An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. Ionospheric data from European ...Space Agency's Swarm satellite constellation mission show prominent 6‐day variations in the dayside low‐latitude region at this time, which can be attributed to forcing from the middle atmosphere by the Rossby normal mode “quasi‐6‐day wave” (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard National Aeronautics and Space Administration's Aura satellite reveal a burst of global Q6DW activity in the mesosphere and lower thermosphere during the SSW, which is one of the strongest in the record. The Q6DW is apparently generated in the polar stratosphere at 30–40 km, where the atmosphere is unstable due to strong vertical wind shear connected with planetary wave breaking. These results suggest that an Antarctic SSW can lead to ionospheric variability through wave forcing from the middle atmosphere.
Plain Language Summary
A sudden stratospheric warming (SSW) is an extreme wintertime polar meteorological phenomenon occurring mostly over the Arctic region. Studies have shown that Arctic SSW can influence the entire atmosphere. In September 2019, a rare SSW event occurred in the Antarctic region, providing an opportunity to investigate its broader impact on the whole atmosphere. We present observations from the middle atmosphere and ionosphere during this event, noting unusually strong wave activity throughout this region. Our results suggest that an Antarctic SSW can have a significant impact on the whole atmosphere system similar to those due to Arctic events.
Key Points
An Antarctic sudden stratospheric warming (SSW) occurred in September 2019
Swarm observations reveal prominent 6‐day variations in the dayside low‐latitude ionosphere
A burst of quasi‐6‐day wave activity is observed in the middle atmosphere during the SSW
Abstract
Observed supercell updrafts consistently produce the fastest mid- to upper-tropospheric vertical velocities among all modes of convection. Two hypotheses for this feature are investigated. ...In the dynamic hypothesis, upward, largely rotationally driven pressure gradient accelerations enhance supercell updrafts relative to other forms of convection. In the thermodynamic hypothesis, supercell updrafts have more low-level inflow than ordinary updrafts because of the large vertical wind shear in supercell environments. This large inflow makes supercell updrafts wider than that of ordinary convection and less susceptible to the deleterious effects of entrainment-driven updraft core dilution on buoyancy. These hypotheses are tested using a large suite of idealized supercell simulations, wherein vertical shear, CAPE, and moisture are systematically varied. Consistent with the thermodynamic hypothesis, storms with the largest storm-relative flow have larger inflow, are wider, have larger buoyancy, and have faster updrafts. Analyses of the vertical momentum forcing along trajectories shows that maximum vertical velocities are often enhanced by dynamic pressure accelerations, but this enhancement is accompanied by larger downward buoyant pressure accelerations than in ordinary convection. Integrated buoyancy along parcel paths is therefore a strong constraint on maximum updraft speeds. Thus, through a combination of processes consistent with the dynamic and thermodynamic hypotheses, supercell updrafts are able to realize a larger percentage of CAPE than ordinary updrafts.
Madden‐Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical troposphere, has a significant impact on global weather and climate. Here we present that the ...year‐to‐year variation of the MJO activity shows significant changes with the quasi‐biennial oscillation (QBO) in the tropical stratosphere. Specifically, the boreal winter MJO amplitude, evaluated by various metrics, is typically stronger than normal during the QBO easterly phase at 50 hPa and weaker than normal during the QBO westerly phase at 50 hPa. This relationship, which is possibly mediated by the QBO‐related static stability and/or vertical wind shear changes in the tropical upper troposphere and lower stratosphere, is robust whether or not the activeness of the MJO or QBO is taken into account. This result suggests a new potential route from the stratosphere that regulates the organized tropical convection, helping to improve the prediction skill of the boreal winter MJO.
