Bjorn STEVENS, Claudia ACQUISTAPACE, Akio HANSEN, Rieke HEINZE, Carolin KLINGER, Daniel KLOCKE, Harald RYBKA, Wiebke SCHUBOTZ, Julia WINDMILLER, Panagiotis ADAMIDIS, Ioanna ARKA, Vasileios BARLAKAS, Joachim BIERCAMP, Matthias BRUECK, Sebastian BRUNE, Stefan A. BUEHLER, Ulrike BURKHARDT, Guido CIONI, Montserrat COSTA-SURÓS, Susanne CREWELL, Traute CRÜGER, Hartwig DENEKE, Petra FRIEDERICHS, Cintia Carbajal HENKEN, CATHY Hohenegger, Marek JACOB, Fabian JAKUB, Norbert KALTHOFF, Martin KÖHLER, Thirza W. van LAAR, Puxi LI, Ulrich LÖHNERT, Andreas MACKE, Nils MADENACH, Bernhard MAYER, Christine NAM, Ann Kristin NAUMANN, Karsten PETERS, Stefan POLL, Johannes QUAAS, Niklas RÖBER, Nicolas ROCHETIN, Leonhard SCHECK, Vera SCHEMANN, Sabrina SCHNITT, Axel SEIFERT, Fabian SENF, Metodija SHAPKALIJEVSKI, Clemens SIMMER, Shweta SINGH, Odran SOURDEVAL, Dela SPICKERMANN, Johan STRANDGREN, Octave TESSIOT, Nikki VERCAUTEREN, Jessica VIAL, Aiko VOIGT, Günter ZÄNGL
More than one hundred days were simulated over very large domains with fine (0.156 km to 2.5 km) grid spacing for realistic conditions to test the hypothesis that storm (kilometer) and large-eddy (hectometer) resolving simulations would provide an improved representation of clouds and precipitation in atmospheric simulations. At scales that resolve convective storms (storm-resolving for short), the vertical velocity variance becomes resolved and a better physical basis is achieved for representing clouds and precipitation. Similarly to past studies we found an improved representation of precipitation at kilometer scales, as compared to models with parameterized convection. The main precipitation features (location, diurnal cycle and spatial propagation) are well captured already at kilometer scales, and refining resolution to hectometer scales does not substantially change the simulations in these respects. It does, however, lead to a reduction in the precipitation on the time-scales considered – most notably over the ocean in the tropics. Changes in the distribution of precipitation, with less frequent extremes are also found in simulations incorporating hectometer scales. Hectometer scales appear to be more important for the representation of clouds, and make it possible to capture many important aspects of the cloud field, from the vertical distribution of cloud cover, to the distribution of cloud sizes, and to the diel (daily) cycle. Qualitative improvements, particularly in the ability to differentiate cumulus from stratiform clouds, are seen when one reduces the grid spacing from kilometer to hectometer scales. At the hectometer scale new challenges arise, but the similarity of observed and simulated scales, and the more direct connection between the circulation and the unconstrained degrees of freedom make these challenges less daunting. This quality, combined with already improved simulation as compared to more parameterized models, underpins our conviction that the use and further development of storm-resolving models offers exciting opportunities for advancing understanding of climate and climate change.
This study investigates a new possible process linking the quasi-stationary Rossby wave propagation (SWP) over Eurasia along the Asian jet and the Pacific-Japan (PJ) pattern through the Rossby wave breaking (RWB) near the jet exit region during boreal summer using a reanalysis dataset. To assess the statistical significance of the process, we conduct a lag composite analysis of the past 44 RWB events east of Japan. The result of the lag composite analysis shows that the SWP along the Asian jet induces the RWB accompanied by an amplified anomalous anticyclone east of Japan. The associated “inverse-S” shaped overturning of the upper-level potential vorticity (PV) distribution causes the southwestward intrusion of the high PV toward the subtropical western North Pacific (WNP). The Q-vector diagnosis and vorticity budget analysis indicate that the upper-level positive vorticity advection associated with the RWB is an important factor dynamically inducing ascent and reinforcing convection over the subtropical WNP, which in turn excites the subsequent PJ pattern. Classification of the cases by RWB strength indicates that the stronger RWB is significantly related to the stronger preceding SWP and subsequent enhanced PJ pattern, and vice versa. A partial correlation analysis of all the cases quantitatively shows the greater contribution of the upper-level positive vorticity advection over the subtropical WNP to the enhanced convection in this area and the formation of the PJ pattern, compared to that of the anomalous warm sea surface temperature condition. These results show that the SWP along the Asian jet can excite the PJ pattern, through the RWB east of Japan and the consequent intrusion of the high PV toward the subtropical WNP.