T-mode Principal Component Analysis Research Articles

Influence of Atmospheric Circulation on Aerosol and its Optical Characteristics in the Pearl River Delta Region

At present, few studies have focused on the impact of circulation patterns on aerosol pollution in the Pearl River delta region (PRD) region based on the objective circulation classification method. Based on PM2.5 observation data, meteorological observation data, Aerosol Robotic Network (AERONET) aerosol observation data and European Center for Medium-Range Weather Forecasting (ECMWF) ERA (European Reanalysis)-Interim data in the PRD during 2013 to 2017, the air pollution level, meteorological conditions, and aerosol optical and radiation characteristics in different circulation patterns are studied in this paper. Using ECMWF ERA-Interim sea level pressure, nine circulation patterns were determined based on the T-mode principal component analysis (PCA) combined with the k-means clustering method. There were significant differences in PM2.5 values under different circulation patterns, indicating that the change of atmospheric circulation is an important factor driving the change of air quality. The prevailing wind in winter (northerly wind) facilitates the transport of pollutants from the north of the PRD and forms severe air pollution, while the prevailing wind in summer (southerly wind) brings clean air from the South China Sea; additionally, a high frequency of precipitation benefits the wet scavenging of pollutants, resulting in improved air quality in the PRD region. The impact of circulation patterns on the AOD (aerosol optical depth) is basically similar to that of the PM2.5 concentration. The study also found that the annual average total radiation was negatively correlated with the annual average PM2.5 concentration. In future, we plan to identify which methods and data are suitable for circulation classification in the PRD region.

Impacts of meteorology and emission control on the abnormally low particulate matter concentration observed during the winter of 2017

As the capital of China, Beijing is the subject of great concern regarding atmospheric pollution, especially that of fine particulate matter (PM2.5). In winter 2017 (i.e., November 2017 to February 2018), PM2.5 concentration in Beijing decreased to its lowest value in the past ten years. In this study, we sought to investigate the impacts of meteorology and emission control on this abnormally low pollution phenomenon. Weather Research and Forecasting coupled with Chemistry (WRF-Chem) simulations show that sensitivity to meteorological factors accounts for approximately 51% of the observed change between winters 2016 and 2017 in Beijing. We further compared the pollution dispersion condition of winters 2016 and 2017 in terms of large-scale circulation patterns and local mixing layer heights (MLHs). The meteorological condition in 2017 was characterized by stronger wind speed (WS) and higher MLH. We classified circulation patterns into two types using the obliquely rotated Principal Component Analysis in T-mode (T-PCA) method. Active weather was found to be associated with high MLH, high near-surface WS, and high ventilation coefficient (VC), which favors the dispersion of pollutants. In contrast, the changes of meteorological variables and pollution conditions are almost opposite for stable weather. Active weather amounted to 73 and 85 days for winters 2016 and 2017, respectively. Stable weather conditions prevailed on 46 days in winter 2016, and 32 days in 2017. Less stable weather during winter 2017 compared with that of winter 2016 possibly favored efficient ventilation of boundary-layer pollution. A lack of persistent stable weather also contributed to low PM2.5 concentration in 2017. This research is important for air-pollution assessment and regional environmental management.

Dominant synoptic patterns during wintertime and their impacts on aerosol pollution in Central China

Abstract Meteorological conditions are largely driven by synoptic circulations, affecting pollutant transport, dispersion, and even emissions. Partly due to the lack of systematic investigations of meteorological impact at large- and local-scale, aerosol pollution in Central China is not yet fully understood. This study conducted synoptic classification based on the obliquely rotated principal component analysis in T-mode, and investigated the relationship between aerosol pollution and the prevailing synoptic patterns over Wuhan during wintertime from 2014 to 2018. Among five identified patterns, three driven by the northerly high-pressure systems, accompanied by weak surface winds, low boundary layer height, and high relative humidity, were found to be closely related to heavy pollution. Notably, the winter aerosol pollution in Wuhan showed a declining trend in recent years, which also slightly benefited from the interannual variation of synoptic patterns besides emission control. However, with the increase in humidity, pollution caused by secondary particles cannot be ignored. The oxidation rate of sulfur and nitrogen in the atmosphere was about 0.1 higher in 2018 than in 2014. This study is significant for pollution control in Wuhan and its surrounding areas of Central China.

