Abstract
Recent research has demonstrated that additional winter radiosonde observations in Arctic regions enhance the predictability of mid-latitude weather extremes by reducing uncertainty in the flow of localised tropopause polar vortices. The impacts of additional Arctic observations during summer are usually confined to high latitudes and they are difficult to realize at mid-latitudes because of the limited scale of localised tropopause polar vortices. However, in certain climatic states, the jet stream can intrude remarkably into the mid-latitudes, even in summer; thus, additional Arctic observations might improve analysis validity and forecast skill for summer atmospheric circulations over the Northern Hemisphere. This study examined such cases that occurred in 2016 by focusing on the prediction of the intensity and track of tropical cyclones (TCs) over the North Atlantic and North Pacific, because TCs are representative of extreme weather in summer. The predictabilities of three TCs were found influenced by additional Arctic observations. Comparisons with ensemble reanalysis data revealed that large errors propagate from the data-sparse Arctic into the mid-latitudes, together with high-potential-vorticity air. Ensemble forecast experiments with different reanalysis data confirmed that additional Arctic observations sometimes improve the initial conditions of upper-level troposphere circulations.
Highlights
The forecast skill of tropical cyclone (TC) tracks has improved substantially in recent decades, extending the tropical cyclones (TCs) forecast period to five days[1,2]
It is expected that improvement in the reproducibility of atmospheric circulations at high latitudes would contribute to improved accuracy of severe weather forecasts for the mid-latitudes, even in summer
Dropsonde observations were taken over the North Atlantic during 20 and 25 September using the National Aeronautics and Space Administration (NASA) Global Hawk unmanned aircraft to improve track forecasts for Tropical storm Karl (Fig. 1)
Summary
Kazutoshi Sato 1,2, Jun Inoue[1,3,4], AkiraYamazaki[3], Joo-Hong Kim 5, Alexander Makshtas[6], Vasilli Kustov[6], Marion Maturilli 7 & Klaus Dethloff[7]. In OSEMIDf, overestimated southerly winds caused by large errors in upper-troposphere circulations resulted in faster northward movement of Lionrock at 1200 UTC 29 August (Supplementary Fig. 4e,k), revealing that the Arctic radiosonde observations had an impact on the Lionrock track forecast, as did the routine radiosondes at mid-latitudes and in the tropics. The amplitudes of the meandering jet stream (calculated using a “zonal index”) over the Pacific Ocean during August and over the Atlantic Ocean during September 2016 were not the largest during 1979–2016 (not shown) This implies that additional Arctic radiosonde observations sometimes affect the forecasts of tropical and mid-latitude cyclone tracks, even in summer. Field campaigns scheduled during the Year Of Polar Prediction[35] and the Years of Maritime Continent from mid-2017 to mid-2019 should provide great opportunity to increase evidence of the effect of additional summer radiosonde observations over the Northern Hemisphere on the predictability of weather extremes at mid-latitudes
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