What is STASH?
STASH is the Unified Model's storage handling and diagnostic system. It is designed to cope with the many different configurations that the UM can be used in, but still provide output in a consistent and standard way.
Prognostic and Diagnostic Fields
The UM considers variables (or fields) to be of two different types, prognostic or diagnostic. Prognostic fields are ones which the model must have values for, prior to each timestep, as the equations of motion the model solves require these fields (these are fields such as specific humidity or potential temperature) so they must exist in the model start dump. Diagnostic fields are all other fields that are derrived from prognostic ones, and as such the model does not need to have prior values for these. Ancillary files (such as emissions, SSTs etc) contain prognostic fields.
From a user's perspective, STASH is used to output fields during the run, and from this point of view it does not matter if these are prognostic or diagnostic fields. However, you will need to consider these differences when you add new chemical tracers.
STASH Sections and Items
Each field that is considered by STASH has a unique address which is given by a section and an item number. Prognostic fields are mostly held in section 0 (with the exception of tracers) and diagnostics are organised by areas of the code, e.g. short-wave radiation diagnostics are held in section 1, long-wave radiation diagnostics are held in section 2 etc. Some sections will always be on, and some sections will only be on if a certain process is selected, e.g. the interactive land-surface scheme. Each section can hold up to 512 items, where each item is a separate prognostic or diagnostic field, and can be either 2D or 3D.
Each field has its own entry in a STASHmaster file. There is a master list of fields which is held in the STASHmaster_A file, which is located at
/projects/um1/vn8.2/ctldata/STASHmaster/STASHmaster_A
on MONSooN, and at
/work/n02/n02/hum/vn8.2/ctldata/STASHmaster/STASHmaster_A
on HECToR. This is also a handy list of all the fields that can be outputted from the model, which is easier to search than by going through the UMUI panels.