Spatial variability analysis of precipitation and its concentration in Chaharmahal and Bakhtiari province, Iran

The spatial variability of annual precipitation and precipitation concentration has been analyzed in Chaharmahal and Bakhtiari province using data from 42 stations between 1981 and 2012. For this purpose, the correlation decay distance (CDD) and the coefficient of variation of annual precipitation have been computed at each station to show the inter-annual variability of precipitation. Results showed an increasing gradient from the southwest to the northeast in inter-annual precipitation variability. The degree of seasonality of precipitation and the year-to-year precipitation concentration variability were also investigated using the precipitation concentration index (PCI) and the coefficient of variation of PCI, respectively. The spatial distributions of intra-annual precipitation variability (or precipitation concentration) and inter-annual precipitation concentration showed a decreasing gradient from the southeast to the northwest. The T-mode principal component analysis (PCA) in association with cluster analysis (CA) was used to delineate homogeneous precipitation sub-zones for simplifying the spatial structure of precipitation variability over the region during the considered period. Results suggest three distinct sub-regions, including the northeastern part (the driest sub-region with the highest precipitation variability), the central and southern parts (the sub-region with the highest degree of seasonality of precipitation), and the northwestern part (the rainiest sub-region with the lowest inter-annual and intra-annual precipitation variability).

Spatio-Temporal Assessment of Vegetation Resource Dynamics in Nigeria from SPOT Satellite Imageries

Vegetation resources in Nigeria are of vital importance for the sustainable development of the country. However, this essential resource is in danger due to the effect of anthropogenic and climate induced impacts. Currently desert encroachment which cuts across the Sahel is affecting most of the states in the northern part of the country particularly the eleven states considered by the Federal Ministry of Environment in Nigeria as the frontline states. Several studies on the Nigerian environment have shown that there are serious threats to the general environment particularly vegetation. Due to population growth and the need for housing as well as the expansion of the over-utilised farmlands across these states, places considered as reserved areas across the country are being exploited to the detriment of the vegetal resources particularly the forest and rangeland areas. This study utilized Idrisi TerrSet (version 18) raster-based remote sensing and GIS software to analyse seventy two (72) dekadal Normalised Vegetation Index (NDVI) imageries from SPOT satellite covering Nigeria in order to assess the anthropogenic and likely climatic impacts on the vegetal resources using the forward t-mode Principal Component Analysis (PCA) with standardised principal components. Results indicated that Component 1 which explains about 69% of the 72 time-series NDVI imageries shows typical vegetation cover over the study area within the time period under study. While component two indicated a cyclic trend differentiating the ENSO events of 1999 and 2009; component three indicated positive anomaly pattern of vegetation NDVI mostly within Sokoto, Kebbi, Kano, Jigawa and the northern parts of Bauchi, Yobe and Borno states. However, Component four imagery indicated a likely link to the 2009 flood that affected Kainji dam and rivers Niger and Benue. For a better result, the integration of socio-economic and high spatial resolution data into an assessment of this kind in future studies is encouraged.

Analyses of winter circulation types and their impacts on haze pollution in Beijing

