Synoptic Weather Types Research Articles

Classification of synoptic weather types using the self-organising map and its application to climate and air quality data visualisation

Classification of synoptic weather types using the self-organising map and its application to climate and air quality data visualisation

Influence of large‐scale climate modes on daily synoptic weather types over New Zealand

AbstractThis article examines the influence of El Niño Southern Oscillation (ENSO), Southern Annular Mode (SAM) and Interdecadal Pacific Oscillation (IPO) on the synoptic weather types over New Zealand. The effects of ENSO (indicated by the Southern Oscillation Index, SOI), SAM and IPO on the occurrence of synoptic types are estimated in a holistic framework using the maximum likelihood method via applications of generalized linear models, both annually and by seasons. The average within‐class variations in the intensity and air‐mass characteristics (as expressed in the Auckland Airport meteorological variables) of synoptic systems were examined for individual types at the annual level. The results show that ENSO, SAM and IPO have significant effects on the probability of occurrence and to lesser degree the intensity and air‐mass characteristics of some synoptic types. The effects vary considerably with seasons, synoptic types and phases of SOI, SAM and IPO and it is the confounding effects of different large‐scale modes that lead to the observed changes in the type frequencies. The findings, with respect to ENSO and SAM in all cases and for IPO at the seasonal level, are in good agreement with the literature. However, the annual changes in type frequencies associated with the 1976/1977 IPO phase shift do not support the observed changes in the strength of anomalous southwesterly windflows over New Zealand, and the influence of the recent negative IPO phase also appears different from what we expect based on the existing research. It appears that the interaction between IPO and ENSO is important for understanding the observed climatic effects of IPO on New Zealand's weather and climate. This aspect deserves further attention in future studies. The findings from this work have important methodological and practical implications for New Zealand climate research. Copyright © 2012 Royal Meteorological Society

Aerosol climatology and planetary boundary influence at the Jungfraujoch analyzed by synoptic weather types

Abstract. Fourteen years of meteorological parameters, aerosol variables (absorption and scattering coefficients, aerosol number concentration) and trace gases (CO, NOx, SO2) measured at the Jungfraujoch (JFJ, 3580 m a.s.l.) have been analyzed as a function of different synoptic weather types. The Schüepp synoptic weather type of the Alps (SYNALP) classification from the Alpine Weather Statistics (AWS) was used to define the synoptic meteorology over the whole Swiss region. The seasonal contribution of each synoptic weather type to the aerosol concentration was deduced from the aerosol annual cycles while the planetary boundary layer (PBL) influence was estimated by means of the diurnal cycles. Since aerosols are scavenged by precipitation, the diurnal cycle of the CO concentration was also used to identify polluted air masses. SO2 and NOx concentrations were used as precursor tracers for new particle formation and growth, respectively. The aerosol optical parameters and number concentration show elevated loadings during advective weather types during the December–March period and for the convective anticyclonic and convective indifferent weather types during the April–September period. This study confirms the consensus view that the JFJ is mainly influenced by the free troposphere during winter and by injection of air parcels from the PBL during summer. A more detailed picture is, however, drawn where the JFJ is completely influenced by free tropospheric air masses in winter during advective weather types and largely influenced by the PBL also during the night in summer during the subsidence weather type. Between these two extreme situations, the PBL influence at the JFJ depends on both the time of year and the synoptic weather type. The fraction of PBL air transported to the JFJ was estimated by the relative increase of the specific humidity and CO.

Unexpected increase in precipitation intensity with temperature — A result of mixing of precipitation types?

Using synoptic weather types and comparing high-resolution precipitation and temperature station data, a separation of large-scale and convective precipitation events is performed. We present percentiles of both types and their superposition for varying precipitation accumulation timescales. In some temperature ranges, large-scale, convective and total precipitation percentiles follow increases with temperature at rates higher than that of the saturation humidity increase of the atmosphere of roughly 7% per degree Kelvin — as given by the Clausius–Clapeyron (CC) relation. However, the increase in total precipitation is found to be due to the transition between the corresponding percentiles of the large-scale and convective types, rather than their individual sections of steep increase. Furthermore, convective precipitation displays a leveling-off towards higher temperatures. This poses further challenges to reconcile arguments brought forward elsewhere — namely those suggesting convective precipitation as the driver of the super-CC increase — with the present observational data.

