Upper-tropospheric Wave Research Articles

Southern Hemispheric jet swing linked to Arctic stratospheric polar vortex

Our study reframes our understanding of stratosphere–troposphere interactions, traditionally thought to be confined within individual hemispheres, by introducing a novel cross-hemispheric link. We demonstrate that strong boreal winter Arctic stratospheric polar vortex (APV) boosts the transmission of upper tropospheric waves from Northern Hemisphere’s mid-high latitudes to the equator. Facilitated by the tropical central and eastern Pacific’s ‘westerly bridge’, these waves reach Southern Hemisphere’s mid-high latitudes. The entire process shows a ‘semicircular road’. Waves reaching the Southern Hemisphere affect the circulation through wave-flow interaction, causing a southward swing of the Southern Hemispheric westerly jet center. This displacement weakens the subtropical jet and strengthens the polar jet, resulting in increased subtropical precipitation and decreased mid-latitude precipitation in the Southern Hemisphere during austral summer. Correspondingly, a weak APV may lead to the opposite result. Our findings underscore APV’s broader impact on the tropospheric atmosphere, extending beyond prior knowledge.

A climatological-dynamical analysis of tropopause folds over Southwest Asia in the period of 1989–2018

Tropopause folding is believed to be connected to the development of cyclonic systems in midlatitudes as well as the larger-scale, lower-frequency processes in the lower latitudes. To investigate such connections, a statistical and dynamical analysis of tropopause folds occurred in Southwest Asia is carried out for the period of 1989–2018. Tropopause folds are identified using the Era-Interim reanalysis dataset from the European Centre for Medium-Range Weather Forecasts. The spatial–temporal distributions of tropopause folding show that the frequency of folding is higher in the winter hemisphere than in the summer hemisphere and greater values of the frequency are found mostly within an elongated zonal band which is situated between 20 and 40 degrees latitude. Southwest Asia also has strong positive anomalous values of tropopause folding frequency and achieves the values more than five times the average of the Northern Hemisphere in the summer, especially in the northern latitudes of this region including Afghanistan, Iran and the eastern Mediterranean. This can be attributed mainly to the formation of summer monsoon in the subtropical latitudes of the Indian Ocean. In this regard, the examination of the effects of El Niño and La Niña on the spatial–temporal distributions of tropopause folding reveals that the values of 3-month running average of tropopause folding frequency coincident with La Niña tend to be larger than those with El Nino in both the hemispheres and Southwest Asia. This result is consistent with the previous findings on the tendency for higher occurrence of anticyclonic (cyclonic) baroclinic wave life cycles during La Niña (El Niño). Dynamically, the folds occurred in the study area are associated with the formation of waves at 500 hPa level and subsequently the creation of surface cyclones. In areas of high frequency of folding events, strong meridional shear of the zonal winds and jet stream amplification are observed at 200 hPa level. The analysis of wave activity flux and its divergence for the selected month of June 2015 shows two regions of wave propagation, one with a wave-packet like signature of flux divergence/convergence centers in the upper troposphere and one over the lower troposphere. The upper-tropospheric wave packet ends with an area of flux convergence related with the receipt and decay of Rossby waves, while in the lower troposphere, a flux divergence region as a source of stationary waves associated with the Indian monsoon activity is identified.

Future changes in North Atlantic winter cyclones in CESM-LE – Part 1: Cyclone intensity, potential vorticity anomalies, and horizontal wind speed

Abstract. Strong low-level winds associated with extratropical cyclones can have substantial impacts on society. The wind intensity and the spatial distribution of wind maxima may change in a warming climate; however, the involved changes in cyclone structure and dynamics are not entirely clear. Here, such structural changes of strong North Atlantic cyclones in a warmer climate close to the end of the current century are investigated with storm-relative composites based on Community Earth System Model Large Ensemble (CESM-LE) simulations. Furthermore, a piecewise potential vorticity inversion is applied to associate such changes in low-level winds to changes in potential vorticity (PV) anomalies at different levels. Projected changes in cyclone intensity are generally rather small. However, using cyclone-relative composites, we identify an extended wind footprint southeast of the center of strong cyclones, where the wind speed tends to intensify in a warmer climate. Both an amplified low-level PV anomaly driven by enhanced diabatic heating and a dipole change in upper-level PV anomalies contribute to this wind intensification. On the contrary, wind changes associated with lower- and upper-level PV anomalies mostly compensate for each other upstream of the cyclone center. Wind changes at upper levels are dominated by changes in upper-level PV anomalies and the background flow. Altogether, our results indicate that a complex interaction of enhanced diabatic heating and altered non-linear upper-tropospheric wave dynamics shape future changes in near-surface winds in North Atlantic cyclones.

