Afternoon Minimum Research Articles

Development and evaluation of processes affecting simulation of diel fine particulate matter variation in the GEOS-Chem model

Abstract. The capability of chemical transport models to represent fine particulate matter (PM2.5) over the course of a day is of vital importance for air quality simulation and assessment. In this work, we used the nested GEOS-Chem model at 0.25∘×0.3125∘ resolution to simulate the diel (24 h) variation in PM2.5 mass concentrations over the contiguous United States (US) in 2016. We evaluate the simulations with in situ measurements from a national monitoring network. Our base case simulation broadly reproduces the observed morning peak, afternoon dip, and evening peak of PM2.5, matching the timings of these features within 1–3 h. However, the simulated PM2.5 diel amplitude in our base case was 106 % biased high, relative to observations. We find that temporal resolution of emissions, subgrid vertical gradient between surface model-level center and observations, and biases in boundary layer mixing and aerosol nitrate are the major causes for this inconsistency. We applied an hourly anthropogenic emission inventory, converted the PM2.5 mass concentrations from the model-level center to the height of surface measurements by correcting for aerodynamic resistance, adjusted the boundary layer heights in the driving meteorological fields using aircraft observations, and constrained nitrate concentrations using in situ measurements. The bias in the PM2.5 diel amplitude was reduced to −12 % in the improved simulation. Gridded hourly emissions rather than diel scaling factors applied to monthly emissions reduced biases in simulated PM2.5 overnight. Resolving the subgrid vertical gradient in the surface model level aided the capturing of the timings of the PM2.5 morning peak and afternoon minimum. Based on the improved model, we find that the mean observed diel variation in PM2.5 for the contiguous US is driven by (1) building up of PM2.5 by 10 % in early morning (04:00–08:00 local time, LT), due to increasing anthropogenic emissions into a shallow mixed layer; (2) decreasing PM2.5 by 22 % from mid-morning (08:00 LT) through afternoon (15:00 LT), associated with mixed-layer growth; (3) increasing PM2.5 by 30 % from mid-afternoon (15:00 LT) though evening (22:00 LT) as emissions persist into a collapsing mixed layer; and (4) decreasing PM2.5 by 10 % overnight (22:00–04:00 LT) as emissions diminish.

Diurnal cycle of precipitation over the southeastern Tibetan Plateau and surrounding regions: Insights from intensity–frequency structure

AbstractBesides the respective importance of the frequency, intensity and diurnal cycle of precipitation, the diurnal cycle of intensity–frequency structure could reveal more physical details about the evolution process of precipitation systems. Here, an objective method to quantitatively depict the diurnal cycle of intensity–frequency structure of precipitation is proposed, and applied to complex terrains of the southeastern Tibetan Plateau (TP) and its surroundings. Results show the diurnal cycle of precipitation over the high terrain has a unimodal night peak, while low terrain with a frequency‐dominated night peak and an intensity‐dominated afternoon peak. Both weak and intense precipitation are more frequent at night than those in the afternoon over unimodal regions compared to bimodal regions. Further analysis reveals different diurnal evolution process of intensity–frequency structure within unimodal and bimodal regions, respectively, which reflects different spatiotemporal traits and evolution process of precipitation. Among unimodal regions, the elevated southeastern TP is featured by the synchronous decrease in both weak and intense precipitation from the night peak to the afternoon minimum. While the decrease of total precipitation in regions with lower terrain is dominated by the decrease of intense precipitation, but with an increase in weak precipitation simultaneously. Regarding bimodal regions, the intense precipitation over southern Hengduan Mountains has a more prominent role in leading the diurnal cycle of precipitation amount, compared with that over the southeastern Yungui Plateau. The proposed method provides a new perspective with more physical details to investigate precipitation characteristics in complex terrains, which could be further used to reasonably evaluate and improve the performance of numerical models.