On two different objective procedures for classifying synoptic weather types over east Australia

AbstractSynoptic weather typing depends on the method and its implementation. This paper demonstrates the utility of a 2‐phase batch self‐organising map procedure (CP2) in comparison with a procedure (CP1) consisting of T‐mode principal component analysis followed by convergent K‐means clustering, for classifying the synoptic weather types over east Australia. Four classifications were obtained for the 52‐year NCEP/NCAR dataset, one from CP1 and three from CP2. These classifications were examined in terms of grouping quality, mean type maps, type frequencies, lifetime and transitions, and in relation to the Southern Oscillation Index (SOI).The results showed that the classifications derived from different procedures are inter‐confirmative and capture a similar set of major synoptic situations influencing east Australia, each having counterparts in previous studies and conforming well to local synoptic experience. In particular, this study demonstrates that CP2 is a promising tool for the purpose of weather typing with a two‐fold utility: CP2 can perform data projection (neighbourhood width > = 1) and provide a flexible means for visualising the broad distribution of the daily weather patterns in the dataset; and the procedure can also conduct cluster analysis (neighbourhood width = 0) and produce results equivalent to those from CP1. It was found that performing data clustering or data projection may affect the inference associated with type frequencies. While focusing on methodology, the analysis has also shown that the frequency of synoptic weather types fluctuates on the seasonal, interannual, and decadal scales. From the 1970s to 1995 there was a decline in the occurrence of a few weather types associated with an east‐coast trough extending over New South Wales from the north, and a slight increase in the activity of some anticyclonic types. Significant correlations were also identified between SOI and frequencies of synoptic weather types on the annual and seasonal timescales. Copyright © 2011 Royal Meteorological Society

A new objective procedure for classifying New Zealand synoptic weather types during 1958–2008

AbstractA set of 12 daily weather types has been obtained for the New Zealand region from the 51‐year NCAR/NCEP dataset by using a two‐stage classification procedure (rotated T‐mode principal component analysis followed by convergent K‐means clustering). Analysis of this classification confirms previous studies and provides further insight into the variation features of the regional synoptic weather on the seasonal, interannual and longer‐time scales.There are seasonal variations in both intensity and frequency of synoptic weather types. For example, southern highs are more frequent in summer and autumn, while troughs and lows occur more often in winter and secondarily spring. The high and low centres of weather types tend to be more intense in winter than summer.The frequency and lifetime of synoptic weather types fluctuate on the interannual and decadal scales; there exist significant correlations between frequencies of individual weather types and Southern Oscillation Index (SOI). In all seasons, the frequencies of weather types can change with phases of SOI. The most significant changes in weather type frequencies during the negative SOI phase occur in spring, and characterized by more frequent occurrence of weather types which provide prevailing southwesterly to westerly winds and reduced prominence of anticyclonic situations. The most significant changes in type frequencies during the positive SOI phase occur in summer, with more frequent anticyclonic types but less frequent cyclonic types. The frequency change in winter seems different from other seasons, although consistent with previous studies. In addition, the probability of extremely frequent occurrence or lack of occurrence of certain weather types during a month also varies significantly with SOI phases and with seasons, indicating that the chance for extreme weather conditions to occur is conditional on SOI.The results can be useful for climate forecasts, studies of climate extremes, air pollution, climatic impacts on human health and other environmental problems. Copyright © 2010 Royal Meteorological Society

Atmospheric circulation patterns associated with strong wind events in Catalonia

Abstract. The benefit of having a daily synoptic weather type catalogue and even more, a detailed catalogue for high impact weather events is well recognised by both climatologist and meteorologist communities. In this way the Meteorological Service of Catalonia (SMC) has produced some accurate classifications for extreme events, such as hailstorms or strong winds (SW). Within the framework of the MEDEX project, the SMC has been collaborating to increase the level of awareness about these events. Following this line of work, the aim of this study is to characterise the SW events in Catalonia. According to the guidelines of the MEDEX project we worked with its SW event database for the period June 1995 to May 2004. We also used the period 2005–2009 to test the methodology. The methodology is based on principal component, cluster and discriminant analyses and applied to four variables: SLP, temperature at 850 hPa and geopotential at 500 hPa on a synoptic-scale and local gust wind. We worked with ERA-Interim reanalysis and applied discriminant analysis to test the quality of the methodology and to classify the events of the validation period. We found seven patterns for the SW events. The strongest event corresponds to NW-Flow with the Azores Anticyclone and the passing of a low pressure through the Pyrenees. This methodology has distinguished the summer events in an independent cluster. The results obtained encourage us to follow this line of work.

