A parameterization of slantwise convection in the WRF model

In this study, we introduce a parameterization scheme for slantwise convection (SC) to be considered for models that are too coarse to resolve slantwise convection explicitly (with a horizontal grid spacing coarser than 15 km or less). This SC scheme operates in a locally defined 2D cross-section perpendicular to the deep-layer-averaged thermal wind. It applies momentum tendency to adjust the environment toward slantwise neutrality with a prescribed adjustment timescale. Condensational heating and the associated moisture loss are also considered. To evaluate the added value of the SC scheme, we implement it in the Weather Research and Forecasting (WRF) model to supplement the existing cumulus parameterization schemes for upright convection and test for two different numerical setups: a 2D idealized, unforced release of conditional symmetric instability (CSI) in an initially conditionally stable environment, and a 3D real-data precipitation event containing both CSI and conditional instability along the cold front of a cyclonic storm near the UK. Both test cases show significant improvements for the coarse-gridded (40-km) simulations when parameterizing slantwise convection. Compared to the 40-km simulations with only the upright convection scheme, the counterparts with the additional SC scheme exhibit a larger extent of CSI neutralization, generate a stronger grid-resolved slantwise circulation, and produce greater amounts of precipitation, all in better agreement with the corresponding fine-gridded reference simulations. Given the importance of slantwise convection in midlatitude weather systems, our results suggest that there exist potential benefits of parameterizing slantwise convection in general circulation models.

The influence of front strength on the development and equilibration of symmetric instability. Part 1. Growth and saturation

Submesoscale fronts with large horizontal buoyancy gradients and $O(1)$ Rossby numbers are common in the upper ocean. These fronts are associated with large vertical transport and are hotspots for biological activity. Submesoscale fronts are susceptible to symmetric instability (SI) – a form of stratified inertial instability which can occur when the potential vorticity is of the opposite sign to the Coriolis parameter. Here, we use a weakly nonlinear stability analysis to study SI in an idealised frontal zone with a uniform horizontal buoyancy gradient in thermal wind balance. We find that the structure and energetics of SI strongly depend on the front strength, defined as the ratio of the horizontal buoyancy gradient to the square of the Coriolis frequency. Vertically bounded non-hydrostatic SI modes can grow by extracting potential or kinetic energy from the balanced front and the relative importance of these energy reservoirs depends on the front strength and vertical stratification. We describe two limiting behaviours as ‘slantwise convection’ and ‘slantwise inertial instability’ where the largest energy source is the buoyancy flux and geostrophic shear production, respectively. The growing linear SI modes eventually break down through a secondary shear instability, and in the process transport considerable geostrophic momentum. The resulting breakdown of thermal wind balance generates vertically sheared inertial oscillations and we estimate the amplitude of these oscillations from the stability analysis. We finally discuss broader implications of these results in the context of current parameterisations of SI.

The influence of front strength on the development and equilibration of symmetric instability. Part 2. Nonlinear evolution

In Part 1 (Wienkers, Thomas & Taylor, J. Fluid Mech., vol. 926, 2021, A6), we described the theory for linear growth and weakly nonlinear saturation of symmetric instability (SI) in the Eady model representing a broad frontal zone. There, we found that both the fraction of the balanced thermal wind mixed down by SI and the primary source of energy are strongly dependent on the front strength, defined as the ratio of the horizontal buoyancy gradient to the square of the Coriolis frequency. Strong fronts with steep isopycnals develop a flavour of SI we call ‘slantwise inertial instability’ by extracting kinetic energy from the background flow and rapidly mixing down the thermal wind profile. In contrast, weak fronts extract more potential energy from the background density profile, which results in ‘slantwise convection.’ Here, we extend the theory from Part 1 using nonlinear numerical simulations to focus on the adjustment of the front following saturation of SI. We find that the details of adjustment and amplitude of the induced inertial oscillations depend on the front strength. While weak fronts develop narrow frontlets and excite small-amplitude vertically sheared inertial oscillations, stronger fronts generate large inertial oscillations and produce bore-like gravity currents that propagate along the top and bottom boundaries. The turbulent dissipation rate in these strong fronts is large, highly intermittent and intensifies during periods of weak stratification. We describe each of these mechanisms and energy pathways as the front evolves towards the final adjusted state, and in particular focus on the effect of varying the dimensionless front strength.

