2021 Hinweis Die Berichte zur Erdsystemforschung werden vom Max-Planck-Institut für Meteorologie in Hamburg in unregelmäßiger Abfolge herausgegeben. Sie enthalten wissenschaftliche und technische Beiträge, inklusive Dissertationen. Die Beiträge geben nicht notwendigerweise die Auffassung des Instituts wieder. Die "Berichte zur Erdsystemforschung" führen die vorherigen Reihen "Reports" und "Examensarbeiten" weiter. Foreword Why should one name a land surface model after a composer? This funny

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... a was brought up around 1999 by Colin Prentice, a former director of the Max Planck Institute for Biogeochemistry in Jena, when he and his coworker Wolfgang Knorr realized in 1999 that climate models should be "biologized". After the achievements made in the 90s by coupling atmospheric general circulation models with ocean circulation models, and after the successful coupling with models for ocean biogeochemistry, it was the logical next step to include the so far missing variability from the interrelation between the land carbon cycle and the climate. By that time, Guy Brasseur had just taken one of the three director positions at the Max Planck Institute for Meteorology in Hamburg (MPI-M), determined to couple the ozone chemistry model MOZART originating from NCAR with the Hamburg atmospheric circulation model ECHAM. Clearly, from the Jena perspective, the Earth's biology is at least equally important as atmospheric chemistry. Hence, in order not to stand back against the MOZART coupling initiative, the name of an at least equally famous composer was needed for the new land surface component. And since it was clear from the outset that this "biologization" could only be achieved by establishing an appropriate research group of scientists from Jena at the home of ECHAM in Hamburg, the German composer J.S. Bach made the run: most conveniently, his name contains the letters "J" and "H", like Jena and Hamburg. So JSBACH stands now for Jena Scheme for Biosphere-Atmosphere Coupling in Hamburg. But from a name to a working model takes a while. Fortunately, Wolfgang's clever marketing of his Biosphere-Energy-Transfer-HYdrology model BETHY in applying for money at different funding agencies turned out to be very successful so that, around the end of 2001, the so-called "Jena group" became established at the MPI-M, financed from different national and European sources. Although well supported by Erich Roeckner and his ECHAM group, the "veggies" from Jena, talking about weird things like Rubisco production and sapwood diameter, remained a bit of "outsiders" at the MPI-M -best pictured by the fact that they remained located in the "Pavilion" (an euphemism for a kind of provisional building, better termed "barrack"), when the whole institute moved into the brand new MPI building. But steadily, the Hamburg institute started relying on "having" a vegetation group -first asking only for presentation material but finally showing pride in hosting such a group. No wonder: carbon cycle studies started to be a must for a first-class climate research institute. Of course, this recognition depended on the realization of JSBACH, which was tackled by four postdocs: Karl-Georg Schnitzler, Thomas Raddatz, and Christian Reick from the "Jena group", and Reiner Schnur from Erich Roeckner's group at MPI-M . The first and most difficult task was to re-code ECHAM5 in order to collect all land surface processes in one place and to sort them into a modular structure. This allowed to test Erich's requirement not to change the simulated climate by running the revised model in a kind of twin set-up -calculating all land surface processes twice, with the old ECHAM5 code and concurrently with the new JSBACH code. Thereafter, a first version of JSBACH was finalized by transferring the photosynthesis v vi code of BETHY, integrating a new phenology model, and adding five land carbon pools. This setup was used to participate in the Coupled Climate Carbon Cycle Model Intercomparison Project (C 4 MIP) in 2005, which led to the first mentioning in the scientific literature [45], a paper that was well recognized in the fourth Assessment Report of the IPCC and being the first real success of the JSBACH project. Nevertheless, the JSBACH group shrank as Colin Prentice moved to Bristol to take the lead in the QUEST project. Full integration into the MPI-M was accomplished not before the arrival of Martin Claussen in 2005 who, as a new director, established a whole department at MPI-M devoted to land processes in the Earth system. With this long-term perspective for JSBACH at MPI-M, with the development of further vegetation components (land use change (2006), dynamic vegetation (2008) ), and with a growing number of publications, the wish for a JSBACH documentation developed. But, as always, advancing to new horizons is much more exciting than consolidating the ground behind. Writing documentation was therefore rather slow and, in view of the ever ongoing code changes, the rudimentary writings were often outdated before they could be finalized. Hence, for years the documentation existed only as a draft, always trying to catch up with the JSBACH development. Nevertheless, this 'living document' helped generations of PhD students to get acquainted with the innards of JSBACH, keeping them fully aware that the final instance to answer questions on model details is always the code. Therefore, in retrospect, this permanent draft state might have been fully appropriate for documenting a model undergoing permanent change. This situation has now changed because the ECHAM-based development line of JSBACH has come to an end. The next-generation JSBACH (JSBACH 4) will be a complete re-write in the context of ICON 1 . Therefore, without the threat of ever getting outdated, now the time has come to publish a final documentation of the latest ECHAM-based JSBACH version 3.2. The JSBACH developers, Hamburg, December 2020