Tapio J. Tuomi

Inverse theory for occultation measurements: 1. Spectral inversion

In this paper we investigate methods to solve the inverse problem for the optical occultation measurements from a planet-orbiting satellite. Our emphasis is on the stellar occultation technique which offers a promising method to monitor the global vertical distributions of the crucial trace gases like ozone in the Earths stratosphere. The occultation measurement principle is self-calibrating because the star (or Sun) signal is also measured when the light is not attenuated by the atmosphere. This feature is crucial in developing the long-term space-borne monitoring systems of the Earths trace gases. The inverse problem for an occultation measurement can be divided into a spectral inversion part and into a spatial inversion part. In the former problem we aim at finding the tangential column densities of the different gases which are involved in the attenuating of the starlight traversing through the atmosphere onto our detector. In the latter part we bring together measurements from different altitudes and geographical locations to construct the global distributions of the trace gases in the Earths atmosphere. This paper deals with the first problem; the second problem will be treated in a subsequent paper.

Attachment of Natural Lightning Flashes to Trees: Preliminary Statistical Characteristics

Lightning attachment to trees was studied based on 37 trees that were struck in Finland in summer 2007 and 2008. The type and severity of lightning damage was correlated with multiple parameters related to the flash, the meteorological characteristics of the strike time, and the surroundings of the tree. Damage was classified into three categories: bark-loss (minor), wood-loss (extensive), end explosive (complete loss of tree material). Four statistically significant parameters were found. The absolute value of the peak flash current is strongly correlated with damage; also, an indirect argument suggests that positive flashes cause more damage than negative flashes. The amount of damage is inversely correlated with the rainfall in the previous three hours, indicating that a wet ground and tree surface protect trees against damage by providing a conducting path to the ground. The ground type also has a weaker statistically significant effect, with poorly conducting ground leading to more extensive damage. Old and rotten trees are statistically most likely to experience explosive damage. Other parameters are inconclusive. The distribution of tree heights points to the possibility that the electrogeometric method does not necessarily predict the strike probability to a given tree. In many cases, the struck trees were clearly within the protective radius of a higher structure, although this is often difficult to determine from photographs. It is therefore suggested that models of lightning flashes to trees should include both the conductivity and height of the tree. The results may be significant for remote prediction of lightning damage, as well as for understanding lightning protection of structures which include trees.

The atmospheric electrode effect over snow

Abstract The atmospheric electrode effect is treated both in the turbulent and non-turbulent cases. For the non-turbulent case it is shown by geometrical considerations that field-free regions near the ground act as an effective ion supply reducing the electrode effect, except in winter when the snow forms a smooth ground. The equations with turbulence are linearized and solved analytically assuming that the charged aerosol remains uniformly distributed owing to its long time of recovery from turbulent fluctuations. Data from continuous measurement of the difference of positive and negative conductivity, which is proportional to the space charge density, show good agreement with theory in both cases, provided that the air is stable near the ground when the positive conductivity is less than about 6 fS m −1 during fair weather.

Atmospheric electrode effect: Approximate theory and wintertime observations

SummaryThe steady-state equations of the atmospheric electrode effect, i.e. the depletion of negative ions near the ground, are solved analytically in an approximate way. The presence of monodisperse aerosol is included. The method is based on the assumption that the positive ion density remains nearly, although not strictly, constant with height. The results show explicitly the effect of various parameters on the electrode layer. In particular, the height is directly proportional to the current density, and it depends on aerosol density only through the influence of the aerosol on the current density. The results are used to give a tentative explanation, in terms of the electrode effect, to an observed wintertime difference of the two polar conductivities.

Light scattering by aerosols with layered humidity-dependent structure

Abstract Computed light scattering by inhomogeneous spherical particles is compared with scattering by equivalent homogeneous spheres. The inhomogeneous particles, called onions, consist of an insoluble spherical core surrounded by a soluble shell which is partially or wholly dissolved into solution depending on the relative humidity. Two equivalent homogeneous spheres are defined as having either the mean refractive index or the outermost (solution) refractive index of the onion. Scattering diagrams are computed for a distribution of optically important particle sizes. The results show that inhomogeneous spheres of this size class may be approximated by homogeneous spheres with the mean refractive index.

Flash Cells in Thunderstorms

When convection develops into a thundercloud stage of at least moderate intensity, its cellular appearance may be monitored not only with weather radars but also with lightning location systems. For intense thunderstorms, the ground-flash rate alone is sufficient for making apparent the underlying convection-cell structure. if flash cells can be properly identified, they show longitudinal development complementary to the usual view of advancing transverse front. A method is described to analyse the flash-cell structure of thunderstorms and is illustrated with an example from Finland. With proper choice of parameters, the largest, most interesting flash cells generally have realistic sizes, lifetimes and motion.

Magnetospheric—ionospheric effect on the ground-level atmospheric electric field at Helsinki

Abstract The magnetospheric—ionospheric influence on the ground-level atmospheric electric field measured at Helsinki is investigated on the basis of comparison with a numerical model of the high-latitude ionospheric electric field of solar wind of magnetospheric origin. It is found that the contribution of the magnetospheric-ionospheric generator to the ground-level atmospheric electric field varies with the local time, the season and the conditions of the magnetosphere, and the large-scale electrodynamic features of the ionosphere. The latter are characterized as disturbances in the east and north components of the interplanetary magnetic field. The atmospheric electric field values can differ in winter by 100–150 Vm −1 between extremely quiet and extremely disturbed magnetospheric conditions.

Backscatter of light by aerosols at high relative humidity

Abstract The optical backscattering as well as extinction coefficients of a model aerosol are computed for relative humidity between 0.80 and 0.995. The model aerosol at this humidity range consists of droplets of salt solution surrounding a water-insoluble nucleus. This layered structure is taken into account in determining the refractive index of the solution, but the scattering is approximated by the ordinary Mie theory of homogeneous particles. The size distribution consists of the sum of two logarithmic normal distributions corresponding to the size ranges of small and large particles. The resulting scattering coefficients are smooth and steadily growing functions of relative humidity, with some dependence on wavelength and size distribution.