Diabatic processes modulating the vertical structure of the jet stream above the cold front of an extratropical cyclone: sensitivity to deep convection schemes

The effect of deep convection parameterization on the jet stream above the cold front of an explosive extratropical cyclone is investigated in the global numerical weather prediction model ARPEGE, operational at Météo-France. Two hindcast simulations differing only in the deep convection scheme used are systematically compared with each other, with (re)-analysis datasets and with NAWDEX airborne observations. The deep convection representation has an important effect on the vertical structure of the jet stream above the cold front at one-day lead time. The simulation with the less active scheme shows a deeper jet stream, associated with a stronger potential vorticity (PV) gradient in the jet core in middle troposphere. This is due to a larger deepening of the dynamical tropopause on the cold-air side of the jet and a higher PV destruction on the warm-air side, near 600 hPa. To better understand the origin of this stronger PV gradient, Lagrangian backward trajectories are computed. On the cold-air side of the jet, numerous trajectories undergo a rapid ascent from the boundary layer to the mid levels in the simulation with the less active deep convection scheme, whereas they stay at mid levels in the other simulation. This ascent explains the higher PV noted on that side of the jet in the simulation with the less active deep convection scheme. These ascending air masses form mid-level ice clouds that are not observed in the microphysical retrievals from airborne radar-lidar measurements. On the warm-air side of the jet, in the warm conveyor belt (WCB) ascending region, the Lagrangian trajectories with the less active deep convection scheme undergo a higher PV destruction due to a stronger heating occurring in the lower and middle troposphere. In contrast, in the simulation with the most active deep convection scheme, both the heating and PV destruction extend further up in the upper troposphere.

The Dynamical Structure of a Warm Core Ring as Inferred from Glider Observations and Along-Track Altimetry

This study investigates the vertical structure of the dynamical properties of a warm-core ring in the Gulf of Mexico (Loop Current ring) using glider observations. We introduce a new method to correct the glider’s along-track coordinate, which is, in general, biased by the unsteady relative movements of the glider and the eddy, yielding large errors on horizontal derivatives. Here, we take advantage of the synopticity of satellite along-track altimetry to apply corrections on the glider’s position by matching in situ steric height with satellite-measured sea surface height. This relocation method allows recovering the eddy’s azimuthal symmetry, precisely estimating the rotation axis position, and computing reliable horizontal derivatives. It is shown to be particularly appropriate to compute the eddy’s cyclo-geostrophic velocity, relative vorticity, and shear strain, which are otherwise out of reach when using the glider’s raw traveled distance as a horizontal coordinate. The Ertel potential vorticity (PV) structure of the warm core ring is studied in details, and we show that the PV anomaly is entirely controlled by vortex stretching. Sign reversal of the PV gradient across the water column suggests that the ring might be baroclinically unstable. The PV gradient is also largely controlled by gradients of the vortex stretching term. We also show that the ring’s total energy partition is strongly skewed, with available potential energy being 3 times larger than kinetic energy. The possible impact of this energy partition on the Loop Current rings longevity is also discussed.

The Dynamical Structure of a Warm Core Ring as Inferred from Glider Observations and Along-Track Altimetry

This study investigates the vertical structure of the dynamical properties of a warm-core ring in the Gulf of Mexico (Loop Current ring) using glider observations. We introduce a new method to correct the glider’s along-track coordinate which is, in general, biased by the unsteady relative movements of the glider and the eddy, yielding large errors on horizontal derivatives. Here, we take advantage of the synopticity of satellite along-track altimetry to apply corrections on the glider’s position, by matching in situ steric height with satellite-measured sea surface height. This relocation method allows to recover the eddy’s azimuthal symmetry, to precisely estimate the rotation axis position, and to compute reliable horizontal derivatives. It is shown to be particularly appropriate to compute the eddy’s cyclo-geostrophic velocity, relative vorticity, and shear strain, which are otherwise out of reach when using the glider’s raw traveled distance as an horizontal coordinate. The Ertel potential vorticity (PV) structure of the warm core ring is studied in details, and we show that the PV anomaly is entirely controlled by vortex stretching. Sign reversal of the PV gradient across the water column suggests that the ring might be baroclinically unstable. The PV gradient is also largely controlled by gradients of the vortex stretching term. We also show that the ring’s total energy partition is strongly skewed, with available potential energy being 3 times larger than kinetic energy. The possible impact of this energy distribution on the Loop Current rings longevity is also discussed.

