scholarly journals Coherent Potential Vorticity Maxima and Their Relationship to Extreme Summer Rainfall in the Australian and North African Tropics

2016 ◽  
Vol 66 (4) ◽  
pp. 424
Author(s):  
Lam P. Hoang ◽  
Michael J. Reeder ◽  
Gareth. J. Berry ◽  
Juliane Schwendike
Keyword(s):  
Potential Vorticity ◽  
West African Monsoon ◽  
Lower Troposphere ◽  
Extreme Rainfall ◽  
Summer Rainfall ◽  
Horizontal Wind ◽  
Synoptic Scale ◽  
North African ◽  
Eastern Australia ◽  
The Tropics

Extreme rainfall in the tropics is frequently linked with coherent synoptic-scale potential vorticity (PV) disturbances. Here, an objective technique is used to identify coherent synoptic-scale cyclonic PV maxima with a focus on those that occur during summer over the African and Australian tropics. These two regions are chosen for comparison because of their geographical and climatological similarities. In particular, in both regions oceans lie equatorward and extensive deserts lie pole-ward, a juxtaposition that produces a reversal in the mean north-south temperature gradient and, through thermal wind, a low level easterly jet.In general, in the lower troposphere there are more coherent PV maxima in the tropics in the summer hemisphere than the winter hemisphere. These coherent PV maxima generally move with the background flow in the lower troposphere. The largest meridional flux of coherent PV maxima lies along eastern Australia with about half of the coherent PV maxima generated through the filamentaton and eventual isolation of midlatitude PV. In contrast, in the north African tropics, coherent PV maxima are generated mostly in the tropics and move westward through the west African monsoon region.Composites based on the extreme rainfall days for two regions are broadly similar with large, statistically significant PV maxima to the east of the maximum positive rainfall anomalies. The vertical structures of the PV fields in the two regions reveal a cyclonic PV maximum in the mid-troposphere collocated with the maximum of diabatic heating. The composite horizontal wind structures in the Australian tropics show structures similar to mesoscale convective systems (MCSs), whereas in the African tropics, they are similar to easterly waves.

scholarly journals North African vegetation-precipitation feedback in early and mid-Holocene climate simulations with CCSM3-DGVM

2014 ◽  
Vol 10 (3) ◽  
pp. 2055-2086 ◽  
Author(s):  
R. Rachmayani ◽  
M. Prange ◽  
M. Schulz
Keyword(s):  
Regional Scale ◽  
West African Monsoon ◽  
Coupled Model ◽  
Summer Precipitation ◽  
Summer Rainfall ◽  
Precipitation Anomaly ◽  
Multiple Equilibrium ◽  
North African ◽  
African Region ◽  
Sahel Region

Abstract. The present study analyses the sign, strength and working mechanism of the vegetation-precipitation feedback over North Africa in middle (6 ka BP) and early Holocene (9 ka BP) simulations using the comprehensive coupled climate-vegetation model CCSM3-DGVM. The coupled model simulates enhanced summer rainfall and a northward migration of the West African monsoon trough along with an expansion of the vegetation cover for the early and middle Holocene compared to pre-industrial. It is shown that dynamic vegetation enhances the orbitally triggered summer precipitation anomaly by approximately 20% in the Sahara/Sahel region (10° N–25° N, 20° W–30° E) in both the early and mid-Holocene experiments compared to their fixed-vegetation counterparts. The primary vegetation-rainfall feedback identified here operates through surface latent heat flux anomalies by canopy evaporation and transpiration and their effect on the mid-tropospheric African Easterly Jet, whereas the effects of vegetation changes on surface albedo and local water recycling play a negligible role. Even though CCSM3-DGVM simulates a positive vegetation-precipitation feedback in the North African region, this feedback is not strong enough to produce multiple equilibrium climate-ecosystem states on a regional scale.

scholarly journals A vertical wind structure that leads to extreme rainfall and major flooding in southeast Australia

