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Bjørn Røsting
and
Jón Egill Kristjánsson

Abstract

It is today widely accepted that potential vorticity (PV) thinking is a highly useful approach for understanding important aspects of dynamic meteorology and for validation of output from state-of-the-art numerical weather prediction (NWP) models. Egger recently presented a critical view on piecewise potential vorticity inversion (PPVI). This was done by defining a PV anomaly by retaining the observed PV field in a specific region, while changing the observed PV fields to zero elsewhere. Inversion of such a modified PV field yields a flow vastly different from the observed. On the basis of this result it was argued that PPVI is useless for understanding the dynamics of the flow.

The present paper argues that the results presented by Egger are incomplete in the context of PPVI, since the complementary cases were not considered and that the results also depend on the idealized model formulations. The complementary case is defined by changing the observed PV to zero in the specific region, while retaining the observed PV field elsewhere.

By including the complementary cases, it can be demonstrated that the streamfunction fields associated with the PV and boundary temperature anomalies presented by Egger add up to yield the observed streamfunction field, as expected if PPVI is to be valid. It follows that PPVI is indeed valid and useful in these cases.

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Bjørn Røsting
and
Jón Egill Kristjánsson
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Camilla W. Stjern
and
Jón Egill Kristjánsson

Abstract

Over the last few decades, aerosol loadings have increased greatly over Southeast Asia, while Europe and North America have experienced huge reductions. Previous studies have suggested that these changes may have influenced the temperature trends as well as precipitation patterns due to the direct and semidirect aerosol effects. Here, an Earth system model with parameterized aerosol–radiation and aerosol–cloud interactions is used to investigate changes in cloud properties and precipitation between 1975 and 2005. This is done globally as well as for the two focus areas Europe and East Asia. Despite systematic changes in cloud droplet number concentration and cloud droplet size, changes in stratiform precipitation are less clear. In both regions there is a dominance of autoconversion over liquid water accretion as the primary precipitation release mechanism, which alone should imply a strong sensitivity to changes in cloud droplet size. However, in these areas liquid water paths are relatively low and background concentrations are high, which produce low simulated precipitation susceptibilities. High susceptibilities are instead found over remote ocean regions, in agreement with expectations. For convective precipitation, both regions show statistically significant changes that are consistent with oppositely signed changes in direct aerosol forcing over Europe and East Asia, respectively.

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Erik Berge
and
Jón Egill Kristjánsson

Abstract

This study investigates the effect on short-range weather prediction of using different numerical advection schemes for humidity and cloud water. Comparisons are made between predictions using the basic centered and upstream schemes and the more sophisticated Smolarkiewicz and Bott schemes. The main purpose of these alternative schemes is to reduce numerical diffusion and dispersion errors encountered in the basic schemes and thereby preserve the shape of features such as fronts. At the same time those schemes are efficient enough to be a realistic choice for operational models.

The simulations are made with a numerical weather prediction model with 150-km horizontal resolution and ten levels. Frontal movements over large distances have been investigated in a domain that extends from eastern North America through Europe. The simulations yield little sensitivity to the choice of advection scheme for cloud water. On the other hand, a large sensitivity to the treatment of the humidity advection is found. This is connected to a substantial feedback with the model dynamics through release of latent heat. Larger numerical errors are identified with the basic schemes than with the alternative schemes, which yield a better shape preservation of the frontal zones. The centered scheme tends to give the heaviest precipitation and the deepest cyclones, while the upstream scheme gives least precipitation. Exaggerated low-level cloudiness is found with the Bott scheme and, to some extent, the Smolarkiewicz scheme. This appears to be caused by insufficient adjustment of other parts of the model to the new, more accurate, transport formulation. The Bott scheme has less numerical diffusion than the Smolarkiewicz scheme, but is somewhat more expensive computationally.

