scholarly journals Large-scale drivers of the mistral wind: link to Rossby wave life cycles and seasonal variability

2021 ◽  
Vol 2 (3) ◽  
pp. 609-630
Author(s):  
Yonatan Givon ◽  
Douglas Keller Jr. ◽  
Vered Silverman ◽  
Romain Pennel ◽  
Philippe Drobinski ◽  
...  
Keyword(s):  
Rossby Wave ◽  
Clustering Algorithm ◽  
Life Cycles ◽  
Heat Fluxes ◽  
Extreme Weather Events ◽  
Turbulent Heat ◽  
Low Level ◽  
Lee Cyclogenesis ◽  
The Mediterranean ◽  
Upper Level

Abstract. The mistral is a northerly low-level jet blowing through the Rhône valley in southern France and down to the Gulf of Lion. It is co-located with the cold sector of a low-level lee cyclone in the Gulf of Genoa, behind an upper-level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the lower-tropospheric mistral and lee cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life cycle, from broad troughs to thin PV streamers to distinguished cutoffs. Each of these PV patterns exhibits a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident, and transitions between the clusters correspond to different Rossby-wave-breaking processes. This analysis provides a new perspective on the variability of the mistral and of the Genoa lee cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

scholarly journals Synoptic-scale drivers of the Mistral wind: link to Rossby wave life cycles and seasonal variability

2021 ◽  
Author(s):  
Yonatan Givon ◽  
Douglas Keller Jr. ◽  
Romain Pennel ◽  
Philippe Drobinski ◽  
Shira Raveh-Rubin
Keyword(s):  
Rossby Wave ◽  
Clustering Algorithm ◽  
Life Cycles ◽  
Extreme Weather Events ◽  
Turbulent Heat ◽  
Synoptic Scale ◽  
Low Level ◽  
Lee Cyclogenesis ◽  
The Mediterranean ◽  
Upper Level

Abstract. The mistral is a northerly low level jet blowing through the Rhône valley in southern France, and down to the Gulf of Lions. It is co-located with the cold sector of a low level lee-cyclone in the Gulf of Genoa, behind an upper level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the low-tropospheric mistral and lee-cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life-cycle, from broad troughs, thin PV streamers, to distinguished cut-offs. Each of these PV patterns exhibit a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident and transitions between the clusters correspond to different Rossby wave-breaking processes. This analysis provides a new perspective on the variability of the mistral, and of the Genoa lee-cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

scholarly journals The Role of Sea Surface Temperature Forcing in the Life-Cycle of Mediterranean Cyclones

2020 ◽  
Vol 12 (5) ◽  
pp. 825 ◽  
Author(s):  
Christos Stathopoulos ◽  
Platon Patlakas ◽  
Christos Tsalis ◽  
George Kallos
Keyword(s):  
Life Cycle ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Early Warning Systems ◽  
Life Cycles ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Mediterranean Cyclones ◽  
The Mediterranean ◽  
The Impact

Air–sea interface processes are highly associated with the evolution and intensity of marine-developed storms. Specifically, in the Mediterranean Sea, the air–ocean temperature deviations have a profound role during the several stages of Mediterranean cyclonic events. Subsequently, this enhances the need for better knowledge and representation of the sea surface temperature (SST). In this work, an analysis of the impact and uncertainty of the SST from different well-known datasets on the life-cycle of Mediterranean cyclones is attempted. Daily SST from the Real Time Global SST (RTG_SST) and hourly SST fields from the Operational SST and Sea Ice Ocean Analysis (OSTIA) and the NEMO ocean circulation model are implemented in the RAMS/ICLAMS-WAM coupled modeling system. For the needs of the study, the Mediterranean cyclones Trixi, Numa, and Zorbas were selected. Numerical experiments covered all stages of their life-cycles (five to seven days). Model results have been analyzed in terms of storm tracks and intensities, cyclonic structural characteristics, and derived heat fluxes. Remote sensing data from the Integrated Multi-satellitE Retrievals (IMERG) for Global Precipitation Measurements (GPM), Blended Sea Winds, and JASON altimetry missions were employed for a qualitative and quantitative comparison of modeled results in precipitation, maximum surface wind speed, and wave height. Spatiotemporal deviations in the SST forcing rather than significant differences in the maximum/minimum SST values, seem to mainly contribute to the differences between the model results. Considerable deviations emerged in the resulting heat fluxes, while the most important differences were found in precipitation exhibiting spatial and intensity variations reaching 100 mm. The employment of widely used products is shown to result in different outcomes and this point should be taken into consideration in forecasting and early warning systems.

Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby Wave Energy Dispersion*

2010 ◽  
Vol 138 (1) ◽  
pp. 42-54 ◽  
Author(s):  
Xuyang Ge ◽  
Tim Li ◽  
Melinda S. Peng
Keyword(s):  
Rossby Wave ◽  
Wave Energy ◽  
Large Scale ◽  
Wave Train ◽  
Energy Dispersion ◽  
Sensitivity Experiment ◽  
Low Level ◽  
Filtering Technique ◽  
Upper Level

Abstract The genesis of Typhoon Prapiroon (2000), in the western North Pacific, is simulated to understand the role of Rossby wave energy dispersion of a preexisting tropical cyclone (TC) in the subsequent genesis event. Two experiments are conducted. In the control experiment (CTL), the authors retain both the previous typhoon, Typhoon Bilis, and its wave train in the initial condition. In the sensitivity experiment (EXP), the circulation of Typhoon Bilis was removed based on a spatial filtering technique of Kurihara et al., while the wave train in the wake is kept. The comparison between these two numerical simulations demonstrates that the preexisting TC impacts the subsequent TC genesis through both a direct and an indirect process. The direct process is through the conventional barotropic Rossby wave energy dispersion, which enhances the low-level wave train, the boundary layer convergence, and the convection–circulation feedback. The indirect process is through the upper-level outflow jet. The asymmetric outflow jet induces a secondary circulation with a strong divergence tendency to the left-exit side of the outflow jet. The upper-level divergence boosts large-scale ascending motion and promotes favorable environmental conditions for a TC-scale vortex development. In addition, the outflow jet induces a well-organized cyclonic eddy angular momentum flux, which acts as a momentum forcing that enhances the upper-level outflow and low-level inflow and favors the growth of the new TC.

scholarly journals Structure and Mechanisms of the Southern Hemisphere Summertime Subtropical Anticyclones

2010 ◽  
Vol 23 (8) ◽  
pp. 2115-2130 ◽  
Author(s):  
Takafumi Miyasaka ◽  
Hisashi Nakamura
Keyword(s):  
Southern Hemisphere ◽  
Rossby Wave ◽  
Sensible Heat Flux ◽  
Wave Activity ◽  
Convective Heating ◽  
Surface Cooling ◽  
Near Surface ◽  
Low Level ◽  
Zonal Wavenumber ◽  
Upper Level

Abstract The three-dimensional structure and dynamics of the climatological-mean summertime subtropical anticyclones in the Southern Hemisphere (SH) are investigated. As in the Northern Hemisphere (NH), each of the surface subtropical anticyclones over the South Pacific, South Atlantic, and South Indian Oceans is accompanied by a meridional vorticity dipole aloft, exhibiting barotropic and baroclinic structures in its poleward and equatorward portions, respectively, in a manner that is dynamically consistent with the observed midtropospheric subsidence. Their dynamics are also similar to their NH counterpart. It is demonstrated through the numerical experiments presented here that each of the SH surface anticyclones observed over the relatively cool eastern oceans can be reproduced as a response to a local near-surface cooling–heating couplet. The cooling is mainly due to radiative cooling associated with low-level maritime clouds, and the heating to the east is due to sensible heat flux over the dry, heated continental surface. The low-level clouds act to maintain the coolness of the underlying ocean surface, which is also maintained by the alongshore surface southerlies. As in the NH, the presence of a local atmosphere–ocean–land feedback loop is thus suggested, in which the summertime subtropical anticyclones and continental cyclones to their east are involved. Both the model experiments conducted here and the diagnosed upward flux of Rossby wave activity suggest that, in addition to continental deep convective heating, the land–sea heating–cooling contrasts across the west coasts of the three continents can contribute to the formation of the summertime upper-level planetary wave pattern observed in the entire subtropical SH, characterized by the zonal wavenumber-3 component. Though rather subtle, there are some interhemispheric differences in the summertime subtropical anticyclones, including their smaller magnitudes in the SH and the stronger equatorward propagation of upper-level Rossby wave activity emanating from the SH surface anticyclones.