The 2021 Western North American Heatwave and Its Subseasonal Predictions

AbstractRecord‐breaking above‐normal temperatures were observed across western North America in June–July 2021. In this study, our ability to predict this heatwave 2–3 weeks in advance is assessed based on 10 Subseasonal‐to‐Seasonal prediction models. It is found that the above normal temperature in Western Canada during June 28–July 4 was predicted by most models as early as June 10. However, for the forecasts initialized earlier than June 17, not a single ensemble member of all the models is able to capture the magnitude of the observed temperature anomaly. We identify two important processes: an upper tropospheric wave train associated with the boreal summer intraseasonal oscillation in Southeast Asia and an anomalous North Pacific atmospheric river leading to high moisture conditions. Most models are able to predict the wave train across the North Pacific. A realistic representation of moisture transport and its pattern appears crucial for the extended‐range forecast of this heatwave.

The aftermath of convective events near Jupiter’s fastest prograde jet: Implications for clouds, dynamics and vertical wind shear

The 24° N jet borders the North Tropical Belt and North Tropical Zone, and is the fastest prograde jet on Jupiter, reaching speeds above 170m∕s. In this region, observations have shown several periodic convective plumes, likely from latent heat release from water condensation, which affect the cloud and zonal wind structure of the jet. We model this region with the Explicit Planetary hybrid-Isentropic Coordinate model using its active microphysics scheme to study the phenomonology of water and ammonia clouds within the jet region. On perturbing the atmosphere, we find that an upper tropospheric wave develops that directly influences the cloud structure within the jet. This wave travels at a net (eastward) phase speed of ∼75m∕s in our model, and leads to periodic chevron-shaped features in the ammonia cloud deck. These features travel with the wave speed, and are subsequently much slower than the zonal wind at the cloud deck. This unique morphology of the ammonia cloud, and the slower drift rate, were both observed following the convective outbreak in this region in 2016 and 2020. We find that an upper level circulation is responsible for these cloud features in the aftermath of the convective outbursts. The comparatively slower observed drift rates of these features, relative to the wind speed of the jet, provides constraints on the vertical wind shear above the cloud tops, and we suggest that wind velocities determined from cloud tracking should correspond to a different altitude compared to the 680 hPa pressure level. We also diagnose the convective potential of the atmosphere due to water condensation, and find that it is strongly coupled to the wave.

A Rossby wave breaking-induced enhancement in the tropospheric ozone over the Central Himalayan region

The high-altitude regions in the Himalayas are prone to high ozone concentrations frequently resulting from diverse dynamical and transport mechanisms. Here, we report an unusual enhancement in the surface and tropospheric ozone concentrations over the central Himalayan region from ground-based and space-borne measurements in the month of December 2010. The surface ozone levels (~80 ppbv) on 18–19 December 2010 is observed to be two-fold higher relative to the seasonal average (December-January-February) of about 40–50 ppbv in the central Himalayan region. The space-borne measurements from Tropospheric Emission Spectrometer and Ozone Monitoring Instrument onboard Aqua satellite also show higher values in the tropospheric column ozone over this region. The satellite observations indicate an increase in tropopause temperature of about 5 °C and decrease in tropopause altitude about 1 km during 18–19 December 2010 resulting in the occurrence of tropopause fold facilitating the stratospheric-tropospheric exchange processes over the study region. The plausible reason for the occurrence of tropopause fold and subsequent enhancement of tropospheric and surface ozone is found to be associated with the breaking Rossby waves in the upper troposphere. The wave breaking leads to the advection of high-PV (potential vorticity) air, with magnitudes of about 3–4 PVU, towards the central Himalayan region from high-latitudes. The vertical component of PV advection also shows a deep stratospheric intrusion of high-PV air into the troposphere. The isentropic transport of ozone across the folding tropopause due to the wave breaking is clearly depicted from the satellite and reanalysis datasets. Therefore, the present study has strong implications of upper tropospheric wave dynamics to the tropospheric and surface ozone over the Himalayan regions having complex topography.

