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  1. Abstract

    On 10 August 2020, a derecho caused widespread damage across Iowa and Illinois. Des Moines station data show that the arrival of the gust front was characterized by an abrupt shift to northerly flow, exceeding 22 m/s for ~ 20 min. To test the hypothesis that this northerly jet is associated with a horizontal potential vorticity (PV) dipole in the lower troposphere, we investigated the structure of PV in the University of Wisconsin Nonhydrostratic Modeling System (UWNMS) and of absolute vorticity in High Resolution Rapid Refresh (HRRR) forecast analyses.

    This structure is described here for the first time. The negative PV member coincides with the downdraft, while the positive PV member coincides with the updraft, with a northerly jet between. The westerly inflow jet descends anticyclonically in the downdraft, joining with northerly flow from the surface anticyclone. The resulting northerly outflow jet creates the trailing comma-shaped radar echo.

    The speed of propagation of the derecho is similar to the westerly wind maximum in the 3-5 km layer associated with the approaching synoptic cyclone, which acts as a steering level for resonant amplification. Idealized diagrams and 3D isosurfaces illustrate the commonality of the PV dipole / northerly jet structure. Differences in this structure among three model states are related to low-level wind shear theory. The PV dipole coincides with the pattern of diabatic stretching tendency, which shifts westward and downward relative to the updraft/downdraft with increasing tilt. The PV dipole can contribute toward dynamical stability in a derecho.

     
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    Free, publicly-accessible full text available April 19, 2025
  2. On 10 August 2020, a derecho caused widespread damage across Iowa and Illinois. Des Moines station data show that the arrival of the gust front was characterized by an abrupt shift to northerly flow, exceeding 22 m/s for ~ 20 min. To test the hypothesis that this northerly jet is associated with a horizontal potential vorticity (PV) dipole in the lower troposphere, we investigated the structure of PV in the University of Wisconsin Nonhydrostratic Modeling System (UWNMS) and of absolute vorticity in High Resolution Rapid Refresh (HRRR) forecast analyses. This structure is described here for the first time. The negative PV member coincides with the downdraft, while the positive PV member coincides with the updraft, with a northerly jet between. The westerly inflow jet descends anticyclonically in the downdraft, joining with northerly flow from the surface anticyclone. The resulting northerly outflow jet creates the trailing comma-shaped radar echo. The speed of propagation of the derecho is similar to the westerly wind maximum in the 3-5 km layer associated with the approaching synoptic cyclone, which acts as a steering level for resonant amplification. Idealized diagrams and 3D isosurfaces illustrate the commonality of the PV dipole / northerly jet structure. Differences in this structure among three model states are related to low-level wind shear theory. The PV dipole coincides with the pattern of diabatic stretching tendency, which shifts westward and downward relative to the updraft/downdraft with increasing tilt. The PV dipole can contribute toward dynamical stability in a derecho. 
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    Free, publicly-accessible full text available April 3, 2025
  3. The present study investigates dynamical coupling between the equatorial stratospheric Quasi31 biennial oscillation (QBO) and the boreal winter surface climate of the Northern Hemisphere mid and high latitudes using 42 years data (1979–2020). For neutral El Niño Southern Oscillation (ENSO) periods, QBO westerlies (W) at 70 hPa favor high sea level pressure in the polar region, colder conditions and deeper snow over Eurasia and North America, and the opposite effects for QBO easterlies (E). When QBO anomalies arrive in the upper troposphere and lower stratosphere (UTLS), it is observed that planetary wave activity is enhanced in the extratropical UTLS during QBO W and diminished during QBO E. This QBO teleconnection pathway along the UTLS to the high latitude surface is independent of the “stratospheric pathway” (Holton-Tan mechanism). Diagnosis of this pathway can help to improve understanding of internal sub-seasonal to seasonal variations, and long-range forecasting over Eurasia and North America. 
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    Free, publicly-accessible full text available April 15, 2025
  4. Abstract

    The influence of solar forcing and Galactic Cosmic Rays (GCR) ionization on the global distribution of clouds is investigated using 42 years ERA-5 data (1979–2020). In the mid-latitudes over Eurasia, GCR and cloudiness are negatively correlated, which argues against the ionization theory of enhanced cloud droplet nucleation due to increased GCR during minima in the solar cycle. In the tropics, the solar cycle and cloudiness are positively correlated in regional Walker circulations below 2 km altitude. The phase relationship between amplification of regional tropical circulations and the solar cycle is consistent with total solar forcing, rather than modulation of GCR. However, in the intertropical convergence zone, changes in the cloud distribution are consistent with a positive coupling with GCR in the free atmosphere (2–6 km). This study opens some future challenges and research directions, and clarifies how atmospheric circulation at the regional scale can help in understanding solar-induced climate variability.

