Sea Level Pressure Patterns Research Articles

Large Scale Processes for Understanding Extreme Rain Storm Over Rangamati on 12 June 2017- A Case Study

Attempts have been made to study the large-scale surface and upper air synoptic processes associated with a monsoon depression during 11-12 June 2017. In this study, Grid Analysis and Display System (GrADS) software has been used to prepare the large-scale sea level pressure and upper flow patterns by analyzing the FNL re-analysis data. In this analysis, FNL dataset is used to characterize the rainstorms, with key hydrometeorological variables describing the prior conditions of the very heavy rainfall event presented the study. National Center for Environmental Prediction (NCEP) Final (FNL) analysis data of 1o by 1o grids for every 6 hours are used for large scale synoptic analysis. The disastrous event was a strong monsoon depression in the early period of southwest monsoon 2017. Due to this depression, very heavy rainfall occurred in the southeastern Bangladesh. Rangamati recorded 343 mm of rainfall in the 24 hours on 12 June 2017. Massive landslides occurred in three districts such as Rangamati, Bandarban and Chittagong. The analysis of surface and upper air synoptic conditions has revealed that a well-marked low was formed over the northwest Bay of Bengal within the low-pressure belt passing from Somalia coast extending through southern Pakistan, and India up to east central Bay of Bengal and adjoining Bangladesh. The wind speed is calculated from the pressure distribution and is found to be 24.23ms-1, which is at par with the observed one. The well-marked low was subsequently intensified into a depression and moved northeastwards over Bangladesh. Strong southsouthwesterly winds were associated in the eastern side of the depression, especially over Chittagong Hill Tracts. The depression was found to extend up to 500 hPa level as seen from the distribution of geopotential and strong circulation around the centre. Winds were advecting from large continental and Ocean areas over the South Asia. Strong winds and moisture influx, strong narrow coma-like trough from a micro low at the surface to 500 hPa level as well as strong wind shear were responsible for the heavy rainfall, disastrous effects and massive landslides over Rangamati and adjoining areas.
 Journal of Engineering Science 11(2), 2020, 87-97

Synoptic analysis of unexampled super-heavy rainfall on April 1, 2019, in west of Iran

The aim of this study is to analyze the synoptic and analyzing the mechanisms of flood occurrence on April 1, 2019, in the western half of Iran. In order to statistically analyze super-heavy precipitation in western Iran, the daily meteorological data from April 2019 for 13 synoptic stations in Kermanshah, Hamadan, and Lorestan Provinces were received and used by the Meteorological Organization. The results show that on the peak day of precipitation in the western half of the country, most stations recorded more than 100 mm of precipitation, the highest of which, 127 mm, occurred at Nahavand station. The arrangement of sea level pressure patterns also indicates the existence of severe pressure gradients between the low pressure in northern Saudi Arabia and western Iran and the high pressure of Siberian and European cold cores, which have created contradictory weather conditions and a strong front at sea level. Also, in the middle and upper levels of the troposphere, with the penetration of cold air flow and cyclonic from Russia and its settlement in West Asia and the region in the form of cutoff low-pressure blocking, severe positive vorticity and divergent and humid ascent flows at all analyzed levels in the western half of the country have been created. Humidity advection maps of 500–1000 hPa levels show that at the 850 and 1000 hPa levels a humidity supply is provided from various sources in the Gulf of Aden, Red Sea, and Mediterranean Sea. But at levels of 500 and 700 hPa, only the Mediterranean Sea plays a key role in supplying and injecting humidity.

Productivity of Jute Influenced by Some Major Climatic Variables in South -Western Coastal Region of Bangladesh

Due to be in disaster prone geographic area because of climate change, Bangladesh is one of the vulnerable largest deltas in the world. Agriculture is the main influential factor for Bangladesh economy. Jute and various vendible of jute are good sources of earning foreign currency of Bangladesh. But in the last decades it seems that our Agricultural sector especially coastal belt has been affected drastically by climate change. It is to scientifically assess the vulnerability of the golden fibre of Bangladesh; Jute production due to the unfavourable climatic variables fluctuation in South Western coastal belt, a original field research has been done considering the influential climatic factors of temperature, rainfall, humidity, sun shine hours and sea level pressure of last 30 years (1981-2011) and their impacts on the production of Jute. During the producing season of Jute, it has been increased the maximum and average temperature. And it was about unchanged pattern of sea-level pressure of here but the most important factor for production rainfall has been increased by 53, 12 and 142 mm in Khulna, Satkhira, Bagerhat respectively. The production proclivity of the jute shows most of the time as increasing except in Khulna in 2009 it was affected by demolishing natural disaster; AILA, so the production was drastically felled down in 2009.