Intraseasonal Periodicity in the Southern Hemisphere Circulation on Regional Spatial Scales

Abstract Wave activity in the Southern Hemisphere extratropical atmosphere exhibits robust periodicity on time scales of ~20–25 days. Previous studies have demonstrated the robustness of the periodicity in hemispheric averages of various eddy quantities. Here the authors explore the signature of the periodicity on regional spatial scales. Intraseasonal periodicity in the Southern Hemisphere circulation derives from out-of-phase anomalies in wave activity that form in association with extratropical wave packets as they propagate to the east. In the upper troposphere, the out-of-phase anomalies in wave activity form not along the path of extratropical wave packets, but in their wake. The out-of-phase anomalies in wave activity give rise to periodicity not only on hemispheric scales, but also on synoptic scales when the circulation is sampled along an eastward path between ~5 and 15 m s−1. It is argued that 1) periodicity in extratropical wave activity derives from two-way interactions between the heat fluxes and baroclinicity in the lower troposphere and 2) the unique longitude–time structure of the periodicity in upper-tropospheric wave activity derives from the contrasting eastward speeds of the source of the periodicity in the lower troposphere (~10 m s−1) and wave packets in the upper troposphere (~25 m s−1).

Barotropic and Baroclinic Eddy Feedbacks in the Midlatitude Jet Variability and Responses to Climate Change–Like Thermal Forcings

Abstract Studies have suggested that the persistence in the meridional vacillation of the midlatitude jet (i.e., annular mode time scale) in comprehensive climate models is related to the model biases in climatological jet latitude, with important implications for projections of future climates and midlatitude weather events. Through the use of the recently developed finite-amplitude wave activity formalism and feedback quantifying techniques, this paper has quantified the role of barotropic and baroclinic eddy feedbacks in annular mode time scales using an idealized dry atmospheric model. The eddy–mean flow interaction that characterizes the persistent anomalous state of the midlatitude jet depends on processes associated with the lower-tropospheric source of vertically propagating Rossby waves, baroclinic mechanisms, and processes associated with upper-tropospheric wave propagation and breaking, barotropic mechanisms. A variety of climate change–like thermal forcings are used to generate a range of meridional shifts in the midlatitude eddy-driven jet. The idealized model shows a reduction in annular mode time scale associated with an increase in jet latitude, similar to comprehensive climate models. This decrease in time scale can be attributed to a similar decrease in the strength of the barotropic eddy feedback, which, in the positive phase of the annular mode, is characterized by anomalous potential vorticity (PV) mixing on the equatorward flank of the climatological jet. The decrease in subtropical PV mixing is, in turn, associated with a stronger subtropical jet as the eddy-driven jet is more distant from the subtropics. These results highlight the importance of subtropical eddy–mean flow interactions for the persistence of an eddy-driven jet.

On the link between Barents‐Kara sea ice variability and European blocking

AbstractThis study examines the connection between the variability of sea ice concentration in the Barents and Kara (B‐K) seas and winter European weather on an intraseasonal time scale. Low sea ice regimes in autumn and early winter over the B‐K seas are shown to affect the strength and position of the polar vortex, and increase the frequency of blocking regimes over the Euro‐Atlantic sector in late winter. A hypothesis is presented on the mechanism that links sea ice over the B‐K seas and circulation regimes in the North Atlantic, and is investigated considering 34 years of European Centre for Medium‐Range Weather Forecasts reanalysis data. Four key steps have been identified, starting from a local response of the near‐surface fluxes and modification of the upper tropospheric wave pattern, to the stratospheric adjustment and the tropospheric response in the North Atlantic. The proposed mechanism explains the delayed, late winter response of the North Atlantic Oscillation to the late autumn sea ice reduction, which has been found both in observations and model experiments. It also provides valuable insights on how the reduction of Arctic sea ice can influence the position of the tropospheric jet in the Euro‐Atlantic sector.

