68 research outputs found
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The sensitivity of convective aggregation to diabatic processes in idealized radiative-convective equilibrium simulations
Idealized explicit convection simulations of the Met Office Unified Model exhibit spontaneous self-aggregation in radiative-convective equilibrium, as seen in other models in previous studies. This self-aggregation is linked to feedbacks between radiation, surface fluxes, and convection, and the organization is intimately related to the evolution of the column water vapor field. Analysis of the budget of the spatial variance of column-integrated frozen moist static energy (MSE), following Wing and Emanuel [2014], reveals that the direct radiative feedback (including significant cloud longwave effects) is dominant in both the initial development of self-aggregation and the maintenance of an aggregated state. A low-level circulation at intermediate stages of aggregation does appear to transport MSE from drier to moister regions, but this circulation is mostly balanced by other advective effects of opposite sign and is forced by horizontal anomalies of convective heating (not radiation). Sensitivity studies with either fixed prescribed radiative cooling, fixed prescribed surface fluxes, or both do not show full self-aggregation from homogeneous initial conditions, though fixed surface fluxes do not disaggregate an initialized aggregated state. A sensitivity study in which rain evaporation is turned off shows more rapid self-aggregation, while a run with this change plus fixed radiative cooling still shows strong self-aggregation, supporting a “moisture memory” effect found in Muller and Bony [2015]. Interestingly, self-aggregation occurs even in simulations with sea surface temperatures (SSTs) of 295 K and 290 K, with direct radiative feedbacks dominating the budget of MSE variance, in contrast to results in some previous studies
Climate Modelling and Sub-seasonal to Seasonal Prediction: Opportunities and Challenges
Dr Steve Woolnough is a Principal Research Fellow in the Climate directorate of the National Centre for Atmospheric Science, and leads their Tropical Group. His interests are in the variability of the Tropical Climate System on intraseasonal to seasonal timescales, and the representation of the tropical climate system in weather and climate prediction models. He is a member of three international panels of the WMO including the Steering Group of their sub-seasonal to seasonal prediction project. Dr Woolnough will discuss the current state of climate modelling and introduce some of the uncertainties in prediction of regional climate change, and the opportunities to narrow these uncertainties. He will also discuss the current state of sub-seasonal to seasonal prediction and introduce the WCRP/WWRP Sub-seasonal Prediction Project, a new WMO project to promote research into and application of operational prediction systems
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Isolating the effects of moisture entrainment on convectively coupled equatorial waves in an aquaplanet GCM
The rate of humidity entrainment in the convective parametrization scheme in a general circulation model affects the simulation of convectively-coupled waves. However, it is unclear whether this is caused directly by the effects of entrainment on waves or indirectly through associated impacts such as on the basic state. Therefore, using an aquaplanet model, we employ a novel framework in which we entrain a weighted average of the resolved humidity field and a prescribed zonally symmetric field, with the weighting controlled by a decoupling parameter. Hence, we can vary the entrainment rate of basic state humidity independently of the entrainment of humidity perturbations, simultaneously minimizing changes in basic state. Thus we isolate the effect of moisture entrainment on the waves. Enhancing entrainment rate increases spectral power over all zonal wavenumbers and frequencies, with an increase in the ratio of eastward-to-westward power. The Kelvin wave speed decreases as entrainment increases, which can be partially accounted for by an associated change in basic state humidity. Increasing the decoupling parameter reduces spectral power in Kelvin waves relative to the background, with only long waves still prominent when entrainment is almost fully decoupled from the resolved moisture field, suggesting the wave structure in humidity is required for convection to organize into short wave structures. For long waves the increase in the ratio of eastward-to-westward power as entrainment rate increases cannot be explained by the changes in the coupling with the wave structure in humidity, but is consistent with the changes in the basic state
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Cloud-resolving model simulations with one- and two-way couplings via the weak temperature gradient approximation
A cloud-resolving model is modified to implement the weak temperature gradient approximation in order to simulate the interactions between tropical convection and the large-scale tropical circulation. The instantaneous domain-mean potential temperature is relaxed toward a reference profile obtained from a radiative–convective equilibrium simulation of the cloud-resolving model. For homogeneous surface conditions, the model state at equilibrium is a large-scale circulation with its descending branch in the simulated column. This is similar to the equilibrium state found in some other studies, but not all. For this model, the development of such a circulation is insensitive to the relaxation profile and the initial conditions.