2016 ◽  
Vol 66 (4) ◽  
pp. 380
Author(s):  
Jeff Callaghan ◽  
Scott B. Power
Keyword(s):  
Coastal Zone ◽  
Heavy Rainfall ◽  
Extreme Rainfall ◽  
Analysis Data ◽  
Atmospheric Moisture ◽  
Southeastern Australia ◽  
Wind Structure ◽  
Eastern Australia ◽  
The Tropics ◽  
Atmospheric Moisture Content

Here we examine winds associated with extreme rainfall and major flooding in coastal catchments and more broadly over southeastern Australia. Both radio-sonde and re-analysis data are examined. In every case (i) atmospheric moisture content is high and (ii) the low-level winds are onshore, and in almost every case (iii) the wind-direction turns anti-cyclonically with increasing height up to 500 hPa. Data from Brisbane extending back more than 50 years is consistent with this behavior: winds turn anti-cyclonically with increasing height on days with heavy rainfall, whereas winds turn cyclonically with increasing height on days with light or no rainfall. In the coastal zone, extreme rainfall rarely occurs without (i), (ii) and (iii). In eastern Australia beyond the coastal zone, conditions (i) and (iii) are also associated with extreme rainfall. We found very few cases where such conditions were not associated with extreme rainfall in this broader region. This study extends previous work by showing that the link between turning winds and rainfall exists in both the tropics and subtropics, and the link applies in cases of extreme rainfall and associated major flooding.

Atmospheric Turbulence and Clouds in the Tropics: Shipborne Lidar Measurements of Dynamics and Thermodynamics During EUREC4A

2021 ◽  
Author(s):  
Diego Lange ◽  
Andreas Behrendt ◽  
Christoph Senff ◽  
Florian Späth ◽  
Volker Wulfmeyer
Keyword(s):  
Water Vapor ◽  
Lower Troposphere ◽  
Horizontal Wind ◽  
Trade Wind ◽  
Data Set ◽  
Maritime Boundary ◽  
Cloud Development ◽  
Water Vapor Mixing Ratio ◽  
Cloud Evolution ◽  
The Tropics

<p>During the EUREC4A campaign (Bony et al., 2017, Stevens et al, 2020), a unique combination of lidar systems was operated to study ocean-atmosphere interaction on the German research vessel R/V Maria S Merian between 18 January and 18 February 2020. These systems observed the maritime boundary layer (MBL) and its relation to cloud development in the trade wind alley east of Barbados and in the "Boulevard des Tourbillons" east of Venezuela with turbulence resolving resolution.</p><p>For this purpose, for the first time, the Atmospheric Raman Temperature and Humidity Sounder (ARTHUS) (Lange et al. 2019; Lange et al. this conference) was operated on a shipborne platform in vertically staring mode. This system is capable of measuring water-vapor, temperature, and aerosol profiles with unprecedented resolution of 7.5 m and 10 s in the lower troposphere. ARTHUS was combined with one Doppler lidar in vertically staring mode and a second one in a 6-beam scanning mode.</p><p>For studying the above mentioned processes, a data set was collected, which includes profiles of water vapor mixing ratio, temperature, relative humidity, vertical and horizontal wind as well as the statistics of higher-order moments of these parameters. Synergetic parameters from the combination of the data are turbulent kinetic energy (TKE), momentum flux, dissipation rate, sensible and latent heat flux profiles (Behrendt et al. 2020). At the conference, highlights of the measurements will be presented which show the dependence of cloud evolution on sea surface temperature and MBL properties as well as the interaction with the trade wind layer.</p><p> </p><p><strong>References</strong></p><p> </p><p>Behrendt et al. 2020, https://doi.org/10.5194/amt-13-3221-2020</p><p>Bony et al. 2017, https://doi.org/10.1007/s10712-017-9428-0</p><p>Lange et al. 2019, https://doi.org/10.1029/2019GL085774</p><p>Stevens et al. 2020, submitted to ESSD</p>

scholarly journals A Climatology of the Tropospheric Thermal Stratification Using Saturation Potential Vorticity

