Stable Marine Boundary Layer Research Articles

Frontal effects on the rapid formation of a deep layer of marine fog and cloud in the NW Atlantic

AbstractRapid formation of a deep layer of fog and clouds extending to a height of 2 km was observed over Sable Island off Nova Scotia on 13–14 July 2019. This fog and cloud event was dominantly caused by the rapid intrusion of a trough from a cyclone over NE Canada. The trough quickly moved from SW–W to E–NE and encroached on the coastal waters of Nova Scotia. Due to the warm front in the trough, the air temperature increased up to 4 km in height with increased stability, while the sea‐level pressure at Sable Island dropped by 11 hPa within 12 hours. The moderate to strong winds turned counterclockwise from the N to the SE, converging with the SE winds maintained by the ridge located to the east. There was no indication of an advection‐type fog mechanism because the surface air temperature was not adjusted to the colder sea‐surface temperature that could have resulted in saturation of the surface air. The leading portion of a massive cloud band in the trough passing over Sable Island included precipitation before fog formation. The rain that evaporated in the unsaturated and warm subcloud layer effectively reduced the air temperature and significantly increased the wet‐bulb temperature in this layer. The cloud extended into the moist subcloud layer, while fog formed at 1400 UTC 13 July and lasted until 2327 UTC 13 July (with two brief periods of mist lasting less than two hours). The development of this deep layer of fog and cloud in the NW Atlantic was caused by evolving synoptic conditions through the simultaneous effects of a rapid trough intrusion over coastal waters with a massive cloud band, induced precipitation causing cloud deepening into the subcloud layer, and surface wind convergence forcing vertical mixing in a stable marine boundary layer.

The Sundowner Winds Experiment (SWEX) in Santa Barbara, California: Advancing Understanding and Predictability of Downslope Windstorms in Coastal Environments

Abstract Coastal Santa Barbara is among the most exposed communities to wildfire hazards in Southern California. Downslope, dry, and gusty windstorms are frequently observed on the south-facing slopes of the Santa Ynez Mountains that separate the Pacific Ocean from the Santa Ynez valley. These winds, known as “Sundowners,” peak after sunset and are strong throughout the night and early morning. The Sundowner Winds Experiment (SWEX) was a field campaign funded by the National Science Foundation that took place in Santa Barbara, California, between 1 April and 15 May 2022. It was a collaborative effort of 10 institutions to advance understanding and predictability of Sundowners, while providing rich datasets for developing new theories of downslope windstorms in coastal environments with similar geographic and climatic characteristics. Sundowner spatiotemporal characteristics are controlled by complex interactions among atmospheric processes occurring upstream (Santa Ynez valley), and downstream due to the influence of a cool and stable marine boundary layer. SWEX was designed to enhance spatial measurements to resolve local circulations and vertical structure from the surface to the midtroposphere and from the Santa Barbara Channel to the Santa Ynez valley. This article discusses how SWEX brought cutting-edge science and the strengths of multiple ground-based and mobile instrument platforms to bear on this important problem. Among them are flux towers, mobile and stationary lidars, wind profilers, ceilometers, radiosondes, and an aircraft equipped with three lidars and a dropsonde system. The unique features observed during SWEX using this network of sophisticated instruments are discussed here.

Ozone production efficiency of a ship-plume: ITCT 2K2 case study

Ozone production efficiency (OPE) of ship plume was first evaluated in this study, based on ship-plume photochemical/dynamic model simulations and the ship-plume composition data measured during the ITCT 2K2 (Intercontinental Transport and Chemical Transformation 2002) aircraft campaign. The averaged instantaneous OPEs (OPEi‾) estimated via the ship-plume photochemical/dynamic modeling for the ITCT 2K2 ship-plume ranged between 4.61 and 18.92, showing that the values vary with the extent of chemical evolution (or chemical stage) of the ship plume and the stability classes of the marine boundary layer (MBL). Together with OPEi‾, the equivalent OPEs (OPEe‾) for the entire ITCT 2K2 ship-plume were also estimated. The OPEe‾ values varied between 9.73 (for the stable MBL) and 12.73 (for the moderately stable MBL), which agreed well with the OPEe‾ of 12.85 estimated based on the ITCT 2K2 ship-plume observations. It was also found that both the model-simulated and observation-based OPEe‾ inside the ship-plume were 0.29–0.38 times smaller than the OPEe‾ calculated/measured outside the ITCT 2K2 ship-plume. Such low OPEs insides the ship plume were due to the high levels of NO and non-liner ship-plume photochemistry. Possible implications of this ship-plume OPE study in the global chemistry-transport modeling are also discussed.

Low Ozone Episodes at Amphitrite Point Marine Boundary Layer Observatory, British Columbia, Canada

ABSTRACTAt Amphitrite Point, ozone (O3) mixing ratios are observed to drop steadily to 5–15 ppb over a period of 12 hours or less with a frequency approaching one event per week (with highest frequencies occurring in summer and fall). Analysis of 47 such O3 depletion events reveals that low O3 episodes are a predominantly nocturnal phenomenon associated with anticyclonic conditions characterized by light onshore or alongshore winds and an absence of fog and mist. Back-trajectories show air carried to the Amphitrite Point Observatory (APO) during depletion events remains in the marine boundary layer and is not brought to the surface from aloft. There is no strong correlation with other “criteria” pollutants (CO, NOx, SO2, PM2.5) that might be indicative of a mechanism for O3 destruction linked to human, terrestrial, or marine pollutant sources. However, CO2 mixing ratios are observed to increase, coincident with O3 depletion. Together, these results point to a natural marine boundary layer phenomenon in which O3 destruction dominates O3 production and/or replenishment by vertical mixing. While there are several candidate mechanisms, the conditions for O3 depletion (and CO2 buildup) to occur are set by meteorology and, in particular, development of a stable marine boundary layer in which vertical mixing is suppressed. Support for this interpretation is provided by simultaneous increases in CO2 in the stable marine boundary that are indicative of an important role played by marine biogenic processes (respiration). Future research should be directed at elucidating the chemical mechanisms responsible for O3 destruction in the coastal zone, which means that there would be a need for a much broader range of measurements at APO (including halogenated species) as well as offshore measurements of both chemical and marine boundary layer meteorological variables.

