Jadwiga H. Richter

The Impact of Convection on ENSO: From a Delayed Oscillator to a Series of Events

Abstract The NCAR Community Climate System Model, version 3 (CCSM3) exhibits persistent errors in its simulation of the El Nino–Southern Oscillation (ENSO) mode of coupled variability. The amplitude of the oscillation is too strong, the dominant 2-yr period too regular, and the width of the sea surface temperature response in the Pacific too narrow, with positive anomalies extending too far into the western Pacific. Two changes in the parameterization of deep convection result in a significant improvement to many aspects of the ENSO simulation. The inclusion of convective momentum transport (CMT) and a dilution approximation for the calculation of convective available potential energy (CAPE) are used in development integrations, and a striking improvement in ENSO characteristics is seen. An increase in the periodicity of ENSO is achieved by a reduction in the strength of the existing “short-circuited” delayed-oscillator mode. The off-equatorial response is weaker and less tropically confined, largely as a...

The mean climate of the Community Atmosphere Model (CAM4) in forced SST and fully coupled experiments

AbstractThe Community Atmosphere Model, version 4 (CAM4), was released as part of the Community Climate System Model, version 4 (CCSM4). The finite volume (FV) dynamical core is now the default because of its superior transport and conservation properties. Deep convection parameterization changes include a dilute plume calculation of convective available potential energy (CAPE) and the introduction of convective momentum transport (CMT). An additional cloud fraction calculation is now performed following macrophysical state updates to provide improved thermodynamic consistency. A freeze-drying modification is further made to the cloud fraction calculation in very dry environments (e.g., the Arctic), where cloud fraction and cloud water values were often inconsistent in CAM3. In CAM4 the FV dynamical core further degrades the excessive trade-wind simulation, but reduces zonal stress errors at higher latitudes. Plume dilution alleviates much of the midtropospheric tropical dry biases and reduces the persist...

Toward a Physically Based Gravity Wave Source Parameterization in a General Circulation Model

Abstract Middle atmospheric general circulation models (GCMs) must employ a parameterization for small-scale gravity waves (GWs). Such parameterizations typically make very simple assumptions about gravity wave sources, such as uniform distribution in space and time or an arbitrarily specified GW source function. The authors present a configuration of the Whole Atmosphere Community Climate Model (WACCM) that replaces the arbitrarily specified GW source spectrum with GW source parameterizations. For the nonorographic wave sources, a frontal system and convective GW source parameterization are used. These parameterizations link GW generation to tropospheric quantities calculated by the GCM and provide a model-consistent GW representation. With the new GW source parameterization, a reasonable middle atmospheric circulation can be obtained and the middle atmospheric circulation is better in several respects than that generated by a typical GW source specification. In particular, the interannual NH stratospher...

Effects of Convective Momentum Transport on the Atmospheric Circulation in the Community Atmosphere Model, Version 3

Abstract Transport of momentum by convection is an important process affecting global circulation. Owing to the lack of global observations, the quantification of the impact of this process on the tropospheric climate is difficult. Here an implementation of two convective momentum transport parameterizations, presented by Schneider and Lindzen and Gregory et al., in the Community Atmosphere Model, version 3 (CAM3) is presented, and their effect on global climate is examined in detail. An analysis of the tropospheric zonal momentum budget reveals that convective momentum transport affects tropospheric climate mainly through changes to the Coriolis torque. These changes result in improvement of the representation of the Hadley circulation: in December–February, the upward branch of the circulation is weakened in the Northern Hemisphere and strengthened in the Southern Hemisphere, and the lower northerly branch is weakened. In June–August, similar improvements are noted. The inclusion of convective momentum ...

Dynamics of the middle atmosphere as simulated by the Whole Atmosphere Community Climate Model, version 3 (WACCM3)

The Whole Atmosphere Community Climate Model, version 3 (WACCM3) is a state-of-the-art climate model extending from the Earths surface to the lower thermosphere. In this paper we present a detailed climatology of the dynamics of the middle atmosphere as represented by WACCM3 at various horizontal resolutions and compare them to observations. In addition to the mean climatological fields, we examine in detail the middle atmospheric momentum budget as well as several lower and upper atmosphere coupling phenomena including stratospheric sudden warmings, the 2-day wave, and the migrating diurnal tide. We find that in large part, differences between WACCM3 and observations and the mean state of the model at various horizontal resolutions are related to gravity wave drag, which is parameterized in WACCM3 (and similar models). All three lower and upper atmosphere coupling processes examined show high sensitivity to the models resolution.

On the simulation of the quasi‐biennial oscillation in the Community Atmosphere Model, version 5

The quasi-biennial oscillation (QBO) of the tropical zonal mean wind is a prominent feature of the tropical stratosphere. The easterly and westerly wind regimes alternate with a period of about 28 months. The QBO is believed to be forced by a combination of equatorial waves, in particular, Kelvin and mixed Rossby-gravity waves, as well as smaller-scale gravity waves. Although the QBO is well observed and basic forcing mechanism well understood, it has been a challenge to simulate in General Circulation Models (GCMs). In this paper we examine the role of vertical resolution and gravity wave parameterization on the simulation of the QBO in the Community Atmosphere Model, version 5. We show that in this model vertical resolution of 500 m and adequate gravity wave drag are needed to obtain a realistic QBO. At 500 m vertical resolution, CAM5 generates significantly more mixed Rossby-gravity and Kelvin waves as compared to CAM5 with 700 m or 1200 m vertical resolution. These waves then contribute to the forcing of the easterly and westerly phases of the QBO, respectively. In this work, we also briefly explore the effects of horizontal resolution on the QBO and conclude that the QBO can be adequately represented with horizontal resolution of ∼200 km as long as vertical resolution of the model is fine enough.