Two columns of the cloud-resolving model are fully coupled by relaxing the instantaneous domain-mean potential temperature in both columns toward each other. This configuration is energetically closed in contrast to the reference-column configuration. No mean large-scale circulation develops over homogeneous surface conditions, regardless of the relative area of the two columns. The sensitivity to nonuniform surface conditions is similar to that obtained in the reference-column configuration if the two simulated columns have very different areas, but it is markedly weaker for columns of comparable area. The weaker sensitivity can be understood as being a consequence of a formulation for which the energy budget is closed. The reference-column configuration has been used to study the convection in a local region under the influence of a large-scale circulation. The extension to a two-column configuration is proposed as a methodology for studying the influence on local convection of changes in remote convection
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An assessment of Indian monsoon seasonal forecasts and mechanisms underlying monsoon interannual variability in the Met Office GloSea5-GC2 system
We assess Indian summer monsoon seasonal forecasts in GloSea5-GC2, the Met Office fully coupled subseasonal to seasonal ensemble forecasting system. Using several metrics, GloSea5-GC2 shows similar skill to other state-of-the-art forecast systems. The prediction skill of the large-scale South Asian monsoon circulation is higher than that of Indian monsoon rainfall. Using multiple linear regression analysis we evaluate relationships between Indian monsoon rainfall and five possible drivers of monsoon interannual variability. Over the time period studied (1992-2011), the El Nino-Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) are the most important of these drivers in both observations and GloSea5-GC2. Our analysis indicates that ENSO and its teleconnection with the Indian rainfall are well represented in GloSea5-GC2. However, the relationship between the IOD and Indian rainfall anomalies is too weak in GloSea5-GC2, which may be limiting the prediction skill of the local monsoon circulation and Indian rainfall. We show that this weak relationship likely results from a coupled mean state bias that limits the impact of anomalous wind forcing on SST variability, resulting in erroneous IOD SST anomalies. Known difficulties in representing convective precipitation over India may also play a role. Since Indian rainfall responds weakly to the IOD, it responds more consistently to ENSO than in observations. Our assessment identifies specific coupled biases that are likely limiting GloSea5-GC2 prediction skill, providing targets for model improvement
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Linking African easterly wave activity with equatorial waves and the influence of Rossby waves from the Southern Hemisphere
A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Rossby-gravity (WMRG) waves and equivalent barotropic Rossby waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of equatorial waves is calculated by projection of broad-band filtered ERA-Interim data onto a horizontal structure basis obtained from equatorial wave theory. Mechanisms enabling interaction between the wave types are identified. AEWs are dominated by a vorticity wave which tilts eastwards below the African Easterly Jet and westwards above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centres within AEWs and equatorial WMRG waves. Although the waves do not phase-lock, positive vorticity centres amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southwards flow) and east of this in the lower troposphere (northwards flow). Two mechanisms could explain the vorticity amplification: vortex stretching below the upper-tropospheric divergence and ascent associated with latent heating in convection in the lower-tropospheric moist northwards flow.
In years of strong AEW activity, SH and equatorial upper-tropospheric zonal winds are more easterly. Stronger easterlies have two effects: i) they Doppler shift WMRG waves so that their period varies little with wavenumber (3-4 days) and ii) they enable westward-moving RWs to propagate into the tropical wave guide from the SH. RW phase speeds can match those of WMRG waves, enabling sustained excitation of WMRG. The WMRG waves have an eastwards group velocity with wave activity accumulating over Africa and invigorating AEWs at similar frequencies through the vorticity amplification mechanism
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Abrupt stratospheric vortex weakening associated with North Atlantic anticyclonic wave breaking
The sudden stratospheric warming (SSW) of 12 February 2018 was not forecast by any extended-range model beyond 12 days. From early February, all forecast models that com prise the subseasonal-to-seasonal (S2S) database abruptly transitioned from indicating a strong stratospheric polar vortex (SPV) to a high likelihood of a major SSW. We demonstrate that this forecast evolution was associated with the track and intensity of a cyclone in the north-east Atlantic, with an associated anticyclonic Rossby wave break, which was not well-forecast. The wave break played a pivotal role in building the Ural high, which existing literature has shown was a precursor of the 2018 SSW. The track of the cyclone built an anomalously strong sea-level pressure dipole between Scandinavia and Greenland (termed the S-G dipole) which we use as a diagnostic of the wave break. Forecasts which did not capture the magnitude of this event had the largest errors in the SPV strength and did not show enhanced vertical wave activity. A composite of similarly strong wintertime (November{March) S-G dipoles in reanalysis shows associated anticyclonic wave breaking leading to significantly enhanced vertical wave activity and a weakened SPV in the following days, which occured in 35% of the 15-day periods preceding observed major SSWs. Our results indicate a particular transient trigger for weakening the SPV, complementing existing results on the importance of tropospheric blocking for disruptions to the Northern Hemisphere extratropical stratospheric circulation
