Synoptic Features Research Articles

Synoptic features associated with the winter variability of the subtropical jet stream over Africa and the Middle East

Spatial, vertical and temporal variations in the subtropical jet stream over Africa and the Middle East were studied from 1958 to 2016 using NCEP/NCAR reanalysis meteorological data. The results of this study showed that the jet stream was located at a pressure level of 200 hPa more than 87% of the time, and it was located in the meridional zone between 20°N and 35°N in more than 83% of these cases. Additionally, the annual winter distribution of the jet stream illustrated two distinct trends: the first trend ended in 1987 and had a high rate of decrease, and the second trend began in 1988 and had a small rate of increase. Furthermore, synoptically, the study showed that the southern shift of the jet stream intensified the low-pressure systems over Africa and the Mediterranean, in addition to producing favorable conditions for the northward extension of the Red Sea Trough. However, the northward shift was related to the intensified Azores high-pressure system over Africa and the southern Mediterranean. Moreover, the temporal variations of the jet stream indicated that the intensification of the high-pressure system (i.e., the Azores and Siberian highs) tended to decrease the number of jet stream cases at 200 hPa or vertically shifted them downward to 250 hPa.

Synoptic Characteristics Related to Warm-Sector Torrential Rainfall Events in South China During the Annually First Rainy Season

Warm-sector torrential rainfall (WSTR) events that occur in the annually first rainy season in south China are characterized by high rainfall intensity and low radar echo centroids. To understand the synoptic characteristics related to these features, 16 WSTR events that occurred in 2013-2017 were examined with another 16 squall line (SL) events occurred during the same period as references. Composite analysis derived from ERA-Interim reanalysis data indicated the importance of the deep layer of warm and moist air for WSTR events. The most significant difference between WSTR and SL events lies in their low-level convergence and lifting; for WSTR events, the low-level convergence and lifting is much shallower with comparable or stronger intensity. The trumpet-shaped topography to the north of the WSTR centers is favorable for the development of such shallow convergences in WSTR events. Results in this study will provide references for future studies to improve the predictability of WSTR.

Synoptic characteristics of extreme heatwaves over the Korean Peninsula based on ERA Interim reanalysis data

AbstractThe characteristics of heatwaves (HWs) in South Korea are studied using data from the European Centre for Medium‐Range Weather Forecasts Reanalysis Interim (ERA Interim) dataset and from automatic surface weather stations. The synoptic conditions of three specific years (1994, 2013, and 2016) during which serious HWs affected South Korea are examined. HWs in South Korea are related to the northwestwardly extension of the western North Pacific subtropical high (WNPSH) toward the Korean Peninsula. Examination of the blocking indices revealed widespread blocking over the northern Pacific Ocean and Asia during South Korean HWs, and blocking is related to stationary weather patterns. The severe temperatures associated with HWs in this region are due to prolonged subsidence. Analysis of the moist static energy (MSE) budget indicated that the energy source of subsidence is derived from both MSE advection and the net heat flux. When compared to the synoptic situation during an HW in South Korea, the relative southward movement of the WNPSH is found before and after the HW. The blocking indices also revealed weak signals and changes in vertical motion due to MSE advection.

Synoptic Features Responsible for Heat Waves in Central Africa, a Region with Strong Multidecadal Trends

Abstract Central Africa (CA) is identified as a location of a large positive trend of the occurrence of heat waves (HWs) during 1979–2016, appearing to result mostly from a regime shift around the year 2000. Therefore, we study the evolution of synoptic features associated with the occurrence of HW events in CA. It is found that the HW-related circulation is typically characterized by an anomalous convergence in the upper troposphere but there are important differences for HW events occurring in the south region of CA (CA_S) versus the north region (CA_N). For the occurrence of the HW events in CA_S, the anomalous subsidence associated with upper troposphere anomalous convergence is the dominant factor for their occurrence and magnitude: the strong subsidence leads to warming through greater solar insolation. The HW events in CA_S are also accompanied by an anomalous surface anticyclone in the north with anomalous northerly flow transporting heat into the CA_S region. In contrast, although the HW events in CA_N are also associated with upper troposphere anomalous convergence, the intensity of the convergence is weak with small solar insolation. Instead, the anomalous warm advection is the main factor for determining the magnitude of the HW events in CA_N, induced by the prevailing northerly winds acting on the anomalous temperature gradient. Thus, the synoptic features associated with HW events in the CA_N and CA_S are quite different despite their nearby locations. The discovered dominant factors for the HW events in CA can be used to improve the forecast skill.