The parameterization of slantwise convection in a numerical model

<p>     Slantwise convection and the associated release of conditional symmetric instability (CSI) have been recognized as important baroclinic processes. Recent climatological studies have highlighted its significant association with midlatitude cyclone activities, raising questions about whether large-scale models can resolve slantwise convection and whether it should be parameterized.</p><p>     To address this issue, the present study simulates isolated free moist slantwise convection in an initially statically stable environment using the 2D idealized, non-hydrostatic Weather Research and Forecasting (WRF) Model. We first examined the sensitivity of the slantwise convection to the cross-band grid spacing (Δy; varied from 40 to 1 km) and found that experiments with ∆y> 5 km fail to capture the band dynamics and larger-scale feedbacks robustly and thus require parameterization. As most of the current convective parameterization schemes target upright convection in a local column, we implemented an additional 2D slantwise convective parameterization scheme and evaluated its impact for coarse-grid runs.</p><p>     The slantwise convective parameterization scheme operates along a sloped trajectory on a horizontally-variant cross section perpendicular to the local thermal wind, adjusting the environment toward a natural state to CSI within a given time scale. With the addition of the slantwise convective parameterization scheme, significant improvements are found in precipitation and the strength of the slantwise updraft, bringing the coarser-grid (∆y=40 km) simulation closer to the finer-grid (converged) results than its counterpart with only the upright convection scheme. After testing the slantwise convective parameterization scheme under idealized frameworks, we will further apply it to regional models to evaluate its benefit to the weather forecasting in real cases.</p>

Damaging Convective and Non-Convective Winds in Southwestern Iberia during Windstorm Xola

On 23/12/2009, windstorm Xola struck mainland Portugal, causing serious damage in a small area north of Lisbon (Oeste region) and in the south region, inflicting economic losses of over EUR 100 million. In both areas, several power towers, designed to withstand up to 46 m s−1 winds, were destroyed. The causes of these two distinct damaging wind events were investigated. Xola was revealed to have a prominent cloud head and a split cold front structure. In the southern region, the damages were due to downburst winds, associated with a mesovortex, observed in a bow echo line triggered by an upper cold front. The cloud head presented several dry air intrusion signatures, co-located with tops progressively lowering towards the hooked tip. This tip revealed features consistent with the presence of slantwise convection, the descending branches of which may have been strengthened by evaporating cooling. At the reflectivity cloud head tip, a jet streak pattern was identified on weather radar, with Doppler velocities exceeding 55 m s−1, just 400 m above ground. This signature is coherent with the presence of a Sting jet, and this phenomenon was associated with the strongest wind gusts (over 40 m s−1) and the largest damages in the Oeste region.

Towards the closure of momentum budget analyses in the WRF (v3.8.1) model

Budget analysis of a tendency equation is widely utilized in numerical studies to quantify different physical processes in a simulated system. While such analysis is often post-processed when the output is made available, it is well acknowledged that the closure of a budget is difficult to achieve without temporal and/or spatial averaging. Nevertheless, the development of errors in such calculations has not been systematically investigated. In this study, an inline budget retrieval method is first developed in the WRF v3.8.1 model and tested on a 2D idealized slantwise convection case with a focus on the momentum equations. This method extracts all the budget terms following the model solver, which gives a high accuracy, with a residual term always less than 0.1 % of the tendency term. Then, taking the inline values as truth, several offline budget analyses with different commonly used simplifications are performed to investigate how they may affect the accuracy of the estimation of individual terms and the resultant residual. These assumptions include using a lower-order advection operator than the one used in the model, neglecting grid staggering, or following a mathematically equivalent but transformed format of the governing equations. Errors in these post-processed analyses are found mostly over the area where the dynamics are the most active, thus impairing the subsequent physical interpretation. A maximum 99th percentile residual can reach >50 % of the concurrent tendency term, indicating the danger of neglecting the residual term as done in many budget studies. This work provides general guidance not only for budget diagnoses with the WRF model but also for minimizing the errors in post-processed budget calculations.