The changing physical and ecological meanings of North Pacific Ocean climate indices

Climate change is likely to change the relationships between commonly used climate indices and underlying patterns of climate variability, but this complexity is rarely considered in studies using climate indices. Here, we show that the physical and ecological conditions mapping onto the Pacific Decadal Oscillation (PDO) index and North Pacific Gyre Oscillation (NPGO) index have changed over multidecadal timescales. These changes apparently began around a 1988/1989 North Pacific climate shift that was marked by abrupt northeast Pacific warming, declining temporal variance in the Aleutian Low (a leading atmospheric driver of the PDO), and increasing correlation between the PDO and NPGO patterns. Sea level pressure and surface temperature patterns associated with each climate index changed after 1988/1989, indicating that identical index values reflect different states of basin-scale climate over time. The PDO and NPGO also show time-dependent skill as indices of regional northeast Pacific ecosystem variability. Since the late 1980s, both indices have become less relevant to physical-ecological variability in regional ecosystems from the Bering Sea to the southern California Current. Users of these climate indices should be aware of nonstationary relationships with underlying climate variability within the historical record, and the potential for further nonstationarity with ongoing climate change.

Monthly Variability in Bering Strait Oceanic Volume and Heat Transports, Links to Atmospheric Circulation and Ocean Temperature, and Implications for Sea Ice Conditions

AbstractThe Bering Strait oceanic heat transport influences seasonal sea ice retreat and advance in the Chukchi Sea. Monitored since 1990, it depends on water temperature and factors controlling the volume transport, assumed to be local winds in the strait and an oceanic pressure difference between the Pacific and Arctic oceans (the “pressure head”). Recent work suggests that variability in the pressure head, especially during summer, relates to the strength of the zonal wind in the East Siberian Sea that raises or drops sea surface height in this area via Ekman transport. We confirm that westward winds in the East Siberian Sea relate to a broader central Arctic pattern of high sea level pressure and note that anticyclonic winds over the central Arctic Ocean also favor low September sea ice extent for the Arctic as a whole by promoting ice convergence and positive temperature anomalies. Month‐to‐month persistence in the volume transport and atmospheric circulation patterns is low, but the period 1980–2017 had a significant summertime (June–August) trend toward higher sea level pressure over the central Arctic Ocean, favoring increased transports. Some recent large heat transports are associated with high water temperatures, consistent with persistence of open water in the Chukchi Sea into winter and early ice retreat in spring. The highest heat transport recorded, October 2016, resulted from high water temperatures and ideal wind conditions yielding a record‐high volume transport. November and December 2005, the only months with southward volume (and thus heat) transports, were associated with southward winds in the strait.

Decadal changes in the leading patterns of sea level pressure in the Arctic and their impacts on the sea ice variability in boreal summer