Atmospheric responses to idealized urban land surface forcing in eastern China during the boreal spring

AbstractIn contrast to the impacts of anthropogenic aerosols and greenhouse gases, little is known about the impact of urban land surface forcing (ULSF) on large‐scale atmospheric circulation. This study explores atmospheric responses to idealized ULSF in eastern China during the boreal spring using the Community Atmosphere Model version 5.1 coupled with the Community Land Model version 4. Results show that the ULSF leads to an increased air temperature in northern China both near the surface and in the lower troposphere. Related to a strong thermal feedback loop, a middle‐upper tropospheric cooling is found in eastern China while a relatively strong warming occurs in the middle‐high latitudes, which acts to enhance the meridional temperature gradient to the north of the source region and then shifts the East Asian subtropical jet stream (EASJ) southward. A weakened southwesterly in the lower troposphere in southern China slows down moisture transportation to northern China, and the southward shifted EASJ induces strong anomalous sinking motion to the north of the Yangtze River Valley (YRV). The associated changes in moisture and vertical airflow result in moisture divergence along the YRV and convergence in southern China. Thus, the spring rain belt is shifted southward, as characterized by below‐normal rainfall extending from the Huai River Valley to South Korea and above‐normal rainfall from southern China to the south coast of Japan. In addition, analysis of the upper tropospheric wave activity signifies that large‐scale atmospheric responses due to the ULSF also exert an important influence on local climate.

Investigation of double tropopause spatial and temporal global variability utilizing High Resolution Dynamics Limb Sounder temperature observations

This study examines the seasonal variation of the double tropopause (DT) using data from the High Resolution Dynamics Limb Sounder. The combination of high data density and high vertical resolution available with this satellite instrument allows for the detailed analysis of fine‐scale structures such as the DT. The spatial distribution of DT frequency is examined for all seasons from 2005 to 2007. Global analysis of this distribution reveals that DTs have a strong tendency to follow the zonal wind pattern and are present all year over the Andes. Moreover, during Northern Hemisphere winter/spring, there is a 15% decrease in the DT frequency over the eastern Pacific and western Atlantic, two regions of wave breaking produced from a weakening of the zonal flow. Significant DT thickness values are also present over these regions, but are found to extend further poleward than the corresponding frequency pattern. A time series of DT frequencies is analyzed and shows an increase in daily frequencies during 2006 that highlights the interannual variability of this thermal structure. For the first time, DT duration is investigated in the extratropics utilizing a Hovmöller diagram of DT frequency. This representation highlights two preferred regions of formation, the Pacific and Atlantic. The slope of the feature gives it a speed of 18 m/s in the Northern Hemisphere and a slightly faster speed for the Southern Hemisphere. These speeds and the corresponding structures highlight a potential connection between upper tropospheric waves and DTs.

How Does the Vertical Profile of Baroclinicity Affect the Wave Instability?

Abstract The growth rate of baroclinic instability waves is generalized in terms of wave–mean flow interactions, with an emphasis on the influence of the vertical profile of baroclinicity. The wave energy is converted from the zonal mean kinetic energy and the growth rate is proportional to the mean zonal flow difference between the Eliassen–Palm (E-P) flux convergence and divergence areas. Mass-weighted isentropic zonal means facilitate the expression of the lower boundary conditions for the mass streamfunctions and E-P flux. For Eady waves, intersections of isentropes with lower/upper boundaries induce the E-P flux divergence/convergence. The growth rate is proportional to the mean zonal flow difference between the two boundaries, indicating that baroclinicity at each level contributes evenly to the instability. The reduced zonal mean kinetic energy is compensated by a conversion from the zonal mean available potential energy. Aquaplanet experiments are carried out to investigate the actual characteristics of baroclinic instability waves. The wave activity is shown to be sensitive to the upper-tropospheric baroclinicity, though it may be most sensitive to baroclinicity near 800 hPa, which is the maximal level of the E-P flux. The local wave energy generation rate suggests that the increased upper-tropospheric zonal flow directly enhances the upper-tropospheric wave energy at the midlatitudes. Note that the actual baroclinic instability waves accompany a considerable amount of the equatorward E-P flux, which causes extinction of wave energy in the subtropical upper troposphere.

