Land-surface forcing and anthropogenic heat modulate ozone by meteorology: A perspective from the Yangtze River Delta region

With the rapid advance in urbanization, land-surface forcing related to the urban expansion and anthropogenic heat (AH) release from human activities significantly affect the urban climate and in turn the air quality. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized place with sever ozone (O3) pollution and complex geography, we estimate the impacts of land-surface forcing and AH on meteorology (meteorological factors and local circulations) and O3 using the WRF-chem model, which can enhance our understanding about the formation of O3 pollution in those rapidly developing regions with unique geographical features as most of our results can be supported by previous studies conducted in other regions in the world. Regional O3 pollution episodes occur frequently (26 times per year) in the YRD in recent years. These O3 pollution episodes are usually under calm conditions characterized by high temperature (over 20 °C), low relative humidity (less than 80 %), light wind (less than 3 m s−1) and shallow cloud cover (less than 5). In this case, high O3 mainly appears during the daytime influenced by the local circulations (the sea and the lake breezes). The change in land-surface forcing can cause an increase in 2-m temperature (T2) by maximum 3 °C, an increase in planetary boundary layer height (PBLH) by maximum 500 m and a decrease in 10-m wind speed (WS10) by maximum 1.5 m s−1, and surface O3 can increase by maximum 20 μg m−3 eventually. Furthermore, the expansion of coastal cities enhances the sea-breeze below 500 m. During the advance of the sea-breeze front inland, the upward air flow induced by the front makes well vertical mixing of O3. However, once the sea-breeze is fully formed, further progression inland is stalled, thus the O3 removal by the low sea-breeze will be weakened and surface O3 can be 10 μg m−3 higher in the case with cities than no-cities. The expansion of lakeside cities can extend the lifetime of the lake-breeze from the noon to the afternoon. Since the net effect of the lake-breeze is to accelerate the vertical mixing in the boundary layer, the surface O3 can increase as much as 30 μg m−3 in lakeside cities. Compared with the effects from land-surface forcing, the impacts of AH are relatively small. And the changes mainly appear in and around cities where AH emission is large. There are increases in T2, PBLH, WS10 and surface O3 when AH are taken into account, with the increment about 0.2 °C, 75 m, 0.3 m s−1 and 4 μg m−3, respectively. Additionally, AH can affect the urban-breeze circulations, meteorological factors and O3 concentration, but its effect on local circulations, such as the sea and the lake breezes, seems to be limited.

Lake and Land Breezes at a Mediterranean Artificial Lake: Observations in Alqueva Reservoir, Portugal

The Alqueva reservoir, in the Southeast of Portugal, has significantly changed the landscape of the region, with impacts also on the local climate, as documented in this manuscript, namely the thermal circulation in the form of lake and land breezes. Taking advantage of three strategic meteorological stations, two installed at the shores and another on a floating platform located near the center of the reservoir, a detailed analysis of lake and land breeze occurrences during two years is presented in this study. The thermal gradient between the reservoir and the surroundings is the main driver for the breeze development and the meteorological stations placed in opposite sides of the reservoir allow to establish the criteria in order to detect lake and land breezes. The results showed more land breeze than lake breeze occurrences, in line with the more negative thermal gradient between shores and reservoir in the annual cycle. Lake breezes are more frequent in summer months during daytime and land breezes in turn are more frequent in winter months during night-time.

