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Dilchand Nauth
,
Christopher P. Loughner
, and
Maria Tzortziou

Abstract

The continually changing atmospheric conditions over densely populated coastal urban regions make it challenging to produce models that accurately capture the complex interactions of anthropogenic and environmental emissions, chemical reactions, and unique meteorological processes, such as sea- and land-breeze circulations. The purpose of this study is to determine and identify the influence of synoptic-scale wind patterns on the development of local-scale sea-breeze circulations and air quality over the New York City (NYC), New York, metropolitan area. This study utilizes column-integrated nitrogen dioxide observations made during the Long Island Sound Tropospheric Ozone Study (LISTOS) field campaign, ground-level ozone observations, the HRRR numerical weather prediction model, and trajectory model simulations using the NOAA HYSPLIT model. A cluster analysis within the HYSPLIT modeling system was performed to determine that there were six unique synoptic-scale transport pathways for NYC. Stagnant conditions or weak transport out of the northwest resulted in the worst air quality for NYC. Weak synoptic-scale forcings associated with these conditions allowed for local-scale sea-breeze circulations to develop, resulting in air pollution being able to recirculate and mix with freshly emitted pollutants.

Significance Statement

The purpose of this work is to understand how synoptic-scale wind patterns influence air quality and sea-breeze circulations in the New York City, New York, metropolitan area. This work shows that clean air can be imported into the region from rural New England and over the Atlantic Ocean, whereas polluted air can be transported into the region from the northwest and southwest. This work also shows the importance of the strength in synoptic-scale forcings in the development of sea-breeze circulations. Weak synoptic-scale winds allow for strong sea-breeze circulations to develop over all coastlines in the New York City region, resulting in air pollutants recirculating and mixing with freshly emitted air pollution and contributing to poor air quality.

Open access
Andrew Hoell
,
Martin Hoerling
,
Xiao-Wei Quan
, and
Rachel Robinson

Abstract

October–September runoff increased 6% and 17% in the upper (UMRB) and lower (LMRB) Missouri River basins, respectively, in a recent (1990–2019) climate in comparison with a past (1960–89) climate. The runoff increases were unanticipated, given various projections for semipermanent drought and/or aridification in the North American Great Plains. Here, five transient coupled climate model ensembles are used to diagnose the effects of natural internal variability and anthropogenic climate change on the observed runoff increases and to project UMRB and LMRB runoff to the mid-twenty-first century. The runoff increases observed in the recent climate in comparison with the past climate were not due to anthropogenic climate change but rather resulted mostly from an extreme occurrence of internal multidecadal variability. High runoff resulted from large, mostly internally generated, precipitation increases (6% in the UMRB and 5% in the LMRB) that exceeded simulated increases attributable to climate change forcing alone (0%–2% intermodel range). The precipitation elasticity of runoff, which relates runoff sensitivity to precipitation differences in the recent climate in comparison with the past climate, led to one–threefold and two–fourfold amplifications of runoff versus precipitation in the UMRB and LMRB, respectively. Without the observed precipitation increases in the recent climate in comparison with the past climate, effects of human-induced warming of about 1°C would alone have most likely induced runoff declines of 7% and 13% in the UMRB and LMRB, respectively. Ensemble model simulations overwhelmingly project lower UMRB and LRMB runoff by 2050 when compared with 1990–2019, a change found to be insensitive to whether individual realizations experienced high flows in the recent climate.

Significance Statement

Declines in Missouri River basin runoff under climate change pose serious threats to communities that depend on riverine transport, irrigated agriculture, and aquatic recreation. Concerns arising from reports and projections of semipermanent drought in the basin have yet to be realized; observed runoff was greater in a recent climate (1990–2019) than in a past climate (1960–89). We found that the observed runoff increase from past to recent climates was due not to anthropogenic influences but rather to internal multidecadal variability that led to unlikely precipitation increases (<10% probability) that overwhelmed the drying effect of warming temperatures. Model simulations indicate that a modest reduction in runoff of ∼7%–15% was most likely from the past climate to the recent climate.

Restricted access
Free access
Lanzhi Tang
,
Wenhua Gao
,
Lulin Xue
,
Guo Zhang
, and
Jianping Guo

Abstract

The long-term characteristics of four hydrometeor species (cloud water, cloud ice, rain, and snow) in precipitating clouds over eastern China (divided into South China, Jianghuai, and North China) and their relationships with surface rainfall are first investigated using the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ERA5) hourly dataset from May to August during 1979–2020. The results show that the cloud water path decreases significantly from south to north as a result of the large-scale circulation and water vapor distribution, with the maximum value of 180 g m−2 in South China and only one-half of that value in North China. The slope in linear relationship between rainwater path and precipitation intensity is at the maximum (5.68 h−1) in South China, implying the highest conversion rate from rainwater to precipitation in this region. When the precipitation rate exceeds 15 mm h−1, the ice-phase hydrometeor contents in South China become the largest among the three regions, indicating that the cold-rain process is crucial to heavy rainfall. The moisture-related processes play a dominant role in the precipitation intensity. Although the contribution of hydrometeor advection to precipitation is generally between −5% and 5%, we found that it can jointly modulate the location of heavy rainfall. In addition, the peaks of cloud water path commonly appear 2–3 h ahead of precipitation, whereas the peaks of ice-phase particles occur 2 and 1 h behind the afternoon precipitation onset in South China and Jianghuai, respectively, which is mainly attributed to the different upward velocity and water vapor convergence in the mid–upper troposphere.