Atmospheric River Families: Definition and Associated Synoptic Conditions

Abstract Atmospheric rivers (ARs) can cause flooding when they are strong and stall over an already wet watershed. While earlier studies emphasized the role of individual, long-duration ARs in triggering floods, it is not uncommon for floods to be associated with a series of ARs that strike in close succession. This study uses measurements from an atmospheric river observatory at Bodega Bay (BBY), in Northern California, to identify periods when multiple AR events occurred in rapid succession. Here, an AR “event” is the period when AR conditions are present continuously at BBY. An objective method is developed to identify such periods, and the concept of “AR families” is introduced. During the period studied there were 228 AR events. Using the AR family identification method, a range of aggregation periods (the length of time allowed for ARs to be considered part of a family) was tested. For example, for an aggregation period of 5 days, there were 109 AR families, with an average of 2.7 ARs per family. Over a range of possible aggregation periods, typically there were 2–6 ARs per family. Compared to single AR events, the synoptic environment of AR families is characterized by lower geopotential heights throughout the midlatitude North Pacific, an enhanced subtropical high, and a stronger zonal North Pacific jet. Analysis of water year 2017 demonstrated a persistent geopotential height dipole throughout the North Pacific and a positive anomaly of integrated water vapor extending toward California. AR families were favored when synoptic features were semistationary.

Study of the Synoptic Characteristics of a Severe Dust Storm over Iraq Using Regional Climate Model 4

Study of the Synoptic Characteristics of a Severe Dust Storm over Iraq Using Regional Climate Model 4

Interdecadal Variation in the Synoptic Features of Mei-Yu in the Yangtze River Valley Region and Relationship with the Pacific Decadal Oscillation

Abstract This study introduces a multivariable covariance index (MVCI) to illustrate the synoptic features of mei-yu in the Yangtze River valley (YRV) region, which contains information of three indicators of mei-yu including precipitation, surface relative humidity, and tropospheric vertical motion. The interdecadal variation in the synoptic features of mei-yu during 1961–2016 is investigated using the MVCI. The date of mei-yu peak and the intensity of mei-yu underwent noticeable interdecadal variations over past decades, which are characterized by a delayed (relatively early) mei-yu peak and a relatively large (small) mei-yu intensity during 1985–97 (1961–80 and 2006–16). The mechanisms of these interdecadal variations are further discussed. The interdecadal variation in the date of mei-yu peak is mainly modulated by the meridional water vapor transport over eastern China during June, which may be partially attributed to an influence of the Pacific decadal oscillation (PDO) on the clockwise gyre over the North Pacific during boreal summer. The interdecadal variation in mei-yu intensity is associated with the interdecadal variation of tropospheric vertical motion over the YRV region during boreal summer, which may be partially attributed to an interaction between the PDO and the large-scale tropical east–west circulation during boreal summer. In addition, the interdecadal variation in the water vapor flux budget and relative humidity over the YRV region also exerted an impact on the interdecadal variation of mei-yu intensity in the YRV region.

Cut‐off low systems over Iraq: Contribution to annual precipitation and synoptic analysis of extreme events