Generation and Trapping of Gravity Waves from Convection with Comparison to Parameterization

Some parameterizations of gravity wave mean flow forcing in global circulation models (GCMs) add realism by describing wave generation by tropospheric convection. Because the convection in GCMs is itself a parameterized process, these convectively generated wave parameterizations necessarily use many simplifying assumptions. In this work, the authors use a realistic simulation of wave generation by convection described in previous work, which was validated by observations from the Darwin Area Wave Experiment (DAWEX), to test these assumptions and to suggest some possible improvements to the parameterizations. In particular, the authors find that wave trapping in the troposphere significantly modifies the spectrum of vertically propagating waves entering the stratosphere above convective wave sources, and offer a linear method for computing wave transmission and reflection effects on the spectrum suitable for inclusion in the parameterizations. The wave fluxes originate from both a time-varying heating mechanism and an obstacle effect mechanism acting in the simulation. Methods for including both mechanisms in the parameterizations are described. Waves emanating from the obstacle effect remain very sensitive to the depth of penetration of latent heating cells into an overlying shear zone, which will continue to make it difficult to accurately parameterize in a GCM where the convective cells are not resolved.

First Simulations of Designing Stratospheric Sulfate Aerosol Geoengineering to Meet Multiple Simultaneous Climate Objectives

We describe the first simulations of stratospheric sulfate aerosol geoengineering using multiple injection locations to meet multiple simultaneous surface temperature objectives. Simulations were performed using CESM1(WACCM), a coupled atmosphere-ocean general circulation model with fully interactive stratospheric chemistry, dynamics (including an internally generated quasi-biennial oscillation), and a sophisticated treatment of sulfate aerosol formation, microphysical growth, and deposition. The objectives are defined as maintaining three temperature features at their 2020 levels against a background of the RCP8.5 scenario over the period 2020–2099. These objectives are met using a feedback mechanism in which the rate of sulfur dioxide injection at each of the four locations is adjusted independently every year of simulation. Even in the presence of uncertainties, nonlinearities, and variability, the objectives are met, predominantly by SO_2 injection at 30°N and 30°S. By the last year of simulation, the feedback algorithm calls for a total injection rate of 51 Tg SO_2 per year. The injections are not in the tropics, which results in a greater degree of linearity of the surface climate response with injection amount than has been found in many previous studies using injection at the equator. Because the objectives are defined in terms of annual mean temperature, the required geongineering results in “overcooling” during summer and “undercooling” during winter. The hydrological cycle is also suppressed as compared to the reference values corresponding to the year 2020. The demonstration we describe in this study is an important step toward understanding what geoengineering can do and what it cannot do.

Effects of vertical resolution and nonorographic gravity wave drag on the simulated climate in the Community Atmosphere Model, version 5

Horizontal resolution of general circulation models (GCMs) has significantly increased during the last decade, however these changes were not accompanied by similar changes in vertical resolution. In our study, the Community Atmosphere Model, version 5 (CAM5) is used to study the sensitivity of climate to vertical resolution and nonorographic gravity wave drag. Nonorographic gravity wave drag is typically omitted from low-top GCMs, however as we show, its influence on climate can be seen all the way to the surface. We show that an increase in vertical resolution from 1200 to 500 m in the free troposphere and lower stratosphere in CAM5 improves the representation of near-tropopause temperatures, lower stratospheric temperatures, and surface wind stresses. In combination with nonorographic gravity waves, CAM5 with increased vertical resolution produces a realistic Quasi-Biennial Oscillation (QBO), has an improved seasonal cycle of temperature in the extratropics, and represents better the coupling between the stratosphere and the troposphere.

Effects of stratospheric variability on El Nino teleconnections

The effects of the tropical Pacific El Nino Southern Oscillation (ENSO) phenomenon are communicated to the rest of the globe via atmospheric teleconnections. Traditionally, ENSO teleconnections have been viewed as tropospheric phenomena, propagating to higher latitudes as Rossby waves. Recent studies, however, suggest an influence of the stratosphere on extra-tropical ENSO teleconnections. The stratosphere is highly variable: in the tropics, the primary mode of variability is the quasi-biennial oscillation (QBO), and in the extra-tropics sudden stratospheric warmings (SSWs) regularly perturb the mean state. Here, we conduct a 10-member ensemble of simulations with a stratosphere-resolving atmospheric general circulation model forced with the observed evolution of sea surface temperatures during 1952–2001 to examine the effects of the QBO and SSWs on the zonal-mean circulation and temperature response to El Nino, with a focus on the northern extra-tropics during winter. We find that SSWs have a larger impact than the QBO on the composite El Nino responses. During El Nino winters with SSWs, the polar stratosphere shows positive temperature anomalies that propagate downward to the surface where they are associated with increased sea-level pressure over the Arctic. During El Nino winters without SSWs, the stratosphere and upper troposphere show negative temperature anomalies but these do not reach the surface. The QBO modulates the El Nino teleconnection primarily in winters without SSWs: the negative temperature anomalies in the polar stratosphere and upper troposphere are twice as large during QBO West compared to QBO East years. In addition, El Nino winters that coincide with the QBO West phase show stronger positive sea-level pressure anomalies over the eastern Atlantic and Northern Europe than those in the QBO East phase. The results imply that the stratosphere imparts considerable variability to ENSO teleconnections.