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Trude Storelvmo
,
Jón Egill Kristjánsson
, and
Ulrike Lohmann

Abstract

A new treatment of mixed-phase cloud microphysics has been implemented in the general circulation model, Community Atmosphere Model (CAM)-Oslo, which combines the NCAR CAM2.0.1 and a detailed aerosol module. The new treatment takes into account the aerosol influence on ice phase initiation in stratiform clouds with temperatures between 0° and −40°C. Both supersaturation and cloud ice fraction, that is, the fraction of cloud ice compared to the total cloud water in a given grid box, are now determined based on a physical reasoning in which not only temperature but also the ambient aerosol concentration play a role. Included in the improved microphysics treatment is also a continuity equation for ice crystal number concentration. Ice crystal sources are heterogeneous and homogeneous freezing processes and ice multiplication. Sink terms are collection processes and precipitation formation, that is, melting and sublimation. Instead of using an idealized ice nuclei concentration for the heterogeneous freezing processes, a common approach in global models, the freezing processes are here dependent on the ability of the ambient aerosols to act as ice nuclei. Additionally, the processes are dependent on the cloud droplet number concentration and hence the aerosols’ ability to act as cloud condensation nuclei. Sensitivity simulations based on the new microphysical treatment of mixed-phase clouds are presented for both preindustrial and present-day aerosol emissions. Freezing efficiency is found to be highly sensitive to the amount of sulphuric acid available for ice nuclei coating. In the simulations, the interaction of anthropogenic aerosols and freezing mechanisms causes a warming of the earth–atmosphere system, counteracting the cooling effect of aerosols influencing warm clouds. The authors find that this reduction of the total aerosol indirect effect amounts to 50%–90% for the specific assumptions on aerosol properties used in this study. However, many microphysical processes in mixed-phase clouds are still poorly understood and the results must be interpreted with that in mind.

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Hilding Sundqvist
,
Erik Berge
, and
Jón Egill Kristjánsson

Abstract

This paper presents the implementation of a parameterization scheme for convective and stratiform condensation (with cloud water as a prognostic variable) into a fine mesh numerical weather prediction model.

The results from a 36 h integration of the model, with grid distance 50 km, indicate that the new condensation scheme contributes to an improved forecast compared to that obtained by the original model. Furthermore, from a qualitative comparison with satellite pictures, it is found that the prediction of condensation-cloud parameters is quite realistic.

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Bjørn Egil Kringlebotn Nygaard
,
Jón Egill Kristjánsson
, and
Lasse Makkonen

Abstract

In-cloud icing on aircraft and ground structures can be observed every winter in many countries. In extreme cases ice can cause accidents and damage to infrastructure such as power transmission lines, telecommunication towers, wind turbines, ski lifts, and so on. This study investigates the potential for predicting episodes of in-cloud icing at ground level using a state-of-the-art numerical weather prediction model. The Weather Research and Forecasting (WRF) model is applied, with attention paid to the model’s skill to explicitly predict the amount of supercooled cloud liquid water content (SLWC) at the ground level at different horizontal resolutions and with different cloud microphysics schemes. The paper also discusses how well the median volume droplet diameter (MVD) can be diagnosed from the model output. A unique dataset of direct measurements of SLWC and MVD at ground level on a hilltop in northern Finland is used for validation. A mean absolute error of predicted SLWC as low as 0.08 g m−3 is obtained when the highest model resolution is applied (grid spacing equal to 0.333 km), together with the Thompson microphysics scheme. The quality of the SLWC predictions decreases dramatically with decreasing model resolution, and a systematic difference in predictive skill is found between the cloud microphysics schemes applied. A comparison between measured and predicted MVD shows that when prescribing the droplet concentration equal to 250 cm−3 the model predicts MVDs ranging from 12 to 20 μm, which corresponds well to the measured range. However, the variation from case to case is not captured by the current cloud microphysics schemes.

