Abstract
This paper investigates the accuracy of simulated long-duration super-pressure balloon trajectories in the lower stratosphere. The observed trajectories were made during the (tropical) Pre-Concordiasi and (polar) Concordiasi campaigns in 2010, while the simulated trajectories are computed using analyses and forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System model. In contrast with the polar stratosphere situation, modelling accurate winds in the tropical lower stratosphere remains challenging for numerical weather prediction systems. The accuracy of the simulated tropical trajectories are quantified with the operational products of 2010 and 2016 in order to understand the impact of model physics and vertical resolution improvements. The median errors in these trajectories are large (typically ≳ 250 km after 24 h), with a significant negative bias in longitude, for both model versions. In contrast, using analyses in which the balloon-borne winds have been assimilated reduces the median error in the balloon position after 24 h to ∼60 km. For future campaigns, we describe operational strategies that take advantage of the geographic distribution and the episodic nature of large error events to anticipate the amplitude of error in trajectory forecasts. We finally stress the importance of a high vertical resolution in the model, given the intense shears encountered in the tropical lower stratosphere.
Highlights
The tropical tropopause layer (TTL) is a striking example of a transition region between two circulation regimes that is challenging to accurately model [1]
The present study uses these runs (CTL-2016 and BAL-2016) as an opportunity to explore the impacts of model improvement and balloon-borne wind assimilation on the accuracy of balloon trajectory calculations; those runs were primarily designed to evaluate whether assimilating long-duration balloon observations could reduce errors reported in [16], so that forecasts were archived on pressure levels only, i.e., at a significantly reduced resolution with respect to model levels
00 UT (t0 ), and the following forecast steps (t0 + 6, 12, 18, 24, 30 . . . h) This time difference between the forecast initialization time and the trajectory starting time mimics the time needed for the numerical weather prediction (NWP) center to compute the forecast set in real-time operations
Summary
The tropical tropopause layer (TTL) is a striking example of a transition region between two circulation regimes that is challenging to accurately model [1]. This study highlighted errors as large as 8 ms−1 in winds provided by both the European Center for Medium-range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) analyses and the Modern-Era Retrospective analysis for Research and Applications (MERRA) reanalyses for periods of time as long as one month Such episodes preferably occurred over regions where wind observations are very sparse such as the Indian. Strateole-2 has been notably designed to investigate processes involved in tropical dehydration, since those quasi-Lagrangian balloon platforms provide a unique opportunity to measure gravity-wave induced fast temperature fluctuations experienced by air parcels [21] These fluctuations contribute to shaping the characteristics of crystals that form in ice clouds (e.g., [22,23]), as well as the vertical distribution of these crystals [24].
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