2007 ◽  
Vol 20 (24) ◽  
pp. 5977-5991 ◽  
Author(s):  
Robert L. Korty ◽  
Tapio Schneider
Keyword(s):  
Potential Vorticity ◽  
Thermal Stratification ◽  
Large Scale ◽  
Lower Troposphere ◽  
Reanalysis Data ◽  
Reanalysis Dataset ◽  
Air Masses ◽  
Slantwise Convection ◽  
The Tropics ◽  
Global Reanalysis

Abstract The condition of convective neutrality is assessed in the troposphere by calculating the saturation potential vorticity P* from reanalysis data. Regions of the atmosphere in which saturation entropy is constant along isosurfaces of absolute angular momentum, a state indicative of slantwise-convective neutrality, have values of P* equal to zero. In a global reanalysis dataset spanning the years 1970–2004, tropospheric regions are identified in which P* is near zero, implying that vertical convection or slantwise convection may be important in determining the local thermal stratification. Convectively neutral air masses are common not only in the Tropics but also in higher latitudes, for example, over midlatitude continents in summer and in storm tracks over oceans in winter. Large-scale eddies appear to stabilize parts of the lower troposphere, particularly in winter.

scholarly journals Tropical Upper-Tropospheric Potential Vorticity Intrusions during Sudden Stratospheric Warmings

2016 ◽  
Vol 73 (6) ◽  
pp. 2361-2384 ◽  
Author(s):  
John R. Albers ◽  
George N. Kiladis ◽  
Thomas Birner ◽  
Juliana Dias
Keyword(s):  
Potential Vorticity ◽  
Wave Breaking ◽  
Polar Vortex ◽  
Synoptic Scale ◽  
Mean Flow ◽  
Rossby Wave Breaking ◽  
Planetary Scale ◽  
The Pacific ◽  
Geographic Distributions ◽  
The Tropics

Abstract The intrusion of lower-stratospheric extratropical potential vorticity into the tropical upper troposphere in the weeks surrounding the occurrence of sudden stratospheric warmings (SSWs) is examined. The analysis reveals that SSW-related PV intrusions are significantly stronger, penetrate more deeply into the tropics, and exhibit distinct geographic distributions compared to their climatological counterparts. While climatological upper-tropospheric and lower-stratospheric (UTLS) PV intrusions are generally attributed to synoptic-scale Rossby wave breaking, it is found that SSW-related PV intrusions are governed by planetary-scale wave disturbances that deform the extratropical meridional PV gradient maximum equatorward. As these deformations unfold, planetary-scale wave breaking along the edge of the polar vortex extends deeply into the subtropical and tropical UTLS. In addition, the material PV deformations also reorganize the geographic structure of the UTLS waveguide, which alters where synoptic-scale waves break. In combination, these two intrusion mechanisms provide a robust explanation describing why displacement and split SSWs—or, more generally, anomalous stratospheric planetary wave events—produce intrusions with unique geographic distributions: displacement SSWs have a single PV intrusion maximum over the Pacific Ocean, while split SSWs have intrusion maxima over the Pacific and Indian Oceans. It is also shown that the two intrusion mechanisms involve distinct time scales of variability, and it is highlighted that they represent an instantaneous and direct link between the stratosphere and troposphere. This is in contrast to higher-latitude stratosphere–troposphere coupling that occurs indirectly via wave–mean flow feedbacks.

scholarly journals Coastal flood risk within a peri-urban area: SUSSEX Inlet district, SE Australia

2021 ◽  
Author(s):  
E. F. Asbridge ◽  
D. Low Choy ◽  
B. Mackey ◽  
S. Serrao-Neumann ◽  
P. Taygfeld ◽  
...  
Keyword(s):  
Flood Risk ◽  
Flood Hazard ◽  
Extreme Rainfall ◽  
Cost Effective ◽  
Extreme Rainfall Events ◽  
Hazard Exposure ◽  
Rate Of Increase ◽  
Eastern Australia ◽  
Planning Strategies ◽  
Natural Resource Managers

