The study and simulation of the Urban Heat Island (UHI) and Heat Index (HI) effects in the Houston-Galveston metropolitan area demand special attention, particularly in considering moist processes aloft. During the warm season, the afternoon sea breeze phenomenon in this coastal city acts as a natural air conditioner for city residents, facilitating the dispersion of pollutants, moisture, and heat. To delve into the intricate relationships among urbanization, clouds, and land-sea interaction, we conducted cloud- and urban-resolving simulations at a 900 m grid resolution. Results show that urbanization correlates with the presence of shallower cumulus clouds, cloud bases at higher altitudes, and increased cloud duration over the Galveston-Houston region compared to rural areas. These urban clouds benefit from the enhanced sensible heat and dynamic drag imparted by the urban landscape, thereby intensifying vertical mixing and moisture flux convergence. This dynamic interplay uplifts heat and moisture convergence, contributing to the enhancement of moist static energy that sustains the additional urban convection. Interestingly, our findings suggest that urbanization augments the mean HI while mitigating its afternoon high. An urban circulation dome emerges, overpowering the influence of land-sea circulations. Contrary to expectations, urbanization doesn’t seem to promote a stronger sea breeze that would favor moist and cooler air mass to the city. Instead, the influence of urbanization on cloud enhancement emerges as a crucial pathway responsible for reducing the afternoon HI values. Moreover, uncertainties in SSTs are closely linked to the sensitivities of land-sea circulations, which in turn modulate UHI and extreme heat indicators.

The study and simulation of the Urban Heat Island (UHI) and Heat Index (HI) effects in the Houston-Galveston metropolitan area demand special attention, particularly in considering moist processes aloft. During the warm season, the afternoon sea breeze phenomenon in this coastal city acts as a natural air conditioner for city residents, facilitating the dispersion of moisture, heat, and pollutants. To delve into the intricate relationships among urbanization, clouds, and land-sea interactions, we conducted cloud- and urban-resolving simulations at a 900 m grid resolution. Results show that urbanization correlates with the presence of shallow cumulus clouds, higher cloud bases, and increased cloud duration over the Galveston-Houston region compared to rural areas. These urban clouds benefit from the enhanced sensible heat and dynamic drag imparted by the urban landscape, thereby intensifying vertical mixing and moisture flux convergence. This dynamic interplay uplifts heat and moisture convergence, contributing to the enhancement of moist static energy that sustains the additional urban convection. Interestingly, our findings suggest that urbanization augments the mean HI while mitigating its afternoon high. An urban circulation dome emerges, overpowering the influence of land-sea circulations. Contrary to expectations, urbanization doesn’t seem to promote a stronger sea breeze that would favor moist and cooler air mass advection to the city. Instead, the influence of urbanization on cloud enhancement emerges as a crucial pathway responsible for reducing the afternoon HI values. Moreover, uncertainties in SSTs are closely linked to the sensitivities of land-sea circulations, which in turn modulate UHI and extreme heat indicators.

This research study shows the ability of cloud resolving models (CRM) to simulate Mesoscale Convective Systems (MCS) over the far Eastern Pacific region, off the coast of Colombia and Panama. The simulation period coincides with the newly developed OTREC field campaign (August-September, 2019), which provided enhanced upper-air soundings, NSF/NCAR G-V dropsonde and HIAPER Cloud Radar data to help evaluate the model and diagnose the environmental conditions favoring MCS development. We tested the model sensitivity to three different microphysics schemes: two popular bulk schemes (Thomson and Morrison) and one spectral bin (SBM) scheme. The models are diagnosed on their ability to simulate the observed large-scale and mesoscale environments associated with MCSs development, including the ChocoJet and Caribbean low-level jets, the semi-permanent Panama low, vertical shear, and mid-level diurnal gravity waves. We also examined the vertical distribution of hydrometeors concentrations and diabatic heat and cooling profiles. Results show that not only the SBM represents better the spatial and vertical distribution of precipitation, but also simulates better MCSs characteristics (intensity, duration, organization) and their predominant westward movement. We hypothesize that the success of the SBM in producing better organized and more long-lasting MCS stands in the stronger diabatic heating, related to a top-heavier mass profile that helps support upper-level convergence, and more intense low-level diabatic cooling that helps support stronger gravity currents. OTREC observations and CRM results shed light on the role of MCSs in the generation of enhanced mid-level mesoscale vorticity, which has been related to generation of easterly waves or enhancement of existing ones. Although the SBM is unpractical due to its computation cost (fast version takes about 10-12 times longer), it represents an important step forward in cloud modeling, with suggestive results indicating that SBM improves confidence of the physical basis of the elusive and challenging simulation of realistic tropical MCSs.

According to TRMM and GPM satellite precipitation composites, a broad maritime area over the far eastern Tropical Pacific and western Colombia houses one of the rainiest spots on Earth. This study aims to present a suite of mechanistic drivers that help create such a world-record breaking rainy spot. Previous research has shown that this oceanic and nearly-continental precipitation maximum has a strong early morning precipitation peak and development of a high density of mesoscale convective systems. We examined new and unique observational evidence highlighting the role of both dynamical and thermodynamical drivers in the activation and duration of organized convection. Results show the existence of a rather large combination of mechanisms, including: (1) dynamics of the Choco (ChocoJet) and Caribbean Low-Level Jets along their confluence zone, including the Panama semi-permanent low; (2) land breeze favors ChocoJet deceleration offshore, enhancing the nighttime and early morning low-level convergence; (3) vertical wind shear and tilting of vertical wind shear into vorticity lines that interact with convective outflows; (4) action of mid-level gravity waves, which support the strong diurnal variability; (5) mesoscale convective vortices related to subsidence in the stratiform region in long lasting MCSs reinforcing (3); and (6) the likely role of land surface-atmosphere interactions and the rainforest over western Colombia. This study emphasizes the multi-scale environmental processes associated with the formation of one of the rainiest spots on Earth and showcases new observations gathered during the Organization of Tropical East Pacific Convection (OTREC; August-September, 2019) which support the outlined mechanisms.