Abstract. Besides its negative trend, the interannual and the interdecadal changes in the Arctic sea ice have also been pronounced in recent decades. The three leading modes in the sea level pressure (SLP) variability in the Arctic (70–90∘ N) – the Arctic Oscillation (AO), the Arctic Dipole (AD), and the third mode (A3) – are analyzed to understand the linkage between sea ice variability and large-scale atmospheric circulation in boreal summer (June–August). This study also compares the decadal changes of the modes between the early (1982–1997) and the recent (1998–2017) periods and their influences on the Arctic sea ice extent (SIE). Only the AD mode shows a significant correlation increase with SIE in summer (JJA) from −0.05 in the early period to 0.57 in the recent period. The AO and the A3 modes show a less significant relationship with SIE for the two periods. The AD is characterized by a dipole pattern of SLP, which modulates the strength of meridional surface winds and the Transpolar Drift Stream (TDS). The major circulation change in the late 1990s is that the direction of the wind has been changed more meridionally over the exit region of the Fram Strait, which causes sea ice drift and discharge through that region. In addition, the response of surface albedo and the net surface heat flux becomes larger and much clearer, suggesting a positive sea-ice–albedo feedback in the sea ice variability associated with the AD. The analysis also reveals that the zonal shift of the centers of SLP anomalies and associated circulation change affects a significant reduction in sea ice concentration over the Pacific sector of the Arctic Ocean. This study further suggests that the Pacific Decadal Oscillation (PDO) phase change could influence the spatial pattern change in the AD.

Understanding future changes in tropical cyclogenesis using Self-Organizing Maps

Future changes in tropical cyclone (TC) genesis locations and frequency are explored by identifying relationships between TC genesis and dominant daily large-scale patterns, and evaluating the strength of these relationships under a climate change scenario. Self-Organizing Maps (SOMs) are used to characterize the dominant large-scale patterns in reanalysis data and in a regional climate model ensemble simulation of current climate. The main features on the resulting sea level pressure (SLP) SOMs are nodes that resemble both the negative and positive phases of the North Atlantic Oscillation, as well as blocking and ridging regimes. The frequency of the NAO-like nodes is strongly linked to TC genesis frequency and preferred genesis locations. This link is used to develop a statistical relationship between the frequency of large scale SLP patterns and TC genesis. The application of this relationship to an ensemble regional climate simulation under a future climate forcing scenario predicts fewer TCs, which is consistent with the regional climate model that explicitly simulates fewer TCs. This demonstrates the strength of the relationships and their use in assessing future changes in TC genesis locations and frequency.

Two Distinct Types of Extratropical Circulation Anomalies Associated with Cold Surges over the South China Sea

Abstract This study investigated the extratropical circulation anomalies responsible for cold surges over the South China Sea in winter. The surge events were identified by the intensity of northerly winds over 110°–117.5°E along 15°N at 925 hPa. Two distinct patterns of sea level pressure (SLP) anomalies in East Asia were found to have a crucial role in inducing cold surges over the South China Sea. Accordingly, the cold surge events were classified into two types. The first type of cold surge is characterized by a pair of SLP anomalies with positive and negative ones centered over China and Japan, respectively, whereas the second type of cold surge is characterized by widespread and persistent positive SLP anomalies over East Asia. Furthermore, the first type of cold surge is accompanied by a deepened East Asian trough and precursory Rossby wave trains across the Eurasian continent in the mid- and upper troposphere, but the latter is not. Prior to both types of the cold surges, the Siberian high is significantly intensified. However, diagnosis of the SLP tendency indicates that the intensification is related to different physical processes. In the first type of cold surge, the Rossby wave trains favor negative vorticity advection and cold advection, inducing intensification of the Siberian high. By contrast, in the second type of cold surge, vorticity advection can be ignored due to the lack of Rossby wave trains, and only the lower-tropospheric cold advection induced by anomalous northerly winds, resulting from the anomalous Siberian high, contributes to the further intensification of the Siberian high.

Arctic Summer Sea Ice Melt and Related Atmospheric Conditions in Coupled Regional Climate Model Simulations and Observations

AbstractObservations from 1979 to 2014 show a positive trend in the summer sea ice melt rate with an acceleration particularly in June and August. This is associated with atmospheric circulation changes such as a tendency toward a dipole pattern in the mean sea level pressure (SLP) trend with an increase over the Arctic Ocean and a decrease over Siberia. Consistent with previous studies, we here show the statistical relationship between the summer sea ice melt rate and SLP and that more than one SLP pattern is associated with anomalously high melt rates. Most high melt rates occur during high pressure over the Arctic Ocean accompanied by low pressure over Siberia, but a strong Beaufort High and advection of warm air associated with a cyclone located over the Taymyr Peninsula can also trigger anomalous high ice melt. We evaluate 10‐member ensemble simulations with the coupled atmosphere‐ice‐ocean Arctic regional climate model HIRHAM‐NAOSIM. The simulations have systematically low acceleration of sea ice melt rate in August, related to shortcomings in representing the strengthening pressure gradient from the Barents/Kara Sea toward Northern Greenland in recent decades. In general, the model shows the same classification of SLP patterns related to anomalous melt rates as the observations. However, the evolution of sea ice melt‐related cloud‐radiation feedback over the summer reveals contrary effects from low‐level clouds in the reanalysis and in the simulations.