Extent of diurnal cycle of rainfall and its intra seasonal variation over coastal Tamil Nadu during north east monsoon season

The diurnal variation of north east monsoon rainfall of coastal Tamil Nadu represented by four coastal stations Chennai Nungambakkam (Nbk), Chennai Meenambakkam (Mbk), Nagapattinam (Npt) and Pamban (Pbn) was studied in detail based on hourly rainfall data of rainy days only, for the period 1 Oct-31 Dec for the 47/48 year period 1969-2016/2017. Mean Octet rainfall and its anomaly were computed for the 8 octets 00-03,…., 21-24 hrs of the day and the anomaly was tested for statistical significance. Various analysis for the individual months of Oct, Nov, Dec and the entire period Oct-Dec were separately conducted. The basic technique of evolutionary histogram analysis supplemented by harmonic analysis of octet mean rainfall anomaly was used to detect the diurnal cycle signal. Two indices named as diurnal variation of rainfall index and coefficient of mean absolute octet rainfall anomaly representing the intensity of diurnal variation in dimensionless numbers were defined, computed and interpreted.
 The analysis based on the above techniques revealed that the diurnal signal which shows an early morning maximum and late afternoon minimum of octet rainfall is well defined in Oct, decreases in Nov and further decreases in Dec for all the 4 stations. Though the diurnal variation manifests a well defined pattern in Dec the signal is not statistically significant in most cases. For Nbk and Mbk there is a weak secondary peak of octet rainfall anomaly occurring in the forenoon and afternoon respectively in Oct and Dec suggesting the presence of semi-diurnal variation of rainfall. Stationwise, the diurnal signal is most well defined for each month/season in Pbn followed by Npt, Nbk and then Mbk.
 
 The physical causes behind the diurnal signal and its decrease as the north east monsoon season advances from Oct to Dec have been deliberated. The well known feature of nocturnal maximum of oceanic convection influencing a coastal station with maritime climate and the higher saturation at the lower levels of the upper atmosphere in the early morning hours have been advanced as some of the causes. For the much more complex feature of decrease of diurnal signal with the advancement of the season, the decrease of minimum surface temperature over coastal Tamil Nadu from Oct to Dec causing an early morning conceptual land breeze has been shown as one of the plausible causes based on analysis of temperature and wind. Scope for further work based on data from automatic weather stations, weather satellites and Doppler Weather Radars has been discussed.

On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10\u201318 August 2020

A sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air, often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall.

Total gaseous mercury in a coastal city (Qingdao, China): Influence of sea-land breeze and regional transport

Abstract Atmospheric mercury cycling in coastal urban areas in East China is complex due to the combined influence of both intensive inland anthropogenic emission sources and marine air masses. In this study, continuous measurements of total gaseous mercury (TGM) concentrations were conducted from December 2017 to December 2018 at a coastal site, Shilaoren, in Qingdao, East China. Concentrations of TGM ranged from 0.59 ng m−3 to 24.46 ng m−3 with a mean value of 2.28 (±0.87) ng m−3. TGM exhibited a distinct diurnal cycle mostly with an early morning peak and an afternoon minimum. The daily minimum and mean TGM concentrations with the occurrence of sea-land breeze circulation were higher than those without. In addition, TGM concentrations exhibited a strong dependence on wind direction with an average value of 2.41 ng m−3 in air masses from inland, ~24% higher than that from the ocean. Correlation analysis suggested that mobile combustion was a major anthropogenic emission source to TGM at Shilaoren, and coal-fired power plants became a significant contributor in winter. Potential source contribution function analysis and episodic studies attributed high TGM concentrations (>90th percentile value = 3.28 ng m−3) to regional transport from anthropogenic emission sources in the neighboring cities or provinces in eastern China. Frequently occurring typhoons contributed to the high TGM episodes in summer. Prior to the arrival of a typhoon, TGM concentrations rose sharply due to transport from polluted regions in East Asia as well as downdrafts at the front and lateral edges of a typhoon, whereas dropped to a mean concentration of 1.57 ng m−3, comparable to the globally background value, as the typhoon arrived. This study contributes to a better understanding of the effects of meteorological processes including sea-land breeze and typhoons as well as regional transport on atmospheric mercury cycling in eastern China's coastal environments.