To better understand the interannual variation of winter haze pollution, this paper investigates winter circulation types and their impacts on local meteorological conditions and haze pollution during 38 winters from 1980 to 2017 in Beijing. Circulation types were classified by T-mode principal component analysis combined with the K-means cluster method using European Centre for Medium-range Weather Forecasts ERA-interim sea level pressure data. The results can significantly distinguish the cold air process, a degeneration of cold air, and stagnant weather. The cold air process over Beijing was accompanied by a low temperature, high relative humidity, large pressure gradient and near-surface wind speed, and deep mixing layer. The cold air process facilitated pollutant dispersion and transport to the outside (such as East China, Bohai Sea, and Yellow Sea) and formed low PM2.5 concentrations and low frequencies of haze days. In contrast, the local meteorology and haze pollution were almost the inverse for stagnant weather. The local meteorological conditions and haze pollution for the degeneration of cold air are between the previous circulation types. Considering PM2.5 observation during 2010–2017, the occurrence frequency of cold air was low in the recent winters of 2013, 2014 and 2017, and resulted in severe PM2.5 pollution. High frequency of stagnant weather (48.4%) was one of the reasons that haze pollution reached 37% during 38 winters from 1980 to 2017 over Beijing. The time series of haze frequency was negatively correlated with that of cold air frequency. Winter haze pollution was affected by climate change over Beijing. During 38 winters from 1980 to 2017, a decreased trend of haze days was found, which was partly related to an increased trend of cold air frequency. However, the trends of haze days and cold air in Beijing were not significant based on regression analysis.

Elucidating the relationship between aerosol concentration and summertime boundary layer structure in central China

Wuhan, a megacity in central China, suffers from frequent aerosol pollution and is accompanied by meteorological factors at both synoptic and local scales. Partly due to the lack of appropriate observations of planetary boundary layer (PBL), the associations between synoptic conditions, PBL, and pollution there are not yet fully understood. Thus, systematic analyses were conducted using the fine-resolution soundings, surface meteorological measurements, and aerosol observations in Wuhan during summer for the period 2013–2016, in combination with T-mode principal component analysis and simulations of backward trajectory. The results showed that the variations of boundary layer height (BLH) not only modulated the diurnal variation of PM2.5 concentration in Wuhan, but also the daily pollution level. Five different synoptic patterns during summer in Wuhan were identified from reanalysis geopotential height fields. Among these synoptic patterns, two types characterized by northeasterly prevailing winds, were found to be associated with heavy pollution in Wuhan. Driven by the northeasterly winds, the polluted air mass from the heavily polluted regions could be easily transported to Wuhan, such as North China Plain and Yangtze River Delta. Such regional transports of pollutants must be partly responsible for the aerosol pollution in Wuhan. In addition, these two synoptic patterns were also featured by the relatively high cloud cover and low boundary layer height in Wuhan, which would favor the occurrence of pollution there. Overall, this study has important implications for understanding the important roles of meteorological factors in modulating aerosol pollution in central China.

Effects of synoptic circulation patterns on air quality in Nanjing and its surrounding areas during 2013–2015

The Yangtze River Delta region is the most active of China's economic regions; however, industrialization and urbanization have seriously affected air quality in the region and air pollution is becoming increasingly serious. Based on the NCEP/NCAR FNL reanalysis data of sea level pressure (SLP) from 2013 to 2015, an objective quantitative method of synoptic circulation classification, T-mode principal component analysis (PCA), was used to analyze the influence of synoptic circulation pattern on air quality in East China. Circulation was divided into nine types: (1) Northwest High Pressure (NWH); (2) Front of Low Pressure (FL); (3) North High Pressure (NH); (4) Northeast High Pressure (NEH); (5) South Low pressure (SL); (6) Weak pressure (Unique); (7) Southwest Low Pressure (SWL); (8) Northeast Low Pressure (NEL); and (9) Low Pressure (L). The air quality in the Nanjing area is discussed in this paper, as well as the relationship between synoptic circulation patterns and local meteorological conditions. The results show that there is a close relationship between synoptic circulation patterns and local meteorological conditions that greatly affects the air quality of the Nanjing area. By the quantitative analysis of the concentrations of five air pollutants (PM2.5, CO, NO2, SO2 and O3) under different synoptic circulation types, it was found that circulation type 1 (CT 1, NWH), circulation type 3 (CT 3, NH) and circulation type 4 (CT 4, NEH) were associated with poor air quality; the mean PM2.5 concentrations during 2013, 2014 and 2015 were 83.8 μgm-3, 77.6 μgm-3 and 78.4 μgm-3, respectively. The local meteorological conditions corresponding to the three circulation types were low temperature and low wind speed. In contrast, circulation type 2 (CT 2, FL) and circulation type 8 (CT 8, NEL), which were associated with better air quality, had opposite local meteorological conditions; the mean PM2.5 concentrations were 53.2 μgm-3 and 57.7 μgm-3, respectively. This case study shows that the Nanjing region was mainly controlled by CT 1 and CT 3 during the major pollution event in January 2015. The regional transport paths associated with specific circulation types were identified by trajectory analysis in the haze episodes. It was found that two rainfall events played a positive role in improving air quality in the Nanjing area.