Prescribing Zonally Asymmetric Ozone Climatologies in Climate Models: Performance Compared to a Chemistry-Climate Model.

Three different methods of specifying ozone in an atmosphere‐only version of the HadGEM3‐A global circulation model are compared to the coupled chemistry configuration of this model. These methods include a specified zonal‐mean ozone climatology, a specified 3‐D ozone climatology, and a calculated‐asymmetry scheme in which a specified zonal‐mean ozone field is adapted online to be consistent with dynamically produced zonal asymmetries. These simulations all use identical boundary conditions and, by construction, have the same climatological zonal‐mean ozone, that of the coupled chemistry configuration of the model. Prescribing ozone, regardless of scheme, results in a simulation which is 3–4 times faster than the coupled chemistry‐climate model (CCM). Prescribing climatological zonal asymmetries leads to a vortex which is the correct intensity but which is systematically displaced over regions with lower prescribed ozone. When zonal asymmetries in ozone are free to evolve interactively with model dynamics, the modeled wintertime stratospheric vortex shape and mean sea level pressure patterns closely resemble that produced by the full CCM in both hemispheres, in terms of statistically significant differences. Further, we separate out the two distinct pathways by which zonal ozone asymmetries influence modeled dynamics. We present this interactive‐ozone zonal‐asymmetry scheme as an inexpensive tool for accurately modeling the impacts of dynamically consistent ozone fields as seen in a CCM which ultimately influence mean sea level pressure and tropospheric circulation (particularly during wintertime in the Northern Hemisphere, when ozone asymmetries are generally largest), without the computational burden of simulating interactive chemistry.

Record-low coastal sea levels in the Northeast Pacific during the winter of 2013\u20132014

During the winter of 2013–2014, the averaged tide gauge (TG) coastal sea level (CSL) anomaly north of 40°N was a record low of −107 mm for the period of 1948–2016. Statistical analysis indicates that this large drop was a once-in-a-century event and closely related to an unusual ocean warming event known as “The Blob”. The Blob developed in the NE Pacific during the winter of 2013–2014. Both the Blob and record-low CSL can be attributed to wind changes associated with an unusually high sea level pressure (SLP) pattern over the NE Pacific. The anomalous local longshore winds induced by the positive SLP anomalies caused strong offshore Ekman transport along the coast of NE Pacific, thereby leading to the record-low CSL. In addition, the steric sea level changes also contributed a significant part (17%) to the record-low CSL. The Pacific Decadal Oscillation (PDO), as the primary variability mode in the NE Pacific on decadal time scales, did not contribute to the emergence of this extreme CSL event.

Synoptic analysis of sea level pressure patterns and Vertically Integrated Moisture Flux Convergence VIMFC during the occurrence of durable and pervasive rainfall in Iran

To carry out this research, interpolated data of daily rainfall from Iran’s Asfazari data base during 1/1/1979–31/12/2013 is used. The day along with pervasive rainfall considered a day that at least 50% of Iran’s territory has received more than 1 mm for at least two consecutive days. Based on mentioned thresholds, 224 days selected for statistical analysis. The sea level pressure data, zonal and meridional wind components and specific humidity with spatial resolution of 0.25*0.25 Gaussian degree in spatial domain of 10 °N to 60 °N and 15 °E to 75 °E obtained from the European Center for Medium range Weather Forecasting (ECMWF) ERA-Interim for selected days. Then on the data matrix of sea level pressure, the cluster analysis by Ward linkage method done and 4 sea level pressure patterns with different configuration of synoptic systems were identified. The findings showed that in the sea level, the interaction between southern thermal low pressure systems (Arabia low pressure) with Europe and Siberia cold immigrant high pressure both by individual and integration and anticyclone circulation of Arab sea from the low level of 1000–500 hPa of troposphere have the most role on occurrence of durable and pervasive rainfall of Iran. The most Vertically Integrated Moisture Flux Convergence in the first layer of troposphere (1000–850 hPa) observed in low height regions, in the second layer of troposphere (775–700 hPa) on Zagros Mountains and in third layer of troposphere (600–500 hPa) is seen in mountains leeward of Iran. Also the results showed that the maximum rainfall cores has the most coordination with Vertically Integrated Moisture Flux Convergence (VIMFC) in the second layer of troposphere (775–700 hPa) on the Zagros heights in the southwest of Iran.