Characterizing NWP Model Errors Using Doppler-Lidar Measurements of Recurrent Regional Diurnal Flows: Marine-Air Intrusions into the Columbia River Basin

Abstract Ground-based Doppler-lidar instrumentation provides atmospheric wind data at dramatically improved accuracies and spatial/temporal resolutions. These capabilities have provided new insights into atmospheric flow phenomena, but they also should have a strong role in NWP model improvement. Insight into the nature of model errors can be gained by studying recurrent atmospheric flows, here a regional summertime diurnal sea breeze and subsequent marine-air intrusion into the arid interior of Oregon–Washington, where these winds are an important wind-energy resource. These marine intrusions were sampled by three scanning Doppler lidars in the Columbia River basin as part of the Second Wind Forecast Improvement Project (WFIP2), using data from summer 2016. Lidar time–height cross sections of wind speed identified 8 days when the diurnal flow cycle (peak wind speeds at midnight, afternoon minima) was obvious and strong. The 8-day composite time–height cross sections of lidar wind speeds are used to validate those generated by the operational NCEP–HRRR model. HRRR simulated the diurnal wind cycle, but produced errors in the timing of onset and significant errors due to a premature nighttime demise of the intrusion flow, producing low-bias errors of 6 m s−1. Day-to-day and in the composite, whenever a marine intrusion occurred, HRRR made these same errors. The errors occurred under a range of gradient wind conditions indicating that they resulted from the misrepresentation of physical processes within a limited region around the measurement locations. Because of their generation within a limited geographical area, field measurement programs can be designed to find and address the sources of these NWP errors.

Sq solar variation at Medea Observatory (Algeria), from 2008 to 2011

This paper presents the regular variations of terrestrial magnetic field recorded by a new magnetic Observatory Medea, Algeria (geographic latitude: 36.85°N, geographic longitude: 2.93°E, geomagnetic latitude: 27.98°N, geomagnetic longitude: 77.7°E) during 2008–2011. The diurnal and seasonal variations of the solar quiet (Sq) variations are analyzed. The results show differences in the diurnal pattern of the northward-component Sq variation (SqX) at different seasons. The seasonal variation of SqX is similar in different years. The diurnal pattern of SqX from July through September cannot be explained by an equivalent current system that is symmetric about the noon time sector. The observations indicate that the major axis of the elliptic current system is tilted towards the equator in the morning hours during those months. The diurnal pattern of SqY indicates southward currents in the morning and northward currents in the afternoon, except during February–March 2009 when there is apparently no southward current during the morning. For the other months, the observations indicate that the maximum northward current intensity in the afternoon tends to be greater than the maximum southward current intensity in the morning. This is because of the UT variation of the Sq current system. That is, the pattern and strength of the Sq current system are different when SqY is measured in the morning around 8UT and in the afternoon around 14UT. The amplitude of these extreme varies linearly with the solar cycle. For the SqY component, the changes in the morning maximum have an annual variation while that of the afternoon minimum has a semi-annual variation. These variations are attributed to seasonal variations in the ionospheric E-region conductivity and atmospheric tidal winds. The field-aligned currents can also contribute to the seasonal variation of SqY. However, the two-dimensional approach used in this article does not allow us to quantitatively determine their influences.

Solar daily variation at geomagnetic observatories in Pakistan

A study of solar daily variation is performed using the famous Chapman–Miller method for solar cycles 22 & 23 (1986–2007). The objective is to study the characteristics of Sq variation at Pakistani geomagnetic observatories using solar harmonics and a more traditional five quietest day's method. The data recorded at the Karachi geomagnetic observatory for SC 22 and 23 and data sets from other Pakistani geomagnetic observatories; Sonmiani, Quetta and Islamabad are analyzed for H, D and Z components of the geomagnetic field.Except for the D and Z components at Karachi and Sonmiani and H component at Islamabad, the two solar daily variations correlated well with each other. Also, the synthesized daily variation from the solar harmonics of H, D and Z components explained the equivalent Sq current system reasonably well for all seasons.For H component, the first solar harmonic (s1) obtained from spherical harmonic analysis of the data, appeared as the largest harmonic with no significant changes for the seasonal division of data. However, for D and Z components, amplitudes are comparable, but undergo distinct variations. s1 for H and D components increases with magnetic activity while for Z component it is the largest for the medium phase of magnetic activity. With the sunspot number division of data, the weighted mean of the Wolf ratio of all three components is in good agreement with the previous studies.The synthesized solar daily variation for D component, S(D), at Karachi, Sonmiani, Quetta and Islamabad did not show any signs of winter anomaly for the period studied. However, S(D) variation at Karachi during winter season showed morning minimum followed by a maximum at local noon and another minimum in the afternoon. We suggest this could be the effects of Equatorial Ionospheric Anomaly (EIA) observable at the Karachi observatory only during the winter season. Similarly, much disturbed in equinoctial and summer months, S(Z) illustrated an unwavering daily variation for the winter season at the Karachi observatory for both solar cycles. We find that it is the vertical component which is more strongly correlated with the mean monthly sunspot number and F10.7 solar radio flux. An interesting result obtained at Islamabad is the semi-diurnal variation of H component with a morning maximum and afternoon minimum and the phase reversal noticed for Z component variation. Attributed to an early eastward current this is, usually, observed for stations close to the Sq focus current system.