Characterization of synoptic patterns causing dust outbreaks that affect the Arabian Peninsula

Dust storms pose serious weather hazards in arid and semiarid regions of the earth. Understanding the main synoptic conditions that give rise to dust outbreaks is important for issuing forecasts and warnings to the public in cases of severe storms. The aim of the present study is to determine synoptic patterns that are associated with or even favor dust outbreaks over the Arabian Peninsula. In this respect, red–green–blue dust composite images from the Meteosat Second Generation (MSG) satellite are used to detect dust outbreaks affecting the Arabian Peninsula, with possible influences in southwestern Asia and northeastern Africa, between 2005 and 2013. The Meteosat imagery yielded a sample of 95 dust storm days. Meteorological fields from NCEP/NCAR reanalysis data of wind fields at 10m and 250hPa, mean sea level pressure, and geopotential heights at 850 and 500hPa were obtained for the dust storm days. Using principal component analysis in T-mode and non-hierarchical k-means clustering, we obtained four major atmospheric circulation patterns associated with dust outbreaks during the study days. Cluster 4 had the largest number of days with dust events, which were constrained to summer, and cluster 3 had the fewest. In clusters 1, 2 and 3, the jet stream favored the entry of a low-pressure area or trough that varied in location between the three clusters. Their most northerly location was found in cluster 4, along with an extensive low-pressure area supporting strong winds over the Arabian Peninsula.The spatial distribution of aerosol optical depth for each cluster obtained was characterized using the Moderate Resolution Imaging Spectroradiometer data. Then, using METAR stations, clusters were also characterized in terms of frequency and visibility.

Classification of summertime synoptic patterns in Beijing and their associations with boundary layer structure affecting aerosol pollution

Abstract. Meteorological conditions within the planetary boundary layer (PBL) are closely governed by large-scale synoptic patterns and play important roles in air quality by directly and indirectly affecting the emission, transport, formation, and deposition of air pollutants. Partly due to the lack of long-term fine-resolution observations of the PBL, the relationships between synoptic patterns, PBL structure, and aerosol pollution in Beijing have not been well understood. This study applied the obliquely rotated principal component analysis in T-mode to classify the summertime synoptic conditions over Beijing using the National Centers for Environmental Prediction reanalysis from 2011 to 2014, and investigated their relationships with PBL structure and aerosol pollution by combining numerical simulations, measurements of surface meteorological variables, fine-resolution soundings, the concentration of particles with diameters less than or equal to 2.5 µm, total cloud cover (CLD), and reanalysis data. Among the seven identified synoptic patterns, three types accounted for 67 % of the total number of cases studied and were associated with heavy aerosol pollution events. These particular synoptic patterns were characterized by high-pressure systems located to the east or southeast of Beijing at the 925 hPa level, which blocked the air flow seaward, and southerly PBL winds that brought in polluted air from the southern industrial zone. The horizontal transport of pollutants induced by the synoptic forcings may be the most important factor affecting the air quality of Beijing in summer. In the vertical dimension, these three synoptic patterns featured a relatively low boundary layer height (BLH) in the afternoon, accompanied by high CLD and southerly cold advection from the seas within the PBL. The high CLD reduced the solar radiation reaching the surface, and suppressed the thermal turbulence, leading to lower BLH. Besides, the numerical sensitive experiments show that cold advection induced by the large-scale synoptic forcing may have cooled the PBL, leading to an increase in near-surface stability and a decrease in the BLH in the afternoon. Moreover, when warm advection appeared simultaneously above the top level of the PBL, the thermal inversion layer capping the PBL may have been strengthened, resulting in the further suppression of PBL and thus the deterioration of aerosol pollution levels. This study has important implications for understanding the crucial roles that meteorological factors (at both synoptic and local scales) play in modulating and forecasting aerosol pollution in Beijing and its surrounding area.