Modulation of Arctic Sea Ice Loss by Atmospheric Teleconnections from Atlantic Multidecadal Variability

Abstract Observed September Arctic sea ice has declined sharply over the satellite era. While most climate models forced by observed external forcing simulate a decline, few show trends matching the observations, suggesting either model deficiencies or significant contributions from internal variability. Using a set of perturbed climate model experiments, we provide evidence that atmospheric teleconnections associated with the Atlantic multidecadal variability (AMV) can drive low-frequency Arctic sea ice fluctuations. Even without AMV-related changes in ocean heat transport, AMV-like surface temperature anomalies lead to adjustments in atmospheric circulation patterns that produce similar Arctic sea ice changes in three different climate models. Positive AMV anomalies induce a decrease in the frequency of winter polar anticyclones, which is reflected both in the sea level pressure as a weakening of the Beaufort Sea high and in the surface temperature as warm anomalies in response to increased low-cloud cover. Positive AMV anomalies are also shown to favor an increased prevalence of an Arctic dipole–like sea level pressure pattern in late winter/early spring. The resulting anomalous winds drive anomalous ice motions (dynamic effect). Combined with the reduced winter sea ice formation (thermodynamic effect), the Arctic sea ice becomes thinner, younger, and more prone to melt in summer. Following a phase shift to positive AMV, the resulting atmospheric teleconnections can lead to a decadal ice thinning trend in the Arctic Ocean on the order of 8%–16% of the reconstructed long-term trend, and a decadal trend (decline) in September Arctic sea ice area of up to 21% of the observed long-term trend.

Extreme droughts and human responses to them: the Czech Lands in the pre-instrumental period

Abstract. The Czech Lands are particularly rich in documentary sources that help elucidate droughts in the pre-instrumental period (12th–18th centuries), together with descriptions of human responses to them. Although droughts appear less frequently before 1501, the documentary evidence has enabled the creation of a series of seasonal and summer half-year drought indices (Standardized Precipitation Index, SPI; Standardized Precipitation Evapotranspiration Index, SPEI; Z index) for the Czech Lands for the 1501–2017 period. Based on the calculation of return period for series of drought indices, extreme droughts were selected for inclusion herein if all three indices indicated a return period of ≥20 years. For further analysis, only those from the pre-instrumental period (before 1804) were used. The extreme droughts selected are characterized by significantly lower values of drought indices, higher temperatures and lower precipitation totals compared to other years. The sea-level pressure patterns typically associated with extreme droughts include significantly higher pressure over Europe and significantly lower pressure over parts of the Atlantic Ocean. Extreme droughts with a return period ≥ 50 years are described in detail on the basis of Czech documentary evidence. A number of selected extreme droughts are reflected in other central European reconstructions derived from documentary data or tree rings. Impacts on social life and responses to extreme droughts are summarized; analysis of fluctuations in grain prices with respect to drought receives particular attention. Finally, extreme droughts from the pre-instrumental and instrumental periods are discussed.

