scholarly journals The Structure and Evolution of Heat Waves in Southeastern Australia

2014 ◽  
Vol 27 (15) ◽  
pp. 5768-5785 ◽  
Author(s):  
Teresa J. Parker ◽  
Gareth J. Berry ◽  
Michael J. Reeder
Keyword(s):  
Large Scale ◽  
Rossby Waves ◽  
Heat Waves ◽  
Vertical Motion ◽  
Daily Maximum ◽  
Southeastern Australia ◽  
Hot Air ◽  
Moisture Fluxes ◽  
Upper Level ◽  
Continental Interior

Abstract The underlying large-scale dynamical processes responsible for the development of heat waves in Victoria, southeastern Australia, in summer are presented here. Heat waves are defined as periods of at least three days and two nights for which daily maximum and minimum temperatures exceed the 90th percentile for a particular location and month, using a station daily temperature dataset. Composites of upper-level potential vorticity anomalies from the Interim ECMWF Re-Analysis (ERA-Interim) reveal that heat waves in southeastern Australia are associated with propagating Rossby waves, which grow in amplitude and eventually overturn. The process of overturning generates an upper-level anticyclone over southern Australia and an upper-level trough to the northeast, with maximum amplitudes near the tropopause. The northerly flow associated with the circulation around the surface anticyclone advects hot air from the continental interior over the southeast of Australia, leading to extreme surface temperatures. Composite rainfall shows that precipitation is enhanced in the vicinity of the upper-level trough over northeastern Australia, consistent with adiabatically forced vertical motion, destabilization of the atmosphere, and modified moisture fluxes. Heat waves in the southeast are frequently accompanied by heavy rainfall over the northeast of the continent and adjacent ocean.

scholarly journals Closed Anticyclones of the Subtropics and Midlatitudes: A 54-Yr Climatology (1950–2003) and Three Case Studies

2008 ◽  
Vol 55 ◽  
pp. 349-392 ◽  
Author(s):  
Thomas J. Galarneau ◽  
Lance F. Bosart ◽  
Anantha R. Aiyyer
Keyword(s):  
Case Studies ◽  
Large Scale ◽  
Heat Waves ◽  
The United States ◽  
Hemispheric Differences ◽  
Air Masses ◽  
Hot Air ◽  
Upper Level ◽  
Height Fields ◽  
Subtropical Oceans

Abstract The pioneering large-scale studies of cyclone frequency, location, and intensity conducted by Fred Sanders prompt similar questions about lesser-studied anticyclone development. The results of a climatology of closed anticyclones (CAs) at 200, 500, and 850 hPa, with an emphasis on the subtropics and midlatitudes, is presented to assess the seasonally varying distribution and hemispheric differences of these features. To construct the CA climatology, a counting program was applied to twice-daily 2.5° NCEP–NCAR reanalysis 200-, 500-, and 850-hPa geopotential height fields for the period 1950–2003. Stationary CAs, defined as those CAs that were located at a particular location for consecutive time periods, were counted only once. The climatology results show that 200-hPa CAs occur preferentially during summer over subtropical continental regions, while 500-hPa CAs occur preferentially over subtropical oceans in all seasons and over subtropical continents in summer. Conversely, 850-hPa CAs occur preferentially over oceanic regions beneath upper-level midocean troughs, and are most prominent in the Northern Hemisphere, and over midlatitude continents in winter. Three case studies of objectively identified CAs that produced heal waves over the United States, Europe, and Australia in 1995, 2003, and 2004, respectively, are presented to supplement the climatological results. The case studies, examining the subset of CAs than can produce heat waves, illustrate how climatologically hot continental tropical air masses produced over arid and semiarid regions of the subtropics and lower midlatitudes can become abnormally hot in conjunction with dynamically driven upper-level ridge amplification. Subsequently, these abnormally hot air masses are advected downstream away from their source regions in conjunction with transient disturbances embedded in anomalously strong westerly jets.

scholarly journals Investigating the Local Atmospheric Response to a Realistic Shift in the Oyashio Sea Surface Temperature Front