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The effects of explicit versus parameterized convection on the MJO in a large-domain high-resolution tropical case study. Part II: Processes leading to differences in MJO development
High-resolution simulations over a large tropical domain (∼20◦S–20◦N and 42◦E–180◦E) using both explicit and parameterized convection are analyzed and compared during a 10-day case study of an active Madden-Julian Oscillation (MJO) event. In Part II, the moisture budgets and moist entropy budgets are analyzed. Vertical subgrid diabatic heating profiles and vertical velocity profiles are also compared; these are related to the horizontal and vertical advective components of the moist entropy budget which contribute to gross moist stability, GMS, and normalized GMS (NGMS). The 4-km model with explicit convection and good MJO performance has a vertical heating structure that increases with height in the lower troposphere in regions of strong convection (like observations), whereas the 12-km model with parameterized convection and a poor MJO does not show this relationship. The 4-km explicit convection model also has a more top-heavy heating profile for the troposphere as a whole near and to the west of the active MJO-related convection, unlike the 12-km parameterized convection model. The dependence of entropy advection components on moisture convergence is fairly weak in all models, and differences between models are not always related to MJO performance, making comparisons to previous work somewhat inconclusive. However, models with relatively good MJO strength and propagation have a slightly larger increase of the vertical advective component with increasing moisture convergence, and their NGMS vertical terms have more variability in time and longitude, with total NGMS that is comparatively larger to the west and smaller to the east
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The implications of an idealised large-scale circulation for mechanical work done by tropical convection
A thermodynamic analysis is presented of an overturning circulation simulated by two cloud resolving models, coupled by a weak temperature gradient
parametrisation. Taken together, they represent two separated regions over
different sea surface temperatures, and the coupling represents an idealised
large-scale circulation such as the Walker circulation. It is demonstrated that a
thermodynamic budget linking net heat input to the generation of mechanical
energy can be partitioned into contributions from the large-scale interaction
between the two regions, as represented by the weak temperature gradient
approximation, and from convective motions in the active warm region and
the suppressed cool region. Model results imply that such thermodynamic
diagnostics for the aggregate system are barely affected by the strength of
the coupling, even its introduction, or by the SST contrast between the regions. This indicates that the weak temperature gradient parametrisation does
not introduce anomalous thermodynamic behaviour. We find that the vertical
kinetic energy associated with the large-scale circulation is more than three
orders of magnitude smaller than the typical vertical kinetic energy in each
region. However, even with very weak coupling circulations, the contrast between the thermodynamic budget terms for the suppressed and active regions
is strong and is relatively insensitive to the degree of the coupling. Additionally, scaling arguments are developed for the relative values of the terms in
the mechanical energy budget
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The role of the cloud radiative effect in the sensitivity of the Intertropical Convergence Zone to convective mixing
Studies have shown that the location and structure of the simulated Intertropical Convergence Zone (ITCZ) is sensitive to the treatment of sub-gridscale convection and cloud-radiation interactions. This sensitivity remains in idealised aquaplanet experiments with fixed surface temperatures. However, studies have not considered the role of cloud-radiative effects (CRE, atmospheric heating due to cloud-radiation interactions) in the sensitivity of the ITCZ to the treatment of convection. We use an atmospheric energy input (AEI) framework to explore how the CRE modulates the sensitivity of the ITCZ to convective mixing in aquaplanet simulations. Simulations show a sensitivity of the ITCZ to convective mixing, with stronger convective mixing favoring a single ITCZ. For simulations with a single ITCZ, the CRE maintains the positive, equatorial AEI. To explore the role of the CRE further, we prescribe the CRE as either zero or a meridionally and diurnally varying climatology. Removing the CRE is associated with a reduced equatorial AEI and an increase in the range of convective mixing rates that produce a double ITCZ. Prescribing the CRE reduces the sensitivity of the ITCZ to convective mixing by 50%. In prescribed CRE simulations, other AEIcomponents, in particular the surface latent heat flux, modulate the sensitivity of the AEI to convective mixing. Analysis of the meridional moist static energy transport shows that a shallower Hadley circulation can produce an equatorward energy transport at low latitudes even with equatorial ascent
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