AbstractWe combine daily in situ precipitation data with meteorological reanalysis data in order to explore the contribution of cut‐off low systems to the seasonal and inter‐annual rainfall variations over Baghdad from 2005 to 2016. During these 12 years (average rainfall of 131 ± 67 mm/year), 38 rainy cut‐off lows brought 43% of the total precipitation, with extreme inter‐annual variations. Indeed, precipitation associated with autumn cut‐off lows was the principal factor that turned an arid into a wet year: during the three most arid years cut‐off lows contributed about 25% of the average rainfall (10 out of 40 mm/year) while during the three wettest years they contributed near 67% (171 out of 254 mm/year). The extreme‐rain cut‐off low systems displayed analogous synoptic characteristics: upper‐atmosphere divergence, upwards vertical motions in the middle atmosphere, and lower‐atmosphere winds into central Iraq at times when the surface Red Sea and Persian Gulf waters were warmer than the surface air. During those days previous to an extreme event, the surface waters cooled substantially and the amount of precipitable water increased largely, suggesting high latent heat transfer. In order to characterize those conditions that favour rainfall, we focus on the November 18–20, 2013 cut‐off low system, which led to the largest flooding and wettest year in Baghdad between 2005 and 2016. The distribution of properties in the middle (500 hPa) and upper (250 hPa) troposphere shows that the region was affected by intense horizontal divergence and upwards motions, coinciding with a surface low over the Arabian Peninsula that caused intense northwards winds over the Persian Gulf and brought substantial moisture to central Iraq. The analysis of several stability indexes indicates that convective instability played a secondary role during the episode.

Climatology and change of extreme precipitation events in Taiwan based on weather types

AbstractTaiwan's most significant natural hazards are caused by hydrological extremes resulting from excessive precipitation. The threat of extreme precipitation is posed by several different types of weather patterns that affect Taiwan. This study examined the bi‐decadal changes in rainfall by defining an extreme precipitation occurrence (EPO) for a range of event durations from 1 to 24 hr. Three major weather types affecting EPO in Taiwan were identified from 1993 to 2015: the front type consisting of either a frontal zone or convective systems developing with an apparent Meiyu cloudband, diurnal rainfall events when no apparent synoptic features are present, and a tropical cyclone (TC) type according to the maximum sustained wind radius of a TC. Results show that TC‐type events have the greatest overall contribution to EPO at longer (>6 hr) durations. Diurnal/afternoon convection events contribute most to the shorter (<3 hr) duration EPO, while frontal/Meiyu systems prevail in the medium (3–6 hr) duration. EPO of almost all durations have experienced an increase, with the 3‐ and 12‐hr EPO having increased by 4.6 days each over the 23 years. However, apparent decadal‐scale variability exists in these EPO associated with the decreasing tendency of EPO after the mid‐2000s, particularly the longer duration (>6 hr) EPO associated with the TC‐type events in summer. The distinction between EPO trends for the entire island of Taiwan and for the Taipei metropolitan area alone (northern Taiwan, population of 7 million) were compared, and an intriguing interannual variation is reported in the TC‐type EPO associated with the TC season 1 year to a year and half just before an El Niño–Southern Oscillation event. The analysis here provides refined statistical distributions of extreme rainfall, and these can contribute to the revision of governmental definitions for weather disasters that are used in mitigation and response strategies.

Long-term changes in southern Australian anticyclones and their impacts

Anticyclones are one of the main synoptic features of the global midlatitudes, where they are associated with dry, settled conditions. Previous studies have identified southern Australia as a region where significant long-term changes have occurred in recent decades, including a decline in cool season rainfall and an increase in the frequency of anticyclones. This paper uses several reanalyses to assess the climatology, variability, trends and impacts of anticyclones in southern Australia and several subregions. A robust increase in the frequency of anticyclones in southern Australia is demonstrated between 1960–1979 and 1997–2014, particularly in southeast Australia and the Tasman Sea during the cool months of the year. At least 30% of the recent cool season rainfall decline in southeast Australia can be associated with an increased frequency of anticyclones in this region. However, there are no clear trends in anticyclone frequency near southwestern Western Australia, where long-term rainfall declines are due to changes in other weather systems.

Spatial patterns and time distribution of central European extreme precipitation events between 1961 and 2013