A precipitation regionalization and regime for Iran based on multivariate analysis

Monthly precipitation time series of 155 synoptic stations distributed over Iran, covering 1990–2014 time period, were used to identify areas with different precipitation time variability and regimes utilizing S-mode principal component analysis (PCA) and cluster analysis (CA) preceded by T-mode PCA, respectively. Taking into account the maximum loading values of the rotated components, the first approach revealed five sub-regions characterized by different precipitation time variability, while the second method delineated eight sub-regions featured with different precipitation regimes. The sub-regions identified by the two used methods, although partly overlapping, are different considering their areal extent and complement each other as they are useful for different purposes and applications. Northwestern Iran and the Caspian Sea area were found as the two most distinctive Iranian precipitation sub-regions considering both time variability and precipitation regime since they were well captured with relatively identical areas by the two used approaches. However, the areal extents of the other three sub-regions identified by the first approach were not coincident with the coverage of their counterpart sub-regions defined by the second approach. Results suggest that the precipitation sub-region identified by the two methods would not be necessarily the same, as the first method which accounts for the variance of the data grouped stations with similar temporal variability while the second one which considers a fixed climatology defined by the average over the period 1990–2014 clusters stations having a similar march of monthly precipitation.

Moderate to heavy cold-weather precipitation occurrences in Tehran and the associated circulation types

Large-scale atmospheric circulations associated with 133 moderate to heavy cold-weather precipitation events recorded at Mehrabad station in Tehran, Iran, during the period 1951–2013 are analysed. To this end, the performance of un-rotated, orthogonally rotated and obliquely rotated solutions of T-mode principal component analysis (PCA) is examined in classifying the atmospheric circulations into a few representative circulation types (CTs). The T-mode PCAs were applied to the 500-hPa geopotential height for the events in a domain from 10∘E to 70∘E and from 20∘N to 50∘N. The first six leading principal components were retained and then orthogonally and obliquely rotated using varimax and promax solutions, respectively. Statistical inter-comparison of the CTs obtained using the three solutions suggests that the obliquely rotated solution is the better choice for circulation classification in the present study. The six CTs obtained using the oblique rotation were then linked to the daily total precipitation and daily mean temperature variability at Tehran station as well as to the standardized anomalies of the daily total precipitation and mean daily temperature of a dense network of stations distributed across Iran. It is found that the CTs identified, though generally comparable in producing significant precipitation in Tehran, vary in their potential to bring cold weather and generate snowfall in Tehran specifically and in the country in general. While the first three CTs give rise to regional patterns of standardized precipitation anomalies centred in Tehran, the next three CTs leave a pronounced precipitation signature almost across the whole country. As regards the standardized temperature anomalies, with the exception of one CT that causes deep and widespread negative standardized anomalies over most parts of the country, the other CTs are characterized with a dipolar structure of a deep intrusion of cold weather to the west and prevailing warm weather to the east of the country.

