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<br />GROUND MICROPHYSICAL MEASUREMENTS <br /> <br />Bruno Federer <br />Atmospheric Physics ETH, 8093 Zurich, Switzerland <br /> <br />1. Introduction <br /> <br />Precipitation is generated by a sequence of events which starts with the formation of <br />a cloud by the nucleation of cloud droplets from the vapour. During this nucleation process, <br />the supersaturation of the air passes through a maximum and some fraction of the atmospheric <br />aerosol particles act as condensation nuclei. The concentration of droplets so formed de- <br />pends upon the nature of the aerosol and the updraft speed. In air masses characterized by <br />a maritime aerosol, droplet concentrations can range from about 10 to 100 cm-3, whereas in <br />air masses in which the aerosol is continental in nature, the concentration may vary from <br />200 to 2000 cm-3. Since there is a relatively fixed amount of water to be shared among <br />these droplets, the droplets in maritimE' clouds are on the average distinctly larger than <br />those in continental clouds and the devE,lopment of precipitation proceeds quite differently <br />in these cloud types. Since the effectiveness of the natural precipitation processes is in- <br />fluenced by the "continentality" of the air masses, the degree of "continentality" should <br />be determined on a routine basis by meaE,urements of the cloud condensation nuclei during <br />aircraft traverses below cloud base or on the grourid. <br />As a cloud ascends, its top may eventually be cooled to temperatures below DoC. Ice <br />formation is initiated as a result of the activity of quite rare kinds of particles in the <br />atmospheric aerosol, called ice nuclei. However. attempts over almost 30 years to quantify <br />their activity have met with very limitE,d success. the various ice nucleus counters used <br />for that purpose differing by over an order of magnitude. The basic reason for this is that <br />ice nuclei can act in a number of different ways to form ice and the various instruments <br />favour different modes of action. Since all those modes can occur in varying degrees in dif- <br />ferent clouds. no one instrument is cleElrly preferable to the others. Regardless of the <br />current questionable meaningfulness of ice nuclei measurements, they can provide a range of <br />natural background data against which to compare subsequent observations acquired during <br />periods of seeding for the purpose of tracking AgI plumes. Furthermore, for clouds develop- <br />ing in "continental" air mass conditionE, the concentration of ice crystals developing in <br />the clouds appears to show some correspondence with the concentration of detectable ice <br />nuclei below cloud base. This can justify the routine measurement of ice nuclei on aircraft <br />traverses below cloud base or, as a substitute, on the ground at a remote, unpolluted site. <br />Raindrop size distributions, which express the number of drops per unit size interval <br />per unit volume, provide for a more complete description of the rain than daily rainfall <br />totals or rainfall rate alone. The routine measurement of "drop spectra" at the ground is <br />important in a cloud physics field experiment because it is needed in quantitative radar <br />work and can give clues about the microphysical situation in different types of rain. <br /> <br />The topics described in this chapter are treated in more detail in the following text- <br />books (listed in order of increasing mathematical complexity): Roger3(1976); Wallace and <br />Hobbs (1977); Fletcher (1962); Mason (1971); Twomey (1977); Pruppacher and Klett (1978). <br /> <br />2. Condensation nuclei <br /> <br />One of the oldest and most convenient techniques for measuring the concentrations of <br />atmospheric aerosol is the Aitken nucleus counter (Fig. 1). Subjected to a sufficiently <br />high supersaturation any particle will act as a center for the growth of a drop from the <br />vapour. In the Aitken counter (as well as in the Pollack or Gardner counters) saturated <br />air is expanded rapidly so that it becorres supl~rsaturated by several hundred percent. Dur- <br />ing the expansion the saturation mixing ratio I's (= mass of water vapour/mass of dry air) <br />given by rs = Ees/(P-es] is changed, where esCT] is the saturation vapour pressure, P the <br />air pressure and E = Mw/Md the ratio of the molecular weights of water and dry air. The <br />expansion takes only about 100 milliseconds and is therefore adiabatic from P1 to P2 (Iri- <br />barne and Godson, 1973): <br />