Project C09:
Role of the representation of gravity waves for downward transport into the UTLS

Brief Summary

Include text for the summary here. („Water vapor in the lowermost stratosphere (LMS) has been identified to act as an important positive climate feedback with significant associated circulation feedbacks. However, current global climate models simulate large moist biases in the LMS which, in turn, cause significant biases in the simulated circulation: an upward shift of subtropical jets, poleward shifts of the tropospheric eddy-driven jet and an intensified stratospheric circulation (Charlesworth et al., 2023). The distribution of water vapor in the LMS is in part controlled by transport, and in part by temperature and  microphysics, which introduce large uncertainties to many aspects regarding the climate relevance of the UTLS. The strong horizontal and vertical gradients of water vapor near the tropopause enhance sensitivities and thereby further complicate quantitative estimates of the associated climate effects.

In this project we will leverage the tracer budget framework in isentropic coordinates that we successfully implemented for ozone during phase 1. This budget approach allows insights into contributions from different transport processes: 1) residual/diabatic circulation transport, 2) adiabatic and diabatic eddy mixing (including contributions by different scales). We will work with both ERA5 and climate model output and carefully consider sources and sinks, especially due to microphysics near the tropopause. We will also work toward a statistical bias correction for reanalyses and climate models of water vapor in the LMS.

We will focus on the variability of LMS water vapor on different time scales by using combined high quality aircraft observations of the last 20 years including long-term observations from IAGOS and high numerous research aircraft campaigns. The data are consistently referenced to ERA 5 data along the flight track. We will determine mean LMS distributions and the variability of water vapor on different timescales from synoptic scale to multiannual time scale. We will further analyze its relation to the tropopause and Lagrangian cold point to infer potential transport pathways determining the role of the Lagrangian cold point for the water vapor in the LMS. We will derive the properties and variability of on different time scales. Using the DLM as well as classical regression models we will derive potential trends.“)

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