The meteorological modulation on PM2.5 interannual oscillation during 2013 to 2015 in Shanghai, China

Since the Action Plan for Air Pollution Prevention and Control (the Action Plan) was implemented at the end of 2013, the ambient air quality in China is significantly improved. However, PM2.5 (particles with diameter≤2.5μm) levels in some cities still exhibit clear interannual oscillations. For example, the annual mean PM2.5 levels in Shanghai decreased by 16.1% in 2014, while increased by 2.2% in 2015 according to year-on-year comparisons. To better understand the corresponding causes, the obliquely rotated T-mode principal component analysis (PCA) method and WRF-Chem model are jointly employed in this study. Results show that the west wind frequency and the accumulative wind (<1.8m/s) duration are the key indicators affecting local PM2.5 transport and dispersion significantly. Moreover, four typical synoptic patterns conductive to PM2.5 pollution are illustrated as mid-ward path cold (CM), eastward path cold (CE), L-share high (GL) and near high center (GC), in which GL is the most adverse circulation pattern. The year-on-year changes of meteorology have positive effects on PM2.5 year-on-year variations. The significant decline of PM2.5 levels in 2014 compared with those in 2013, as well as the obvious increase in 2015 compared with 2014, both well correspond to year-on-year variations of meteorological indicators. Model results present that PM2.5 interannual variations result from the changes of meteorology during 2013 to 2015, are consistent with measured oscillations. By comparing measured and modeled PM2.5 year-on-year variabilities, a greater PM2.5 decreasing at 9.4% is estimated under favorable meteorological conditions, while a less increasing at 6% under unfavorable meteorological conditions due to emission reductions, indicating the initial improvement has been achieved by the Action Plan. Otherwise, since the current Action Plan has difficulties in completely offsetting the PM2.5 rise attributed to the adverse weather, more stringent program should be drawn up for unfavorable meteorological conditions.

Spatial variability analysis of precipitation in northwest Iran

The spatial variability of precipitation was investigated in the northwestern corner of Iran using data collected at 24 synoptic stations from 1986 to 2015. Principal component analysis (PCA) and cluster analysis (CA) were used to regionalize precipitation in the study area. Eleven precipitation variables were averaged and arranged as an input matrix for the R-mode PCA to identify the precipitation patterns. Results suggest that the study area can be divided into four spatially homogeneous sub-zones. In addition, the spatial patterns of annual precipitation were identified by applying the T-mode PCA and CA to the annual precipitation data. The delineated spatial patterns revealed three distinct sub-regions. The resultant maps were compared with the spatial distribution of the rotated principal components (PCs). Results pointed out that the delineated clusters are characterized by different precipitation variability; and using different precipitation parameters can lead to different spatial patterns of precipitation over northwest Iran.

Análise da relação entre o δ18O do Nevado Illimani com a distribuição espacial da precipitação sobre a América do Sul

Esse artigo analisa as relações entre a série temporal da razão isotópica do oxigênio dezoito (δ18O) mensurada no testemunho de gelo do Nevado Illimani (Bolívia; 16°37’S, 67°46’W) com a distribuição espacial da precipitação na América do Sul. Foram utilizados dados dos primeiros 50 m do testemunho de gelo e 360 campos espaciais da precipitação observada no continente entre 4°N e 40°S. Os campos de precipitação foram agrupados por meio da Análise das Componentes Principais no Modo-T e a ocorrência temporal de cada grupo (Padrão Espacial Anômalo - PEA) foi identificada quando o índice de correlação entre a componente principal e os campos observados é superior a |0,4|. Constatou-se que há 12 PEA da precipitação e que são originários, predominantemente, da variação na temperatura superficial dos oceanos Pacífico Equatorial e Atlântico Tropical Norte. A análise da ocorrência temporal desses PEA com a série temporal do δ18O evidenciou que os registros de máximo (mínimo) fracionamento isotópico no verão, estão relacionados à seca (chuvas acima da média) na Amazônia ou no nordeste brasileiro. Já no inverno, os registros de máximo (mínimo) fracionamento isotópico estão relacionados à secas (chuvas acima da média no litoral) na região tropical e chuvas acima da média (secas) na região extratropical.