Key Points
MJO strengthens (weakens) during EQBO (WQBO)
The QBO‐MJO connection is significant during boreal wintertime only
The QBO‐MJO connection can be robustly found in various metrics
Many studies have investigated the impacts of aerosol on
the intensity and amount of precipitation, but few have been done so regarding
the impacts of aerosol on the start and peak times of ...precipitation. Using
the high-resolution precipitation, aerosol, and meteorological data in the warm
season of June–August from 2015 to 2020, this study investigates the
influence of aerosol on the start and peak times of precipitation over three
different regions, the North China Plain (NCP), the Yangtze River Delta
(YRD), and the Pearl River Delta (PRD). It shows that the period with the
highest frequency of precipitation start time, defined as the
frequent period (FP) of precipitation start time, is delayed and prolonged
by aerosols in NCP, contributing to the similar durations of precipitation
in NCP, YRD, and PRD. This study also shows that different types of aerosol
(absorbing versus scattering) have caused different influences on the start
and peak times of precipitation over the three study regions. The
precipitation start time is 3 h advanced in NCP but 2 h delayed in
PRD by aerosols during precipitation FP and shows no response to aerosol in
YRD. Compared to stratiform precipitation, the convective precipitation is
more sensitive to aerosol. The start and peak times of convective
precipitation show similar responses to aerosols. This study further shows
that the aerosol impacts on precipitation can vary with meteorological
conditions. Humidity is beneficial to precipitation, which can advance the
precipitation start and peak times and prolong the precipitation duration
time. Correspondingly, the impacts of aerosol on start time of precipitation
are significant under low humidity or weak low tropospheric stability
conditions. The impacts of vertical wind shear (WS) on the start and peak
times of precipitation are contrary to that of aerosols, resulting in the
fact that WS inhibits the aerosol effects on precipitation.
It has been revealed that the vertical wind shear (VWS) can result in the rainfall asymmetry of tropical cyclone (TC), where the VWS is conventionally defined as the environmental horizontal winds ...between 200‐ and 850‐hPa levels within a certain area from the TC centre. The rainfall maximum is generally displaced downshear to downshear‐left side in the Northern Hemisphere or downshear to downshear‐right side in the Southern Hemisphere. Here, we revisit the rainfall asymmetries of TCs over the western North Pacific using the state‐of‐art Global Precipitation Measurement data. Observations pose that using the conventional VWS to interpret the TC rainfall asymmetry is inopportune. The conventional VWS can only largely grasp the rainfall asymmetry in the inner core of TC, but not in the outer core. We, therefore, explicitly propose a new VWS, namely the effective VWS, to fill this gap. The effective shear layer responsible for the rainfall asymmetry is a function of TC intensity and core region. In addition, we suggest that the angle difference between the effective shears for the inner‐core and outer‐core rainfall asymmetries can help to comprehend the spirality of the rainfall asymmetries that are, the fashions, or patterns of the rainbands. Meanwhile, the moisture flux convergence and storm motion show little effect on the rainfall asymmetry. This study advances the understanding of TC rainfall asymmetry and provides scientific support for disaster prediction, prevention, and mitigation.
Using the conventional vertical wind shear (VWS) to interpret the tropical cyclone (TC) rainfall asymmetry is inopportune.
An effective VWS is proposed where it is a function of TC intensity and core region.
The angle difference between the effective shears for the inner‐core and outer‐core rainfall asymmetries can help to comprehend the fashions of the rainfall asymmetries.
This study finds a significant positive correlation between the Pacific meridional mode (PMM) index and the frequency of intense tropical cyclones (TCs) over the western North Pacific (WNP) during ...the peak TC season (June–November). The PMM influences the occurrence of intense TCs mainly by modulating large-scale dynamical conditions over the main development region. During the positive PMM phase, anomalous off-equatorial heating in the eastern Pacific induces anomalous low-level westerlies (and cyclonic flow) and upper-level easterlies (and anticyclonic flow) over a large portion of the main development region through a Matsuno–Gill-type Rossby wave response. The resulting weaker vertical wind shear and larger low-level relative vorticity favor the genesis of intense TCs over the southeastern part of the WNP and their subsequent intensification over the main development region. The PMM index would therefore be a valuable predictor for the frequency of intense TCs over the WNP.
