Abstract
A pragmatic approach for representing partially resolved turbulence in numerical weather prediction models is introduced and tested. The method blends a conventional boundary layer ...parameterization, suitable for large grid lengths, with a subgrid turbulence scheme suitable for large-eddy simulation. The key parameter for blending the schemes is the ratio of grid length to boundary layer depth. The new parameterization is combined with a scale-aware microphysical parameterization and tested on a case study forecast of stratocumulus evolution. Simulations at a range of model grid lengths between 1 km and 100 m are compared to aircraft observations. The improved microphysical representation removes the correlation between precipitation rate and model grid length, while the new turbulence parameterization improves the transition from unresolved to resolved turbulence as grid length is reduced.
Spatial variability of liquid cloud water content and rainwater content is analysed from three different observational platforms: in situ measurements from research aircraft, land‐based remote ...sensing techniques using radar and lidar, and spaceborne remote sensing from CloudSat. The variance is found to increase with spatial scale, but also depends strongly on the cloud or rain fraction regime, with overcast regions containing less variability than broken cloud fields. This variability is shown to lead to large biases, up to a factor of 4, in both the autoconversion and accretion rates estimated at a model grid scale of ≈︁40 km by a typical microphysical parametrization using in‐cloud mean values. A parametrization for the subgrid variability of liquid cloud and rainwater content is developed, based on the observations, which varies with both the grid scale and cloud or rain fraction, and is applicable for all model grid scales. It is then shown that if this parametrization of the variability is analytically incorporated into the autoconversion and accretion rate calculations, the bias is significantly reduced.
ABSTRACT
We present results from 3D radiative-hydrodynamical simulations of HD 209458b with a fully coupled treatment of clouds using the EddySed code, critically, including cloud radiative feedback ...via absorption and scattering. We demonstrate that the thermal and optical structure of the simulated atmosphere is markedly different, for the majority of our simulations, when including cloud radiative effects, suggesting this important mechanism cannot be neglected. Additionally, we further demonstrate that the cloud structure is sensitive to not only the cloud sedimentation efficiency (termed fsed in EddySed), but also the temperature–pressure profile of the deeper atmosphere. We briefly discuss the large difference between the resolved cloud structures of this work, adopting a phase-equilibrium and parametrized cloud model, and our previous work incorporating a cloud microphysical model, although a fairer comparison where, for example, the same list of constituent condensates is included in both treatments is reserved for a future work. Our results underline the importance of further study into the potential condensate size distributions and vertical structures, as both strongly influence the radiative impact of clouds on the atmosphere. Finally, we present synthetic observations from our simulations reporting an improved match, over our previous cloud-free simulations, to the observed transmission, HST WFC3 emission, and 4.5 μm Spitzer phase curve of HD 209458b. Additionally, we find all our cloudy simulations have an apparent albedo consistent with observations.
The London Model: forecasting fog at 333 m resolution Boutle, I. A.; Finnenkoetter, A.; Lock, A. P. ...
Quarterly journal of the Royal Meteorological Society,
January 2016 Part A, Volume:
142, Issue:
694
Journal Article
Peer reviewed
A very high resolution numerical weather prediction model is nested inside the Met Office's main United Kingdom forecast model to investigate whether further enhancements to resolution provide any ...benefit for fog forecasting. The London Model shows similar performance to its lower resolution equivalent at short lead times, but improved performance at longer lead times and an improved frequency bias of forecast fog events. Differences in the model cloud parametrization are the key reason for the differing behaviour, leading to systematically less cloud, colder night time minimum temperatures and therefore more fog in the London Model. Benefit of the enhanced resolution is also found, via an improved representation of how orographic variability enhances turbulence in the stable boundary layer.
Aims. To understand and compare the 3D atmospheric structure of HD 209458 b and HD 189733 b, focusing on the formation and distribution of cloud particles, as well as their feedback on the dynamics ...and thermal profile. Methods. We coupled the 3D Met Office Unified Model (UM), including detailed treatments of atmospheric radiative transfer and dynamics, to a kinetic cloud formation scheme. The resulting model self-consistently solves for the formation of condensation seeds, surface growth and evaporation, gravitational settling and advection, cloud radiative feedback via absorption, and crucially, scattering. We used fluxes directly obtained from the UM to produce synthetic spectral energy distributions and phase curves. Results. Our simulations show extensive cloud formation in both HD 209458 b and HD 189733 b. However, cooler temperatures in the latter result in higher cloud particle number densities. Large particles, reaching 1 μm in diameter, can form due to high particle growth velocities, and sub-μm particles are suspended by vertical flows leading to extensive upper-atmosphere cloud cover. A combination of meridional advection and efficient cloud formation in cooler high latitude regions, results in enhanced cloud coverage for latitudes above 30° and leads to a zonally banded structure for all our simulations. The cloud bands extend around the entire planet, for HD 209458 b and HD 189733 b, as the temperatures, even on the day side, remain below the condensation temperature of silicates and oxides. Therefore, the simulated optical phase curve for HD 209458 b shows no “offset”, in contrast to observations. Efficient scattering of stellar irradiation by cloud particles results in a local maximum cooling of up to 250 K in the upper atmosphere, and an advection-driven fluctuating cloud opacity causes temporal variability in the thermal emission. The inclusion of this fundamental cloud-atmosphere radiative feedback leads to significant differences with approaches neglecting these physical elements, which have been employed to interpret observations and determine thermal profiles for these planets. This suggests that readers should be cautious of interpretations neglecting such cloud feedback and scattering, and that the subject merits further study.