Indo-Western Pacific Climate Variability: ENSO Forcing and Internal Dynamics in a Tropical Pacific Pacemaker Simulation

El Niño–Southern Oscillation (ENSO) peaks in boreal winter but its impact on Indo-western Pacific climate persists for another two seasons. Key ocean–atmosphere interaction processes for the ENSO effect are investigated using the Pacific Ocean–Global Atmosphere (POGA) experiment with a coupled general circulation model, where tropical Pacific sea surface temperature (SST) anomalies are restored to follow observations while the atmosphere and oceans are fully coupled elsewhere. The POGA shows skills in simulating the ENSO-forced warming of the tropical Indian Ocean and an anomalous anticyclonic circulation pattern over the northwestern tropical Pacific in the post–El Niño spring and summer. The 10-member POGA ensemble allows decomposing Indo-western Pacific variability into the ENSO forced and ENSO-unrelated (internal) components. Internal variability is comparable to the ENSO forcing in magnitude and independent of ENSO amplitude and phase. Random internal variability causes apparent decadal modulations of ENSO correlations over the Indo-western Pacific, which are high during epochs of high ENSO variance. This is broadly consistent with instrumental observations over the past 130 years as documented in recent studies. Internal variability features a sea level pressure pattern that extends into the north Indian Ocean and is associated with coherent SST anomalies from the Arabian Sea to the western Pacific, suggestive of ocean–atmosphere coupling.

Large‐Scale Drivers and Seasonal Predictability of Extreme Wind Speeds Over the North Atlantic and Europe

AbstractAs extreme wind speeds are responsible for large socioeconomic losses in the European domain, a skillful prediction would be of great benefit for disaster prevention as well as the actuarial community. Here we evaluate the patterns of atmospheric variability and the seasonal predictability of extreme wind speeds (e.g., >95th percentile) in the European domain in the dynamical seasonal forecast system European Centre for Medium‐Range Weather Forecasts (ECMWF) System 4 and compare to the predictability using a statistical prediction model. Further we compare the seasonal forecast system with ECMWF Re‐Analysis (ERA)‐Interim in order to advance the understanding of the large‐scale conditions that generate extreme winds. The dominant mean sea level pressure patterns of atmospheric variability show distinct differences between reanalysis and System 4 as most patterns in System 4 are extended downstream in comparison to ERA‐Interim. This dissimilar manifestation of the patterns across the two models leads to substantially different drivers associated with the generation of extreme winds: While the prominent pattern of the North Atlantic Oscillation could be identified as the main driver in the reanalysis, extreme winds in System 4 appear to be related to different large‐scale atmospheric pressure patterns. Thus, our results suggest that System 4 does not seem to capture the potential predictability of extreme winds that exists in the real world. This circumstance is likely related to the unrealistic representation of the atmospheric patterns driving these extreme winds. Hence, our study points to potential improvements of dynamical prediction skill by improving the simulation of large‐scale atmospheric variability.

Abrupt Decrease of Wintertime Cold Nights in Korea in the Late 1980s

The decadal change of wintertime cold nights in Korea and its relationship with atmospheric circulation are investigated. Wintertime cold nights, defined as the nights when the daily minimum temperaturesare lower thantheir 10th percentile, are calculated for 10 Korean Meteorological Administration (KMA) stations over the period of 1960–2015. In all stations, the number of cold nights sharply decreased in the late 1980s. Astep-wise changeis largely due to the reduction in long-lasting events persisting for three consecutive nightsor longer. It is further found that the cold nights before and after the late 1980s areassociated with different synoptic and large-scale circulation patterns. While the pre-1980 events aremaintained by aneast-westdipolar sea level pressure (SLP) pattern with an anomalous high over Northern China and an anomalous low in the western North Pacific, the post-1990 events arecharacterized by a north-south dipole with an enhanced impact from Siberia and a reduced influence from the western North Pacific. In accordance with these synoptic patterns, the Pacific-related climate variability indices, such as the Pacific-North American (PNA) teleconnection index, exhibita negligible interannual relationship with the number of cold nights in the post-1990. These results suggest that the wintertime cold extremes over Korea in recent decades are influenced by different circulation patterns from those in the 1970s and 1980s.