     
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    Free, publicly-accessible full text available December 1, 2024
  5. Monthly-mean data of ERA-Interim reanalysis, precipitation, outgoing longwave radiation (OLR) and sea surface temperature(SST) are investigated for 40 years (1979-2018) to reveal the modulation of the global monsoon systems by the equatorial quasi-biennial oscillation (QBO), focusing only on the neutral El Niño-Southern Oscillation (ENSO) periods (in total 374 months). First, the climatology of the global monsoon systems is viewed with longitude-latitude plots of the precipitation, its proxies and lower tropospheric circulations for the annual mean and two solstice seasons, together with the composite differences between the two seasons. In addition to seasonal variations of Intertropical Convergence Zones (ITCZs), several regional monsoon systems are well identified with an anti-phase of the annual cycle between the two hemispheres. Precipitation-related quantities (OLR and specific humidity), surface conditions [i.e., mean sea level pressure (MSLP) and SST] and circulation fields related to moist convection systems show fundamental features of the global monsoon systems. After introducing eight QBO phases based on the leading two principal components of the zonal-mean zonal wind variations in the equatorial lower-stratosphere, the statistical significance of the composite difference in the precipitation and tropospheric circulation is evaluated for the opposite QBO phases. The composite differences show significant modulations in some regional monsoon systems, dominated by zonally asymmetric components, with the largest magnitudes for specific QBO-phases corresponding to traditional indices of the equatorial zonal-mean zonal wind at 20 and 50 hPa. Along the equator, significant QBO influence is characterized by the modulation of the Walker circulation over the western Pacific. In middle latitudes during boreal summer, for a specific QBO-phase, statistically significant modulation of low-pressure cyclonic perturbation is obtained over the Northern-Hemisphere western Pacific Ocean associated with statistically significant features of heavier precipitation over the eastern side of the cyclonic perturbation and the opposite lighter precipitation over the western side. During boreal winter, similar significant low-pressure cyclonic perturbations were found over the Northern-Hemisphere eastern Pacific and Atlantic Oceans for specific phases.

     
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  6. The European Centre for Medium-range Weather Forecasting (ECMWF) Reanalysis v5 (ERA5) data set (1979 – 2020) is used in many climate studies.  The present monthly mean data set of Eliassen-Palm fluxes (EP fluxes) and EP flux divergences, was created from twice-daily gridded values of ERA5 winds and temperature on pressure surfaces and is intended to fill a gap in availability.   It is compatible for use with ERA5 monthly mean data.   The EP flux is a diagnostic tool for assessing wave propagation and wave-mean flow interaction. It is a vector representation for the propagation of synoptic and planetary Rossby wave activity in the meridional plane.  An upward component indicates a poleward heat flux while an equatorward component indicates a poleward momentum flux.   EP flux divergence implies a source of Rossby wave activity, and EP flux convergence implies absorption of Rossby wave activity.  EP flux divergence represents the body force, or net effect of waves on the zonal mean zonal wind, with EP flux convergence causing deceleration of zonal mean westerlies and EP flux divergence causing acceleration. The primary effect of a region of EP flux convergence, however, is to induce poleward motion, with an associated mean meridional circulation and quadrupole of temperature anomalies in the meridional plane. EP fluxes are useful in investigating many planetary and synoptic scale weather phenomena such as the Quasi-Biennial Oscillation (QBO) and Sudden Stratospheric Warmings (SSWs).   The meridional and vertical components of the EP flux are calculated in pressure coordinates using the following initial conditions:  Ground density (rho_0 ):P_0/R_0/T_0 Ground pressure (P_0): 1013hPa Ground temperature (T_0): Temperature at 1000hPa Earth radius (a) = 6378km   Dimensions in the data: Latitude: 90°N-90°S (grid spacing is 0.25°, as in the ERA5 reanalysis)  Pressure: 1 hPa-1000 hPa (levels are the same as in the ERA5 reanalysis)  Time: January 1979 – December 2020; December, January, February (DJF) only; 00z and 12z 
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  7. Abstract The role of differential advection in creating tropopause folds and strong constituent gradients near midlatitude westerly jets is investigated using the University of Wisconsin Non-hydrostatic Modeling System (UWNMS). Dynamical structures are compared with aircraft observations through a fold and subpolar jet (SPJ) during RF04 of the Stratosphere-Troposphere Analyses of Regional Transport (START08) campaign. The observed distribution of water vapor and ozone during RF04 provides evidence of rapid transport in the SPJ, enhancing constituent gradients above relative to below the intrusion. The creation of a tropopause fold by quasi-isentropic differential advection on the upstream side of the trough is described. This fold was created by a southward jet streak in the SPJ, where upper tropospheric air displaced the tropopause eastward in the 6-10 km layer, thereby overlying stratospheric air in the 3-6 km layer. The subsequent superposition of the subtropical and subpolar jets is also shown to result from quasi-isentropic differential advection. The occurrence of low values of ozone, water vapor, and potential vorticity on the equatorward side of the SPJ can be explained by convective transport of low-ozone air from the boundary layer, dehydration in the updraft, and detrainment of inertially-unstable air in the outflow layer. An example of rapid juxtaposition with stratospheric air in the jet core is shown for RF01. The net effect of upstream convective events is suggested as a fundamental cause of the strong constituent gradients observed in midlatitude jets. Idealized diagrams illustrate the role of differential advection in creating tropopause folds and constituent gradient enhancement. 
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  8. Abstract