Several of the best track datasets reveal the earlier end of the tropical cyclone (TC) season over the main WNP (the east of the Philippine Islands) from 1980 to 2019, especially by the date of the ...upper decile of the storm days (constructed by adding one to each date if any TC exists). The enhanced vertical wind shear (VWS) during the late season (October and November) is consistent with this earlier end of the TC season. In October, the enhanced VWS is the result of the increased zonal and meridional shears, which are affected by the earlier circulation transition of the East Asian summer monsoon recessed and winter monsoon onset during late October. During November, the zonal wind shear dominates the enhanced VWS, which vertically is the opposite trend of air temperature below and above the tropopause and the meridionally opposite trend of air temperature in the north and south of 30°N. The opposite air temperature trends induce the thermal wind balance, which causes enhanced westerlies in the mid-latitudes and strong zonal shear. Three pathways increase VWS in the late season over the WNP, leading to the earlier end of the TC season. The strong impact of VWS limits TC formation, although the ambient warming underlying the surface (supports a broader and stronger potential intensity). The effect of the thermodynamical parameter, depending on relative humidity at the low and middle troposphere, is uncertain because of the inconsistent and weak trends in October between ERA5 and MERRA-2, but they both become unfavorable to TC genesis during November. The practically important meaning of the VWS for the TC season provides a possibility for future projections of the TC season.
The wind shear theory is widely accepted as an explanation for the formation of a sporadic E (Es) layer, but the direct comparison of Es with the local wind shear has been limited due to the lack of ...neutral wind measurements. This study examines the role of the vertical wind shear for Es, using signal‐to‐noise ratio profiles from COSMIC‐2 radio occultation measurements and concurrent measurements of neutral wind profiles from the Ionospheric Connection Explorer. It is observed that the Es occurrence rate and average S4 index are correlated with the negative vertical shear of the eastward wind, providing observational support for the wind shear theory. Es can be observed even when the vertical wind shear is positive, which is interpreted as metallic ion layers generated at an earlier time.
Plain Language Summary
Sporadic E (Es) is anomalous radio propagation resulting from intense clouds of ionization at heights of the E‐region ionosphere (90–120 km). The formation of an Es layer is generally attributed to the vertical wind shear, which can move metallic ions in the vertical direction by the Lorentz force. According to the wind shear theory, a negative shear of the eastward wind is effective in converging the metallic ions into a thin layer to produce Es. Although previous observations and modeling studies have supported the theory to various degrees, the direct comparison of Es with the vertical wind shear has been limited due to sparse observations of neutral winds at E‐region heights. Neutral wind profiles from the Ionospheric Connection Explorer mission, together with Es data from COSMIC‐2 radio occultation measurements, provide an opportunity to fill this knowledge gap. Direct comparisons of these measurements reveal that the Es occurrence rate is higher and lower for larger negative and positive wind shears, respectively, providing observational evidence for the wind shear theory.
Key Points
Conjunction observations of sporadic E (Es) from COSMIC‐2 and neutral wind profiles from Ionospheric Connection Explorer/Michelson Interferometer for Global High‐Resolution Thermospheric Imaging are analyzed
Es occurrence rate correlates with the negative vertical shear of eastward wind, providing observational evidence for the wind shear theory
Es can be observed even when the vertical shear of the local eastward wind is positive
A recent study showed that the global average latitude where tropical cyclones achieve their lifetime-maximum intensity has been migrating poleward at a rate of about one-half degree of latitude per ...decade over the last 30 years in each hemisphere. However, it does not answer a critical question: is the poleward migration of tropical cyclone lifetime-maximum intensity associated with a poleward migration of tropical cyclone genesis? In this study we will examine this question. First we analyze changes in the environmental variables associated with tropical cyclone genesis, namely entropy deficit, potential intensity, vertical wind shear, vorticity, skin temperature and specific humidity at 500 hPa in reanalysis datasets between 1980 and 2013. Then, a selection of these variables is combined into two tropical cyclone genesis indices that empirically relate tropical cyclone genesis to large-scale variables. We find a shift toward greater (smaller) average potential number of genesis at higher (lower) latitudes over most regions of the Pacific Ocean, which is consistent with a migration of tropical cyclone genesis towards higher latitudes. We then examine the global best track archive and find coherent and significant poleward shifts in mean genesis position over the Pacific Ocean basins.
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