Mid-term presentation by Jesper Ejlebæk Holm

Title: Firn-Air Modelling in the Community Firn model

Over the last glacial period, a series of 25 rapid climate fluctuations were observed, characterized by rapid warming followed by a longer cooling period. One of these Dansgaard-Oescher (D-O) events happened around 11,500 years ago and heralded the end of the ice age and the beginning of the Holocene period.

This project aims to quantify the timing and magnitude of this transition to the Holocene by reconstructing paleotemperatures obtained with the inversion of coupled densification and diffusion models forced by surface temperatures and accumulation rates.

The Community Firn Model (CFM) is one such example, and it provides an open-source-modular framework to simulate physical processes in the firn. A 1-dimensional Lagrangian grid is used where density is explicitly calculated while diffusion is solved through implicit Finite Volume Method (FVM). The presentation will focus on three experiments to better understand the model’s behavior under different conditions.

First, the halfway time to a new equilibrium following a linear fluctuation is computed for a series of amplitudes and duration. We show that for changes in amplitude, the halfway time tends to decrease, while the behavior is more complex for the duration. In addition, we examine the behavior of the CFM and different air-advection schemes by testing a variety of “DO-like” events. It is observed that higher temperatures and lower accumulation rates lead to a faster onset of the close-off depth.

Finally, noisy d15N2 data, computed from NGRIP, is inverted into noisy temperature data by way of Brent’s root-finding method.