Precipitation extremes always have an area‐related effect, emphasizing the need for a spatial assessment of extreme precipitation events that sometimes show similar behaviour in precipitation patterns due to recurring synoptic features. Our study investigates spatial patterns in the extreme precipitation events (EPEs) that occurred in central Europe between 1961 and 2013. As many as 53 maximum events were selected by the weather extremity index (WEI), reflecting simultaneously the spatial extent and the return periods of t day precipitation totals within an event‐adjusted study area. The extremity of the EPEs is further evaluated at two lower spatial levels, the main river basins (the Rhine, Weser/Ems, Elbe, Danube, and Oder) and 20 smaller subcatchments, which enables a more detailed study of the events’ spatial structure and similarity. A correlation analysis demonstrated that heavy precipitation occurs simultaneously not only in neighbouring subcatchments but also in rather distant regions with similar orientations of mountain ranges (e.g., in the Elbe‐a and Danube‐b subcatchments). In contrast, strong negative correlations appeared between several subcatchments in the Oder and Rhine River basins, which exclude heavy precipitation from occurring simultaneously in both basins. Similar spatial patterns are obvious among precipitation extremes; using the relative WEI values as the similarity measure, agglomerative hierarchical clustering detected two well‐separated groups of events, namely, W‐CE and E‐CE, affecting mainly the western and eastern parts of central Europe, respectively. A finer division of the EPEs distinguished five clusters of events with different spatio‐temporal characteristics. Only two clusters, ED (Elbe‐Danube) and O (Oder), were represented among the top 10 central European EPEs, which occurred exclusively from the end of May to the beginning of September. Three other clusters consisted of generally lower extreme events rather equally distributed throughout the year.

Role of East–West Shear Zone and Wind Confluence on the Occurrence of Intense Heavy Rainfall over North Konkan During Southwest Monsoon Season

An attempt has been made to understand the mechanism in occurrence of intense heavy rainfall events over North Konkan and adjoining areas using observed, satellite and reanalysis datasets. Synoptic conditions associated with Dahanu heavy rainfall event, 2016, Mumbai floods, 2005 and historical heavy rainfall events over North Konkan region are considered in the present study. In presence of East–West shear zone with upper air cyclonic circulations extending up to mid-tropospheric levels located at west central Bay of Bengal, west Arabian Sea and anti-cyclonic circulation over east-central Arabian Sea were the major synoptic features observed during these events. Results suggest that the confluence of moisture-laden winds from these circulations over North Konkan lead to the occurrence of intense heavy rainfall events.

Extreme precipitation events over north-western Europe: getting water from the tropics

Our capability to adapt to extreme precipitation events is linked to our skill in predicting their magnitude and timing. Synoptic features (such as Atmospheric Rivers) developing over the North Atlantic Ocean are known as the source of the majority of water vapour transport into European mid-latitudes, and are associated with episodes of heavy and prolonged rainfall over UK and north western Europe. Thus, a better understanding of the North Atlantic atmospheric conditions prior the occurrence of extreme precipitation events over Europe could help in improving our capability to predict them. We build on atmospheric re-analyses at high spatial resolution (such as JRA-55 and MERRA), on a daily time scale, to highlight the anomalous path of the vertically integrated water content, transferring water from the western tropical North Atlantic to high latitudes and fuelling the storms developing in the North Atlantic sector, bound to affect Europe as responsible of the most intense precipitation events. The systematic link between anomalous north-eastward transport of vertically integrated water (precipitable water) from the western North Atlantic and anomalously high pressure patterns in the central North Atlantic, developing 5 days prior the extreme precipitation occurrence, suggest the central North Atlantic surface pressure as a potential precursor of extreme precipitation events.

Simulation of synoptic features during summer monsoon onset over GWB, India, with CFSv2 coupled model: skill and bias assessment

The aim of this study is to examine the skill of climate forecast system version 2 (CFSv2) in simulating the synoptic features of Bay of Bengal (BOB) branch of summer monsoon (SM) during the onset over Gangetic West Bengal (GWB), India. Precise prediction of the onset time and the synoptic features associated with the onset is a major challenge in SM study. Better understanding of the synoptic and intra-seasonal variability during the propagation along with the mean simulation of monsoon features is crucial, especially for the operational models. The earlier studies focused mainly on the mean simulation of SM during June–September (JJAS) period. However, the main objective of the present study is to improve the understanding of CFSv2 model biases in simulating the synoptic features during the propagation of BOB branch of SM system till the onset over GWB. The skill of the coupled model is estimated for the years 2011 to 2015 with tropospheric temperature (TT), sea surface temperature (SST), mean sea level pressure (MSLP), winds at 850 and 500 hPa pressure levels, and rainfall rate (RR). The result shows that the observed characteristics are simulated, reasonably well, by CFSv2 model with quite high reliability unlike other coupled models. The CFSv2 has been able to simulate the position/variation during the onset; however, the model has not been able to estimate the intensity in some occasions. The gradients of pressure and SST have been slightly overestimated by the model. The model has not been able to simulate the winds at 850 and 500 hPa pressure levels in some occasions. The CFSv2 model in simulating the features during propagation of BOB branch of SM system shows disparity from observation in some occasions during 2011 to 2015. The result also reveals that the model biases remain unaltered during El Nino episode of 2011.