Kidson’s Synoptic Weather Types and Surface Climate Variability over New Zealand

Kidson’s Synoptic Weather Types and Surface Climate Variability over New Zealand

Linking synoptic weather types to daily rainfall in Auckland

Linking synoptic weather types to daily rainfall in Auckland

Synoptic weather types in the south of South America and their relationship to daily rainfall in the core crop-producing region in Argentina

Synoptic weather types in the south of South America and their relationship to daily rainfall in the core crop-producing region in Argentina

Synoptic Weather Types and Long-range Transport Patterns for Ozone Precursors during High-ozone Events in Southern Sweden

We studied long-range transport patterns and related weather types in relation to high-ozone events in southern Sweden. The aim was to deepen the understanding of the relationship between Lamb-Jenkinson weather types and surface ozone concentration variation, thus widening the application of the weather type analysis of air quality at 4 sites in this region. The long-range transport patterns associated with high-ozone events were classified into trajectories from Western Europe, Eastern Europe, and in the vicinity of southern Sweden (VIC). The VIC type, characterized by short and whirling curves, represented more than 40% of the high-ozone events at the studied rural sites. More than half of the high-ozone events occurred under high-pressure conditions, belonging to weather type A (anticyclones). The high correlation coefficient between annual counts of weather type A and those of long-range transport pattern VIC confirmed the strong link between stagnant weather conditions and high-ozone events, especially during the summer. Furthermore, a strong linear anticorrelation was detected between high-ozone events and annual counts of weather type C (cyclones) during the summer. This relationship implies that the frequency of weather type C is a useful indicator for low risk of summertime high-ozone events in southern Sweden. Moreover, the relationship between the weather type and high ozone risk may be useful in examining the potential effect of climate change on the regional air quality.

Synoptic circulation control on wild fire occurrence

Wild fires have gained increasing interest during the last decade worldwide. Millions of hectares are burned out every year in various countries in all continents. Most of them are burned by the villagers to increase the areas which are available to cultivation but many fires also start by natural causes. Especially in the Mediterranean regions the demise of the traditional landscape management practices along the last decades is another relevant cause of wild fires, since large amounts of fuels are accumulated in the forests, increasing the risk of forest fires. The continuous drought, the rarity of precipitation and the intensity of the winds play a significant role in the onset and evolution of a wild fire. In this work we are trying to link synoptic weather types with events of forest fires that took place in Greece during the 20-year period 1985–2004. To find possible associations with the weather systems prevailing in an area we used the synoptic classification proposed by the ESF’s COST 733 action-release 1.2 and the classification software v.17-01. Since the onset of a wild fire is a rather complex phenomenon and is largely determined by the combination of high air temperatures and absence (or rareness) of precipitation during a certain period before the event, we used the sequential option of the classifications. Specifically we applied 3, 7 and 15 days sequences, for 9 and 18 classes, and for KMEANS, DKMEANS, KHC, LUND, HCl and SANDRAS synoptic catalogues in Domain 10 of the COST 733 scheme, in which Greece is located. The analysis shows that if we use KMEANS and HCl for both the 9 and 18 classes and for all sequences the number of synoptic types that are associated with wild fires is reduced to 2–3. These 2–3 categories are associated with almost 70–90% of the total wild fires events in Greece. On the other hand the KHC and LUND catalogues are not very successful since wild fire events are almost evenly dispersed among the categories.

Synoptic weather typing and typhoon with an application to Chiayi, Taiwan: potential for future climate change impact analysis

In this study, an automated synoptic weather typing was employed to identity the weather types most likely associated with daily typhoon/typhoon-related heavy rainfall events for Chiayi, Taiwan. The synoptic weather typing was developed using principal components analysis, an average linkage clustering procedure, and discriminant function analysis. The classification results showed that the synoptic weather typing was successful at identifying typhoon-related weather types. Five synoptic weather types (Weather Types 1–5) were identified over the past 11-year period as the primary typhoon-related weather types. These five typhoon-related weather types can capture 34 out of 36 total typhoon-related heavy rainfall days (>50 mm/d) and all nine cases with typhoon-related daily rainfall >200 mm during the period March 1998–December 2008. This result suggests that synoptic weather typing can be useful to identify historical typhoon/typhoon-related heavy rainfall events. Moreover, the method has potential to assess climate change impacts on the frequency/intensity of future typhoon/typhoon-related heavy rainfall events using future downscaled GCM climate data.