Convectively Coupled Equatorial Waves in High-Resolution Hadley Centre Climate Models

Abstract A methodology for diagnosing convectively coupled equatorial waves is applied to output from two high-resolution versions of atmospheric models, the Hadley Centre Atmospheric Model, version 3 (HadAM3), and the new Hadley Centre Global Atmospheric Model, version 1 (HadGAM1), which have fundamental differences in dynamical formulation. Variability, horizontal and vertical structures, and propagation characteristics of tropical convection and equatorial waves, along with their coupled behavior in the models, are examined and evaluated against a previous comprehensive study of observed convectively coupled equatorial waves using the 15-yr ECMWF Re-Analysis (ERA-15) and satellite observed data. The extent to which the models are able to represent the coupled waves found in real atmospheric observations is investigated. It is shown that, in general, the models perform well for equatorial waves coupled with off-equatorial convection. However, they perform poorly for waves coupled with equatorial convection. Convection in both models contains much-reduced variance in equatorial regions, but reasonable off-equatorial variance. The models fail to simulate coupling of the waves with equatorial convection and the tendency for equatorial convection to appear in the region of wave-enhanced near-surface westerlies. In addition, the simulated Kelvin wave and its associated convection generally tend to have lower frequency and slower phase speed than that observed. The models are also not able to capture the observed vertical tilt structure and signatures of energy conversion in the Kelvin wave, particularly in HadAM3. On the other hand, models perform better in simulating westward-moving waves coupled with off-equatorial convection, in terms of horizontal and vertical structures, zonal propagation, and energy conversion signals. In most cases both models fail to simulate well a key picture emerging from the observations, that some wave modes in the lower troposphere can act as a forcing agent for equatorial convection, and that the upper-tropospheric waves generally appear to be forced by the convection both on and off the equator.

Teleconnection mechanisms of northeast Brazil droughts: modeling and empirical evidence

Targeted numerical modelling experimaents are conducted to complement the previous empirical diagnostics of circulation mechanisms leading from sea surface temperature (SST) departures in the equatorial Pacific in January to anomalies in the March-April rainy season of Brazil's Nordeste. A weak interhemispheric northward directed SST gradient in the Atlantic favors a more southerly position of the hydrostatically controlled low pressure trough, embedded in which is the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste. In addition, anomalously warm waters in the equatorial Pacific in January tend to be followed by Nordeste drought. The underlying chain of causalities has been explored by empirical diagnostics and numerical modelling. During El Nino years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the tropical North Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. This leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, weaker North Atlantic tradewinds, an anomalously far northerly ITCZ position and thus Nordeste drought. The previous empirical diagnostics are overall supported by the modelling experiments.

Intense Rainfall Events Affecting the La Plata Basin

Abstract The circulation features associated with intense precipitation events over the La Plata Basin (LPB) during the austral summers of 2001/02 and 2002/03 are investigated using the Eta Model runs generated at the University of Maryland. Based on the main mode of variability over LPB, two regions were selected: (i) the region of Brazil that is at the core of the South American summer monsoon system (SAMS) and (ii) the central region of LPB in southeastern South America (SESA). First, a comparison between the 24-h total precipitation in the Eta Model and the 24-h observed precipitation was made. Results show that the Eta Model captures well the temporal variability of precipitation events in both regions, although a positive bias is noticed over SAMS. Likewise, the model reproduces the distribution of precipitation rate over SESA, but not over SAMS. Nevertheless, the distribution of the moisture flux convergence intensity, which represents the dynamical forcing, is closer in shape to the observed precipitation distribution, suggesting that the model can be a useful tool in identifying the forcing for heavy precipitation events over both regions. Composites of atmospheric and surface variables were constructed for intense precipitation events during austral summer over both regions. Intense rainfall over the central La Plata Basin (SESA) is linked to an amplified upper-tropospheric midlatitude wave pattern in which rainfall occurs just east of an enhanced cyclonic circulation. Accompanying this circulation pattern, an enhanced low-level jet (LLJ) transports warm, moist air from the Amazon toward the region, contributing to an increase in the thermal contrast over SESA. The combined patterns of thermal and dynamical variables suggest that large-scale systems, like frontal systems, are important in producing intense rainfall events. The SAMS region events have a similar upper-level structure as in SESA, but they are longer lived. In this case, the moisture fluxes are determined by an eastward shift of the LLJ, but also directly from the Amazon Basin to the north. As expected, precipitation events produce large increases of simulated runoff. The largest impact is on the SESA region, affecting the streamflow of the Paraná, Paraguay, and Uruguay, the three main rivers of the LPB.