Regional Climate Model Performance in Simulating Intra-seasonal and Interannual Variability of Indian Summer Monsoon

Establishment of Indian summer monsoon (ISM) rainfall passes through the different phases and is not uniformly distributed over the Indian subcontinent. This enhancement and reduction in daily rainfall anomaly over the Indian core monsoon region during peak monsoon season (i.e., July and August) are commonly termed as ‘active’ and ‘break’ phases of monsoon. The purpose of this study is to analyze REGional Climate Model (RegCM) results obtained using the most suitable convective parameterization scheme (CPS) to determine active/break phases of ISM. The model-simulated daily outgoing longwave radiation (OLR), mean sea level pressure (MSLP), and the wind at 850 hPa of spatial resolution of 0.5°× 0.5° are compared with NOAA, NCEP, and EIN15 data, respectively over the South-Asia Co-Ordinated Regional Climate Downscaling EXperiment (CORDEX) region. 25 years (1986–2010) composites of OLR, MSLP, and the wind at 850 hPa are considered from start to the dates of active/break phase and up to the end dates of active/break spell of monsoon. A negative/positive anomaly of OLR with active/break phase is found in simulations with CPSs Emanuel and Mix99 (Grell over land; Emanuel over ocean) over the core monsoon region as well as over Monsoon Convergence Zone (MCZ) of India. The appearance of monsoon trough during active phase over the core monsoon zone and its shifting towards the Himalayan foothills during break phase are also depicted well. Because of multi-cloud function over oceanic region and single cloud function over the land mass, the Mix99 CPSs perform well in simulating the synoptic features during the phases of monsoon.

A Synoptic analogue model to issue QPF over Gangetic West Bengal and adjoining Jharkhand

In this study the Areal Average Precipitation (AAP) data for each day over each of the six catchments of Gangetic West Bengal (GWB) and adjoining Jharkhand namely river catchments of Mayurakhshi, Ajoy, Kansabati, Damodar, Barakar and Lower Valley of Damodar Valley Corporation during monsoon season for 25 years from 1990 to 2014 have been analyzed by grouping the AAP in three different ranges (11-25 mm, 26-50 mm, 51-100 mm and more), excluding Mainly Dry and 01-10 mm. The associated main synoptic features viz., trough at mean sea level, low pressure area, well marked low pressure area, cyclonic storm and cyclonic circulation for each day and their location with respect to the river catchments, viz., over the catchment, neighbourhood of the catchment (within 200 km South or North) and outside the catchment (more than 200 km South or North) have also been studied. The association of AAP ranges over six catchments with different categories of synoptic features has been examined. The distribution of percentage frequency of AAPs associated with the category of synoptic feature for the period 1990 to 2014 has led to development of a Synoptic Analogue Model (SAM) for issue of Quantitative Precipitation Forecast (QPF). The results obtained from SAM have been verified for rainfall data and calculated AAPs of monsoon season of 2015 over all the catchments and different skills scores also presented in this study.

Climatology of the autumn Red Sea trough

In this study, the Sudan low and the associated Red Sea trough (RST) are objectively identified using the mean sea level pressure (SLP) data from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset covering the period 1955–2015. The Sudan low was detected in approximately 60.6% of the autumn periods, and approximately 83% of the detected low-pressure systems extended into RSTs, with most generated at night and during cold months. The distribution of the RSTs demonstrated that Sudan, South Sudan and Red Sea are the primary development areas of the RSTs, generating 97% of the RSTs in the study period. In addition, the outermost areas affected by RSTs, which include the southern, central and northern Red Sea areas, received approximately 91% of the RSTs originating from the primary generation areas. The synoptic features indicated that a Sudan low developed into an RST when the Sudan low deepened in the atmosphere, while the low pressures over the southern Arabian Peninsula are shallow and the anticyclonic systems are weakened over the northern Red Sea. Moreover, stabile areas over Africa and Arabian Peninsula form a high stability gradient around the Red Sea and the upper maximum winds weaken. The results of the case studies indicate that RSTs extend northward when the upper cyclonic and anticyclonic systems form a high geopotential gradient over Arabian Peninsula. Furthermore, the RST is oriented from the west to the east when the Azores high extends eastward and the Siberian high shrinks eastward or shifts northward.