Diurnal cycle of the South Pacific Convergence Zone in 30 years of satellite images

AbstractA data set of three‐hourly South Pacific Convergence Zone (SPCZ) location and extent, from which IR temperature within the SPCZ may be obtained, for 1980–2012, November–April is used to examine the diurnal cycle of the SPCZ. Maximum SPCZ area occurs at 15:00–18:00 local standard time (LST). Two minima in mean IR temperature are evident, one at 13:00–16:00 LST, nearly coinciding with the maximum in area, and the second in the early morning hours at 05:00–07:00 LST, when area is at a minimum. The morning minimum in mean IR temperature is associated with a peak in deep convection while the afternoon minimum is associated with a peak in midlevel clouds. On average, the morning minimum in IR temperature dominates in the tropical regions of the SPCZ, while the afternoon IR temperature minimum dominates in the subtropical regions of the SPCZ. The relative strength of the two IR temperature minima is affected by the seasonal cycle, intraseasonal variability associated with active Madden Julian Oscillation events, and interannual variability due to the El Niño Southern Oscillation. The morning IR temperature minimum becomes more dominant when the SPCZ is more frequently present or when the SPCZ is shifted toward the equator. In terms of cloud height, midlevel clouds dominate at all times in all regions of the SPCZ and peak in abundance between 15:00–18:00 LST. Low‐level clouds peak near midnight and then transition to high‐level clouds, which peak between 03:00–06:00 LST, just before sunrise.

A one‐year study of the diurnal cycle of meteorology, clouds and radiation in the West African Sahel region

The diurnal cycles of meteorological and radiation variables are analysed during the wet and dry seasons over the Sahel region of West Africa during 2006 using surface data collected by the Atmospheric Radiation Measurement (ARM) programme's Mobile Facility, satellite radiation measurements from the Geostationary Earth Radiation Budget (GERB) instrument aboard Meteosat 8, and reanalysis products from the National Centers for Environmental Prediction (NCEP). The meteorological analysis builds upon past studies of the diurnal cycle in the region by incorporating diurnal cycles of lower tropospheric wind profiles, thermodynamic profiles, integrated water vapour and liquid water measurements, and cloud radar measurements of frequency and location. These meteorological measurements are complemented by 3 h measurements of the diurnal cycles of the top‐of‐atmosphere (TOA) and surface short‐wave (SW) and long‐wave (LW) radiative fluxes and cloud radiative effects (CREs), and the atmospheric radiative flux divergence (RFD) and atmospheric CREs. Cirrus cloudiness during the dry season is shown to peak in coverage in the afternoon, while convective clouds during the wet season are shown to peak near dawn and have an afternoon minimum related to the rise of the lifting condensation level into the Saharan Air Layer. The LW and SW RFDs and CREs exhibit diurnal cycles during both seasons, but there is a relatively small difference in the LW cycles during the two seasons (10 − 30 W m−2 depending on the variable and time of day). Small differences in the TOA CREs during the two seasons are overwhelmed by large differences in the surface SW CREs, which exceed 100 W m−2. A significant surface SW CRE during the wet season combined with a negligible TOA SW CRE produces a diurnal cycle in the atmospheric CRE that is modulated primarily by the SW surface CRE, peaks at midday at ∼150 W m−2, and varies widely from day to day.