2015 ◽  
Vol 28 (3) ◽  
pp. 1126-1147 ◽  
Author(s):  
Dimitry Smirnov ◽  
Matthew Newman ◽  
Michael A. Alexander ◽  
Young-Oh Kwon ◽  
Claude Frankignoul
Keyword(s):  
High Resolution ◽  
Surface Wind ◽  
Lower Troposphere ◽  
Vertical Motion ◽  
Atmospheric Response ◽  
Transient Eddy ◽  
Sst Front ◽  
Moisture Fluxes ◽  
Upper Level ◽  
Heat And Moisture

Abstract The local atmospheric response to a realistic shift of the Oyashio Extension SST front in the western North Pacific is analyzed using a high-resolution (HR; 0.25°) version of the global Community Atmosphere Model, version 5 (CAM5). A northward shift in the SST front causes an atmospheric response consisting of a weak surface wind anomaly but a strong vertical circulation extending throughout the troposphere. In the lower troposphere, most of the SST anomaly–induced diabatic heating is balanced by poleward transient eddy heat and moisture fluxes. Collectively, this response differs from the circulation suggested by linear dynamics, where extratropical SST forcing produces shallow anomalous heating balanced by strong equatorward cold air advection driven by an anomalous, stationary surface low to the east. This latter response, however, is obtained by repeating the same experiment except using a relatively low-resolution (LR; 1°) version of CAM5. Comparison to observations suggests that the HR response is closer to nature than the LR response. Strikingly, HR and LR experiments have almost identical vertical profiles of . However, diagnosis of the diabatic quasigeostrophic vertical pressure velocity (ω) budget reveals that HR has a substantially stronger response, which together with upper-level mean differential thermal advection balances stronger vertical motion. The results herein suggest that changes in transient eddy heat and moisture fluxes are critical to the overall local atmospheric response to Oyashio Front anomalies, which may consequently yield a stronger downstream response. These changes may require the high resolution to be fully reproduced, warranting further experiments of this type with other high-resolution atmosphere-only and fully coupled GCMs.

scholarly journals Detection of Intraseasonal Large-Scale Heat Waves: Characteristics and Historical Trends during the Sahelian Spring

2017 ◽  
Vol 31 (1) ◽  
pp. 61-80 ◽  
Author(s):  
J. Barbier ◽  
F. Guichard ◽  
D. Bouniol ◽  
F. Couvreux ◽  
R. Roehrig
Keyword(s):  
Large Scale ◽  
Heat Waves ◽  
Heat Index ◽  
Daily Maximum ◽  
Historical Trends ◽  
Climatic Trend ◽  
Climatic Trends ◽  
Spatiotemporal Scales ◽  
Time Of Year ◽  
Very High

Abstract In the Sahel very high temperatures prevail in spring, but little is known about heat waves in this region at that time of year. This study documents Sahelian heat waves with a new methodology that allows selecting heat waves at specific spatiotemporal scales and can be used in other parts of the world. It is applied separately to daily maximum and minimum temperatures, as they lead to the identification of distinct events. Synoptic–intraseasonal Sahelian heat waves are characterized from March to July over the period 1950–2012 with the Berkeley Earth Surface Temperature (BEST) gridded dataset. Morphological and temperature-related characteristics of the selected heat waves are presented. From March to July, the further into the season, the shorter and the less frequent the heat waves become. From 1950 to 2012, these synoptic–intraseasonal heat waves do not tend to be more frequent; however, they become warmer, and this trend follows the Sahelian climatic trend. Compared to other commonly used indices, the present index tends to select heat waves with more uniform intensities. This comparison of indices also underlined the importance of the heat index definition on the estimated climatic heat wave trends in a changing climate. Finally, heat waves were identified with data from three meteorological reanalyses: ERA-Interim, MERRA, and NCEP-2. The spreads in temperature variabilities, seasonal cycles, and trends among reanalyses lead to differences in the characteristics, interannual variability, and climatic trends of heat waves, with fewer departures from BEST for ERA-Interim.

