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Kirsti Jylhä
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Kirsti Jylhä

Abstract

The relation between the scavenging coefficient Λ (s−1) for air pollutants in precipitation and the radar reflectivity factor Z (mm6 m−3) is based on the fact that they are both functions of the hydrometeor size distribution. In this paper, which combines the fields of air pollution physics, cloud physics, and radar meteorology, Λ–Z relationships are derived analytically for below-cloud gaseous and particulate pollutants and, with certain restrictions, for pollutants incorporated into cloud droplets. For the types of precipitation and pollutant considered, it can be shown that Λ ≈ aZ b, where the coefficient a has an order of magnitude of 10−7–10−6 for submicron aerosol particles, 10−6–10−5 for highly soluble gases, and 10−5 for pollutants in cloud droplets. In stratiform rain the exponent b ranges between about 0.4 and 0.6, so that an increase of Z by a factor of 10 approximately corresponds to a twofold to fourfold increase in Λ. In snowfall, mainly due to the diversity of solid hydrometeors, the value of b may vary more considerably but probably is somewhat smaller than in rain. Because weather radar estimates the spatial distribution of Z essentially in real time, Λ–Z relationships can be used to monitor and nowcast those areas most significantly exposed to wet deposition.

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Kirsti Jylhä

Abstract

The power-law dependences between the scavenging coefficient Λ for pollutants in precipitation and the radar reflectivity factor Z, theoretically derived in Part I, are discussed here from the point of view of applications. Possible problems in their use are related mainly to the uncertain characteristics of the pollutants involved and to common error sources in weather radar measurements of precipitation. The greatest usefulness of the Λ–Z relationships probably is obtained when the hydrometeor population that is producing the radar signal is the same as the population that is scavenging the pollutants. Because radar estimates Z in real time, the relationships can be utilized in short-term forecasts of the cleansing effect of precipitation and of wet deposition. This use is illustrated in the current paper with the aid of radioactivity and radar measurements in Finland following the Chernobyl accident. The Λ–Z relationships yielded estimates of radioactive fallout that were in good agreement with observations: a logarithmic correlation coefficient of 0.67 was found between gamma radiation dose rates and radar-derived estimates for the time integral of Λ, a quantity that is approximately proportional to the accumulated wet deposition.

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Timo Puhakka, Kirsti Jylhä, Pirkko Saarikivi, Jarmo Koistinen, and Janne Koivukoski

Abstract

After the accident at the Chernobyl nuclear plant on 26 April 1986, much of Europe was affected by radioactive pollution. The first releases were transported toward Scandinavia, where most of the fallout was attributable to wet deposition. This study analyzes the synoptic scale and mesoscale meteorological conditions influencing the transport, and the meteorological factors related to the observed fallout in southern Finland. The study focuses on the role of rainfall in the final deposition onto the ground, studied using weather radar data. The results demonstrate that, although the large scale transport from Chernobyl could be roughly estimated by simple methods using routine synoptic data, sonic essential smaller-scale features could not be understood before an isentropic trajectory analysis, together with the conceptual model of a cyclone and its related conveyor belts, was applied. The main result of the study was the good correlation between the radioactive fallout and the corresponding areal distribution of rainfall measured by a weather radar.

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Kirsti Jylhä, Heikki Tuomenvirta, Kimmo Ruosteenoja, Hanna Niemi-Hugaerts, Krista Keisu, and Juha A. Karhu

Abstract

A Web site questionnaire survey in Finland suggested that maps illustrating projected shifts of Köppen climatic zones are an effective visualization tool for disseminating climate change information. The climate classification is based on seasonal cycles of monthly-mean temperature and precipitation, and it divides Europe and its adjacent land areas into tundra, boreal, temperate, and dry climate types. Projections of future changes in the climatic zones were composed using multimodel mean projections based on simulations performed with 19 global climate models. The projections imply that, depending on the greenhouse gas scenarios, about half or possibly even two-thirds of the study domain will be affected by shifts toward a warmer or drier climate type during this century. The projected changes within the next few decades are chiefly located near regions where shifts in the borders of the zones have already occurred during the period 1950–2006. The questionnaire survey indicated that the information regarding the shifting climatic zones as disseminated by the maps was generally interpreted correctly, with the average percentage of correct answers being 86%. Additional examples of the use of the climatic zones to communicate climate change information to the public are included.

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