Temporal variability and radiative impact of black carbon aerosol over tropical urban station Hyderabad

Time variability of black carbon (BC) aerosols over different timescales (daily, weekly and annual) is studied over a tropical urban location Hyderabad in India using seven channel portable Aethalometer. The results for the 2-year period (January 2009–December 2010) show a daily-mean BC variability from ~1.00±0.12µgm−3 to 12.50±3.06µgm−3, with a remarkable annual pattern of winter high and monsoon low. The BC values maximize during winter (December–January), ~6.67±0.22µgm−3, and drop during summer (June–August), ~2.36±0.09µgm−3, which establishes a large seasonal variation. Furthermore, the BC mass concentration exhibits a well-defined diurnal variation, with a morning peak and early afternoon minimum. The magnitude of the diurnal variations is seasonal dependent, which maximizes during the winter months. Air mass back trajectories indicated several different transport pathways, while the concentration weighted trajectory (CWT) analysis reveals that the most important potential sources for BC aerosols are the Indo-Gangetic plain (IGP), central India and some hot spots in Pakistan, Arabian Peninsula and Persian Gulf. The absorbing Ångström exponent (αabs) estimated from the spectral values of absorption coefficient (σabs) ranges from 0.9 to 1.1 indicating high BC/OC ratio typical of fossil fuel origin. The annual average BC mass fraction to composite aerosols is found to be (10±3) % contributing to the atmospheric forcing by (55±10) %. The BC radiative forcing at the atmosphere shows strong seasonal dependency with higher values in winter (33.49±7.01) and spring (31.78±12.89) and moderate in autumn (18.94±6.71) and summer (13.15±1.66). The BC radiative forcing at the top of the atmosphere (TOA) is positive in all months, suggesting an overall heating of the regional climate over Hyderabad.

Analysis of one year's OC/EC data at a Prague suburban site with 2-h time resolution

The behaviour of elemental and organic carbon (EC and OC) in the atmospheric aerosol fraction PM2.5 was measured at a Prague urban background site in the Czech Republic. The measurements were performed by a semi-online field OC/EC analyzer with a two-hour resolution that sufficiently showed the diurnal variability of OC/EC. The seasonal, daily and diurnal behaviour of the EC and OC were studied using an analysis of the collected data. The results of a one-year campaign (Sep 2009–Aug 2010) provide various seasonal patterns of the EC and OC concentrations characteristic for a suburban site in Central Europe. Different sources of carbonaceous aerosols during the various seasons were identified. The winter main sources were probably traffic (mainly EC) and residential heating (both EC and OC). On the other hand, the main EC source in summer is traffic, while the major OC source may be secondary organic aerosols. Winter concentrations were significantly higher than in other seasons. The reason is a combination of stronger air pollution sources in conjunction with poor mixing of the boundary layer. Daily changes of the boundary layer influence the diurnal variations of both EC and OC, too. Afternoon OC concentrations were lower than those at night owing to better daytime atmospheric mixing. The EC late night minima were only slightly lower than the afternoon minima despite much higher traffic in the afternoon. Higher EC concentrations were observed during morning rush hours during all the seasons.

Long Term Black Carbon Measurements at Two Urban Locations in New York

Measurements of PM2.5 black carbon (BC) over an 8–9 year period are used to characterize temporal patterns at sites in New York City (NYC) and Rochester, NY. Annual mean BC at the NYC location ranges from 1.4 to 2.0 μg/m 3 , whereas mean concentrations at Rochester are approximately a factor of 2–3 lower. BC amounts to 15–20% of PM2.5 mass in NYC compared to 7–10% at Rochester. Seasonal patterns reveal the highest BC concentrations in NYC from November to February versus June to November at Rochester. At both locations, mean weekday (Monday-Friday) BC concentrations are statistically higher compared to weekends (Saturday and Sunday). Weekday BC diurnal profiles exhibit a morning peak between 6–10 AM EST followed by an afternoon minimum, with a secondary peak in the late evening. Sunday BC diurnal profiles show highest concentrations at night, from 8 PM to 2 AM. These patterns are consistent with vehicle counts on nearby roadways and boundary layer dynamics at both locations. Simultaneous measurements of BC at 370 and 880 nm show an enhancement in BC370 relative to the BC880 from October to March. This enhanced signal is most evident at Rochester during late evening and early morning hours (8 PM to 4 AM) on weekends, and is attributed to UV absorbing species (such as wood smoke markers) in the ambient particle mixture. At the NYC site, the levels of Nickel (Ni) and Cobalt (Co) in PM10 are elevated during the heating season due to residual oil combustion. The long term datasets are used to explore the seasonal relationship between BC880 and EC at both sites.