Simulations at a range of resolutions are compared to observations from the South-East Pacific taken during VOCALS-REx. It is found that increased horizontal and vertical resolution make only small ...improvements to the bulk properties of the simulated cloud and drizzle, but the highest resolution simulation is able to realistically represent mesoscale features in the cloud field. We focus on the highest resolution simulation and demonstrate that a poor representation of the cloud microphysics results in excessive drizzle production. This promotes persistent drizzle induced decoupling of the boundary layer, giving a poor representation of the observed diurnal cycle of stratocumulus. Two simple changes to the microphysics scheme are implemented: a modified autoconversion parametrization and a new representation of the rain drop size distribution. This results in a more realistic simulation of boundary-layer diurnal decoupling, and improvements to the cloud liquid water path and surface drizzle rate.
We present significant differences in the simulated atmospheric flow for warm, tidally locked small Neptunes and super Earths (based on a nominal GJ 1214b) when solving the simplified, and commonly ...used, primitive dynamical equations or the full Navier-Stokes equations. The dominant prograde, superrotating zonal jet is markedly different between the simulations, which are performed using practically identical numerical setups, within the same model. The differences arise due to the breakdown of the so-called "shallow-fluid" and traditional approximations, which worsens when rotation rates are slowed, and day-night temperature contrasts are increased. The changes in the zonal advection between simulations solving the full and simplified equations, give rise to significant differences in the atmospheric redistribution of heat, altering the position of the hottest part of the atmosphere and temperature contrast between the daysides and nightsides. The implications for the atmospheric chemistry, and therefore, observations need to be studied with a model including a more detailed treatment of the radiative transfer and chemistry. Small Neptunes and super Earths are extremely abundant and important, potentially bridging the structural properties (mass, radius, and composition) of terrestrial and gas giant planets. Our results indicate care is required when interpreting the output of models solving the primitive equations of motion for such planets.
ABSTRACT
We present results of 3D hydrodynamical simulations of HD209458b including a coupled, radiatively active cloud model (eddysed). We investigate the role of the mixing by replacing the default ...convective treatment used in previous works with a more physically relevant mixing treatment (Kzz) based on global circulation. We find that uncertainty in the efficiency of sedimentation through the sedimentation factor fsed plays a larger role in shaping cloud thickness and its radiative feedback on the local gas temperatures – e.g. hotspot shift and day-to-night side temperature gradient – than the switch in mixing treatment. We demonstrate using our new mixing treatments that simulations with cloud scales that are a fraction of the pressure scale height improve agreement with the observed transmission spectra, the emission spectra, and the Spitzer 4.5 µm phase curve, although our models are still unable to reproduce the optical and ultraviolet transmission spectra. We also find that the inclusion of cloud increases the transit asymmetry in the optical between the east and west limbs, although the difference remains small ($\lesssim 1{{\ \rm per\ cent}}$).
ABSTRACT
We present high-resolution transmission spectra, calculated directly from a 3D radiative hydrodynamic simulation that includes kinetic cloud formation, for HD 209458b. We find that the high ...opacity of our vertically extensive cloud deck, composed of a large number density of sub-$\mu$m particles, flattens the transmission spectrum and obscures spectral features identified in the observed data. We use the pandexo simulator to explore features of our HD 209458b spectrum which may be detectable with the James Webb Space Telescope. We determine that an 8–12$\mu$m absorption feature attributed to the mixed-composition, predominantly silicate cloud particles is a viable marker for the presence of cloud. Further calculations explore, and trends are identified with, variations in cloud opacity, composition heterogeneity, and artificially scaled gravitational settling on the transmission spectrum. Principally, by varying the upper extent of our cloud decks, rainout is identified to be a key process for the dynamical atmospheres of hot Jupiters and shown to dramatically alter the resulting spectrum. Our synthetic transmission spectra, obtained from the most complete, forward atmosphere simulations to date, allow us to explore the model’s ability to conform with observations. Such comparisons can provide insight into the physical processes either missing or requiring improvement.
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