AbstractThe peri-urban interface (PUI) exhibits characteristic qualities of both urban and rural regions, and this complexity has meant that risk assessments and long-term planning for PUI are lagging, despite these areas representing new developing settlement frontiers. This study aims to address this knowledge gap by modifying an existing approach to quantify and assess flood risk. The risk triangle framework was used to map exposure, vulnerability and biophysical variables; however, in a novel application, the risk triangle framework was adapted by presuming that there is a variation in the degree of exposure, vulnerability and biophysical variables. Within Australia and globally, PUIs are often coastal, and flood risk associated with rainfall and coastal inundation poses considerable risk to communities in the PUI; these risks will be further exacerbated should projections of increasing frequency of extreme rainfall events and accelerating sea-level rise eventuate. An indicator-based approach using the risk triangle framework that maps flood hazard, exposure and vulnerability was used to integrate the biophysical and socio-economic flooding risk for communities in PUI of the St Georges Basin and Sussex Inlet catchments of south-eastern Australia. Integrating the flood risk triangle with future scenarios of demographic and climate change, and considering factors that contribute to PUI flood risk, facilitated the identification of planning strategies that would reduce the future rate of increase in flood risk. These planning strategies are useful for natural resource managers and land use planners across Australia and globally, who are tasked with balancing socio-economic prosperity for a changing population, whilst maintaining and enhancing ecosystem services and values. The indicator-based approach used in this study provides a cost-effective first-pass risk assessment and is a valuable tool for decision makers planning for flood risk across PUIs in NSW and globally.

scholarly journals Influential Role of Moisture Supply from the Kuroshio/Kuroshio Extension in the Rapid Development of an Extratropical Cyclone

2015 ◽  
Vol 143 (10) ◽  
pp. 4126-4144 ◽  
Author(s):  
Hidetaka Hirata ◽  
Ryuichi Kawamura ◽  
Masaya Kato ◽  
Taro Shinoda
Keyword(s):  
Diabatic Heating ◽  
Rapid Development ◽  
Kuroshio Extension ◽  
Lower Troposphere ◽  
Active Role ◽  
Conveyor Belt ◽  
Synoptic Scale ◽  
Warm Front ◽  
The Kuroshio

Abstract This study focused on an explosive cyclone migrating along the southern periphery of the Kuroshio/Kuroshio Extension in the middle of January 2013 and examined how those warm currents played an active role in the rapid development of the cyclone using a high-resolution coupled atmosphere–ocean regional model. The evolutions of surface fronts of the simulated cyclone resemble the Shapiro–Keyser model. At the time of the maximum deepening rate, strong mesoscale diabatic heating areas appear over the bent-back front and the warm front east of the cyclone center. Diabatic heating over the bent-back front and the eastern warm front is mainly induced by the condensation of moisture imported by the cold conveyor belt (CCB) and the warm conveyor belt (WCB), respectively. The dry air parcels transported by the CCB can receive large amounts of moisture from the warm currents, whereas the very humid air parcels transported by the WCB can hardly be modified by those currents. The well-organized nature of the CCB plays a key role not only in enhancing surface evaporation from the warm currents but also in importing the evaporated vapor into the bent-back front. The imported vapor converges at the bent-back front, leading to latent heat release. The latent heating facilitates the cyclone’s development through the production of positive potential vorticity in the lower troposphere. Its deepening can, in turn, reinforce the CCB. In the presence of a favorable synoptic-scale environment, such a positive feedback process can lead to the rapid intensification of a cyclone over warm currents.

