analyses) physically consistent with the native model components of the coupled forecast model, a native coupled analysis likely being the optimal initialization method.

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C. J. Donlon, S. J. Keogh, D. J. Baldwin, I. S. Robinson, I. Ridley, T. Sheasby, I. J. Barton, E. F. Bradley, T. J. Nightingale, and W. Emery

Abstract

Satellite sea surface skin temperature (SSST) maps are readily available from precisely calibrated radiometer systems such as the ERS along-track scanning radiometer and, in the near future, from the moderate-resolution imaging spectroradiometer. However, the use of subsurface bulk sea surface temperature (BSST) measurements as the primary source of in situ data required for the development of new sea surface temperature algorithms and the accurate validation of these global datasets is questionable. This is because BSST measurements are not a measure of the sea surface skin temperature, which is actually observed by a satellite infrared radiometer. Consequently, the use of BSST data for validation and derivation of satellite derived “pseudo-BSST” and SSST datasets will limit their accuracy to at least the rms deviation of the BSST–SSST difference, typically about ±0.5 K. Unfortunately, the prohibitive cost and difficulty of deploying infrared radiometers at sea has prevented the regular collection of a comprehensive global satellite SSST validation dataset. In response to this situation, an assessment of the TASCO THI-500L infrared radiometer system as a potential candidate for the widespread validation of satellite SSST observations is presented. This is a low-cost, broadband radiometer that has been commonly deployed in the field to measure SSST by several research groups. A comparison between SSST derived from TASCO THI-500L measurements and contemporaneous scanning infrared sea surface temperature radiometer measurements, which are accurate to better than 0.1 K, demonstrates low bias (0.1 K) and rms (0.08 K) differences between the two instruments. However, to achieve this accuracy, the TASCO THI-500L radiometer must be deployed with care to ensure that the radiometer fore-optics are kept free of salt water contamination and shaded from direct sunlight. When this is done, this type of low-cost radiometer system could form the core of a global SSST validation program.

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