Impact of wind shear and stratification near the tropopause on baroclinic instability

<p>Many weather and climate models fail to represent the sharp vertical changes of vertical wind shear and stratification near the tropopause. This discrepancy results in errors in the horizontal gradient of potential vorticity (PV), which acts as a wave guide for Rossby waves that highly influence surface weather in midlatitudes. In an idealised quasi-geostrophic model developed from the Eady model, we investigate how variations in vertical wind shear and stratification near the tropopause affect baroclinic growth. Comparing sharp and smooth vertical profiles of wind shear and stratification across the tropopause for different tropopause altitudes, we find that both smoothing and tropopause altitude have little impact on the growth rate, wavelength, phase speed, and structure of baroclinic waves, despite a sometimes significant weakening of the maximum PV gradient for extensive smoothing. Instead, we find that baroclinic growth is more sensitive if the vertical integral of the PV gradient is not conserved across the tropopause. Furthermore, including mid-tropospheric latent heating highlights that errors in baroclinic growth related to a misrepresentation of latent heating intensity are typically much larger than those associated with the correct representation of vertical wind shear and stratification in the tropopause region. Our results thus indicate that the correct representation of latent heating in weather forecast models is of higher importance than adequately resolving the tropopause.</p>

Formation of Anticyclones above Topographic Depressions

Long-lived anticyclonic eddies (ACs) have been repeatedly observed over several North Atlantic basins characterized by bowl-like topographic depressions. Motivated by these previous findings, the authors conduct numerical simulations of the spindown of eddies initialized in idealized topographic bowls. In experiments with one or two isopycnal layers, it is found that a bowl-trapped AC is an emergent circulation pattern under a wide range of parameters. The trapped AC, often formed by repeated mergers of ACs over the bowl interior, is characterized by anomalously low potential vorticity (PV). Several PV segregation mechanisms that can contribute to the AC formation are examined. In one-layer experiments, the dynamics of the AC are largely determined by a nonlinearity parameter ϵ that quantifies the vorticity of the AC relative to the bowl’s topographic PV gradient. The AC is trapped in the bowl for low , but for moderate values () partial PV segregation allows the AC to reside at finite distances from the center of the bowl. For higher , eddies freely cross the topography and the AC is not confined to the bowl. These regimes are characterized across a suite of model experiments using ϵ and a PV homogenization parameter. Two-layer experiments show that the trapped AC can be top or bottom intensified, as determined by the domain-mean initial vertical energy distribution. These findings contrast with previous theories of mesoscale turbulence over topography that predict the formation of a prograde slope current, but do not predict a trapped AC.

A Geometric Perspective on the Modulation of Potential Energy Release by a Lateral Potential Vorticity Gradient

The release of available potential energy by growing baroclinic instability requires the slope of the eddy fluxes to be shallower than that of mean density surfaces, where the amount of energy released depends on both the flux angle and the distance of fluid parcel excursions against the background density gradient. The presence of a lateral potential vorticity (PV) gradient is known to affect the growth rate and energy release by baroclinic instability, but often makes the mathematics of formal linear stability analysis intractable. Here the effects of a lateral PV gradient on baroclinic growth are examined by considering its effects on the slope of the eddy fluxes. It is shown that the PV gradient systematically shifts the unstable modes toward higher wavenumbers and creates a cutoff to the instability at large scales, both of which steepen the eddy flux angle and limit the amount of energy released. This effect may contribute to the severe inhibition of baroclinic turbulence in systems dominated by barotropic jets, making them less likely to transition to turbulence-dominated flow regimes.

Overtransmission of Rossby Waves at a Lower-Layer Critical Latitude in the Two-Layer Model

Rossby waves, propagating from the midlatitudes toward the tropics, are typically absorbed by critical latitudes (CLs) in the upper troposphere. However, these waves typically encounter CLs in the lower troposphere first. We study a two-layer linear scattering problem to examine the effects of lower CLs on these waves. We begin with a review of the simpler barotropic case to orient the reader. We then progress to the baroclinic case using a two-layer quasigeostrophic model in which there is vertical shear in the mean flow on which the waves propagate, and in which the incident wave is assumed to be an external-mode Rossby wave. We use linearized equations and add small damping to remove the critical-latitude singularities. We consider cases in which either there is only one CL, in the lower layer, or there are CLs in both layers, with the lower-layer CL encountered first. If there is only a CL in the lower layer, the wave’s response depends on the sign of the mean potential vorticity gradient at this lower-layer CL: if the PV gradient is positive, then the CL partially absorbs the wave, as in the barotropic case, while for a negative PV gradient, the CL is a wave emitter, and can potentially produce overreflection and/or overtransmission. Our numerical results indicate that overtransmission is by far the dominant response in these cases. When an upper-layer absorbing CL is encountered, following the lower-layer encounter, one can still see the signature of overtransmission at the lower-layer CL.

Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear

The transport by materially coherent surface-layer eddies was studied in a two-layer quasigeostrophic model driven by eastward mean shear. The coherent eddies were identified by closed contours of the Lagrangian-averaged vorticity deviation obtained from Lagrangian particles advected by the flow. Attention was restricted to eastward mean flows, but a wide range of flow regimes with different bottom friction strengths, layer thickness ratios, and background potential vorticity (PV) gradients were otherwise considered. It was found that coherent eddies become more prevalent and longer-lasting as the strength of bottom drag increases and the stratification becomes more surface-intensified. The number of coherent eddies is minimal when the shear-induced PV gradient is 10–20 times the planetary PV gradient and increases for both larger and smaller values of the planetary PV gradient. These coherent eddies, with an average core radius close to the deformation radius, propagate meridionally with a preference for cyclones to propagate poleward and anticyclones to propagate equatorward. The meridional propagation preference of the coherent eddies gives rise to a systematic upgradient PV transport, which is in the opposite direction as the background PV transport and not captured by standard Lagrangian diffusivity estimates. The upgradient PV transport by coherent eddy cores is less than 15% of the total PV transport, but the PV transport by the periphery flow induced by the PV inside coherent eddies is significant and downgradient. These results clarify the distinct roles of the trapping and stirring effect of coherent eddies in PV transport in geophysical turbulence.

Generation Mechanism of Tropical Instability Waves in the Equatorial Pacific Ocean

Tropical instability waves (TIWs) are prominent features in the equatorial Pacific, propagating westward at a speed of ~0.5 m s−1 with a wavelength of ~1000 km. In this study, we show that a linear stability analysis using a 1.5-layer shallow water model can predict successfully an unstable mode whose wavelength, phase speed, growth rate, and meridional structure are all consistent with those of the TIWs simulated by an eddy-resolving ocean general circulation model (OGCM). This unstable mode can be interpreted as resulting from the coupling of two Rossby waves, namely, one trapped just north of the equator (~1°–3.5°N) and the other trapped farther north (~3.5°–8°N). Although these two Rossby waves have opposite intrinsic phase propagation directions reflecting the negative and positive local meridional potential vorticity (PV) gradients, respectively, their actual propagation direction can be adjusted through the advection by the South Equatorial Current and the North Equatorial Countercurrent such that they might propagate westward at the same speed and with the same zonal wavenumber yielding the largest growth rate of TIWs. The unstable mode does not appear during the period in which the negative PV gradient is absent, which demonstrates its essential role in generating TIWs. Indeed, the seasonal and interannual variability of the TIWs simulated by the OGCM is shown to be significantly controlled by the strength of the negative PV gradient just north of the equator, suggesting that it could be a key parameter toward a dynamically based parameterization of the heat and momentum transfer associated with TIWs in coarse-resolution OGCMs.

Balancing the Potential Vorticity Seesaw: The Bare Essentials of Baroclinic Instability

This paper bypasses the mathematical technicalities of baroclinic instability and tries to provide a more conceptual, mechanistic explanation for a phenomenon that is fundamentally important to the dynamics of the earth’s atmosphere and oceans. The standard conceptual picture of baroclinic instability is reviewed and stripped down to identify the most essential features. These are: (a) Regions with both positive and negative potential vorticity (PV) gradients, (b) separate Rossby wave perturbations in each region where PV gradients are of different signs, and (c) cooperative phase locking between Rossby waves in regions of opposite PV gradient, which renders them stationary, and allows them to amplify to reduce the background temperature gradient (or baroclinicity) while still conserving total PV. These three factors constitute the “counterpropagating Rossby wave” perspective, and suggest the heuristic picture of a “PV seesaw”, which remains balanced as the instabilities (i.e., the phase-locked PV wave perturbations) grow out along opposite limbs. After reviewing the key characteristics of PV and Rossby waves, the process is illustrated by the spontaneous onset of baroclinic instability during spin-up of the Held–Suarez dynamical core atmospheric model.