Circulation and teleconnection mechanisms of Northeast Brazil droughts

The Northern Nordeste of Brazil has its short rainy season narrowly concentrated around March–April, when the interhemispheric southward gradient of sea surface temperature (SST) is weakest and the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste, reaches its southernmost position in the course of the year. The recurrent Secas (droughts) have a severe socio-economic impact in this semi-arid region. In drought years, the pre-season (October–January) rainfall is scarce, the interhemispheric SST gradient weakened and the basin-wide southerly (northerly) wind component enhanced (reduced), all manifestations of an anomalously far northward ITCZ position. Apart from this ensemble of Atlantic indicators, the Secas also tend to be preceded by anomalously warm equatorial Pacific waters in January. During El Niño years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the northern tropical Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. The altered vertical motion leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, and thus weaker North Atlantic tradewinds. The concomitant reduction of evaporation and wind stirring allows for warmer surface waters in the tropical North Atlantic and thus steeper interhemispheric meridional thermal gradient. Consequently, the ITCZ stays anomalously far North and the Nordeste rainy season becomes deficient.

Sensitivity of last glacial maximum climate to sea ice conditions in the Nordic Seas

Published reconstructions of last glacial maximum (LGM) sea surface temperatures and sea ice extent differ significantly. We here test the sensitivity of simulated North Atlantic climates to two different reconstructions by using these reconstructions as boundary conditions for model experiments. An atmospheric general circulation model has been used to perform two simulations of the (LGM) and a modern-day control simulation. Standard (CLIMAP) reconstructions of sea ice and sea surface temperatures have been used for the first simulation, and a set of new reconstructions in the Nordic Seas/Northern Atlantic have been used for the second experiment. The new reconstruction is based on 158 core samples, and represents ice-free conditions during summer in the Nordic Seas, with accordingly warmer sea surface temperatures and less extensive sea ice during winter as well. The simulated glacial climate is globally 5.7 K colder than modern day, with the largest changes at mid and high latitudes. Due to more intense Hadley circulation, the precipitation at lower latitudes has increased in the simulations of the LGM. Relative to the simulation with the standard CLIMAP reconstructions, reduction of the sea ice in the North Atlantic gives positive local responses in temperature, precipitation and reduction of the sea level pressure. Only very weak signatures of the wintertime Icelandic Low occur when the standard CLIMAP sea surface temperature reconstruction is used as the lower boundary condition in LGM. With reduced sea ice conditions in the Nordic Seas, the Icelandic Low becomes more intense and closer to its present structure. This indicates that thermal forcing is an important factor in determining the strength and position of the Icelandic Low. The Arctic Oscillation is the most dominant large scale variability feature on the Northern Hemisphere in modern day winter climate. In the simulation of the LGM with extensive sea ice this pattern is significantly changed and represents no systematic large scale variability over the North Atlantic. Reduction of the North Atlantic sea ice extent leads to stronger variability in monthly mean sea level pressure in winter. The synoptic variability appears at a lower level in the simulation when standard reconstructions of the sea surface in the LGM are used. A closer inspection of storm tracks in this model experiment shows that that the synoptic lows follow a narrow band along the ice edge during winter. The trajectories of synoptic lows are not constrained to the sea ice edge to the same degree when the sea ice extent is reduced. Seasonally open waters in the Nordic Seas in the new reconstruction apparently act as a moisture source, consistent with the current understanding of the rapid growth of the Fennoscandian and Barents Ice Sheets, during the LGM. The signal from the intensified thermal forcing in the North Atlantic in Boreal winter is carried zonally by upper tropospheric waves, and thus generates non-local responses to the changed sea ice cover.