Outlier Events of Solar Forecasts for Regional Power Grid in Japan Using JMA Mesoscale Model

To realize the safety control of electric power systems under high penetration of photovoltaic power systems, accurate global horizontal irradiance (GHI) forecasts using numerical weather prediction models (NWP) are becoming increasingly important. The objective of this study is to understand meteorological characteristics pertaining to large errors (i.e., outlier events) of GHI day-ahead forecasts obtained from the Japan Meteorological Agency, for nine electric power areas during four years from 2014 to 2017. Under outlier events in GHI day-ahead forecasts, several sea-level pressure (SLP) patterns were found in 80 events during the four years; (a) a western edge of anticyclone over the Pacific Ocean (frequency per 80 outlier events; 48.8%), (b) stationary fronts (20.0%), (c) a synoptic-scale cyclone (18.8%), and (d) typhoons (tropical cyclones) (8.8%) around the Japanese islands. In this study, the four case studies of the worst outlier events were performed. A remarkable SLP pattern was the case of the western edge of anticyclone over the Pacific Ocean around Japan. The comparison between regionally integrated GHI day-ahead forecast errors and cloudiness forecasts suggests that the issue of accuracy of cloud forecasts in high- and mid-levels troposphere in NWPs will remain in the future.

The impacts of the meteorology features on PM2.5 levels during a severe haze episode in central-east China

The most polluted urban agglomeration including 13 cities in Central-East China (112–122°E, 34–42°N) were selected to study the impacts of meteorology features on PM2.5 levels during the severe haze episode by using observational PM2.5 concentration, surface and balloon sounding meteorology data. The study results showed that the temporal changing of PM2.5 in the 13 cities showed well correlation at the haze beginning, maintenance, and ending period due to the similar 500 hPa circulation and surface sea level pressure pattern. The increasing of surface relative humidity (RH) and temperature preceded PM2.5 accumulation when haze began. RH usually reached up to 90–95% during the period of PM2.5 peak, suggesting the possible contribution of high humidity to extreme PM2.5 values. In contrast to the similar circulation of upper air and surface pressure pattern, the divergences of local PBL meteorology, especially their vertical structure, were very obvious, which was the major meteorology cause for the different PM2.5 levels in these cities. The temperature rise at 850 hPa layer was higher than that at 925 hPa, which was higher than that at 1000 hPa, leading to the formation of temperature inversion. This was the most important trigger factor for haze. PM2.5 maximum generally occurred within 12 h after the formation of the strongest inversion in each city. PM2.5 levels in the 13 cities strongly depend on their reverse intensity: for the cities where the inversion was strong and long lasting, its PM2.5 was often the highest. Stable inversions are more likely to form in the transitional area from the northwestern mountains to the southeastern plains because of the mountain's blocking of cold air and the warming of boundary layer by sinking airflow from the mountaintop. This is the major meteorology cause for the frequent occurrence of the extreme PM2.5 levels in middle-south plain of Hebei province. This study reminds us that local boundary layer and inversion conditions, closely related with geographical location and local topography, contributes greatly to local PM2.5 levels and should be fully considered in the emission reduction and industrial layout policy by government.

Attribution of recent warming in Alaska

Alaska has experienced some of the strongest warming rates in the Northern Hemisphere since the mid-20th century. The winter-season warming is especially strong: approximately 4.1 °C since 1950. The atmospheric circulation contributes to interannual variability of Alaska's temperatures through advection and thereby contributes to temperature trends over decadal to multidecadal timescales. In this study, we quantify the contribution of the atmospheric circulation to Alaska's warming by using an analog methodology to identify years with sea level pressure patterns most closely resembling the pressure pattern of each year between 1950 and 2017. The analogs enable a dynamical adjustment of temperature anomalies by removing the contribution of the atmospheric circulation. The dynamical adjustment explains approximately half the variance of Alaska's statewide temperature in winter, and smaller fractions in the other seasons. The unexplained variance, termed the “excess warmth,” shows a systematic increase from 1950 to 2017. The trends in the excess warmth correspond to a warming of 2.1 °C in winter and spring, 1.3 °C in summer, and 0.5 °C in autumn, which are consistent with the trends simulated by global climate models run with historical and projected greenhouse gas concentrations for the same period. The excess warmth accounts for 51% of the Alaska's winter warming and 75% of Alaska's annual mean warming over the 1950–2017 time period.