The Effects of Midlatitude Waves over and around the Tibetan Plateau on Submonthly Variability of the East Asian Summer Monsoon

2009 ◽  
Vol 137 (7) ◽  
pp. 2286-2304 ◽  
Author(s):  
Hatsuki Fujinami ◽  
Tetsuzo Yasunari
Keyword(s):  
Tibetan Plateau ◽  
Rossby Wave ◽  
Large Scale ◽  
Southern China ◽  
Phase Speed ◽  
The Tibetan Plateau ◽  
Low Level ◽  
Zonal Wavenumber ◽  
Upper Level ◽  
Southward Migration

Convective variability at submonthly time scales (7–25 days) over the Yangtze and Huaihe River basins (YHRBs) and associated large-scale atmospheric circulation during the mei-yu season were examined using interpolated outgoing longwave radiation (OLR) and NCEP–NCAR reanalysis data for 12 yr having active submonthly convective fluctuation over the YHRBs within the period 1979–2004. Correlations between convection anomalies over the YHRBs and upper-level streamfunction anomalies at every grid point show two contrasting patterns. One pattern exhibits high correlations along the northern to eastern peripheries of the Tibetan Plateau (defined as the NET pattern), whereas the other has high correlations across the Tibetan Plateau (defined as the AT pattern). Composite analysis of the NET pattern shows slow southward migration of convection anomalies from the northeastern periphery of the Tibetan Plateau to southern China, in relation to southward migration of the mei-yu front caused by simultaneous amplification of upper- and low-level waves north of the YHRBs. In the AT pattern, convection anomalies migrate eastward from the western Tibetan Plateau to the YHRBs. A low-level vortex is created at the lee of the plateau by eastward-moving upper-level wave packets and associated convection from the plateau. Rossby wave trains along the Asian jet characterize the upper-level circulation anomalies in the two patterns. The basic state of the Asian jet during the mei-yu season differs between the two patterns, especially around the Tibetan Plateau. The Asian jet has a northward arclike structure in NET years, while a zonal jet dominates in AT years. These differences could alter the Rossby wave train propagation route. Furthermore, the larger zonal wavenumber of AT waves (∼7–8) than of NET waves (∼6) means faster zonal phase speed relative to the ground in the AT pattern than in the NET pattern. These differences likely explain the meridional amplification of waves north of the YHRBs in the NET pattern and the eastward wave movement across the plateau in the AT pattern.

Numerical Simulation of Baroclinic Waves with a Parameterized Boundary Layer

2007 ◽  
Vol 64 (12) ◽  
pp. 4383-4399 ◽  
Author(s):  
R. S. Plant ◽  
S. E. Belcher
Keyword(s):  
Boundary Layer ◽  
Lower Troposphere ◽  
Conveyor Belt ◽  
Static Stability ◽  
Turbulent Fluxes ◽  
Life Cycles ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Ekman Pumping ◽  
Basic States

Abstract A dry three-dimensional baroclinic life cycle model is used to investigate the role of turbulent fluxes of heat and momentum within the boundary layer on midlatitude cyclones. Simulations are performed of life cycles for two basic states: with and without turbulent fluxes. The different basic states produce cyclones with contrasting frontal and mesoscale flow structures. The analysis focuses on the generation of potential vorticity (PV) in the boundary layer and its subsequent transport into the free troposphere. The dynamic mechanism through which friction mitigates a barotropic vortex is that of Ekman pumping. This has often been assumed to also be the dominant mechanism for baroclinic developments. The PV framework highlights an additional, baroclinic mechanism. Positive PV is generated baroclinically due to friction to the northeast of a surface low and is transported out of the boundary layer by a cyclonic conveyor belt flow. The result is an anomaly of increased static stability in the lower troposphere, which restricts the growth of the baroclinic wave. The reduced coupling between lower and upper levels can be sufficient to change the character of the upper-level evolution of the mature wave. The basic features of the baroclinic damping mechanism are robust for different frontal structures, with and without turbulent heat fluxes, and for the range of surface roughness found over the oceans.

