Alvaro de la Cámara

Modification of the Gravity Wave Parameterization in the Whole Atmosphere Community Climate Model: Motivation and Results

AbstractThe current standard version of the Whole Atmosphere Community Climate Model (WACCM) simulates Southern Hemisphere winter and spring temperatures that are too cold compared with observations. This “cold-pole bias” leads to unrealistically low ozone column amounts in Antarctic spring. Here, the cold-pole problem is addressed by introducing additional mechanical forcing of the circulation via parameterized gravity waves. Insofar as observational guidance is ambiguous regarding the gravity waves that might be important in the Southern Hemisphere stratosphere, the impact of increasing the forcing by orographic gravity waves was investigated. This reduces the strength of the Antarctic polar vortex in WACCM, bringing it into closer agreement with observations, and accelerates the Brewer–Dobson circulation in the polar stratosphere, which warms the polar cap and improves substantially the simulation of Antarctic temperature. These improvements are achieved without degrading the performance of the model i...

Stochastic parameterization: Towards a new view of weather and climate models

AbstractThe last decade has seen the success of stochastic parameterizations in short-term, medium-range, and seasonal forecasts: operational weather centers now routinely use stochastic parameterization schemes to represent model inadequacy better and to improve the quantification of forecast uncertainty. Developed initially for numerical weather prediction, the inclusion of stochastic parameterizations not only provides better estimates of uncertainty, but it is also extremely promising for reducing long-standing climate biases and is relevant for determining the climate response to external forcing. This article highlights recent developments from different research groups that show that the stochastic representation of unresolved processes in the atmosphere, oceans, land surface, and cryosphere of comprehensive weather and climate models 1) gives rise to more reliable probabilistic forecasts of weather and climate and 2) reduces systematic model bias. We make a case that the use of mathematically stri...

Comparison of Gravity Waves in the Southern Hemisphere Derived from Balloon Observations and the ECMWF Analyses

AbstractThe increase of spatial resolution allows the ECMWF operational model to explicitly resolve a significant portion of the atmospheric gravity wave (GW) field, but the realism of the simulated GW field in the ECMWF analyses still needs to be precisely evaluated. Here the authors use data collected during the Concordiasi stratospheric balloon campaign to assess the representation of GWs in the ECMWF analyses over Antarctica and the Southern Ocean in spring 2010. The authors first compare the balloonborne GW momentum fluxes with those in ECMWF analyses throughout the campaign and find a correct agreement of the geographical and seasonal patterns. However, the authors also note that ECMWF analyses generally underestimate the balloon fluxes by a factor of 5, which may be essentially due to the spatial truncation of the ECMWF model. Intermittency of wave activity in the analyses and observations are found comparable. These results are confirmed on two case studies dealing with orographic and nonorographi...

Routes of Transport across the Antarctic Polar Vortex in the Southern Spring

AbstractTransport in the lower stratosphere over Antarctica has been studied in the past by means of several approaches, such as contour dynamics or Lyapunov exponents. This paper examines the problem by means of a new Lagrangian descriptor, which is referred to as the function M. The focus is on the southern spring of 2005, which allows for a comparison with previous analyses based on Lyapunov exponents. With the methodology based on the function M, a much sharper depiction of key Lagrangian features is achieved and routes of large-scale horizontal transport across the vortex edge are captured. These results highlight the importance of lobe dynamics as a transport mechanism across the Antarctic polar vortex.

Isentropic Transport within the Antarctic Polar-Night Vortex: Rossby Wave Breaking Evidence and Lagrangian Structures

AbstractThe trajectories in the lower stratosphere of isopycnic balloons released from Antarctica by Vorcore and Concordiasi field campaigns during the southern springs of 2005 and 2010 showed events of latitudinal transport inside the stratospheric polar vortex, both away from and toward the poleward flank of the polar-night jet. The present paper applies trajectory-based diagnostic techniques to examine mechanisms at work during such events. Reverse domain-filling calculations of potential vorticity (PV) fields from the ECMWF Interim Re-Analysis (ERA-Interim) dataset during the events show irreversible filamentation of the PV fields in the inner side of the polar-night jet, which is a signature of planetary (Rossby) wave breaking. Balloon motions during the events are fairly consistent with the PV filaments. Events of both large (~15° of arc length) and small (~5° of arc length) balloon displacements from the vortex edge are associated, respectively, with deep and shallow penetration into the core of th...