Significance Statement

Reanalysis data and satellite retrievals have been widely used in investigating cloud water and cloud ice in nonprecipitating clouds. However, studies on long-term characteristics of precipitating hydrometeors in precipitating clouds, which are directly connected and crucial to surface rainfall, are still very limited to date because of limitations in observations of precipitating clouds. In this study, the latest ERA5 reanalysis hourly dataset is first used to quantitatively explore the climatological characteristics of four hydrometeors (cloud water, cloud ice, rain, and snow) in precipitating clouds as well as their relationships with precipitation intensity over eastern China from 1979 to 2020. The results advance our understanding of precipitation mechanisms from the perspective of hydrometeor climatology.

Open access
Ali Tokay
,
Liang Liao
,
Robert Meneghini
,
Charles N. Helms
,
S. Joseph Munchak
,
David B. Wolff
, and
Patrick N. Gatlin

Abstract

Parameters of the normalized gamma particle size distribution (PSD) have been retrieved from the Precipitation Image Package (PIP) snowfall observations collected during the International Collaborative Experiment–PyeongChang Olympic and Paralympic winter games (ICE-POP 2018). Two of the gamma PSD parameters, the mass-weighted particle diameter D mass and the normalized intercept parameter NW , have median values of 1.15–1.31 mm and 2.84–3.04 log(mm−1 m−3), respectively. This range arises from the choice of the relationship between the maximum versus equivalent diameter, D mxD eq, and the relationship between the Reynolds and Best numbers, Re–X. Normalization of snow water equivalent rate (SWER) and ice water content W by NW reduces the range in NW , resulting in well-fitted power-law relationships between SWER/NW and D mass and between W/NW and D mass. The bulk descriptors of snowfall are calculated from PIP observations and from the gamma PSD with values of the shape parameter μ ranging from −2 to 10. NASA’s Global Precipitation Measurement (GPM) mission, which adopted the normalized gamma PSD, assumes μ = 2 and 3 in its two separate algorithms. The mean fractional bias (MFB) of the snowfall parameters changes with μ, where the functional dependence on μ depends on the specific snowfall parameter of interest. The MFB of the total concentration was underestimated by 0.23–0.34 when μ = 2 and by 0.29–0.40 when μ = 3, whereas the MFB of SWER had a much narrower range (from −0.03 to 0.04) for the same μ values.

Restricted access
Robert T. Maisha
,
Thando Ndarana
,
Francois A. Engelbrecht
,
Marcus Thatcher
,
Mary-Jane M. Bopape
,
Jacobus van der Merwe
,
Yerdashin Padayachi
, and
Cecilia Masemola

Abstract

The study evaluates the performance of the Conformal Cubic Atmospheric Model (CCAM) when simulating an urban heat island (UHI) over the city of eThekwini, located along the southeast coast of South Africa. The CCAM is applied at a grid length of 1 km on the panel with eThekwini, in a stretched-grid mode. The CCAM is coupled to the urban climate model called the Australian Town Energy Budget (ATEB). The ATEB incorporates measured urban parameters including building characteristics, emissions, and albedo. The ATEB incorporates the land-cover boundary conditions obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite. The CCAM configuration applied realistically captured the orientation of the city and land-cover types. Simulations of meteorological variables such as temperatures and longwave radiation reproduced the spatial distribution and intensity of the UHI. Results show that the UHI is stronger during summer and weaker in all other seasons. The UHI developed because of natural factors (e.g., distribution of longwave radiation) and human factors (e.g., urban expansion, an increase in anthropogenic emissions, and additional heating). Because of the city’s location along the coast, the UHI simulation could be weakened by atmospheric circulations resulting from land and sea breezes. Mitigation methods such as applying reflective paints and revegetation of the city may increase albedo and latent heat fluxes but reduce the sensible heat fluxes and weaken the UHI. However, the UHI may not be completely eliminated since natural factors and emissions constantly influence its development.

Significance Statement

The outcome of this study could be particularly valuable for municipalities in their disaster management planning since the occurrence of UHIs can cause heat-related diseases such as heatstrokes and even fatalities, especially for the elderly, in cities. Increases in temperatures also lead to higher demand for air conditioners, which in the long term lead to higher demand and pressure on the electricity grid system as well as increased costs for the individual. As higher temperatures increase heatwave events, increases in anthropogenic emissions also result in degraded air quality that impacts health. UHIs impact human lives and can cause deterioration in health when individuals experience high temperatures in summer. Warmer temperatures also reduce energy demand (and in the long term assist with global environmental restoration).

