Emergence of a nocturnal low-level jet from a broad baroclinic zone

An analytical model is presented for the generation of a Blackadar-like nocturnal low-level jet in a broad baroclinic zone. The flow is forced from below (flat ground) by a surface buoyancy gradient and from above (free atmosphere) by a constant pressure gradient force. Diurnally-varying mixing coefficients are specified to increase abruptly at sunrise and decrease abruptly at sunset. With attention restricted to a surface buoyancy that varies linearly with a horizontal coordinate, the Boussinesq-approximated equations of motion, thermal energy, and mass conservation reduce to a system of one-dimensional equations that can be solved analytically. Sensitivity tests with southerly jets suggest that (i) stronger jets are associated with larger decreases of the eddy viscosity at sunset (as in Blackadar theory), (ii) the nighttime surface buoyancy gradient has little impact on jet strength, and (iii) for pure baroclinic forcing (no free-atmosphere geostrophic wind), the nighttime eddy diffusivity has little impact on jet strength, but the daytime eddy diffusivity is very important and has a larger impact than the daytime eddy viscosity. The model was applied to a jet that developed in fair weather conditions over the Great Plains from southern Texas to northern South Dakota on 1 May 2020. The ECMWF Reanalysis v5 (ERA5) for the afternoon prior to jet formation showed that a broad north-south-oriented baroclinic zone covered much of the region. The peak model-predicted winds were in good agreement with ERA5 winds and lidar data from the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) central facility in north-central Oklahoma.

Downstream Evolution and Coastal-to-Inland Transition of Landfalling Lake-Effect Systems

The distribution and intensity of lake- and sea-effect (hereafter lake-effect) precipitation are strongly influenced by the mode of landfalling lake-effect systems. Here, we used idealized large-eddy simulations to investigate the downstream evolution and coastal-to-inland transition of two lake-effect modes: 1) a long-lake-axis-parallel (LLAP) band generated by an oval body of water (hereafter lake; e.g., Lake Ontario) and 2) broad-coverage, open-cell convection generated by an open lake (e.g., Sea of Japan).Under identical atmospheric conditions and lake-surface temperatures, the oval lake generates a LLAP band with heavy precipitation along the mid-lake axis, whereas the open lake generates broad-coverage, open-cell convection with widespread, light accumulations. Over the oval lake, the LLAP band features a thermally forced and diabatically enhanced cross-band secondary circulation with convergence and ascent over the mid-lake axis. Downstream of the lake, flanking airstreams that avoid lake modification merge beneath the band where they experience sublimational cooling, producing a cold pool. At the upstream edge of the cold pool, a coastal baroclinic zone forms. Above this zone, ascent and hydrometeor mass growth are maximized, resulting in an inland precipitation maximum due to subsequent hydrometeor transport and fallout. Over the open lake, individual open cells grow larger and stronger with over-water extent, but a convective-to-stratiform transition begins at the coast. Here, convective vigor decays, mesoscale ascent begins, and enhanced hydrometeor growth results in an inland precipitation maximum. These results highlight variations in the coastal-to-inland transition of lake-effect systems that ultimately influence the distribution and intensity of lake-effect precipitation.

Vortex Spinup Process in the Extratropical Transition of Hurricane Sandy (2012)

This study utilizes the quasi-Lagrangian azimuthal momentum equation (i.e., budget calculation) and 1.667-km-resolution numerical simulation data to study the intensity and structural changes in Hurricane Sandy’s extratropical transition. The results indicate that after the onset of extratropical transition, Sandy maintains an eyewall-like convection and warm core in the core region and has a frontal structure in the outer region. In the outer region, baroclinicity-driven frontal convection induces extensive planetary boundary layer (PBL) inflow, causing an inward advection of absolute angular momentum (AAM) per unit radius, which generates outer local wind maxima and expands Sandy’s outer wind field through a spinup process. Moreover, because the outer tangential wind velocity accelerates in a frontal convection, local wind maxima associated with fronts can expand to the outer sides of frontal regions. Frontal convection increases AAM in the outer region, providing the precondition for reintensification; however, the front itself cannot cause Sandy’s reintensification. The eyewall-like convection in the core region still plays an important role in Sandy’s reintensification. When the baroclinic zone, where a strong horizontal temperature gradient exists, approaches the core region, the eyewall-like convection is enhanced because the warm, moist air of the core region is lifted by the cold, dry air associated with the approaching baroclinic zone. Consequently, owing to the enhancement of eyewall-like convection, the PBL inflow, which extends from the outer region to the core region, develops. This inflow increases the inward transportation of the outer frontal region’s high-AAM air, thus leading to spinning up the core region’s wind and reintensification.