GOES‐10 microphysical retrievals in marine warm clouds: Multi‐instrument validation and daytime cycle over the southeast Pacific

The daytime evolution of warm cloud microphysical properties over the southeast Pacific during October–November 2008 is investigated with optical/infrared retrievals from the Tenth Geostationary Operational Environmental Satellite (GOES‐10) imager. GOES‐10 retrievals, produced at NASA Langley Research Center, are validated against in situ aircraft observations and with independent satellite observations. Comparisons with in situ observations reveal high linear correlations (r) for cloud effective radius (re) and optical thickness (τ) (r = 0.89 and 0.69 respectively); nevertheless, a GOES‐10 positive mean re bias of 2.3 μm is apparent, and consistent with other previously reported satellite biases. Smaller biases are found for liquid water path (LWP) and an adiabatic‐based cloud droplet number concentration (Nd), both variables derived by combining re and τ.In addition, GOES‐10 observations are well correlated with their Moderate Resolution Imaging Spectroradiometer (MODIS) counterparts, but with smaller biases and root‐mean‐square errors for the Aqua satellite passes, arguably associated with a better calibrated MODIS‐Aqua instrument relative to MODIS‐Terra. Furthermore, the excellent agreement between GOES‐10 LWP and microwave‐based satellite retrievals, especially at high solar zenith angles (>60°), provide further evidence of the utility of using GOES‐10 retrievals to represent the daytime cloud cycle. In terms of the daytime cycle, GOES‐10 observations show an afternoon minimum in LWP and an increase thereafter, consistent with satellite microwave climatologies. Theτ cycle explains most of the LWP variance with both variables in phase, minima near noon along the coast, and a 13:30–14:00 local solar time (LST) minimum offshore. In contrast, re is not exactly in phase with LWP and τ, having a minimum approximately at 12:30 LST throughout the domain. A unique feature is a striking re maximum along the coast at 16:15 LST, concomitant with a faster τ recovery. An explanation for a coastal reafternoon maximum is lacking although this is consistent with an enhancement of the updraft velocity reported in previous modeling studies. Finally, the GOES‐derived Nd (Nd ∝ τ1/2 re−5/2) shows a complex daytime cycle with maxima at 7:15 and 13:15 LST. While the first maximum is driven by large τ, the second one is mainly explained by a minimum in re.

Observations of Urban Heat Island Influence on Lake-Breeze Frontal Movement

AbstractPredictions of lake and sea breezes are particularly important in large coastal population centers because of the circulations’ influence on heat-wave relief, energy use, precipitation, and dispersion of pollutants. While recent numerical modeling studies have suggested that sea or lake breezes should move more slowly through urban areas than in the surrounding suburbs because of urban heat island (UHI) circulations, there have been few quantitative observational studies to evaluate these results. This study utilizes high-resolution Weather Surveillance Radar-1988 Doppler (WSR-88D) observations to determine the effect of the UHI on lake-breeze frontal movement through Chicago, Illinois, and nearby suburban areas. A total of 44 lake-breeze cases from the April–September 2005 period were examined. The inland movement of the lake-breeze front (LBF) was calculated by tracking “fine lines” of radar reflectivity along several cross sections perpendicular to the Lake Michigan shoreline. The average inland propagation speed of the LBF was 5.0 km h−1; there was substantial spatial and temporal variability in LBF propagation, however. Chicago’s UHI magnitude on lake-breeze days exhibited an average nighttime maximum urban–rural temperature difference near 4.5°C and an afternoon minimum near 0°C. The observed daytime UHI magnitude did not have a significant relationship with lake-breeze frontal movement through Chicago. However, the maximum magnitude of the nighttime UHI preceding lake-breeze development was found to be strongly related to a decrease in speed of LBF movement through Chicago’s southwest (inland) suburbs. This relationship is consistent with previous studies of the diurnal evolution of UHI circulations and may represent a useful method for predicting lake-breeze inland movement.