Characterizing implications of two-dimensional space–time orientations for principal component analysis of geographic time series

This article introduces four new modes of principal component analysis (PCA) to investigate space–time variability in an image time series. Using the concept of tensors, an image time series can be understood as a space–time cube and can be analysed using six different orientations by grouping the basic elements (voxels) of the cube across different dimensions. Voxels grouped across columns or rows of the cube to produce vectors result in profiles. Voxels grouped across different planes to produce matrices result in slices. The traditional S-mode and T-mode PCA are thus the profile modes and slice modes across time and across space, respectively. This research introduces two profile-mode orientations across longitude and latitude and two slice-mode orientations across longitude–time and latitude–time. The research shows that a more complete understanding of the spatio-temporal variability in the data set can be achieved by investigating these different orientation modes, as individual modes have the capability of capturing variability in a particular dimension of a spatio-temporal data set. A case study was carried out using weekly anomalies of the AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data) sea surface height product filtered for tropical instability waves (TIWs) for a three-year time period from 1997 to 1999 in the tropical Pacific region. The results show that PCA with longitude as the dimension of variability and latitude–time as the dimension of variability were able to capture the TIW and barotropic Rossby wave propagation across the equatorial Pacific. The other two orientation modes were able to detect dominant latitudinal locations for TIW.

A synoptic characterization of the dust transport and associated thermal anomalies in the Mediterranean basin

ABSTRACTIn this work the possibility to characterize the daily dust transport and associated thermal anomalies in the Mediterranean through circulation type classification (CTC) methods is explored. The dust loads are estimated through two different sources: the aerosol optical depth (AOD) simulated by the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model in the period 2000–2007 and AOD remote sensed by the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) in the period 2001–2010. The dust transport from the Sahara is identified linking the AOD anomalies to the thermal anomalies into the Mediterranean, studying the covariance modes of AOD and air temperature at 850 hPa from the NASA Modern‐Era Retrospective Analysis for Research and Applications (MERRA) dataset. The time series of the expansion coefficients associated to the first two modes, explaining around 90% of the total covariance, allow to describe the dust transport and thermal anomalies in the eastern, western and central Mediterranean sub‐basins. The circulation types are classified using the MERRA geopotential height at 700 hPa in the period 1979–2010. Two classification methods are tested, based on T‐mode and S‐mode principal component analysis (PCA), with 6, 10 and 14 classes. The performance of the CTC methods in the characterization of the dust and thermal anomalies is evaluated and the best method is selected. Results show that a T‐mode PCA method with 14 classes allows the characterization of dust transport and thermal anomalies in the eastern and western Mediterranean, while the variability in the central Mediterranean is well characterized by a S‐mode PCA method with 10 classes.

The impact of circulation patterns on regional transport pathways and air quality over Beijing and its surroundings

Abstract. This study investigated the air pollution characteristics of synoptic-scale circulation in the Beijing megacity, and provided quantitative evaluation of the impacts of circulation patterns on air quality during the 2008 Beijing Summer Olympics. Nine weather circulation types (CTs) were objectively identified over the North China region during 2000–2009, using obliquely rotated T-mode principal component analysis (PCA). The resulting CTs were examined in relation to the local meteorology, regional transport pathways, and air quality parameters, respectively. The FLEXPART-WRF model was used to calculate 48-h backward plume trajectories for each CT. Each CT was characterized with distinct local meteorology and air mass origin. CT 1 (high pressure to the west with a strong pressure gradient) was characterized by a northwestern air mass origin, with the smallest local and southeasterly air mass sources, and CT 6 (high pressure to the northwest) had air mass sources mostly from the north and east. On the contrary, CTs 5, 8, and 9 (weak pressure field, high pressure to the east, and low pressure to the northwest, respectively) were characterized by southern and southeastern trajectories, which indicated a greater influence of high pollutant emission sources. In turn, poor air quality in Beijing (high loadings of PM10, BC, SO2, NO2, NOx, O3, AOD, and low visibility) was associated with these CTs. Good air quality in Beijing was associated with CTs 1 and 6. The average visibilities (with ±1σ) in Beijing for CTs 1 and 6 during 2000–2009 were 18.5 ± 8.3 km and 14.3 ± 8.5 km, respectively. In contrast, low visibility values of 6.0 ± 3.5 km, 6.6 ± 3.7 km, and 6.7 ± 3.6 km were found in CTs 5, 8, and 9, respectively. The mean concentrations of PM10 for CTs 1, 6, 5, 8, and 9 during 2005–2009 were 90.3 ± 76.3 μg m−3, 111.7 ± 89.6 μg m−3, 173.4 ± 105.8 μg m−3, 158.4 ± 90.0 μg m−3, and 151.2 ± 93.1 μg m−3, respectively. Analysis of the relationship between circulation pattern and air quality during the emission control period suggests that CTs are the primary drivers of day-to-day variations in pollutant concentrations over Beijing and its vicinity. During the Olympics period, the frequency of CT 6 was twice that of the mean in August from 2000 to 2009. This CT had northerly transport pathways and favorable meteorological conditions (e.g. frequent precipitation) for clean air during the Olympics. Assuming that relationships between CTs and air quality parameters in the same season are fixed in different years, the relative contributions of synoptic circulation to decreases in PM10, BC, SO2, NO2, NOx, CO, and horizontal light extinction during the Olympics were estimated as 19 ± 14%, 18 ± 13%, 41 ± 36%, 12 ± 7%, 10 ± 5%, 19 ± 11%, and 54 ± 25%, respectively.