Signatures of midlatitude heat waves in Rossby wave variability spectra 

2021 ◽  
Author(s):  
Iana Strigunova ◽  
Richard Blender ◽  
Frank Lunkeit ◽  
Nedjeljka Žagar
Keyword(s):  
Rossby Wave ◽  
Large Scale ◽  
Rossby Waves ◽  
Heat Waves ◽  
Probability Distributions ◽  
Physical Space ◽  
Mean Flow ◽  
Single Wave ◽  
Fourier Decomposition ◽  
Latitude Belt

<p>This work aims at identifying extreme circulation conditions such as heat waves in modal space which is defined by eigensolutions of the linearized primitive equations. Here, the Rossby waves are represented in terms of Hough harmonics that are an orthogonal and complete expansion set allowing Rossby wave diagnostics in terms of their total (kinetic and available potential) energies. We expect that this diagnostic provides a more clear picture of the Rossby wave variability spectra compared to the common Fourier decomposition along a latitude belt. </p> <p>The probability distributions of Rossby wave energies are analysed separately for the zonal mean flow, for the planetary and synoptic zonal wavenumbers. The robustness is ensured by considering the four reanalyses ERA5, ERA-Interim, JRA-55 and MERRA. A single wave is characterized by Gaussianity in the complex Hough amplitudes and by a chi-square distribution for the energies. We find that the distributions of the energy anomalies in the wavenumber space are non-Gaussian with almost the same positive skewness in the four reanalyses.  The skewness increases during persistent heat waves for all energy anomaly distributions, in agreement with the recent trend of increased subseasonal variance in large-scale Rossby waves and decreased variance at synoptic scales. The new approach offers a selective filtering to physical space. The reconstructed circulation during heat waves is dominated by large-scale anticyclonic systems in northeastern Europe with zonal wavenumbers 2 and 3, in agreement with previous studies, thereby demonstrating physical meaningfulness of the skewness. </p> <p> </p>

scholarly journals Heat Waves in Florida: Climatology, Trends, and Related Precipitation Events

2019 ◽  
Vol 58 (3) ◽  
pp. 447-466 ◽  
Author(s):  
Shealynn R. Cloutier-Bisbee ◽  
Ajay Raghavendra ◽  
Shawn M. Milrad
Keyword(s):  
Heat Wave ◽  
Large Scale ◽  
Climate Model ◽  
Heat Waves ◽  
Heavy Precipitation ◽  
Daily Maximum ◽  
Recent Climate ◽  
Climate Model Simulations ◽  
Southern Florida ◽  
Precipitation Events

AbstractHeat waves are increasing in frequency, duration, and intensity and are strongly linked to anthropogenic climate change. However, few studies have examined heat waves in Florida, despite an older population and increasingly urbanized land areas that make it particularly susceptible to heat impacts. Heavy precipitation events are also becoming more frequent and intense; recent climate model simulations showed that heavy precipitation in the three days after a Florida heat wave follow these trends, yet the underlying dynamic and thermodynamic mechanisms have not been investigated. In this study, a heat wave climatology and trend analysis are developed from 1950 to 2016 for seven major airports in Florida. Heat waves are defined based on the 95th percentile of daily maximum, minimum, and mean temperatures. Results show that heat waves exhibit statistically significant increases in frequency and duration at most stations, especially for mean and minimum temperature events. Frequency and duration increases are most prominent at Tallahassee, Tampa, Miami, and Key West. Heat waves in northern Florida are characterized by large-scale continental ridging, while heat waves in central and southern Florida are associated with a combination of a continental ridge and a westward extension of the Bermuda–Azores high. Heavy precipitation events that follow a heat wave are characterized by anomalously large ascent and moisture, as well as strong instability. Light precipitation events in northern Florida are characterized by advection of drier air from the continent, while over central and southern Florida, prolonged subsidence is the most important difference between heavy and light events.

