Acta Geod. Geoph. Hung., Vol. 34{1 -2) , pp. 191 -195 (1999)

HANS ERTEL- A PIONEER IN GEOPHYSICAL SCIENCES

W SCHRODER! and H-J TREDER2

[Manuscript received April 14, 1998)

Extractsfrom papers byHans Ertel (1904-1971) relating to geophysicsandcosmicphysics are presented here as to give an idea about his contribution to this subjectduring the years 1925 to 1971.

Keywords: cosmic physics; Ertel's potential vorticity

Hans Ertel (1904-1971) was appointed in 1946 professor of Geophysics at theUniversity Berlin, as well as director of the Institute of Meteorology and Geophysicsthere. He died suddenly and unexpectedly in 1971. He held then lectures as visitingprofessor at the University Uppsala (Sweden) and met his old friend, theoreticalmeteorologist Hilding Koehler as well as Markus Bath. Koehler was tutor of BertBolin and contributed significantly to the scientific carrier of Carl-Gustaf Rossby.It is of historical interest that Ertel and Rossby could met again after the war inStockholm and Uppsala, as Uppsala invited Ertel for some lectures. Else Rossbywas the first foreign guest who visited Ertel's institute in Berlin after the war.Rossby's lecture on the waves lat er named after him have shown that this resultwas implicitely included in Ertel's papers but they did not became known to theAnglo-American scientific community. This is exactly the tragic point that manyimportant papers by Hans Ertel published in the forties in the "MeteorologischeZeitschrift" did not get known for many researchers in the United States/Englanddue to the war.

A later publication by Ertel co-authored by Rossby remained similarly unknownfor a certain time. This situation changed when C Truesdall studied his papers andpresented a general survey on Ertel's results in his well-known paper in "Handbuchder Physik" (1960) .

With the re-opening of the Institute of Meteorology, Ertel started most suc­cessful activities both as a university professor and as a researcher. His activitybecame even broader when in 1948 he was appointed director of the Institute ofPhysical Hydrology of the German Academy of Sciences (DAW) in Berlin. In thisinstitute he continued his hydrodynamic research and together with it , he started totackle with his staff important geoecological issues ; these activities are consideredtoday as pioneering works. The fields of research of this institute, as e.g. theoreticalhydrodynamics, special hydrography of Northern German lakes and coasts (coastprotection) , hydraulic nomography, hydrographic cartography, as well as weatherhistory of Europe were successfully elaborated with a small staff and led within a

1 Hechelstrasse 8, D-28777 Bremen-Roennebeck, Germany2Rosa-Luxemburg-Strasse 17a, D-14482 Potsdam, Germany

1217-8977/99/$ 5.00 @1999 Akademiai Kiad6, Budapest

192 W SCHRODER and H-J TREDER

Fig . 1. Ertel's Institute of Physical Hydrography of the German Academy of Sciences in Berlin

short time to a high international reputation of the institute. Many results werepublished in the journal of the institute, "Acta Hydrophysica", and Curt Weikinnsucceeded in publishing a source-book on the weather history. The latter could bepublished due to Ertel's proleptic wisdom and it gained international recognition,too . In later years Ertel was increasingly engaged in problems of coast protectionin the framework of activities of the Academy and this research resulted in newinsights, as e.g. the theory of lake embankment, the problem of "sea bears" , i.e. ofsudden increases of the sea level, as well as many new results in connection withtheoretical geomorphology. Ertel published a considerable part of his papers in"Monatsberichte der Deutschen Akademie der Wissenschaften" (Monthly Reportsof the German Academy of Sciences) being not very well known internationally,thus these important papers became only later known, too (as e.g. in Scheidegger's"Theoretical Geomorphology"). Else Ertel treated meteorological problems too,as shown by his studies in the working group "Influence of the orography on thedevelopment of weather" in the framework of the Conferences on Carpathian Mete­orology. The title of his last paper is "A differential invariant of inertial movementsin the atmosphere and in the oceans" (Z. Meteorologie, 22, 1971,329-331) .

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HANS ERTEL

Fig . 2. Professor Hans Ertel

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Ertel was elected in 1949 an ordinary member of the German Academy of Sci­ences at Berlin and became for ten years its vice president. During this time manynew institutes of the Academy were founded, the German-German co-operation,e.g. in the framework of the International Geophysical Year promoted, and manyother projects were also supported. Ertel and Julius Bartels joined the forces forthese aims. Ertel participated intensively in other international projects, as e.g. inthe Decade of Hydrology. He took part in the Carpathian Co-operation, too, hevisited many conferences and influenced strongly international co-operation in thisfield.