Chemical Analysis of Surface-Level Ozone Exceedances during the 2015 Pan American Games

Surface-level ozone (O3) continues to be a significant health risk in the Greater Toronto Hamilton Area (GTHA) of Canada even though precursor emissions in the area have decreased significantly over the past two decades. In July 2015, Environment and Climate Change Canada (ECCC) led an intensive field study coincident with Toronto hosting the 2015 Pan American Games. During the field study, the daily 1-h maximum O3 standard (80 ppbv) was exceeded twice at a measurement site in North Toronto, once on July 12 and again on July 28. In this study, ECCC’s 2.5-km configuration of the Global Environmental Multi-scale (GEM) meteorological model was combined with the Modelling Air-quality and CHemistry (MACH) on-line atmospheric chemistry model and the Town Energy Balance (TEB) urban surface parameterization to create a new urban air quality modelling system. In general, the model results showed that the nested 2.5-km grid-spaced urban air quality model performed better in statistical scores compared to the piloting 10-km grid-spaced GEM-MACH model without TEB. Model analyses were performed with GEM-MACH-TEB for the two exceedance periods. The local meteorology for both cases consisted of light winds with the highest O3 predictions situated along lake-breeze fronts. For the July 28 case, O3 production sensitivity analysis along the trajectory of the lake-breeze circulation showed that the region of most efficient O3 production occurred in the updraft region of the lake-breeze front, as the precursors to O3 formation underwent vertical mixing. In this updraft region, the ozone production switches from volatile organic compound (VOC)-sensitive to NOx-sensitive, and the local net O3 production rate reaches a maximum. This transition in the chemical regime is a previously unidentified factor for why O3 surface-level mixing ratios maximize along the lake-breeze front. For the July 12 case, differences between the model and observed Lake Ontario water temperature and the strength of lake-breeze opposing wind flow play a role in differences in the timing of the lake-breeze, which impacts the predicted location of the O3 maximum north of Toronto.

Convection Initiation Aided by Lake-Breeze Convergence over the Niagara Peninsula

Observations from the 2015 Environment and Climate Change Canada Pan/Parapan American Science Showcase (ECPASS) and real-case, cloud-resolving numerical simulations with the Weather Research and Forecasting (WRF) Model are used to investigate two cases of moist convection forced by lake-breeze convergence over southern Ontario (18 July and 15 August 2015). The two cases shared several characteristics, including high pressure conditions, similar morning soundings, and isolated afternoon convection along a line of lake-breeze convergence between Lakes Erie and Ontario. However, the convection was significantly stronger in the August case, with robustly deeper clouds and larger radar reflectivities than in the July case. Synoptic and mesoscale analyses of these events reveal that the key difference between them was their large-scale forcing. The July event exhibited a combination of strong warm advection and large-scale descent at midlevels (850–650 hPa), which created an inversion layer that capped cloud tops at 4–6 km. The August case exhibited similar features (large-scale descent and warm advection), but these were focused at higher levels (700–400 hPa) and weaker. As a consequence, the convection in the August case was less suppressed at midlevels and ascended deeper (reaching over 8 km). Although the subcloud updraft along the lake-breeze convergence zone was also found to be stronger in the August case, this difference was found to be an effect, rather than a cause, of stronger moist convection within the cloud layer.

Improving Lake-Breeze Simulation with WRF Nested LES and Lake Model over a Large Shallow Lake

The Weather Research and Forecasting (WRF) Model is used in large-eddy simulation (LES) mode to investigate a lake-breeze case occurring on 12 June 2012 over the Lake Taihu region of China. Observational data from 15 locations, wind profiler radar, and the Moderate Resolution Imaging Spectroradiometer (MODIS) are used to evaluate the WRF nested-LES performance in simulating lake breezes. Results indicate that the simulated temporal and spatial variations of the lake breeze by WRF nested LES are consistent with observations. The simulations with high-resolution grid spacing and the LES scheme have a high correlation coefficient and low mean bias when evaluated against 2-m temperature, 10-m wind, and horizontal and vertical lake-breeze circulations. The atmospheric boundary layer (ABL) remains stable over the lake throughout the lake-breeze event, and the stability becomes even stronger as the lake breeze reaches its mature stage. The improved ABL simulation with LES at a grid spacing of 150 m indicates that the non-LES planetary boundary layer parameterization scheme does not adequately represent subgrid-scale turbulent motions. Running WRF fully coupled to a lake model improves lake-surface temperature and consequently the lake-breeze simulations. Allowing for additional model spinup results in a positive impact on lake-surface temperature prediction but is a heavy computational burden. Refinement of a water-property parameter used in the Community Land Model, version 4.5, within WRF and constraining the lake-surface temperature with observational data would further improve lake-breeze representation.

