Mauro Hermann and his former MSc thesis supervisors (Lukas Papritz, Heini Wernli) recently submitted a study on Greenland melt events in Weather and Climate Dynamics. The use of Lagrangian backward trajectories – calculated with the powerful LAGRANTO tool also used in the INTEXseas project – gave them insight into the dynamical and thermodynamic processes acting on air masses, which end up just above the surface of the Greenland Ice Sheet during extensive melt periods.
The authors found that an upper-tropospheric ridge southeast of Greenland is mostly present during the 77 identified melt events in 1979-2017. Surprisingly, their intuitive idea that subsidence-induced adiabatic warming is an important contribution to the observed warm anomaly, as for air masses associated with near-surface heatwaves in the midlatitudes or the Central Arctic, was not confirmed. Instead, strong poleward transport from a climatologically warmer region (at lower levels and/or at lower latitudes) causes the air mass warmth. Latent heat release occurring as the air masses are forced to ascend orographically or dynamically is of secondary importance. The dynamical perspective helped the INTEXseas team members to systematically identify synoptic patterns and thermodynamic mechanisms, which are associated with air masses causing melt of the Greenland Ice Sheet.
The figure shows 10-day trajectories arriving over melting ice during the most extensive melt event in July 2012, colored according to their pressure, show illustratively the low-level and low-latitude origin of these air masses. Five representative example trajectories represent characteristic airstreams (S, C, E, N1, N2), shown in thicker lines with one circle per day colored from white (t = -240 h) to black (t = 0 h).
Find the discussion paper here.