scholarly journals Diverse Characteristics of U.S. Summer Heat Waves

2017 ◽  
Vol 30 (19) ◽  
pp. 7827-7845 ◽  
Author(s):  
Bradfield Lyon ◽  
Anthony G. Barnston
Keyword(s):  
Heat Wave ◽  
Land Surface ◽  
Large Scale ◽  
Heat Waves ◽  
Primary Data ◽  
Maximum Temperature ◽  
Daily Maximum ◽  
Regional Variations ◽  
Daily Minimum ◽  
Summer Heat

Abstract Heat waves are climate extremes having significant environmental and social impacts. However, there is no universally accepted definition of a heat wave. The major goal of this study is to compare characteristics of continental U.S. warm season (May–September) heat waves defined using four different variables—temperature itself and three variables incorporating atmospheric moisture—all for differing intensity and duration requirements. To normalize across different locations and climates, daily intensity is defined using percentiles computed over the 1979–2013 period. The primary data source is the U.S. Historical Climatological Network (USHCN), with humidity data from the North American Regional Reanalysis (NARR) also tested and utilized. The results indicate that heat waves defined using daily maximum temperatures are more frequent and persistent than when based on minimum temperatures, with substantial regional variations in behavior. For all four temperature variables, heat waves based on daily minimum values have greater spatial coherency than for daily maximum values. Regionally, statistically significant upward trends (1979–2013) in heat wave frequency are identified, largest when based on daily minimum values, across variables. Other notable differences in behavior include a higher frequency of heat waves based on maximum temperature itself than for variables that include humidity, while daily minimum temperatures show greater similarity across all variables in this regard. Overall, the study provides a baseline to compare with results from climate model simulations and projections, for examining differing regional and large-scale circulation patterns associated with U.S. summer heat waves and for examining the role of land surface conditions in modulating regional variations in heat wave behavior.

scholarly journals Precipitation Enhancement via the Interplay between Atmospheric Rivers and Cutoff Lows

2019 ◽  
Vol 147 (7) ◽  
pp. 2451-2466 ◽  
Author(s):  
Hiroki Tsuji ◽  
Yukari N. Takayabu
Keyword(s):  
Large Scale ◽  
Vertical Motion ◽  
Precipitation Event ◽  
Common Mechanism ◽  
Narrow Region ◽  
Disastrous Flood ◽  
Precipitation Area ◽  
Cutoff Low ◽  
Upper Level ◽  
Moist Layer

Abstract A significant enhancement of precipitation can result from the interplay between two independent, large-scale phenomena: an atmospheric river (AR) and a cutoff low. An AR is a long, narrow region with a deep moist layer. A cutoff low is an upper-level cyclonic eddy isolated from the meandering upper-level westerly jet. Herein, we construct composites of cutoff lows both close to an AR (AR-close category) and distant from an AR (AR-distant category) over a 14-yr period across the western North Pacific region. A comparison between the two categories shows an enhanced precipitation area to the northwest of the cutoff low and to the south of the AR axis in the AR-close category. The horizontal formation among the AR, cutoff low, and enhanced precipitation area in the composite coincides with that in a disastrous flood event that occurred in Hiroshima, Japan, in 2014. The deep moist layer associated with the AR, and the destabilization and isentropic up-gliding effect associated with the cutoff low are also observed in both the composite and the Hiroshima cases. We further evaluate the distribution of quasigeostrophic forcing (Q vector) for vertical motion. This shows that warm air advection associated with the AR overcomes the descending forcing inherent in the northwest of the cutoff low and makes the instability and up-gliding effect in that region more effective. These results indicate that the interplay between ARs and cutoff lows is a common mechanism in the enhancement of precipitation and the Hiroshima case is an extreme precipitation event caused by this interplay.

scholarly journals A meteorological overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the southeastern Atlantic during 2016–2018: Part 1 – Climatology

2021 ◽  
Vol 21 (22) ◽  
pp. 16689-16707
Author(s):  
Ju-Mee Ryoo ◽  
Leonhard Pfister ◽  
Rei Ueyama ◽  
Paquita Zuidema ◽  
Robert Wood ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Large Scale ◽  
Coastal Region ◽  
Vertical Motion ◽  
Mixing Ratios ◽  
Low Cloud ◽  
Upper Level ◽  
Upper Level Jet ◽  
The Impact ◽  
Heat Low