Ertel influenced international discussion as editor of the journals Gerlands Bei­trage zur Geophysik, Zeitschrift fiir Meteorologie, Forschungen und Fortschritteand Deutsche Literaturzeitung published in co-operation of the German academies.He also influenced strongly the "Alexander-von-Humboldt-Forschung" for which theDAW founded a special commission on his advice and published in 1959 a Humboldtmemorial volume.

Ertel's later interests were partly determined by his institute, but he returnedoften to meteorological problems and worked on an extension of the geophysicallyoriented hydrodynamics with its wide range of applications.

Ertel considered his field as theoretical mechanics (thus e.g. Theoretical Me­chanics was added in 1960 to his university institute) and mathematical physicsand devoted himself to the task to elaborate laws of physics in a form which isapplicable to the Earth and to the Universe.

Ertel's vorticity theorems are formulated in a form which is valid and directly ap­plicable in all systems of reference and with many complementary conditions. Theymake it possible to discuss the dependence of the continuum on non-mechanical

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parameters, first of all, on thermodynamic parameters (as on temperature) and onphysical-chemical ones (as on air humidity). Thus they enable us to deduce math­ematically the excitation and annihilation of vortices. Ertel's theorem is, however,for all continua valid in which a vortex potential can be defined; thus they are alsovalid in magnetohydrodynamics.

The problems of the validity limits of the continuum physics and the possibil­ity to deduce them from atomistics have been repeatedly discussed by Hans Erteland H J Treder. Ertel supposed that this problem would never be solved "in a finalform". Ertel supposed the stochastic cause in atomistic variations as in the "macro­scopically random" swinging over of instable weather situations, and generally thebasis of the "randomness" both in space and time of the transitions of instable situ­ations into stable ones, as well as cause of the randomness of the location and timeof the transition of the unstable into new, stable states (according to Poincare'sstatility criteria).

Ertel had been known in the thirties in certain circles as a cosmologist (hiscorresponding works were supported among others by von Laue, Einstein andSchrodinger}. In the papers on relativistic universe models with a spherically closedexpanding space and with a value A > 0 of Einstein's cosmological constant, Erteltreated the "great cosmic numbers" which according to A S Eddington and H Weylrefer to relations between cosmos and elementary particles.

It is valid according to the order of magnitude for Planck's quantum h, thegravity constant CT, the velocity of light c and the mass of the proton m:

(GhA)-lC3 = ((hc)j(om2))3 = 10117 .

Eddington considered this connection as the result of a primary stationary, butinstable state of the cosmos with the mass density:

Ae--­- 41]CTC3 •

Ertel remarked that this mass density determines in a very general class ofcosmological solutions of Einstein's gravity equations, namely in the Friedmann­Lemaitre-models, a quasi-stationary state which can survive for a very long timeand that the connections between cosmological and atomistic quantities had todevelop in this quasi-stationary phase ("Previously there was the "big bang").

According to Einstein's self critical remark that in interest of an unambiguousputting of the question it is better to use A = 0, the cosmological models with A > 0and together with them, the great cosmic numbers lost significance for many years.Since a few years astrophysicists reached again and again the conclusions that ithas to be A > 0, and therefore Ertel's work became again important.

Ertel's last publication (1971) was a new about cosmology. Treder remarkedto Ertel that all universe models, both Newtonian and Einsteinian ones togetherwith their modifications set out from the fact that the Hamiltonian of the cosmosis constant. Ertel as a potential theorist suggested at once that this is identicalwith the statement that the average variations of the Newtonian gravity potentialdisappear in cosmic dimensions (Ertel and Treder 1971).

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HANS ERTEL

References

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Ertel H 1942a: Meteorol. Z ., 59, 277-281.Ertel H 1942b: Meteorol. Z., 59, 385-387.Ertel H 1955: Kanonisches Algorithmus hydrodynamischer Wirbelgleichungen. Sitz .-Ber .

Deutsche Akad. Wiss ., Berlin , Akademie-Verlag, 1954/4 .Ertel H 1960: Gerlands Beitr. Geophysik, 69, 357-361.Ertel H 1962: Monatsber. Dt. Akad. Wiss. Berlin, 4, 368-372 .Ertel H 1963: Monatsber. Dt . Akad. Wiss . Berlin, 5, 362-365Ertel H 1971: Z. f. Meteorol., 22, 329-331.Ertel H, Treder H J 1971: Ann. d. Physik, 26, 23-28.Schroder W 1991: Geophysical hydrogynamics and Ertel's potential vorticity (including

english translations of Ertels hydrodynamical theorems) . IAGA/Science, Bremen

Acta Geod. Geoph, Hung. 34, 1999