Severe weather in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere system

<p>Tropical and extratropical cyclones can intensify into the most destructive weather systems that have significant societal and economic impacts. Rapid intensification of such weather systems has been examined in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere (MIA) system. It has been shown [1,2] that explosive extratropical cyclones and rapid intensification of tropical cyclones tend to follow arrivals of high-speed solar wind when the MIA coupling is strongest. The coupling generates atmospheric gravity waves (AGWs) that propagate from the high-latitude lower thermosphere both upward and downward [3,4]. In the upper atmosphere, AGWs are observed as traveling ionospheric disturbances. In the lower atmosphere, they can reach the troposphere and be ducted [4] to low latitudes. Despite significantly reduced wave amplitude, but subject to amplification upon over-reflection in the upper troposphere, these AGWs can trigger/release moist instabilities leading to convection and latent heat release, which is the energy driving the storms. The release of conditional symmetric instability is known to initiate slantwise convection producing rain/snow bands in extratropical cyclones. Severe weather, including severe winter storms, heavy snowfall and rainfall events, have been examined in the context of MIA coupling [5]. The results indicate a tendency of significant weather events, particularly if caused by low pressure systems in winter, to follow arrivals of solar wind high-speed streams from coronal holes. In the present paper we review the published results and provide further evidence to support them. This includes the occurrence of heavy rainfall events and flash floods, as well as the rapid intensification of recent hurricanes and typhoons, with the goal to identify sources of AGWs at high latitudes that may play a role in triggering convective bursts potentially leading to such events.</p><p>[1] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys., 149, 219–231, 2016.</p><p>[2] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys., 183, 36–60, 2019.</p><p>[3] Prikryl P., et al., Ann. Geophys., 23, 401–417, 2005.</p><p>[4] Mayr H.G., et al., J. Geophys. Res., 89, 10929–10959, 1984.</p><p>[5] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys., 171, 94–110, 2018.</p>

Towards the closure of momentum budget analyses in the WRF (v3.8.1) model

Budget analysis of a tendency equation is widely utilized in numerical studies to quantify different physical processes in a simulated system. While such analysis is often post-processed when the output is made available, it is well-acknowledged that the closure of a budget is difficult to achieve without averaging. Nevertheless, the potential rise of the errors in such calculation has not been systematically investigated. In this study, an inline budget retrieval method is first developed in the WRF v3.8.1 model and tested on a 2D idealized slantwise convection case with a focus on the momentum equations. This method extracts all the budget terms following the model solver, which gives a high accuracy with a residual term always less than 0.02 % of the tendency term. Then, taking the inline values as truth, several post-processing budget analyses with different commonly-used simplifications are performed to investigate how they may affect the accuracy of the estimation of individual terms and the resultant residual. These assumptions include using a lower order advection operator than the one used in the model, neglecting the C staggering grids, or following a mathematically-equivalent but transformed format of equation. Errors in these post-processed analyses are found mostly over the area where the dynamics are the most active, impairing the subsequent physical interpretation. A maximum 99th percentile residual can reach 800 % of the concurrent tendency term, indicating the danger of neglecting the residual term as done in many budget studies. This work provides general guidance not only for applying an inline budget retrieval to the WRF model but for minimizing the errors in post-processing budget calculations.

Assessment of Conditional Symmetric Instability from Global Reanalysis Data

Slantwise convection, the process by which moist symmetric instability is released, has often been linked to banded clouds and precipitation, especially in frontal zones within extratropical cyclones. Studies also suggest that the latent heat release associated with slantwise convection can lead to a spinup of surface frontogenesis, which can enhance the rapid intensification of extratropical cyclones. However, most of these studies considered only local areas or short time durations. In this study, we provide a novel statistical investigation of the global climatology of the potential occurrence of slantwise convection, in terms of conditional symmetric instability, and its relationship with precipitating systems. Using the 6-hourly ERA-Interim, two different indices are calculated, namely, slantwise convective available potential energy (SCAPE) and vertically integrated extent of realizable symmetric instability (VRS), to assess the likelihood of occurrence of slantwise convection around the globe. The degree of association is quantified between these indices and the observed surface precipitation as well as the cyclone activity. The susceptibility of midlatitude cyclones to slantwise convection at different stages of their life cycle is also investigated. As compared to the nonexplosive cyclone cases, the time evolution of SCAPE and VRS within rapidly deepening cyclones exhibit higher values before, and a more significant drop after, the onset of rapid intensification, supporting the idea that the release of symmetric instability might contribute to the intensification of storms.