Sensitivity of Sudden Stratospheric Warmings to Previous Stratospheric Conditions

AbstractThe Whole Atmosphere Community Climate Model, version 4 (WACCM4), is used to investigate the influence of stratospheric conditions on the development of sudden stratospheric warmings (SSWs). To this end, targeted experiments are performed on selected modeled SSW events. Specifically, the model is reinitialized three weeks before a given SSW, relaxing the surface fluxes, winds, and temperature below 10 km to the corresponding fields from the free-running simulation. Hence, the tropospheric wave evolution is unaltered across the targeted experiments, but the stratosphere itself can evolve freely. The stratospheric zonal-mean state is then altered 21 days prior to the selected SSWs and rerun with an ensemble of different initial conditions. It is found that a given tropospheric evolution concomitant with the development of an SSW does not uniquely determine the occurrence of an event and that the stratospheric conditions are relevant to the subsequent evolution of the stratospheric flow toward an SSW...

On the Gravity Wave Forcing during the Southern Stratospheric Final Warming in LMDZ

AbstractThe austral stratospheric final warming date is often predicted with substantial delay in several climate models. This systematic error is generally attributed to insufficient parameterized gravity wave (GW) drag in the stratosphere around 60°S. A simulation with a general circulation model [Laboratoire de Meteorologie Dynamique zoom model (LMDZ)] with a much less pronounced bias is used to analyze the contribution of the different types of waves to the dynamics of the final warming. For this purpose, the resolved and unresolved wave forcing of the middle atmosphere during the austral spring are examined in LMDZ and reanalysis data, and a good agreement is found between the two datasets. The role of parameterized orographic and nonorographic GWs in LMDZ is further examined, and it is found that orographic and nonorographic GWs contribute evenly to the GW forcing in the stratosphere, unlike in other climate models, where orographic GWs are the main contributor. This result is shown to be in good ag...

On the Relation between Gravity Waves and Wind Speed in the Lower Stratosphere over the Southern Ocean

AbstractThe relationship between gravity wave momentum fluxes and local wind speed is investigated for oceanic regions at high southern latitudes during austral spring. The motivation is to better describe the gravity wave field by identifying a simple relationship between gravity waves and the large-scale flow. The tools used to describe the gravity waves are probability density functions of the gravity wave momentum fluxes. Three independent datasets covering high latitudes in the Southern Hemisphere springtime are analyzed: simulations with a mesoscale model, analyses from the European Centre for Medium-Range Weather Forecasts, and observations from superpressure balloons of the Concordiasi campaign in 2010. A remarkably robust relation is found, with stronger momentum fluxes much more likely in regions of strong winds. The tails of the probability density functions are well described as lognormal. The median momentum flux increases linearly with background wind speed: for winds stronger than 50 m s−1,...

Non-orographic Gravity Waves: Representation in Climate Models and Effects on Infrasound

Long-range infrasound propagation is controlled by atmospheric waveguides that extend up to the mesosphere and lower thermosphere and whose efficiency is affected by gravity waves (GWs). These GWs are not explicitly represented in the global models often used to calculate infrasound propagation because their spatial scales are well below the models’ resolution. These unresolved GWs also transport momentum and control in good part the large-scale circulation in the middle atmosphere. These two issues make that the GWs need to be parameterized to improve the datasets used to calculate infrasound propagation as well as in the atmospheric general circulation model (AGCMs) that are used to make weather forecasts and climate predictions. These two issues gain in being treated in conjunction. From this, improved infrasound calculations could be made by using a realistic amount of GWs. In return, using infrasound records could help specifying important characteristics of the GWs that are parameterized in the climate models. The paper presents a research framework developed to address these issues. It first presents a non-orographic GWs parameterization used and tested in a well-established AGCM, emphasizing the most recent developments, like the introduction of stochastic techniques and a better specification of the GWs sources. The significance of GWs on the global climate is then illustrated by making sensitivity tests where the frontal and convective GWs parameters are moderately changed. These changes impact the structure of the jets in the midlatitude stratosphere and the intensity of the sudden stratospheric warmings. The paper also presents a method to calculate long-range infrasound propagation, and to incorporate the contribution of the GWs that are parameterized in the AGCM. We then show that the changes in GW parameters tested in the model also impact infrasound propagation. This makes infrasound detection a potential tool to tune GWs parameterization in large-scale models.