Epidemiology and development of forecasting models for White rust of Brassica juncea in India

Experiments were laid out at Bharatpur, New Delhi and Kangra with Indian mustard (Brassica juncea) cvs ‘Varuna’ and an important one in the locality sown on 10 dates at weekly intervals. First appearance of white rust disease (Albugo candida) on leaves and pods (staghead formation) of mustard occurred between 36 and 131 days after sowing (d.a.s.), 60 and 123 d.a.s., respectively. Severity of white rust disease on leaves was favoured by >40% afternoon (minimum) relative humidity (RH), >97% morning (maximum) RH and 16–24°C maximum daily temperature. Staghead formation was significantly and positively influenced by 20–29°C maximum daily temperature and further aided by >12°C minimum daily temperature and >97% morning (maximum) RH. Regional and cultivar specific models devised could predict, at a few weeks after sowing, the crop age at which white rust first appeared on the leaves, as staghead, the highest rust severity on leaves, staghead numbers and the crop age at peak rust severity on leaf, highest staghead numbers at least 1 week ahead of first appearance of the disease on the crop.

Sq field characteristics at Phu Thuy, Vietnam, during solar cycle 23: comparisons with Sq field in other longitude sectors

Abstract. Quiet days variations in the Earth's magnetic field (the Sq current system) are compared and contrasted for the Asian, African and American sectors using a new dataset from Vietnam. This is the first presentation of the variation of the Earth's magnetic field (Sq), during the solar cycle 23, at Phu Thuy, Vietnam (geographic latitudes 21.03° N and longitude: 105.95° E). Phu Thuy observatory is located below the crest of the equatorial fountain in the Asian longitude sector of the Northern Hemisphere. The morphology of the Sq daily variation is presented as a function of solar cycle and seasons. The diurnal variation of Phu Thuy is compared to those obtained in different magnetic observatories over the world to highlight the characteristics of the Phu Thuy observations. In other longitude sectors we find different patterns. At Phu Thuy the solar cycle variation of the amplitude of the daily variation of the X component is correlated to the F.10.7 cm solar radiation (~0.74). This correlation factor is greater than the correlation factor obtained in two observatories located at the same magnetic latitudes in other longitude sectors: at Tamanrasset in the African sector (~0.42, geographic latitude ~22.79) and San Juan in the American sector (~0.03, geographic latitude ~18.38). At Phu Thuy, the Sq field exhibits an equinoctial and a diurnal asymmetry: – The seasonal variation of the monthly mean of X component exhibits the well known semiannual pattern with 2 equinox maxima, but the X component is larger in spring than in autumn. Depending of the phase of the sunspot cycle, the maximum amplitude of the X component varies in spring from 30 nT to 75 nT and in autumn from 20 nT to 60 nT. The maximum amplitude of the X component exhibits roughly the same variation in both solstices, varying from about ~20 nT to 50 nT, depending on the position into the solar cycle. – In all seasons, the mean equinoctial diurnal Y component has a morning maximum Larger than the afternoon minimum i.e. the equivalent current flow over a day is more southward than northward. During winter, the asymmetry is maximum, it erases the afternoon minimum. At the Gnangara observatory, in Asian Southern Hemisphere, the diurnal Y pattern is opposite and the current flow is more northward. It seems that in the Asian sector, the northern and southern Sq current cells both contribute strongly to the equatorial electrojet. The pattern is different in the African and American sectors where the northern Sq current cell contribution to the equatorial electrojet is smaller than the southern one. These observations can explain the unexpected maximum of amplitude of the equatorial electrojet observed in the Asian sector where the internal field is very large. During winter the Y component flow presents an anomaly, it is always southward during the whole day and there is no afternoon northward circulation.

Total gaseous mercury in Pearl River Delta region, China during 2008 winter period

Total gaseous mercury (TGM) was monitored at Wanqingsha, a rural site in the central Pearl River Delta region, south China from 23 November to 22 December, 2008. TGM ranged 0.50–18.67 ng m−3 with an average of 2.94 ng m−3. TGM was found to present a diurnal cycle with the maximum at 7:00–8:00 and the minimum at 14:00–16:00 (local time). The afternoon minimum in TGM were likely related to increased vertical mixing and the TGM oxidation by ozone, which was observed to negative correlation with TGM. A sudden increase in TGM from 6:00–7:00 could be due to activation of local surface emission sources by rising air temperature. Similar variations in TGM and trace gases including CO and SO2 suggested potential anthropogenic sources contribution as well. Wind-rose patterns further indicated that the westerly wind might carry high concentration of TGM from the high mercury emission areas of Southwest China and/or local anthropogenic emission sources.