Abstract. In 2016–2018, the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) project undertook 3-month-long deployments to the southeastern (SE) Atlantic Ocean using research aircraft to better understand the impact of biomass burning (BB) aerosol transport to the SE Atlantic Ocean on climate. In this (part 1 of the meteorological overview) paper, the climatological features at monthly timescales are investigated. The southern African easterly jet (AEJ-S), defined as the zonal easterlies over 600–700 hPa exceeding 6 m s−1 around 5–15∘ S, is a characteristic feature of the mid-level circulation over southern Africa that was also during the deployment months of August 2017, September 2016, and October 2018. Climatologically, the AEJ-S develops at lower altitudes (∼ 3 km; 700 hPa) between 5–10∘ S in August, while it develops at around 4 km (∼ 600 hPa) and further south (5–15∘ S) in September and October, largely driven by the strong sensible heating over the African plateau. Notable meteorological anomalous characteristics during the 3 deployment months, compared to climatology (2000–2018), include the following: (1) during August 2017, the AEJ-S was weaker than the climatological mean, with an additional anomalous upper-level jet aloft (∼ 6 km) around 10∘ S. August 2017 was also drier over the SE Atlantic at 600–700 hPa than climatology, with a stronger Benguela low-level jet (LLJ) at 925–950 hPa along the Namibian coast of the SE Atlantic. Consistent with this, the southern Atlantic anticyclone was also stronger and closer to the coast than the August climatological mean. (2) During September 2016, the AEJ-S intensity was similar to the climatological mean, although the heat low and vertical motion over the land was slightly stronger compared to the September climatology. The LLJ and the large-scale southern Atlantic anticyclone were stronger than the climatological mean. (3) During October 2018, the AEJ-S was slightly weaker compared to the climatological mean, as was the LLJ and the southern Atlantic anticyclone. October 2018 was wetter over the Benguela coastal region at 600 hPa than the climatological mean. During all the deployment months, the sea surface temperatures (SST) over the SE Atlantic were warmer than the climatological means, but the monthly mean low cloud fraction was only noticeably reduced in August 2017. A weak August 2017 AEJ-S can explain low offshore black carbon (BC) mixing ratios within the European Centre for Medium-Range Weather Forecasts (ECMWF) Copernicus Atmosphere Monitoring Service (CAMS) reanalysis, although the BC peak altitude, at 2–3 km, is below that of the AEJ-S. The upper-level wave disturbance and the associated anomalous circulation also explain the weakening of AEJ-S through the reduction of the strength of the heat low over the land during August 2017.

scholarly journals On the heat waves in Bangladesh, their trends and associated large scale tropospheric conditions

2020 ◽  
Vol 11 (1) ◽  
pp. 19-36
Author(s):  
Samarendra Karmakar ◽  
Mohan Kumar Das
Keyword(s):  
Large Scale ◽  
Heat Waves ◽  
Maximum Temperature ◽  
Chittagong Hill Tracts ◽  
The West ◽  
Southwestern Part ◽  
Synoptic Conditions ◽  
Annual Frequency ◽  
Upper Level ◽  
Increasing Trends