Mechanisms for the Development of Locally Low-Dimensional Atmospheric Dynamics

Abstract The complexity of atmospheric instabilities is investigated by a combination of numerical experiments and diagnostic tools that do not require the assumption of linear error dynamics. These tools include the well-established analysis of the local energetics of the atmospheric flow and the recently introduced ensemble dimension (E dimension). The E dimension is a local measure that varies in both space and time and quantifies the distribution of the variance between phase space directions for an ensemble of nonlinear model solutions over a geographically localized region. The E dimension is maximal, that is, equal to the number of ensemble members (k), when the variance is equally distributed between k phase space directions. The more unevenly distributed the variance, the lower the E dimension. Numerical experiments with the state-of-the-art operational Global Forecast System (GFS) of the National Centers for Environmental Prediction (NCEP) at a reduced resolution are carried out to investigate the spatiotemporal evolution of the E dimension. This evolution is characterized by an initial transient phase in which coherent regions of low dimensionality develop through a rapid local decay of the E dimension. The typical duration of the transient is between 12 and 48 h depending on the flow; after the initial transient, the E dimension gradually increases with time. The main goal of this study is to identify processes that contribute to transient local low-dimensional behavior. Case studies are presented to show that local baroclinic and barotropic instabilities, downstream development of upper-tropospheric wave packets, phase shifts of finite amplitude waves, anticyclonic wave breaking, and some combinations of these processes can all play crucial roles in lowering the E dimension. The practical implication of the results is that a wide range of synoptic-scale weather events may exist whose prediction can be significantly improved in the short and early medium range by enhancing the prediction of only a few local phase space directions. This potential is demonstrated by a reexamination of the targeted weather observations missions from the 2000 Winter Storm Reconnaissance (WSR00) program.

Orographic Constraints on a Modeled Siberian Snow–Tropospheric–Stratospheric Teleconnection Pathway

Previous modeling studies have identified a teleconnection pathway linking observation-based early season Siberian snow perturbations to a modulation of the winter Arctic Oscillation (AO) mode. In this study, the key role of orography in producing this modeled teleconnection is explicitly investigated using numerical experiments analogous to the previous studies. The climatic response to the same snow perturbation is investigated under modified orographic barriers in southern and eastern Siberia. Reducing these barriers results in a weakening of the prevailing orographically forced region of stationary wave activity centered over Siberia, as well as the snow-forced upward wave flux anomaly that initiates the teleconnection. This diminished anomaly propagates upward, but does not extend into the stratosphere to weaken the polar vortex. Consequently, poleward refraction of upper-tropospheric waves and downward propagation of coupled wave‐mean flow anomalies, which ultimately produce the negative winter AO response, fail to develop. Thus, the mountains represent an orographic constraint on the snow‐AO teleconnection pathway. By reducing the orographic barrier, the snow-forced influx of wave energy remains in the troposphere and, instead, produces a hemispheric-scale equatorward wave refraction.

The Role of Gravity Waves in the Meso-.BETA.-Scale Cycle of Squall-Line Type Convective Systems

Observed or simulated mesoscale convective systems (MCSs) often show pulsations with time periods of a few hours. In this paper, results from a two-dimensional cloud model are used to determine the dynamical processes responsible for such self-modulating oscillations (meso-β-scale cycle, MBC) in squallline type convective systems. The simulated storms have weak cold pools and include notable pulsations with time periods of 3∼4 hours. These pulsations are manifested as periodic organizations of convective-scale cells into upsheartilted meso-β-scale cells (MCs). A rearward-propagating, dissipating MC and disturbances on the downshear side have the properties of vertically propagating gravity waves, with storm-relative time periods that are about 1.5 hour less than those of the MBC. Dry model simulations driven by prescribed thermal forcings show that the waves arise in response to the reduction of latent heating in the MC in its stationary stage. The evolution of storm structure following a convective burst is related to the phasing of the uppertropospheric wave disturbances relative to the lower-tropospheric disturbances. The storm exhibits multicellular structure so long as the downward and frontward acceleration phase of gravity waves exerts suppressing effects on convections behind the gust front. As the phase propagates to the rear, it in turn enhances the convergence at the terminus of the steady lower-tropospheric front-to-rear flow, aiding the cell reinvigoration there. The time period of MBC increases as MCs tilt more horizontally in their stationary growth stage, and as the storm-relative rear-to-front upper-tropospheric wind increases. The former effect increases the intrinsic period of excited waves, and the latter increases the storm-relative period of the waves by Doppler-shifting the phase speed. These results support the argument that gravity waves generated by tilted MCs are prominent in the dynamics of MBCs.