Soya bean yield variability in the Argentine Pampas in relation to synoptic weather types: monitoring implications

AbstractThis paper analyses the soya bean yield variability in the Argentine Pampas in relation to the atmospheric circulation patterns in southern South America. The region studied is the main soya bean production area in Argentina and contributes a high percentage to world trade in soya beans and their by‐products.Two datasets were used: the soya bean yield series of 58 provincial districts and daily average fields of 1000 and 500 hPa geopotential height corresponding to the NCEP_Reanalysis 2 in the period 1979–2001. Adverse impacts on final yield may be mainly associated to weather types (WTs) related to intense rainfall during harvest time (April and May) and to structures which favour stability and warm advection in the summer at lower levels. The thermal effect on soya bean may be observed in the WTs whose configuration of anomalies favours the incursion of cold air at the lower levels contributing to low temperatures during seed germination and early vegetative growth stages. Positive impacts may be associated with WTs that reduce thermal stress during flowering and pod set stages (summer) and WTs which favour stability at harvest time (April).In general terms, atmospheric circulation structures have been identified as elements which may be added to the set of climate predictors to explain a part of interannual yield variability. The structures found have direct implications for short and middle term forecasts. They constitute a primary element for the monitoring of the growing season of the soya bean crop and for planning strategies. Copyright © 2009 Royal Meteorological Society

Atmospheric forcing of the Beaufort Sea ice gyre: Surface pressure climatology and sea ice motion

The Beaufort Gyre (BG) typically rotates anticyclonically and exerts an important control on Arctic Sea ice dynamics. Previous studies have shown reversals in the BG to rotate cyclonically during summer months and, in recent decades, throughout the annual cycle. In this investigation, we explore the synoptic climatology of atmospheric forcing and its relationship to sea ice motion and BG reversals. A catalog of daily synoptic weather types is generated for the Beaufort Sea Region covering the period 1979 to 2006 using NCEP/NCAR reanalysis mean sea level pressure data, principle components, and k‐means cluster analyses. Mean synoptic type frequency, persistence, and duration values are calculated for each synoptic type and contrasted between the summer and winter seasons. Daily synoptic types are linked to changes in sea ice vorticity by using correlation analysis on lagged sea ice vorticity data. Lag correlations are found between synoptic types and sea ice vorticity smoothed over a 12‐week running mean and show that cyclonic types, which promote southerly or easterly atmospheric circulation over the southern Beaufort Sea, commonly precede summer reversals. Furthermore, significant seasonal within‐type variability in sea ice vorticity is detected within the synoptic types illustrating the importance of seasonal variability on these processes.

A multi-model ensemble approach for assessment of climate change impact on surface winds in France

Statistical downscaling of 14 coupled atmosphere-ocean general circulation models (AOGCM) is presented to assess potential changes of the 10 m wind speeds in France. First, a statistical downscaling method is introduced to estimate daily mean 10 m wind speed at specific sites using general circulation model output. Daily 850 hPa wind field has been selected as the large scale circulation predictor. The method is based on a classification of the daily wind fields into a few synoptic weather types and multiple linear regressions. Years are divided into an extended winter season from October to March and an extended summer season from April to September, and the procedure is conducted separately for each season. ERA40 reanalysis and observed station data have been used to build and validate the downscaling algorithm over France for the period 1974–2002. The method is then applied to 14 AOGCMs of the coupled model intercomparison project phase 3 (CMIP3) multi-model dataset. Three time periods are focused on: a historical period (1971–2000) from the climate of the twentieth century experiment and two climate projection periods (2046–2065 and 2081–2100) from the IPCC SRES A1B experiment. Evolution of the 10 m wind speed in France and associated uncertainties are discussed. Significant changes are depicted, in particular a decrease of the wind speed in the Mediterranean area. Sources of those changes are investigated by quantifying the effects of changes in the weather type occurrences, and modifications of the distribution of the days within the weather types.

Seasonal and Synoptic Variations in Near-Surface Air Temperature Lapse Rates in a Mountainous Basin

