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Ian P. White, Chaim I. Garfinkel, and Peter Hitchcock

Abstract

An idealized model is used to examine the tropospheric response to sudden stratospheric warmings (SSWs), by imposing transient stratospheric momentum torques tailored to mimic the wave-forcing impulse associated with spontaneously occurring SSWs. Such an approach enables us to examine both the ∼2–3-week forcing stage of an SSW during which there is anomalous stratospheric wave-activity convergence, as well as the recovery stage during which the wave forcing abates and the stratosphere radiatively recovers over 2–3 months. It is argued that applying a torque is better suited than a heating perturbation for examining the response to SSWs, due to the meridional circulation that is induced to maintain thermal-wind balance (i.e., the “Eliassen adjustment”); an easterly torque yields downwelling at high latitudes and equatorward flow below, similar to the wave-induced circulation that occurs during spontaneously occurring SSWs, whereas a heating perturbation yields qualitatively opposite behavior and thus cannot capture the initial SSW evolution. During the forcing stage, the meridional circulation in response to an impulse comparable to the model’s internal variability is able to penetrate down to the surface and drive easterly-wind anomalies via Coriolis torques acting on the anomalous equatorward flow. During the recovery stage, after which the tropospheric flow has already responded, the meridional circulation associated with the stratosphere’s radiative recovery appears to provide the persistent stratospheric forcing that drives the high-latitude easterly anomalies, whereas planetary waves are found to play a smaller role. This is then augmented by synoptic-wave feedbacks that drive and amplify the annular-mode response.

Open access
Tobias Selz, Michael Riemer, and George C. Craig

Abstract

This study investigates the transition from current practical predictability of midlatitude weather to its intrinsic limit. For this purpose, estimates of the current initial condition uncertainty of 12 real cases are reduced in several steps from 100% to 0.1% and propagated in time with a global numerical weather prediction model (ICON at 40 km resolution) that is extended by a stochastic convection scheme to better represent error growth from unresolved motions. With the provision that the perfect model assumption is sufficiently valid, it is found that the potential forecast improvement that could be obtained by perfecting the initial conditions is 4–5 days. This improvement is essentially achieved with an initial condition uncertainty reduction by 90% relative to current conditions, at which point the dominant error growth mechanism changes: With respect to physical processes, a transition occurs from rotationally driven initial error growth to error growth dominated by latent heat release in convection and due to the divergent component of the flow. With respect to spatial scales, a transition from large-scale up-amplitude error growth to a very rapid initial error growth on small scales is found. Reference experiments with a deterministic convection scheme show a 5%–10% longer predictability, but only if the initial condition uncertainty is small. These results confirm that planetary-scale predictability is intrinsically limited by rapid error growth due to latent heat release in clouds through an upscale-interaction process, while this interaction process is unimportant on average for current levels of initial condition uncertainty.

Significance Statement

Weather predictions provide high socioeconomic value and have been greatly improved over the last decades. However, it is widely believed that there is an intrinsic limit to how far into the future the weather can be predicted. Using numerical simulations with an innovative representation of convection, we are able to confirm the existence of this limit and to demonstrate which physical processes are responsible. We further provide quantitative estimates for the limit and the remaining improvement potential. These results make clear that our current weather prediction capabilities are not yet maxed out and could still be significantly improved with advancements in atmospheric observation and simulation technology in the upcoming decades.

Open access
Paul E. Roundy

Abstract

A robust linear regression algorithm is applied to estimate 95% confidence intervals on the background wind associated with Madden–Julian oscillation (MJO) upper-tropospheric atmospheric circulation signals characterized by different phase speeds. Data reconstructed from the ERA5 to represent advection by the upper-tropospheric background flow and MJO-associated zonal wind anomalies, together with satellite outgoing longwave radiation anomalies, all in the equatorial plane, are regressed against advection data filtered for zonal wavenumber 2 and phase speeds of 3, 4, 5, and 7 m s−1. The regressed advection by the background flow is then divided by the negative of the zonal gradient of regressed zonal wind across the central Indian Ocean base longitude at 80°E to estimate the associated background wind that leads to the given advection. The median estimates of background wind associated with these phase speeds are 13.4, 11.2, 10.5, and 10.3 m s−1 easterly. The differences between estimated values at neighboring speeds suggests that advection acts most strongly in slow MJO events, indicating that the slowest events happen to be slow because they experience stronger easterly advection by the upper-tropospheric background wind.