The Eddy-Driven Jet and Storm-Track Responses to Boundary Layer Drag: Insights from an Idealized Dry GCM Study

Simulations using a dry, idealized general circulation model (GCM) are conducted to systematically investigate the eddy-driven jet’s sensitivity to the location of boundary layer drag. Perturbations of boundary layer drag solely within the baroclinic zone reproduce the eddy-driven jet responses to global drag variations. The implications for current theories of eddy-driven jet shifts are discussed. Hemispherically asymmetric drag simulations in equinoctial and solstitial thermal conditions show that perturbations of surface drag in one hemisphere have negligible effects on the strength and latitude of the eddy-driven jet in the opposite hemisphere. Jet speed exhibits larger sensitivities to surface drag in perpetual winter simulations, while sensitivities in jet latitude are larger in perpetual summer simulations. Near-surface drag simulations with an Earthlike continental profile show how surface drag may facilitate tropical–extratropical teleconnections by modifying waveguides through changes in jet latitude. Longitudinally confined drag simulations demonstrate a novel mechanism for localizing storm tracks. A theoretical analysis is used to show that asymmetries in the Bernoulli function within the baroclinic zone are important for the eddy-driven jet latitude responses because they directly modulate the sensitivity of the zonal-mean zonal wind to drag in the boundary layer momentum balance. The simulations contained herein provide a rich array of case studies against which to test current theories of eddy-driven jet and storm-track shifts, and the results affirm the importance of correct, well-constrained locations and intensities of boundary layer drag in order to reduce jet and storm-track biases in climate and forecast models.

Ontario Winter Lake-effect Systems (OWLeS): Bulk Characteristics and Kinematic Evolution of Misovortices in Long-Lake-Axis-Parallel Snowbands

During the Ontario Winter Lake-effect Systems (OWLeS) field campaign, 12 long-lake-axis-parallel (LLAP) snowband events were sampled. Misovortices occurred in 11 of these events, with characteristic diameters of ~800 m, differential velocities of ~11 m s−1, and spacing between vortices of ~3 km. A detailed observational analysis of one such snowband provided further insight on the processes governing misovortex genesis and evolution, adding to the growing body of knowledge of these intense snowband features. On 15–16 December 2013, a misovortex-producing snowband was exceptionally well sampled by ground-based OWLeS instrumentation, which allowed for integrated finescale dual-Doppler and surface thermodynamic analyses. Similar to other studies, horizontal shearing instability (HSI), coupled with stretching, was shown to be the primary genesis mechanism. The HSI location was influenced by snowband-generated boundaries and location of the Arctic front relative to the band. Surface temperature observations, available for the first time, indicated that the misovortices formed along a baroclinic zone. Enhanced mixing, higher radar reflectivity, and increased precipitation rate accompanied the vortices. As the snowband came ashore, OWLeS participants indicated an increase in snowfall and white out conditions with the passage of the snowband. A sharp, small-scale pressure drop, coupled with winds of ~16 m s−1, marked the passage of a misovortex and may be typical of snowband misovortices.

Rainfall Characteristics of Recurving Tropical Cyclones over the Western North Pacific

A dataset of 88 recurving western North Pacific tropical cyclones from 2004 to 2015 is investigated for rainfall characteristics during their period of recurvature. The TCs are categorized into two groups based on different large-scale patterns from empirical orthogonal function analysis. Group 1 is characterized by an intense midlatitude baroclinic zone and close distance between the zone and TC, while Group 2 is characterized by a weaker midlatitude baroclinic zone and more remote distance between the zone and TC at the time of recurvature. The results show the large-scale environment has substantial impact on TC rainfall patterns. In Group 1, as the TC approaches and is embedded into the baroclinic zone, a relatively strong interaction between the TC and midlatitudes occurs, which is reflected by a rapid increase of environmental vertical wind shear and TC translation speed, the alignment of the shear vector and motion vector, and a sharp contrast of temperature and moisture. Higher rainfall and wider coverage of rainfall tends to be produced along the track after recurvature, and the rainfall pattern turns from a right-of-track (ROT) to a left-of-track (LOT) preference. Conversely, in Group 2, a relatively weak interaction between the TC and midlatitude circulation occurs, which is reflected by weaker vertical wind shear and slower TC motion, a separation of the shear vector and motion vector, and a weak gradient of temperature and moisture. The corresponding rainfall swath for Group 2 exhibits a narrower rainfall swath after recurvature. The rain pattern changes from a LOT to ROT preference.