Abstract To accurately estimate near-surface (2 m) air temperatures in a mountainous region for hydrologic prediction models and other investigations of environmental processes, the authors evaluated daily and seasonal variations (with the consideration of different weather types) of surface air temperature lapse rates at a spatial scale of 10 000 km2 in south-central Idaho. Near-surface air temperature data (Tmax, Tmin, and Tavg) from 14 meteorological stations were used to compute daily lapse rates from January 1989 to December 2004 for a medium-elevation study area in south-central Idaho. Daily lapse rates were grouped by month, synoptic weather type, and a combination of both (seasonal–synoptic). Daily air temperature lapse rates show high variability at both daily and seasonal time scales. Daily Tmax lapse rates show a distinct seasonal trend, with steeper lapse rates (greater decrease in temperature with height) occurring in summer and shallower rates (lesser decrease in temperature with height) occurring in winter. Daily Tmin and Tavg lapse rates are more variable and tend to be steepest in spring and shallowest in midsummer. Different synoptic weather types also influence lapse rates, although differences are tenuous. In general, warmer air masses tend to be associated with steeper lapse rates for maximum temperature, and drier air masses have shallower lapse rates for minimum temperature. The largest diurnal range is produced by dry tropical conditions (clear skies, high solar input). Cross-validation results indicate that the commonly used environmental lapse rate [typically assumed to be −0.65°C (100 m)−1] is solely applicable to maximum temperature and often grossly overestimates Tmin and Tavg lapse rates. Regional lapse rates perform better than the environmental lapse rate for Tmin and Tavg, although for some months rates can be predicted more accurately by using monthly lapse rates. Lapse rates computed for different months, synoptic types, and seasonal–synoptic categories all perform similarly. Therefore, the use of monthly lapse rates is recommended as a practical combination of effective performance and ease of implementation.

Synoptic weather types that have caused heavy precipitation in Estonia in the period 1961–2005

The subjects of the study are precipitation and weather types, caused by it, in Estonia in the period 1961-2005. Precipitation is labeled heavy when its 24-hour accumulated sum is at least 50 mm. The work was concentrated on manual analysis and classification of synoptic situations as well as on the definition of tracks of cyclones, fronts and troughs in order to better understand the circumstances leading to the rainfalls. Most of rainfalls have been caused by the passage of different depressions and frontal systems. The role played by convective precipitation is less signif icant. Most frequently, rainfall is brought to Estonia by the southern cyclones, born in the region of the Black Sea or the Mediterranean Sea.

Factors affecting the concentrations of PM 10 in central Taiwan

In this study, the synoptic weather types that have high probability and low probability of producing PM 10 episode are referred to as HPE and LPE, respectively. Multiple linear regressions analysis showed that NO 2 was the most important contributor (35.61%) to the concentrations of PM 10 for HPE weather. For LPE weather, the season factor had the greatest contribution (48.11%) to the concentrations of PM 10. Using the correlation coefficients between the concentrations of PM 10 and SO 2 or NO 2 on HPE and LPE to calculate the increase of PM 10 from LPE to HPE, we found that the increase of PM 10 owing to the increase of SO 2 and NO 2 from LPE to HPE was 12.93 μg/m 3 which was about 51% of the total amount of PM 10 increased from LPE to HPE. Results of factor analysis showed that the first component could be attributed to the result of local pollution especially for the weather patterns of types P3 and P6, while the secondary component for the weather patterns of types P1 and P4 can be attributed to the long-range transport of SO 2 pollutants from China.

The influence of an upper‐level frontal zone on the Mack Lake Wildfire environment

AbstractMeteorological assessment of wildland fire danger has traditionally involved the identification of several synoptic weather types empirically determined to influence wildfire spread. Specifically, in the Great Lakes Region, high wildfire danger is often witnessed in association with northwesterly synoptic‐scale flow aloft. Such synoptic‐scale flow is regularly associated with the development of upper‐level frontal zones also known as upper‐level jet/front systems, which are often characterised by intrusions of stratospheric air into the troposphere. The notion that upper‐frontal development can play an important role in promoting wildfire spread is advanced through interrogation of the output from a fine‐scale numerical simulation of a documented explosive wildfire case; the Mack Lake Fire of May 1980.The Mack Lake case was characterised by a developing upper‐level front embedded within a shortwave trough in the vicinity of the fire location. The upper‐level front originated in northwesterly flow in central Canada as an upper‐tropospheric ridge amplified over western North America. A thermally indirect circulation at the jet exit region both contributed to the intensification of the front and was associated with a maximum in quasi‐geostrophic descent at mid‐levels upstream of the fire region. The subsidence ushered dry air from the middle and upper‐troposphere downward along sloping isentropes adiabatically warming and drying it along the way. A well‐developed dry air intrusion associated with the operation of these processes extended to nearly the 750 hPa level far downstream from the actual upper‐frontal zone supplying the fire environment with dry air that originated in the upper‐troposphere/lower stratosphere. The organised subsidence was also responsible for downward advection of high momentum air from within the frontal zone into the fire environment, further influencing the wildfire spread. We conclude that upper‐frontal processes, characteristic of northwesterly synoptic‐scale flow, are likely a contributing factor to the prevalence of wildfire spread under such synoptic‐scale conditions. Copyright © 2007 Royal Meteorological Society