The COsmic-ray Soil Moisture Interaction Code (COSMIC) for use in data assimilation

Soil moisture status in land surface models (LSMs) can be updated by assimilating cosmic-ray neutron intensity measured in air above the surface. This requires a fast and accurate model to calculate the neutron inten- sity from the profiles of soil moisture modeled by the LSM. The existing Monte Carlo N-Particle eXtended (MCNPX) model is sufficiently accurate but too slow to be practical in the context of data assimilation. Consequently an alter- native and efficient model is needed which can be calibrated accurately to reproduce the calculations made by MCNPX and used to substitute for MCNPX during data assimilation. This paper describes the construction and calibration of such a model, COsmic-ray Soil Moisture Interaction Code (COS- MIC), which is simple, physically based and analytic, and which, because it runs at least 50 000 times faster than MC- NPX, is appropriate in data assimilation applications. The model includes simple descriptions of (a) degradation of the incoming high-energy neutron flux with soil depth, (b) cre- ation of fast neutrons at each depth in the soil, and (c) scat- tering of the resulting fast neutrons before they reach the soil surface, all of which processes may have parameterized de- pendency on the chemistry and moisture content of the soil. The site-to-site variability in the parameters used in COS- MIC is explored for 42 sample sites in the COsmic-ray Soil Moisture Observing System (COSMOS), and the compar- ative performance of COSMIC relative to MCNPX when applied to represent interactions between cosmic-ray neu- trons and moist soil is explored. At an example site in Ari- zona, fast-neutron counts calculated by COSMIC from the average soil moisture profile given by an independent net- work of point measurements in the COSMOS probe foot- print are similar to the fast-neutron intensity measured by the COSMOS probe. It was demonstrated that, when used within a data assimilation framework to assimilate COS- MOS probe counts into the Noah land surface model at the Santa Rita Experimental Range field site, the calibrated COSMIC model provided an effective mechanism for trans- lating model-calculated soil moisture profiles into above- ground fast-neutron count when applied with two radically different approaches used to remove the bias between data and model.