Prathap Ramamurthy

The article investigates the interaction between the sea-breeze circulation and the urban heat island effect in Houston, Texas, located on the Gulf of Mexico coast. The analysis focuses on the summer period in 2022 during the Convective-cloud Urban Boundary-layer Experiment and the Tracking Aerosol Cloud Convection Interactions Experiment. The work, which exclusively used ground-based observations, found a high degree of intra-urban variability in both the temperature and the humidity field during the sea-breeze days. Near-surface virtual potential temperature during the sea-breeze episodes in the metropolitan area varied by almost 10 K, with urban regions near the coast experiencing lower temperature than the inland region. The urban heat island effect was strong enough to create persistent hot spots even during the sea-breeze episodes. On land-breeze days, both the temperature and the humidity fields were more uniform with little spatial variability. The depth of the sea-breeze circulation was captured using X-band radar and radiosondes; it averaged around 1500–2000 m above ground level. The thermal gradient due to the interaction between the sea-breeze circulation and the urban heat island effect led to secondary flows that influenced local convective activity. This article is part of the theme issue ‘Urban heat spreading above and below ground’.

Air quality remains a serious threat to urban residents; it is the primary contributor to respiratory illnesses. Those living in densely populated neighborhoods are particularly vulnerable. Despite considerable advancements in our overall understanding of urban air pollution, our insights into neighborhood- and street-level pollution remain insufficient. Dense urban neighborhoods are dominated by tall buildings and high levels of traffic, leading to complex flow patterns and uneven distribution of primary pollutants. Herein, we delve into the street and neighborhood-scale pollution dynamics in New York City, harnessing both mobile and in-situ observations. We used a backpack fitted with research-grade instruments to monitor particulate matter (PM2.5) and ozone in the summer of 2023. Between July and August, nearly 50 runs were conducted, sampling multiple neighborhoods during various time periods. In-situation observations from multiple public air quality networks were also included in our analysis. Our results show a high degree of uniformity in street-level ozone concentration in NYC, while the particulate matter concentration varied significantly. On days impacted by synoptic disturbances, both ozone and particulate matter concentrations were nearly uniform throughout the city. The fixed ground stations were able to adequately capture the median PM2.5 concentration. However, they missed the extremes, which were, in some cases, two to five times the median value. The observations were also used to validate an urbanized WRF-Chem model and satellite-derived measurements. The numerical simulations conducted at 4km X 4km resolution performed better than the current forecast model in predicting both PM2.5 and ozone concentration.

Coastal cities face critical challenges in managing ozone pollution due to their high population density and complex interactions between urbanization and coastal meteorology. This study uses New York City (NYC) as a case study to investigate how urban surfaces influence heatwaves, land-sea breezes, and planetary boundary layer dynamics, and how these factors shape nitrogen dioxide (NO₂) distribution and ozone formation. Using data from the 2018 Long Island Sound Tropospheric Ozone Study (LISTOS), we leverage an airborne high-spatial-resolution ultraviolet-visible spectrometer to measure NO₂ tropospheric column densities. These observations provide valuable insights into precursor dynamics and serve as a benchmark for evaluating the performance of the urban Weather Research and Forecasting model coupled with Chemistry (uWRF-Chem), which simulates interactions between urban meteorology and chemical transport. Our findings highlight key features of chemical dynamics in NYC during a high ozone and heatwave episode. Early morning urban heat island effects trigger sea-breeze fronts, concentrating ozone precursors in the city center. Building-induced drag reduces wind flow, altering advection patterns and prolonging the residence time of these precursors over the urban area. These processes ultimately result in elevated ozone levels both within the city center and in downwind regions. Together, these insights enhance our understanding of urban air quality challenges and provide a foundation for more effective pollution mitigation strategies in densely populated coastal cities.

This work is motivated by the accelerated pathways to decarbonize our cities. New York City (NYC) is a key example with local law 97 which calls to decarbonize the city by 2050. One of the major sources of carbon emissions is the usage of natural gas and oil for space heating. Air source heat pump (ASHP) systems can replace the natural gas-based heating systems due in large to their deployable in existing buildings. However, a major drawback of ASHP systems is that the performance decreases significantly when outdoor temperature becomes extremely low. It is therefore imperative for comprehensive studies on the performance of ASHP systems in cold climates to determine deployability. In this study, the performance of variable capacity ASHP using R410A refrigerant was investigated in the Northeastern US winter climate, specifically for NYC. A complete laboratory setup was built to simulate the extreme outdoor winter conditions similar to NYC temperatures in January 2022. First law and second law analyses were conducted including a comprehensive exergy analysis. The study reveals that the coefficient of performance (COP) increases from 2.53 to 3.81 when the outdoor temperature increases from 16 °F to 50 °F. The compressor has the highest exergy losses followed by the condenser, expansion valve, and evaporator. The power consumption of the system decreases significantly with the increase in outdoor temperature. This study provides insights into the feasibility and challenges of deploying air source heat pump systems in the NYC winter climates.

Significant knowledge gaps exist in our understanding of urban boundary layer processes, particularly the hygrothermal state. The earth system community has successfully used microwave radiometers for several decades. However, the applicability in complex urban environments has never been adequately tested. Here, observations from a microwave radiometer are compared to radiosonde readings in a densely urbanized site in Houston, Texas. The site was influenced by both an urban heat island and the sea breeze phenomenon. The analysis showed significant disagreement between the virtual potential temperature predicted by the microwave radiometer and the radiosonde for all periods within the boundary layer. However, the values were reasonably comparable above the boundary layer. The microwave radiometer incorrectly predicted an inversion layer instead of a mixed layer during convective periods. The microwave radiometer measurements deviated from the radiosonde measurements throughout the lower troposphere for the relative humidity.