Climate dynamics

Abstract:

This study evaluates convective cell properties and their relationships with convective and stratiform rainfall within a season‐long convection‐permitting weather research and forecasting simulation over central Argentina using radar, satellite, and radiosonde measurements from the RELAMPAGO‐CACTI field campaign. The simulation slightly underestimates radar‐estimated rainfall over the ∼3.5‐month evaluation period but underestimates stratiform rainfall by 46% and overestimates convective rainfall by 43%. As convective available potential energy (CAPE) increases, the convective rainfall overestimation decreases, but the stratiform rainfall underestimation increases such that the contribution of convective to total rainfall remains constantly high biased by ∼26%. Overestimated convective rainfall arises from the simulation generating 2.6 times more precipitating convective cells (14,299) than observed by radar (5,662) despite similar observed and simulated cell growth processes, with relatively wide cells contributing mostly to excessive convective rainfall. Relatively shallow cells, typically reaching heights of 4–7 km, contribute most to the cell number bias. This cell number bias increases as CAPE decreases, potentially because cells and their updrafts become narrower and more under‐resolved as CAPE decreases. The gross overproduction of precipitating shallow cells leads to overly efficient precipitation and inadequate detrainment of ice aloft, thereby diminishing the formation of robust stratiform rainfall regions. Decreasing model horizontal grid spacing from 3 to 1 or 0.333 km for low (<300 J kg−1) and high CAPE (>1,000 J kg−1) cases results in minimal change to cell number, depth, and convective‐to‐stratiform partitioning biases. This suggests that improving prediction of these convective properties depends on factors beyond solely increasing model resolution.

Abstract:

Purpose of Review: Extratropical jets and associated storm tracks significantly influence weather and regional climate across various timescales. Understanding jet responses to climate change is essential for reliable regional climate projections. This review serves two main purposes: (1) to provide an accessible overview of extratropical jet dynamics and a comprehensive examination of current challenges and uncertainties in predicting jet responses to greenhouse gas increases and (2) to suggest innovative experiments to advance our understanding of these responses. Recent Findings: While successive generations of climate model ensembles consistently project a mean poleward shift of the midlatitude zonal-mean maximum winds, there remains considerable intermodel spread and large uncertainty across seasonal and regional jet responses. Of particular note is our limited understanding of how these jets respond to the intricate interplay of multiple concurrent drivers, such as the strong warming in polar and tropical regions, and the relative importance of each factor. Furthermore, the difficulty of simulating processes requiring high resolution, such as those linked to sharp sea surface temperature gradients or diabatic effects related to tropical convection and extratropical cyclones, has historically hindered progress. Summary: We advocate for a collaborative effort to enhance our understanding of the jet stream response to climate change. We propose a series of new experiments that take advantage of recent advances in computing power and modelling capabilities to better resolve small-scale processes such as convective circulations, which we consider essential for a good representation of jet dynamics.

Abstract:

The Quasi-Biennial Oscillation (QBO) is one of the most cyclic phenomena in the atmosphere except for the annular and diurnal cycles, which provide the predictability source for subseasonal-to-seasonal forecasts on the globe. The QBO is generated by the interaction between the background circulation and the equatorial waves, which cover a wide spectrum consisting of those that are eastward- and westward-propagating. The QBO can affect the climate in both the Northern and Southern Hemispheres through at least three dynamic pathways, including the stratospheric polar vortex pathway, the subtropical downward-arching zonal wind pathway, and the tropical convection pathway. The impact of the QBO on the extratropics is projected to strengthen in future scenario experiments, although the maximum QBO wind magnitude gradually decreased in recent decades. As a newly emerging feature, the QBO disruption during the westerly phase is mainly caused by the extremely active Rossby waves from the extratropics. The QBO disruptions are likely to increase in a warmer climate background.