Attempts have been made to study the heat waves along with the trends in Bangladesh and the large-scale tropospheric conditions over Bangladesh and neighbourhood responsible for generating heat waves in the country. Maximum temperature (Tmax) for the period 1981-2016 has been used in the study. Based on the definition of heat waves of Bangladesh Meteorological Department (BMD), the frequencies of days with Tmax≥360C and ≥ 380C have been computed and their trends are investigated to delineate the areas of heat waves in Bangladesh. During the period 1981-2016, Tmax≥360C is found to prevail annually for 79 days at Rajshahi, 51 days at Mongla, 50 days at Khulna in 2014 whereas it is found to prevail annually for 85 days at Chuadanga in 1992, 78 days at Jessore in 2010, 72 days at Satkhira in 1986 and 68 days at Ishurdi in 1995. In 2014, Tmax≥360C is found to exist annually for many days in western pat of Bangladesh in comparison to the eastern and costal region of the country. That is why year 2014 has been selected to study waves extensively. The study reveals that the monthly, seasonal and annual frequency of Tmax≥360C have increasing trends in Bangladesh except a very few places, having the highest increasing trends over southwestern part of the country with increasing rates of 0.816 day/year and 1.02 day/year, respectively at Mongla. The highest increasing trends over southwestern part may be due to the advection and penetration of higher Tmax due to northwesterly winds and less rainfall over the area. The seasonal and annual frequencies of Tmax≥380C have increasing trends at less than 50% stations in Bangladesh; some of the increasing trends of the seasonal and annual frequency of maximum temperature ≥380C are statistically significant up to 95-99% level of significance. The large-scale synoptic conditions show that heat waves are found to enter Bangladesh from the west/northwest due to the advection of higher Tmax from the west. Heat waves extend from west to east up to about central Bangladesh and a separate area of heat waves develop over the Chittagong Hill Tracts, the reason of which may be due to the diverging pattern of wind flows near the Chittagong Hill Tracts in the year of heat waves. Heat waves are absent along Sandwip-M.Court-Feni-Comilla region. In 2014, heat waves are found to be due to the influence of sub-tropical high over India and its extension over Bangladesh at the surface and at 850 hPa level with strong westerly/northwesterly winds at 850 hPa, influence of anticyclones persisting for many days over the Bay of Bengal at 300 hPa level, absence of upper level westerly troughs over India and Bangladesh Journal of Engineering Science 11(1), 2020, 19-36

scholarly journals A Study on Synoptic Conditions Leading to the Extreme Rainfall in Taiwan during 10–12 June 2012

Atmosphere ◽  
2021 ◽  
Vol 12 (10) ◽  
pp. 1255
Author(s):  
An-Hsiang Wang ◽  
Chung-Chieh Wang ◽  
George Tai-Jen Chen
Keyword(s):  
South China ◽  
Large Scale ◽  
Vertical Motion ◽  
Daily Rainfall ◽  
Dynamic Effects ◽  
Comparable Amount ◽  
Low Level ◽  
Synoptic Conditions ◽  
Upper Level ◽  
Pressure Fall

During 10–12 June 2012, heavy rainfall occurred three days in a row in southern and central Taiwan, with daily rainfall maxima exceeding 500 mm on each day. In the Mei-yu season (May–June) during 1993–2000, only two other rainfall events had a comparable amount and duration, but this case was the only one that occurred well before the arrival of the Mei-yu front. The synoptic conditions and their evolution leading to this unique event are thus important and are the foci of this study. Our analysis indicates that the 10–12 June 2012 event in Taiwan was caused by the strong and persistent west-southwesterly low-level jet (LLJ) that transported warm, moist, and unstable air from upstream and then impinged on the island. The LLJ developed due to the enhanced horizontal pressure (or height) gradient when the pressure at low-levels fell significantly (by ~8 hPa) in South China (north of the jet) during 8–10 June, but the subtropical high to the southeast maintained its strength. Further, through a diagnosis using the pressure tendency equation, it is found that both warm air advection and the dynamic effects (column divergence and transport of mass by vertical motion) contributed to the pressure fall in South China. The warm air advection occurred in the southern part of a large-scale confluent pattern in China, and the persistent west-southwesterly flow through deep layer (mainly above 800 hPa) in South China transported warmer and less dense air into the region from lower latitudes. On the other hand, South China was also located under the diffluent zone in the northeastern quadrant of the South Asian upper-level anticyclone, which strengthened during 5–10 June and provided divergence aloft, which exceeded the low-level convergence and upward transport of mass (at a fixed height) into the column by vertical motion on 9 June. As a result, the dynamic effects also contributed to the pressure fall, although secondary to the warm air advection. The destabilization process in South China during 8–10 June was also helpful to increase convective activity and upper-level divergence.

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