Diurnal Cycle of Convection in the Peruvian Highlands

This manuscript examines from the diurnal convection cycle (CDC) to the interdecadal variability in the region of the Peruvian Altiplano (RAP). Currently, estimating precipitation using satellites is an alternative which can be used to study the spatio-temporal evolution of precipitation systems. Herein CPC data Morphing technique - CMORPH (Joyce et al, 2004) was used between 2002 and 2014 to analyze the CDC in RAP. The CMOPRH data were compared with rainfall data series measured by rain gauges of meteorological stations (EMS) in the RAP. The results indicate that the CDC shows high variability in the Titicaca Basin and is associated with patterns of lake breeze (day), land breeze (night) and mountain - valley circulation. The CDC starts at 1800 HL (local time) in the northern region of Lake Titicaca, lasting between 2 h and 6 h, and most of 2000 HL. The CDC over the dry surface (ST) of Titicaca Basin starts early at around 1200 HL, lasting 4 h to 7 h, and maximum at 1800 HL.

Characteristics of a Hailstorm over the Andean La Paz Valley

The iconic hailstorm and flash flood episode of 19 February 2002 over La Paz city is numerically investigated in this article. Large scale atmospheric circulation is dynamically downscaled in order to take into account the complex orography forcing and local features. Satellite observations suggests late morning shallow convection over the Altiplano that becomes deep convection in the early afternoon around complex orography. The control simulation captures well the cloud evolution and suggest a two-stage precipitation mechanism. First, early convection occurred around 1200 LST and originated from thermodynamic instability combined with lake breeze and orographic lifting. Rainfall discharge then generated cold pools. During the second stage, cold pools around complex orography were propagated by lake breeze and encountered the La Paz Valley breeze, triggering the deep convection near La Paz city around 1400 LST. We assess the importance of local features through numerical experiments, which include modification of orography, suppression of surface heat fluxes, changes of surface lake temperature and removal of the lake. We show the importance of orographic configuration as triggering mechanism for convection initiation and for mesoscale circulation, the role of lake temperature for frontal breeze and propagation of cold pools, and of surface heat fluxes for atmospheric instability. This study highlights the complex interaction between lakes, surface heating and orography that favour deep convection and hailstorm formation, which is especially relevant around the Titicaca lake region.

Breeze effects at a large artificial lake: summer case study

Natural lakes and big artificial reservoirs can affect the weather regime of surrounding areas but, usually, consideration of all aspects of this impact and their quantification is a difficult task. The Alqueva reservoir, the largest artificial lake in western Europe, located on the south-east of Portugal, was filled in 2004. It is a large natural laboratory that allows the study of changes in surface and in landscape and how they affect the weather in the region. This paper is focused on a 3-day case study, 22–24 July 2014, during which an intensive observation campaign was carried out. In order to quantify the breeze effects induced by the Alqueva reservoir, two simulations with the mesoscale atmospheric model Meso-NH coupled to the FLake freshwater lake model has been performed. The difference between the two simulations lies in the presence or absence of the reservoir on the model surface. Comparing the two simulation datasets, with and without the reservoir, net results of the lake impact were obtained. Magnitude of the impact on air temperature, relative humidity, and other atmospheric variables are shown. The clear effect of a lake breeze (5–7 m s−1) can be observed during daytime on distances up to 6 km away from the shores and up to 300 m above the surface. The lake breeze system starts to form at 09:00 UTC and dissipates at 18:00–19:00 UTC with the arrival of a larger-scale Atlantic breeze. The descending branch of the lake breeze circulation brings dry air from higher atmospheric layers (2–2.5 km) and redistributes it over the lake. It is also shown that despite its significant intensity the effect is limited to a couple of kilometres away from the lake borders.