Heavy precipitating events in satellites and rain-gauge products over the Sahel

2021 ◽  
Author(s):  
Sidiki Sanogo ◽  
Philippe Peyrillé ◽  
Romain Roehrig ◽  
Françoise Guichard ◽  
Ousmane Ouedraogo
Keyword(s):  
Burkina Faso ◽  
West African Monsoon ◽  
Extreme Rainfall ◽  
Rain Gauge ◽  
Reference Dataset ◽  
Extreme Rainfall Events ◽  
Microwave Sensor ◽  
Sahel Region ◽  
Distribution Intensity ◽  
Rain Gauge Network

<p>The Sahel has experienced an increase in the frequency and intensity of extreme rainfall events over the recent decades. These trends are expected to continue in the future. However the properties of these events have so far received little attention. In the present study, we define a heavy precipitating event (HPE) as the occurrence of daily-mean precipitation exceeding a given percentile (e.g., 99<sup>th</sup> and higher) over a 1°x1° pixel and examine their spatial distribution, intensity, seasonality and interannual variability. We take advantage of an original reference dataset based on a rather high-density rain-gauge network over Burkina Faso (142 stations) to evaluate 22 precipitation gridded datasets often used in the literature, based on rain-gauge-only measurements, satellite measurements, or both. Our reference dataset documents the HPEs over Burkina Faso. The 99<sup>th</sup> percentile identifies events greater than 26 mm d<sup>-1</sup> with a ~2.5 mm confidence interval depending on the number of stations within a 1°x1° pixel. The HPEs occur in phase with the West African monsoon annual cycle, more frequently during the monsoon core season and during wet years. The evaluation of the gridded rainfall products reveals that only two of the datasets, namely the rain-gauge-only based products GPCC-DDv1 and REGENv1, are able to properly reproduce all of the HPE features examined in the present work. A subset of the remaining rainfall products also provide satisfying skills over Burkina Faso, but generally only for a few HPE features examined here. In particular, we notice a general better performance for rainfall products that include rain-gauge data in the calibration process, while estimates using microwave sensor measurements are prone to overestimate the HPE intensity. The agreement among the 22 datasets is also assessed over the entire Sahel region. While the meridional gradient in HPE properties is well captured by the good performance subset, the zonal direction exhibit larger inter-products spread. This advocates for the need to continue similar evaluation with the available rain-gauge network available in West Africa, both to enhance the HPE documentation and understanding at the scale of the region and to help improve the rainfall dataset quality.</p>

scholarly journals Filamentation Time Diagnosis of Thinning Troughs and Cutoff Lows

2010 ◽  
Vol 138 (6) ◽  
pp. 2327-2335 ◽  
Author(s):  
Yu-Ming Tsai ◽  
Hung-Chi Kuo ◽  
Wayne H. Schubert
Keyword(s):  
Potential Vorticity ◽  
Vertical Direction ◽  
Spherical Coordinates ◽  
Curvature Effects ◽  
Velocity Gradients ◽  
Shearing Deformation ◽  
Cutoff Low ◽  
The Tropics ◽  
Improved Accuracy ◽  
Time Diagnosis

Abstract This paper analyzes synoptic-scale trough-thinning processes using a filamentation time diagnostic. The filamentation time diagnostic is derived from the potential vorticity equation expressed in spherical coordinates in the horizontal plane and the isentropic coordinate in the vertical direction. The diagnostic is an accurate measure of stirring processes under the condition of “slowly varying velocity gradients.” Trough-thinning processes are analyzed for one tropical example and two midlatitude examples. The results indicate that the filamentation time for the tropical trough-thinning event is generally longer than those for the midlatitude trough events. In addition to the effects of stretching and shearing deformation, the filamentation time diagnostic contains the effects of divergence. For the calculation of filamentation time on isentropic surfaces in spherical coordinates, it is acceptable to ignore the curvature effects in the tropics; however, in both the midlatitudes and the tropics, isentropic divergence effects should be retained for improved accuracy. Combining an analysis of cross potential vorticity contour flows on isentropic surfaces with the filamentation time analysis gives a more complete description of the dynamics. The results show that the filamentation time diagnostic can serve as a useful aid in the analysis and prediction of trough thinning and cutoff-low formation.

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