Significance Statement

The Madden–Julian oscillation (MJO) is the dominant subseasonal rainfall signal of the tropical atmosphere. This project shows that the background wind of the tropical atmosphere most especially slows down the slowest MJO events. Understanding what controls its speed might help scientists better predict events.

Open access
Jun-Ichi Yano

Abstract

The present study shows by a linear hydrodynamic stability analysis that an unstable mixed-layer deep circulation can be generated in the dry convective well-mixed layer by the entrainment from the top. The newly identified instability arises under the two competing processes induced by the top entrainment: the destabilization by generating thermal perturbations and the damping by mechanical mixing. The former and the latter, respectively, dominate over the other in the limits of large and small scales. As a result, the instability is realized at the horizontal scales larger than the order of the mixed-layer depth (ca. 1 km), and the time scale for the growth is about 1 day. This study has been motivated from a question of whether the cloud-top entrainment instability (CTEI) can induce a transition of the stratocumulus-topped well-mixed boundary layer into trade cumulus. The present study intends to extend the previous studies based on the local parcel analyses to a full analysis based on the hydrodynamics. Unfortunately, being based on a dry formulation, the present result does not apply directly to the CTEI problem. Especially, the evaporative cooling is totally neglected. Nevertheless, the present result can still be applied to moist systems, to some extent, by redefining certain terms in the formulation.

Open access
Ramon Padullés, Yi-Hung Kuo, J. David Neelin, F. Joseph Turk, Chi O. Ao, and Manuel de la Torre Juárez

Abstract

The transition to deep convection and associated precipitation is often studied in relationship to the associated column water vapor owing to the wide availability of these data from various ground or satellite-based products. Based on radiosonde and ground-based global navigation satellite system (GNSS) data examined at limited locations and model comparison studies, water vapor at different vertical levels is conjectured to have different relationships to convective intensity. Here, the relationship between precipitation and water vapor in different free-tropospheric layers is investigated using globally distributed GNSS radio occultation (RO) temperature and moisture profiles collocated with GPM IMERG precipitation across the tropical latitudes. A key feature of the RO measurement is its ability to directly sense in and near regions of heavy precipitation and clouds. Sharp pickups (i.e., sudden increases) of conditionally averaged precipitation as a function of water vapor in different tropospheric layers are noted for a variety of tropical ocean and land regions. The layer-integrated water vapor value at which this pickup occurs has a dependence on temperature that is more complex than constant RH, with larger subsaturation at warmer temperatures. These relationships of precipitation to its thermodynamic environment for different layers can provide a baseline for comparison with climate model simulations of the convective onset. Furthermore, vertical profiles before, during, and after convection are consistent with the hypothesis that the lower troposphere plays a causal role in the onset of convection, while the upper troposphere is moistened by detrainment from convection.

Open access
Clément Soufflet, François Lott, and Bruno Deremble

Abstract

The boundary layer theory for nonhydrostatic mountain waves presented in is extended to include upward-propagating gravity waves and trapped lee waves. To do so, the background wind with constant shear used in is smoothly curved and becomes constant above a “boundary layer” height d, which is much larger than the inner layer scale δ. As in , the pressure drag stays well predicted by a gravity wave drag when the surface Richardson number J > 1 and by a form drag due to nonseparated sheltering when J < 1. As in also, the sign of the Reynolds stress is predominantly positive in the near-neutral case (J < 1) and negative in the stable case (J > 1) but situations characterized by positive and negative Reynolds stress now combine when J ∼ 1. In the latter case, and even when dissipation produces positive stress in the lower part of the inner layer, a property we associated with nonseparated sheltering in , negative stresses are quite systematically found aloft. These negative stresses are due to upward-propagating waves and trapped lee waves, the first being associated with negative vertical flux of pseudomomentum aloft the inner layer, the second to negative horizontal flux of pseudomomentum downstream the obstacle. These results suggest that the significance of mountain waves for the large-scale flow is more substantial than expected and when compared to the form drag due to nonseparated sheltering.

Open access
Chiung-Yin Chang and Isaac M. Held

Abstract

Diffusive theories for the meridional atmospheric energy transport can summarize our understanding of this central aspect of the general circulation. They can also be utilized in simple models of Earth’s energy balance to help interpret the response of the system to perturbations. A theory for this diffusivity of eddy heat transport is described based on Rhines scaling and the global entropy budget, each of which provides a constraint between the kinetic energy dissipation and the diffusivity. An expression for the diffusivity is then obtained by eliminating the dissipation from this set of two constraints. The theory can be thought of as a generalization of the theories of Held–Larichev and Barry–Craig–Thuburn. The theory is compared to simulations of the Held–Suarez idealized dry atmospheric model. Limitations of the theory are emphasized. The form of the theory allows it to be readily generalized to a moist atmosphere.