Restricted access
Yoshiaki Miyamoto
,
Ayako Matsumoto
, and
Soshi Ito

Abstract

This study examined the statistics of aviation turbulence that occurred in Japan between 2006 and 2018 by analyzing pilot reports (PIREP). In total, 81 639 turbulence events, with moderate or greater intensity, were reported over this period. The monthly number of turbulence cases has an annual periodic variation as observed in different regions by previous studies. The number of turbulence cases is high from March to June and low in July and August. Higher numbers of turbulence cases are experienced along the major flight routes in Japan, especially around Tokyo, for the active period between 0900 and 2000 local time. The number of cases of turbulence peaks when the flight reaches an altitude of 33 000 ft (FL330; 1000 ft ≈ 300 m), whereas it decreases when the flight altitude is above FL380 and below FL280. The statistical features are not largely different among the four seasons; however, there are some exceptions. For instance, the number of turbulence cases is large in high altitudes in summer and small in low altitudes in winter. Considering the number of flights, it is evident that the frequency of turbulence is higher in altitudes between FL200 and FL350, although the number of flights is low in this altitude region. The number of convectively induced turbulence cases is relatively large during the daytime in summer in comparison with the other seasons. A large amount of mountain-wave turbulence is observed around the mountainous region in autumn and winter when the jet stream flows over Japan.

Significance Statement

This study examines the statistics of aviation turbulence reported over Japan from 2008 to 2018.

Open access
Chenxi Wang
,
Dong Zheng
,
Yijun Zhang
,
Wen Yao
, and
Wenjuan Zhang

Abstract

Using hail records at national meteorological stations for 2014–18, ERA-Interim reanalysis data, and Doppler weather radar data, the spatiotemporal distribution of hail events (HEs) in the Beijing–Tianjin–Hebei region is revealed, and the environmental conditions and hailstorm structures corresponding to large hail (diameter ≥ 20 mm) events (LHEs) and small hail (2 ≤ diameter < 20 mm) events (SHEs) are compared. It is found that, although HEs may be more frequent in mountainous areas, most LHEs occur in the plains and near the foot of the mountains. The HE frequency peaks in June, and the average hailstone size is larger during May and June. According to daytime records, the HEs predominantly occur in the afternoon and evening, whereas LHE tends to be more in the evening. Comparison of environmental parameters suggests that, relative to SHEs, LHEs tend to correspond to higher 2-m temperature, a wetter lower layer, a larger difference in relative humidity between 925 and 500 hPa, greater unstable energy, and stronger wind shear. Hailstorms associated with LHEs tend to feature greater mesoscale rotation velocity than those associated with SHEs. Hailstorms usually show rapid increase (RI) in vertically integrated liquid (VIL) before hailstones are observed. A significant difference between the hailstorms associated with LHEs and SHEs is that the former has an obviously longer time interval between the end of VIL RI and the occurrence of hailfall, indicating that the large hail size benefits from the constant supply of liquid water and the hail can be lifted by updrafts for a long time.

Significance Statement

Whereas previous studies have predominantly focused on large hail (diameter ≥ 20 mm) events (LHEs) and their yielding conditions, this study was devoted to examining the difference between the LHEs and small hail (2 ≤ diameter < 20 mm) events in their associated atmospheric environments and storm structures. The interesting new insight is that the hailstorms yielding LHEs tend to feature a significantly longer time interval after the rapid increase of vertically integrated liquid and before hailfall. This study can provide a reference for the early warning of the scale of hail, which is one of the difficulties of weather services.

Restricted access
Souad Lagmiri
and
Salem Dahech

Abstract

Daily atmospheric concentrations of the pollutants PM10 and O3 vary according to weather types. This study aims to identify the weather patterns associated with PM10 and O3 pollution episodes from 2009 to 2020. Episodes characterized by exceedance of World Health Organization standards were identified, and their duration and persistence were studied. The results show that air pollution days are associated with three atmospheric patterns for PM10 and four for O3. The dominant weather pattern corresponds to an anticyclonic situation in central and Eastern Europe with a ridge of high pressure over France at the surface and 500-hPa geopotential height. For PM10, the persistent high-concentration sequences were found to be associated with a thermal inversion constraining the vertical dispersion of pollutants. For O3, the four weather types responsible for ozone pollution all have a higher occurrence in summer. The highest percentage (46% of days) is associated with the presence of a ground-level barometric marsh (an area of the atmosphere between two weather systems where the pressure varies slightly but is slightly low) and a ridge at 500 hPa (weather type T1). Similarly, thermal inversions and thermal winds cause pollution to persist beyond 8 consecutive days.

Significance Statement

Air quality is not only influenced by ground-level emissions, but also by complex meteorological processes that can contribute to pollutant accumulations. The importance of this research is that the prediction of these processes helps to prevent the development of extreme concentrations near the surface. The results of this study provide a better understanding of how characteristic weather patterns in the Cergy-Pontoise conurbation impact PM10 and O3 pollutant levels. These impacts are expressed by the intensity and frequency of pollution episodes.

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Free access