Turbulence over the Atlantic ITCZ - Is the upper troposphere above doldrums less turbulent?

Abstract. Flight level wind measurements from the German research aircraft HALO are used to investigate the turbulence in the upper troposphere over the inter-tropical convergence zone (ITCZ) of the eastern and western tropical Atlantic. During the 2024 PERCUSION mission, 23 research flights were conducted to investigate mesoscale drivers of the inner life cycle of the ITCZ and to provide a rich data set for validating EarthCARE satellite measurements. From these flights, 10  Hz BAHAMAS measurements along 183 constant altitude legs were extracted and analyzed to characterize the turbulence over the tropical Atlantic. A key focus was on investigating whether there is indeed a link between the recently discovered subsidence over the doldrums - calm regions within the ITCZ - and reduced turbulence activity in the upper troposphere. The ECMWF's meteorological analyses of 89 circular flight legs identified 13 doldrums in which the area-averaged wind speed at a height of 10 m was less than 3  m  s^-1. These regions were further characterized by a column-integrated divergence, i.e. by mass spreading out above the doldrums.

The turbulence observed during the PERCUSION mission was generally stronger than that recorded in previous HALO measurements in the stratosphere, but was moderate in absolute terms; strong turbulence events remained rare. Turbulence intensity was higher in the eastern Atlantic (near Cape Verde) and closer to the equator, often associated with large convective systems. The relationship between the observed turbulent kinetic energy (TKE) and the energy dissipation rate ε matches the theoretical expectations ε ∝ TKE^3/2, allowing estimates of the outer scale of turbulence. There is a preliminary, weak indication that upper-tropospheric turbulence is indeed reduced above the doldrums. The relationship is not exclusive; low-level calms do not always guarantee reduced upper-level turbulence, and significant atmospheric variability exists. While the thermal stability in the upper troposphere above the doldrums is comparable to that of all circular flight legs, ECMWF's meteorological analyses indicate a reduction of vertical shear above the doldrums. This tentative link requires further investigation: High-resolution modeling or additional data are needed for robust conclusions.