Synoptic patterns associated with wildfires caused by lightning in Castile and Leon, Spain

Abstract. The Iberian Peninsula presents the highest number of wildfires in Europe. In the NW of Spain in particular, wildfires are the natural risk with the greatest economic impact in this region. Wildfires caused by lightning are closely related to the triggering of convective phenomena. The prediction of thunderstorms is a very complex task because these weather events have a local character and are highly dependent on mesoscale atmospheric conditions. The development of convective storms is directly linked to the existence of a synoptic environment favoring convection. The aim of this study is to classify the atmospheric patterns that provide favorable environments for the occurrence of wildfires caused by lightning in the region of Castile and Leon, Spain. The database used for the study contains 376 wildfire days from the period 1987–2006. NCEP data reanalysis has been used. The atmospheric fields used to characterise each day were: geopotential heights and temperatures at 500 hPa and 850 hPa, relative humidity and the horizontal wind at 850 hPa. A Principal Component Analysis in T-mode followed by a Cluster Analysis resulted in a classification of wildfire days into five clusters. The characteristics of these clusters were analysed and described, focusing particularly on the study of those wildfire days in which more than one wildfire was detected. In these cases the main feature observed was the intensification of the disturbance typical of the cluster to which the wildfire belongs.

Spatial fields of Antarctic sea-ice concentration anomalies for summer–autumn and their relationship to Southern Hemisphere atmospheric circulation during the period 1979–2009

AbstractSummer–autumn monthly sea-ice concentration anomaly (SICA) fields in Antarctica obtained from satellite data for the period 1979–2009 were analysed with Varimax-rotated T-mode principal component analysis (PCA). the first three PCA scores described the SICA spatial behaviour and explained 38.07% of the total variance. the related atmospheric circulation characteristics were analysed using 850 hPa height and surface air-temperature anomalies for the months clustered by the corresponding SICA composites, which were based on PCA loadings above a ±0.3 threshold. the principal characteristics of SICA can be seen between the Ross and Weddell Seas, areas that remained ice-covered during the analysis period. Elsewhere around Antarctica, small distinct characteristics occur mostly in embayments. the leading summer–autumn SICA pattern shows a structure with two centres of equal sign located one over the Weddell and the other over the Ross Sea–southwest Pacific Ocean sector and a centre of opposite sign over the Bellingshausen and Amundsen Seas. the second SICA pattern is represented by a dipole over the Weddell Sea as a result of an increase (decrease) in sea-ice concentration in the northern sector (positive phase) and a decrease (increase) in the southern region, together with a positive (negative) centre over the Ross and Amundsen Seas. the latter pattern is characterized by equal-sign anomalies on both sides of the Antarctic Peninsula and opposite-sign centres all around Antarctica with the highest intensity over the Ross Sea.