Multiscale Characteristics of Surface Winds in an Area of Complex Terrain in Northwest Utah

Climatological features of the surface wind on diurnal and seasonal time scales over a 17-yr period in an area of complex terrain at Dugway Proving Ground in northwestern Utah are analyzed, and potential synoptic-scale, mesoscale, and microscale forcings on the surface wind are identified. Analysis of the wind climatology at 26 automated weather stations revealed a bimodal wind direction distribution at times when thermally driven circulations were expected to produce a single primary direction. The two modes of this distribution are referred to as the “northerly” and “southerly” regimes. The northerly regime is most frequent in May, and the southerly regime is most frequent in August. January, May, and August constitute a “tripole seasonality” of the wind evolution. Although both regimes occur in all months, the monthly changes in regime frequency are related to changes in synoptic and mesoscale phenomena including the climatological position of the primary synoptic baroclinic zone in the western United States, interaction of the large-scale flow with the Sierra Nevada, and thermal low pressure systems that form in the Intermountain West in summer. Numerous applications require accurate forecasts of surface winds in complex terrain, yet mesoscale models perform relatively poorly in these areas, contributing to poor operational forecast skill. Knowledge of the climatologically persistent wind flows and their potential forcings will enable relevant model deficiencies to be addressed.

Forward and Reverse Shear Environments during Polar Low Genesis over the Northeast Atlantic

The synoptic and subsynoptic environments associated with polar low genesis are examined. Ambient pre–polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind. Forward shear environments are associated with a synoptic-scale ridge over Scandinavia, featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause. Similar to typical midlatitude cyclogenesis, concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward, parallel to isolines of sea surface temperature. Reverse shear environments exhibit a distinctly different structure and are characterized by a trough over Scandinavia, associated with a synoptic-scale, occluded cyclone. The genesis area exhibits strong cold air advection on its right-hand side and polar low development occurs on the warm side of an intense low-level jet. The environment resembles the characteristics conducive to secondary development associated with frontal instability. Polar lows developing in this configuration propagate mainly southward, perpendicular to isolines of sea surface temperature. The two genesis environments exhibit similar temperature differences between the sea surface and atmosphere near the surface, yet the magnitude of the surface fluxes is approximately double during reverse shear conditions due to stronger low-level winds. The ratio between surface sensible and latent heat fluxes is close to unity for both shear environments.

The Latitudinal Dependence of Atmospheric Jet Scales and Macroturbulent Energy Cascades

The latitudinal width of atmospheric eddy-driven jets and scales of macroturbulence are examined latitude by latitude over a wide range of rotation rates using a high-resolution idealized GCM. It is found that for each latitude, through all rotation rates, the jet spacing scales with the Rhines scale. These simulations show the presence of a “supercriticality latitude” within the baroclinic zone, where poleward (equatorward) of this latitude, the Rhines scale is larger (smaller) than the Rossby deformation radius. Poleward of this latitude, a classic geostrophic turbulence picture appears with a − spectral slope of inverse cascade from the deformation radius up to the Rhines scale. A shallower slope than the −3 slope of enstrophy cascade is found from the deformation radius down to the viscosity scale as a result of the broad input of baroclinic eddy kinetic energy. At these latitudes, eddy–eddy interactions transfer barotropic eddy kinetic energy from the input scales of baroclinic eddy kinetic energy up to the jet scale and down to smaller scales. For the Earth case, this latitude is outside the baroclinic zone and therefore an inverse cascade does not appear. Equatorward of the supercriticality latitude, the − slope of inverse cascade vanishes, eddy–mean flow interactions play an important role in the balance, and the spectrum follows a −3 slope from the Rhines scale down to smaller scales, similar to what is observed on Earth. Moreover, the length scale of the energy-containing zonal wavenumber is equal to (larger than) the jet scale poleward (equatorward) of the supercriticality latitude.

Separation of Climatological Imprints of the Kuroshio Extension and Oyashio Fronts on the Wintertime Atmospheric Boundary Layer: Their Sensitivity to SST Resolution Prescribed for Atmospheric Reanalysis

Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio–Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.