Contribution of large-scale midlatitude disturbances to hourly precipitation extremes in the United States

Midlatitude synoptic weather regimes account for a substantial portion of annual precipitation accumulation as well as multi-day precipitation extremes across parts of the United States (US). However, little attention has been devoted to understanding how synoptic-scale patterns contribute to hourly precipitation extremes. A majority of 1-h annual maximum precipitation (AMP) across the western US were found to be linked to two coherent midlatitude synoptic patterns: disturbances propagating along the jet stream, and cutoff upper-level lows. The influence of these two patterns on 1-h AMP varies geographically. Over 95% of 1-h AMP along the western coastal US were coincident with progressive midlatitude waves embedded within the jet stream, while over 30% of 1-h AMP across the interior western US were coincident with cutoff lows. Between 30–60% of 1-h AMP were coincident with the jet stream across the Ohio River Valley and southeastern US, whereas a a majority of 1-h AMP over the rest of central and eastern US were not found to be associated with either midlatitude synoptic features. Composite analyses for 1-h AMP days coincident to cutoff lows and jet stream show that an anomalous moisture flux and upper-level dynamics are responsible for initiating instability and setting up an environment conducive to 1-h AMP events. While hourly precipitation extremes are generally thought to be purely convective in nature, this study shows that large-scale dynamics and baroclinic disturbances may also contribute to precipitation extremes on sub-daily timescales.

Subantarctic cyclones identified by 14 tracking methods, and their role for moisture transports into the continent

Extra-tropical cyclones in the subantarctic play a central role in the poleward transport of heat and moisture into Antarctica, with the latter being a key component of the mass balance of the Antarctic ice sheet. As the climate in this region undergoes substantial changes, it is anticipated that the character of these synoptic features will change. There are a number of different methods used to identify and track cyclones, which can potentially lead to different conclusions as to cyclone variability and trends, and mechanisms which drive these features. Given this, it is timely to assess the level of consensus among 14 state-of-the-art cyclone identification and tracking methods. We undertake this comparison with the ERA-Interim data-set for the period 1979–2008 and find large differences in the number of tracks identified by different methods, but the spatial patterns of the system density broadly agree. Links between large-scale modes of variability, such as the Southern Annular Mode (SAM), and subantarctic cyclones as suggested in the literature are confirmed by our analysis. Trends in the number of cyclone tracks show a more diverse picture. Robust trends are identified by almost all methods for austral summer over the region south to 60°S, mainly due to the strong relation to SAM, whereas in austral winter the methods disagree in the statistical significance of the trends. The agreement among the methods is greater when the comparison is confined to the stronger cyclones. This is confirmed by a moisture flux analysis associated with these strong synoptic systems. Our results indicate that multiple cyclone identification and tracking methods should be used to obtain robust conclusions for trends in cyclone characteristics as well as their relation to the large-scale circulation in the subantarctic region.

A classification of synoptic patterns inducing heavy precipitation in Saudi Arabia during the period 2000-2014

This study investigates the synoptic patterns associated with the development of heavy rainfall in five different regions of Saudi Arabia. Different synoptic patterns were derived for the heavy rainfall events using a correlation-based map of pattern classifications applied to 1000 hPa gridded NCEP/UCAR reanalysis data for the period 2000-2014. The study summarizes six major synoptic features and classifies 28 detailed synoptic patterns emerging from 186 cases. The derived synoptic patterns differ significantly at lower levels but are relatively similar at upper levels. Heavy rainfall events were most frequent in winter and then spring, while a small percentage occurred during fall in all the regions of the country. In summer, heavy precipitation was restricted to the southern region, which is influenced by an upslope flow.