    The joint influence of the stratospheric quasi‐biennial oscillation (QBO) and the El Niño Southern Oscillation (ENSO) on the polar vortex, subtropical westerly jets (STJs), and wave patterns during boreal winter is investigated in 40 years (1979–2018) of monthly mean ERA‐Interim reanalyses. The method of Wallace et al. (1993),https://doi.org/10.1175/15200469(1993)050<1751:ROTESQ>2.0.CO;2is used to conduct a QBO phase angle sweep. QBO westerly (W) and easterly (E) composites are then segregated by the phase of ENSO. Two pathways are described by which the QBO mean meridional circulation (MMC) influences the northern winter hemisphere. The “stratospheric pathway” modulates stratospheric planetary wave absorption via the Holton‐Tan mechanism. The “tropospheric pathway” modulates the tropical and subtropical upper troposphere and lower stratosphere. QBO MMC anomalies exhibit a checkerboard pattern in temperature and arched structures in zonal wind which extend into midlatitudes, and are stronger on the winter side. During QBO W, the polar vortex and STJs are enhanced. QBO signals in the polar vortex are amplified during La Niña. During El Niño and QBO W, the strongest STJs occur, and a warm pole/wave two pattern is found. During El Niño and QBO E, a trough is found over Eurasia and a ridge over the North Atlantic, in a wave one pattern. El Niño diminishes QBO anomalies in the tropical stratosphere and reduces the poleward extent and amplitude of the QBO MMC, thereby influencing the stratospheric pathway. Effects on the boreal winter hemisphere are attributed to the combined influence of the QBO and ENSO via both pathways.

     
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  9. null (Ed.)
  10. null (Ed.)
    Abstract The stalling and rapid destruction of a potential vorticity (PV) anomaly in the upper troposphere–lower stratosphere (UTLS) by convectively detrained inertially unstable air is described. On 20 August 2018, 10–15 in. (~0.3–0.4 m) of rain fell on western Dane County, Wisconsin, primarily during 0100–0300 UTC 21 August (1900–2100 CDT 20 August), leading to extreme local flooding. Dynamical aspects are investigated using the University of Wisconsin Nonhydrostratic Modeling System (UWNMS). Results are compared with available radiosonde, radar, total rainfall estimates, satellite infrared, and high-resolution European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. Using ECMWF analyses, the formation of the UTLS PV anomaly is traced to its origin a week earlier in a PV streamer over the west coast of North America. The rainfall maximum over southern Wisconsin was associated with this PV anomaly, whereby convection forming in the warm-upglide sector rotated cyclonically into the region. The quasi-stationarity of this rainfall feature was aided by a broad northeastward surge of inertially unstable convective outflow air into southeastern Wisconsin, which coincided with stalling of the eastward progression of the PV anomaly and its diversion into southern Wisconsin, extending heavy rainfall for several hours. Cessation of rainfall coincided with dilution of the PV maximum in less than an hour (2100–2200 CDT), associated with the arrival of negative PV in the upper troposphere. The region of negative PV was created when convection over Illinois transported air with low wind speed into northeastward shear. This feature is diagnosed using the convective momentum transport hypothesis. 
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