scholarly journals Life Cycle Study of a Diabatic Rossby Wave as a Precursor to Rapid Cyclogenesis in the North Atlantic—Dynamics and Forecast Performance

2011 ◽  
Vol 139 (6) ◽  
pp. 1861-1878 ◽  
Author(s):  
Maxi Boettcher ◽  
Heini Wernli
Keyword(s):  
Life Cycle ◽  
Rossby Wave ◽  
North Atlantic ◽  
Vertical Motion ◽  
Environmental Parameters ◽  
Forecast Performance ◽  
Low Level ◽  
The North ◽  
Upper Level ◽  
The Impact

Abstract The life cycle of a North Atlantic cyclone in December 2005 that included a rapid propagation phase as a diabatic Rossby wave (DRW) is investigated by means of operational analyses and deterministic forecasts from the ECMWF. A quasigeostrophic omega diagnostic has been applied to assess the impact of upper-level forcing during the genesis, propagation, and intensification phase, respectively. The system was generated in the Gulf of Mexico as a mesoscale convective vortex (MCV) influenced by vertical motion forcing from a nearby upper-level trough. The DRW propagation phase was characterized by a shallow, low-level, diabatically produced potential vorticity (PV) anomaly that rapidly propagated along the southern border of an intense baroclinic zone. No significant upper-level forcing could be identified during this phase of the development. Eventually, explosive intensification occurred as the region of vertical motion forced by an approaching upper-level trough reached the position of the DRW. The rapid intensification of 34 hPa in 24 h led to a mature extratropical cyclone in the central North Atlantic with marked frontal structures associated with a pronounced PV tower. The performance of four operational deterministic ECMWF forecasts has been investigated for the DRW propagation and cyclone intensification. The forecasts showed a highly variable skill. Despite the fact that the DRW was initially well represented in all forecasts, two of them failed to capture the explosive intensification. By applying a DRW tracking tool, the low-level baroclinicity downstream of the DRW and the moisture supply to the south of the DRW could be identified as the key environmental parameters during DRW propagation. The subsequent cyclone intensification went wrong in two of the forecasts because of the missing interaction of the DRW and the upper-level trough. It is shown that this interaction can fail if the intensity of the DRW and/or the approaching upper-level wave are too weak, or in case of an erroneous structure of the upper-level trough leading to a phasing problem of the vertical interaction with the DRW. Therefore, the DRW intensification bears similar characteristics and forecast challenges as the extratropical reintensification of tropical cyclones.

scholarly journals Evaluation of Near-Surface Parameters in the Two Versions of the Atmospheric Model in CESM1 using Flux Station Observations

2013 ◽  
Vol 26 (1) ◽  
pp. 26-44 ◽  
Author(s):  
Jenny Lindvall ◽  
Gunilla Svensson ◽  
Cecile Hannay
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Diurnal Cycle ◽  
Surface Wind ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Seasonal Temperature ◽  
Near Surface ◽  
Surface Parameters ◽  
Low Level

Abstract This paper describes the performance of the Community Atmosphere Model (CAM) versions 4 and 5 in simulating near-surface parameters. CAM is the atmospheric component of the Community Earth System Model (CESM). Most of the parameterizations in the two versions are substantially different, and that is also true for the boundary layer scheme: CAM4 employs a nonlocal K-profile scheme, whereas CAM5 uses a turbulent kinetic energy (TKE) scheme. The evaluation focuses on the diurnal cycle and global observational and reanalysis datasets are used together with multiyear observations from 35 flux tower sites, providing high-frequency measurements in a range of different climate zones. It is found that both model versions capture the timing of the diurnal cycle but considerably overestimate the diurnal amplitude of net radiation, temperature, wind, and turbulent heat fluxes. The seasonal temperature range at mid- and high latitudes is also overestimated with too warm summer temperatures and too cold winter temperatures. The diagnosed boundary layer is deeper in CAM5 over ocean in regions with low-level marine clouds as a result of the turbulence generated by cloud-top cooling. Elsewhere, the boundary layer is in general shallower in CAM5. The two model versions differ substantially in their representation of near-surface wind speeds over land. The low-level wind speed in CAM5 is about half as strong as in CAM4, and the difference is even larger in areas where the subgrid-scale terrain is significant. The reason is the turbulent mountain stress parameterization, only applied in CAM5, which acts to increase the surface stress and thereby reduce the wind speed.