A whole atmosphere model simulation of the impact of a sudden stratospheric warming on thermosphere dynamics and electrodynamics

A Whole Atmosphere Model (WAM) has been used to explore the possible physical connection between a sudden stratospheric warming (SSW) and the dynamics and electrodynamics of the lower thermosphere. WAM produces SSWs naturally without the need for external forcing. The classical signatures of an SSW appear in the model with a warming of the winter polar stratosphere, a reversal of the temperature gradient, and a breakdown of the stratospheric polar vortex. Substantial changes in the amplitude of stationary planetary wave numbers 1, 2, and 3 occur as the zonal mean zonal wind evolves. The simulations also show a cooling in the mesosphere and a warming in the lower thermosphere consistent with observations. The magnitude of this particular SSW is modest, belonging to the category of minor warming. In the lower thermosphere the amplitude of diurnal, semidiurnal, and terdiurnal, eastward and westward propagating tidal modes change substantially during the event. Since the magnitude of the warming is minor and the tidal interactions with the mean flow and planetary waves are complex, the one‐to‐one correspondence between tidal amplitudes in the lower thermosphere and the zonal mean and stationary waves in the stratosphere is not entirely obvious. The increase in the magnitude of the terdiurnal tide (TW3) in the lower thermosphere has the clearest correlation with the SSW, although the timing appears delayed by about three days. The fast group velocity of the long vertical wavelength TW3 tide would suggest a faster onset for the direct propagation of the tide from the lower atmosphere. It is possible that changes in the magnitude of the diurnal and semidiurnal tides, with their slower vertical propagation, may interact in the lower thermosphere to introduce a terdiurnal tide with a longer delay. An increase in TW3 in the lower thermosphere would be expected to alter the local time variation of the electrodynamic response. The day‐to‐day changes in the lower thermosphere winds from WAM are shown to introduce variability in the magnitude of dayside low latitude electric fields, with a tendency during the warming for the dayside vertical drift to be larger and occur earlier, and for the afternoon minimum to be smaller. The numerical simulations suggest that it is quite feasible that a major SSW, with a magnitude seen in January 2009, could cause large changes in lower thermosphere electrodynamics and hence in total electron content.

Numerical Simulations of Island Effects on Airflow and Weather during the Summer over the Island of Oahu

Abstract The high-resolution (1.5 km) nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and an advanced land surface model (LSM) are used to study the island-induced airflow and weather for the island of Oahu, Hawaii, under summer trade wind conditions. Despite Oahu’s relatively small area (1536 km2), there are considerable spatial variations in horizontal distribution of thermodynamic fields related to terrain, airflow, rain, cloud, and ground cover. The largest diurnal variations in temperature and moisture occur in the lee sides of mountains, especially along the western leeside coast. The island-scale surface airflow is also significantly affected by terrain and land surface forcing. The downslope winds above the leeside slopes of both the Ko’olau and Waianae Mountains are simulated with significant diurnal variations with the strongest downslope winds just before sunrise. The timing of diurnal rainfall maxima over the Ko’olau Mountains is closely related to vertical motions. The early morning rainfall maximum on the windward side is caused by anomalous rising motion due to significant flow deceleration when the land surface is the coolest. The evening rainfall maximum after sunset is related to anomalous orographic lifting due to stronger winds aloft. In the early afternoon, winds aloft are relatively weak with a relatively high level of free convection (LFC) because of vertical mixing. As a result, the rainfall over the Ko’olau Mountains exhibits an afternoon minimum. The westerly reversed flow off the western leeside coast in the afternoon is mainly thermally driven and related to land surface heating superimposed by latent heat release of persistent orographic precipitation over the Ko’olau Mountains. Rainfall along the western leeside slopes has a late afternoon maximum due to the development of the onshore/upslope flow.