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GuĐún Nína Petersen
,
Haraldur Ólafsson
, and
Jón Egill Kristjánsson

Abstract

A series of idealized simulations of flow impinging on large mountains is conducted to investigate the impact of the mountain on the flow far downstream and to shed some light on the effects that Greenland may have on airflow over the North Atlantic. The upstream profiles of wind and stability are kept constant, there is no surface friction, the Rossby number is 0.4, and the nondimensional mountain height (ĥ = Nh/U) is varied from 1 to 6. The maximum sea level pressure deficit, the maximum geopotential height deficit, and the orographically generated potential vorticity all increase with increased ĥ, showing no signs of abrupt change as the flow enters the regime of upstream blocking. The potential vorticity produced at the mountain is accumulated in vortices that are advected downstream. The vortices are associated with a larger pressure gradient to the south of the wake, giving rise to stronger westerlies at the surface as well as at upper levels.

This process can explain how Greenland may affect cyclones moving far outside the mountain wake. An example from Fronts and Atlantic Storm Track Experiment (FASTEX) shows that a cyclone moving from the southwest toward Scotland becomes shallower and slower if the Greenland topography is removed.

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Corinna Hoose
,
Jón Egill Kristjánsson
,
Jen-Ping Chen
, and
Anupam Hazra

Abstract

An ice nucleation parameterization based on classical nucleation theory, with aerosol-specific parameters derived from experiments, has been implemented into a global climate model—the Community Atmosphere Model (CAM)-Oslo. The parameterization treats immersion, contact, and deposition nucleation by mineral dust, soot, bacteria, fungal spores, and pollen in mixed-phase clouds at temperatures between 0° and −38°C. Immersion freezing is considered for insoluble particles that are activated to cloud droplets, and deposition and contact nucleation are only allowed for uncoated, unactivated aerosols. Immersion freezing by mineral dust is found to be the dominant ice formation process, followed by immersion and contact freezing by soot. The simulated biological aerosol contribution to global atmospheric ice formation is marginal, even with high estimates of their ice nucleation activity, because the number concentration of ice nucleation active biological particles in the atmosphere is low compared to other ice nucleating aerosols. Because of the dominance of mineral dust, the simulated ice nuclei concentrations at temperatures below −20°C are found to correlate with coarse-mode aerosol particle concentrations. The ice nuclei (IN) concentrations in the model agree well overall with in situ continuous flow diffusion chamber measurements. At individual locations, the model exhibits a stronger temperature dependence on IN concentrations than what is observed. The simulated IN composition (77% mineral dust, 23% soot, and 10−5% biological particles) lies in the range of observed ice nuclei and ice crystal residue compositions.

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Sigurdur Thorsteinsson
,
Vidar Erlingsson
,
Jón Egill Kristjánsson
,
Bjørn Røsting
, and
Gudmundur Freyr Ulfarsson

Abstract

The evolution of a deep North Atlantic cyclone, which caused devastating avalanches in northwest (NW) Iceland in October 1995, was investigated. As the main tool for this investigation, potential vorticity analysis was used. This allows the quantification and comparison of the roles of different processes that contribute to the cyclone deepening at different stages.

Interpretation of potential vorticity inversions and isentropic air trajectories yields the following picture of the cyclone development. The thermal field over the North Atlantic had acquired strong west–east gradients due to a combination of advection of cold air southeastward from a cold cyclonic gyre south of Iceland and advection of warm air northward on the westward flank of a warm anticyclonic ridge over central Europe. A low-level baroclinic wave forming just south of Ireland was rapidly reinforced due to interaction with a descending, high-value, upper-level potential vorticity anomaly and was isentropically advected from the low south of Iceland. As the wave deepened, diabatic heating in association with the frontal systems became a major source of cyclonic vorticity. Cross sections of the height fields associated with potential vorticity anomalies reveal the baroclinic nature of some of the anomalies.

The isentropic trajectory analysis shows strong ascent of warm air taking place over Iceland and thereby explaining the heavy precipitation in NW Iceland. The advection of rather warm, humid air overlying very cold air from a persistent high over Greenland, together with orographic lifting, seems to be responsible for the snowfall that together with heavy winds produced the unusual avalanches in Iceland.

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