The impact of southward propagation of the upper-tropospheric Rossby wave activity on the Red Sea trough

2020 ◽  
Author(s):  
Zakieh Alizadeh ◽  
Alireza Mohebalhojeh ◽  
Farhang Ahmadi-Givi ◽  
Mohammad Mirzaei ◽  
Sakineh Khansalari
Keyword(s):  
Red Sea ◽  
Rossby Wave ◽  
North Atlantic ◽  
Storm Track ◽  
Wave Activity ◽  
The Mediterranean ◽  
Northeast Africa ◽  
Upper Level ◽  
Critical Phases ◽  
Wave Activity Flux

<p>The Red Sea Trough (RST) is an inverted trough of low-pressure system at lower tropospheric levels over the northeast Africa and the Red Sea. The previous research conducted on the RST suggests that when this system is activated, heavy rainfall occurs in large parts of the eastern Mediterranean and southwest Asia. The main aim of this article is to investigate the way Rossby wave activity at the upper level troposphere and its interaction with the lower tropospheric circulation activate the RST.</p><p>This study was carried out in three stages: first, the climatological behavior of RST in winter (December to February) was studied and then, cyclones were identified and tracked in the northeast Africa and the Red Sea using a cyclone tracking scheme. In the second stage, the Rossby wave activity flux at the 300 hPa level was considered in the region. Finally, the interaction between the wave activity flux and the RST was investigated. Two critical phases for the wave flux entering the region were considered. The positive (negative) critical phase corresponds to the period when the highest (lowest) values of the wave activity flux enter the northeast Africa and Red Sea regions. The results show that, during the positive critical phase, the RST strengthens and extends to the northeast of the Mediterranean Sea and cyclogenesis is increased in the northeast of Africa and especially in the northeast of the Red Sea. The main reasons for this phenomenon can be deduced as follows:</p><p>With regard to the divergence of wave activity flux and its southward flux, the source of energy and activity needed for cyclogenesis and reinforcement of the RST is provided by the flux convergence core of the North Atlantic storm track. The results of the wave activity time series show that part of the activity from the northeast is integrated with the convergence core of the Mediterranean storm track, leading to enhancement of the cyclones in the northeast of the Red Sea and the extension of the RST to the northeast. But most of the activity joins the flux divergence core of the Mediterranean storm track in the west of the region and results in amplification of Sudan’s cyclones and activation of the RST along both the meridional and zonal directions; the important point to consider is that the wave activity flux entering the region is greater in the zonal direction. In addition to the southward propagation of the wave activity, the packets of flux convergence and divergence in the central North Atlantic are tilted in the northeast–southwest direction, indicating the dominance of anticyclonic Rossby wave breaking. Associated with the upper-level wave activity fluxes entering the region, there is jet enhancement and low-level cold advection from higher latitudes to the tropical and subtropical regions. The difference of RST between the positive and negative critical phases is turned out to be statistically significant with confidence levels of greater than or equal to 90%.</p>

Local and remote Rossby wave responses to an anomalously dry or wet Australian continent

2020 ◽  
Author(s):  
Olivia Martius ◽  
Kathrin Wehrli ◽  
Sonia Seneviratne
Keyword(s):  
Soil Moisture ◽  
Rossby Wave ◽  
Wave Train ◽  
Heat Fluxes ◽  
Linear Wave ◽  
Main Mode ◽  
Near Surface ◽  
Wave Dynamics ◽  
Australian Continent ◽  
Upper Level

<p>An ensemble of CESM atmosphere only experiments with varying soil moisture anomalies over Australia (+1 , 0 ,-1 STD) is analysed with respect to the atmospheric response. Locally an intensification of the surface heat low and an upper-level anticyclone is found for the negative anomaly. The local response to the low soil moisture content is driven by increase sensible heat fluxes and associated positive near-surface temperature anomalies.</p><p>A remote response of the upper-level flow consists of a downstream Rossby wave train extending along the jet waveguide and an upstream response projecting upon the main mode of variability the southern annular. The downstream response is driven by linear wave dynamics while the upstream response is modulated by non-linear wave dynamics and associated eddy fluxes. The sensitivity of the response to the background flow, i.e., different phases of ENSO is explored.</p>

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