Quantification of internal variability of the arctic summer atmosphere based on HIRHAM5 ensemble simulations The non linear behaviour of the atmospheric dynamics is not well understood and makes the evaluation and usage of regional climate models (RCMs) difficult. Due to these non linearities chaos and internal variability (IV) within the RCMs are induced leading to a sensitivity of RCMs to their initial conditions (IC). The IV is the ability of RCMs to realise different solutions of simulations that differ in their IC but have the same lower and lateral boundary conditions (LBC) hence can be defined as the across member spread between the ensemble members. For the investigation of the IV and the dynamical and diabatic contributions generating the IV four ensembles of RCM simulations are performed with the atmospheric regional model HIRHAM5. The integration area is the Arctic and each ensemble consists of 20 members. The ensembles cover the time period from July to September for the years 2006 2007 2009 and 2012. The ensemble members have the same LBC and differ in their IC only. The different IC are arranged by an initialisation time…