Open access
Željka Fuchs-Stone and Kerry Emanuel

Abstract

Two analytical models with different starting points of convective parameterizations, the Fuchs and Raymond model on one hand and the Khairoutdinov and Emanuel model on the other, are used to develop “minimal difference” models for the MJO. The main physical mechanisms that drive the MJO in both models are wind-induced surface heat exchange (WISHE) and cloud–radiation interactions (CRI). The dispersion curves for the modeled eastward-propagating mode, the MJO mode, are presented for an idealized case with zero meridional wind and for the realistic cases with higher meridional numbers. In both cases, the two models produce eastward-propagating modes with the growth rate greatest at the largest wavelengths despite having different representations of cumulus convection. We show that the relative contributions of WISHE and CRI are sensitive to how the convection and entropy/moisture budgets are represented in models like these.

Significance Statement

The Madden–Julian oscillation is the largest weather disturbance on our planet. It propagates eastward encompassing the whole tropical belt. It influences weather all around the globe by modulating hurricanes, atmospheric rivers, and other phenomena. Numerical models that forecast the Madden–Julian oscillation need improvement. Here we explore the physics behind the Madden–Julian oscillation using simple analytical models. Our models are based on the assumption that surface enthalpy fluxes and cloud–radiation interactions are responsible for the Madden–Julian oscillation but it should be borne in mind that other physical mechanisms have been proposed for the MJO. The impact of this research is to better understand the Madden–Julian oscillation mechanism.

Open access
Ryusuke Masunaga and Niklas Schneider

Abstract

Satellite observations have revealed that mesoscale sea surface temperature (SST) perturbations can exert distinct influence on sea surface wind by modifying the overlying atmospheric boundary layer. Recently, spectral transfer functions have been shown to be useful to elucidate the wind response features. Spectral transfer functions can represent spatially lagged responses, their horizontal scale dependence, and background wind speed dependence. By adopting the transfer function analysis, the present study explores seasonality and regional differences in the wind response over the major western boundary current regions. Transfer functions estimated from satellite observations are found to be largely consistent among seasons and regions, suggesting that the underlying dominant dynamics are ubiquitous. Nevertheless, the wind response exhibits statistically significant seasonal and regional differences depending on background wind speed. When background wind is stronger (weaker) than 8.5 m s−1, the wind response is stronger (weaker) in winter than in summer. The Agulhas Retroflection region exhibits stronger wind response typically by 30% than the Gulf Stream and Kuroshio Extension regions. Although observed wind distributions are reasonably reconstructed from the transfer functions and observed SST, surface wind convergence zones along the Gulf Stream and Kuroshio Extension are underrepresented. The state-of-the-art atmospheric reanalysis and regional model represent well the structure of the transfer functions in the wavenumber space. The amplitude is, however, underestimated by typically 30%. The transfer function analysis can be adapted to many other atmospheric responses besides sea surface wind, and thus provide new insights into the climatic role of the mesoscale air–sea coupling.

Open access
Enoch Jo and Sonia Lasher-Trapp

Abstract

Supercell thunderstorms can produce heavy precipitation, and an improved understanding of entrainment may help to explain why. In Part I of this series, various mechanisms of entrainment were identified in the rotating stage of a single simulated supercell thunderstorm. The current study examines the strength and effectiveness of these mechanisms as a function of the environmental vertical wind shear in eight different supercell simulations. Entrainment is calculated directly as fluxes of air over the surface of the storm core; tracers are used to assess the resulting dilution of the moistest air ingested by the storm. Model microphysical rates are used to compare the impacts of entrainment on the efficiency of condensation/deposition of water vapor on hydrometeors within the core, and ultimately, upon precipitation production. Results show that the ascending “ribbons” of horizontal vorticity wrapping around the updraft contribute more to entrainment with increasing vertical wind shear, while turbulent eddies on the opposite side of the updraft contribute less. The storm-relative airstream introduces more low-level air into the storm core with increasing vertical wind shear. Thus, the total entrainment increases with increasing vertical wind shear, but the fractional entrainment decreases, yielding an increase in undiluted air within the storm core. As a result, the condensation efficiency within the storm core also increases with increasing vertical wind shear. Due to the increase in hydrometeors detrained aloft and the resulting enhanced evaporation as they fall, the precipitation efficiency evaluated using surface rainfall decreases with increasing vertical wind shear, as found in past studies.

Open access