Modeling ozone plumes observed downwind of New York City over the North Atlantic Ocean during the ICARTT field campaign

Abstract. Transport and chemical transformation of well-defined New York City (NYC) urban plumes over the North Atlantic Ocean were studied using aircraft measurements collected on 20–21 July 2004 during the ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) field campaign and WRF-Chem (Weather Research and Forecasting-Chemistry) model simulations. The strong NYC urban plumes were characterized by carbon monoxide (CO) mixing ratios of 350–400 parts per billion by volume (ppbv) and ozone (O3) levels of about 100 ppbv near New York City on 20 July in the WP-3D in-situ and DC-3 lidar aircraft measurements. On 21 July, the two aircraft captured strong urban plumes with about 350 ppbv CO and over 150 ppbv O3 (~160 ppbv maximum) about 600 km downwind of NYC over the North Atlantic Ocean. The measured urban plumes extended vertically up to about 2 km near New York City, but shrank to 1–1.5 km over the stable marine boundary layer (MBL) over the North Atlantic Ocean. The WRF-Chem model reproduced ozone formation processes, chemical characteristics, and meteorology of the measured urban plumes near New York City (20 July) and in the far downwind region over the North Atlantic Ocean (21 July). The quasi-Lagrangian analysis of transport and chemical transformation of the simulated NYC urban plumes using WRF-Chem results showed that the pollutants can be efficiently transported in (isentropic) layers in the lower atmosphere (<2–3 km) over the North Atlantic Ocean while maintaining a dynamic vertical decoupling by cessation of turbulence in the stable MBL. The O3 mixing ratio in the NYC urban plumes remained at 80–90 ppbv during nocturnal transport over the stable MBL, then grew to over 100 ppbv by daytime oxidation of nitrogen oxides (NOx = NO + NO2) with mixing ratios on the order of 1 ppbv. Efficient transport of reactive nitrogen species (NOy), specifically nitric acid (HNO3), was confirmed through the comparison of the CO/NOy ratio in photochemically fresh and aged NYC plumes, implying the possibility of long-range transport of O3 over the stable MBL over the North Atlantic Ocean in association with NOx regeneration mechanism. The impact of chemical initial and boundary conditions (IC/BCs) on modelled O3 urban plumes was investigated in terms of the background O3 level and the vertical structure of the urban plumes. Simulations with dynamic ("time-variant") chemical IC/BCs enhanced the O3 level by 2–12 ppbv on average in the atmospheric layer below 3 km, showing better agreement with the observed NYC plumes and biomass-burning plumes than the simulation with prescribed static IC/BCs. The simulation including MOZART-4 chemical IC/BCs and Alaskan/Canadian wildfire emissions compared better to the observed O3 profiles in the upper atmospheric layer (>~3 km) than models that only accounted for North American anthropogenic/biogenic and wildfire contributions to background ozone. The comparison between models and observations show that chemical IC/BCs must be properly specified to achieve accurate model results.

The Spatial and Temporal Variability of Convective Storms over the Northeast United States during the Warm Season

Abstract A spatial and temporal climatology of convective storms over the Northeast United States during the warm season (April–September) is presented using composite National Operational Weather radar (NOWrad) data at 2-km grid spacing from 1996 to 2007 as well as cloud-to-ground lightning from the National Lightning Data Network (NLDN) on a 10-km grid from 2001 to 2007. There are preferred regions for convective storms within New York’s Hudson River Valley, western and southeastern Pennsylvania, central New Jersey, and across the Delmarva Peninsula. A favored initiation area during the early afternoon is the immediate lee of the Appalachians, with a tendency for these convective systems to move eastward to the coast by late evening. There is a sharp gradient in convective frequency within 20 km of the coast on average as a result of the relatively stable marine boundary layer, but as the sea surface warms by midsummer this convective activity increases near the coast, with a nocturnal (0600–1200 UTC) convective maximum over the coastal waters. Convective frequency can vary by more than 40% interannually across subregions of the Northeast. There was 40%–50% more convection across southern New England and Long Island during 1998–2001 than in 2002–05, which was partially the result of more frequent and amplified trough activity in 1998–2001, which helped to trigger convective storms. Spatial composites using the North American Regional Reanalysis (NARR) highlight some of the synoptic flow patterns associated with the enhanced convective frequencies. Convective storms tend to weaken rapidly from west to east across southern New England when there is low-level southerly flow from the relatively cool ocean. Severe convection is favored over the populated New York City and Long Island coastal regions when there is warm, moist, and unstable air extending northward along the mid-Atlantic coastal plain, with west-southwesterly flow at low levels and an approaching shortwave trough at midlevels.

Structure and formation of the highly stable marine boundary layer over the Gulf of Maine

A shallow, stable boundary layer is ubiquitous over the cool waters of the Gulf of Maine in summer. This layer affects pollutant transport throughout the region by isolating overlying flow from the surface. In this paper, we explore how the stable boundary layer is formed and describe its characteristics. The temperature profile of the lowest 1–2 km of the atmosphere over the Gulf of Maine is remarkably similar regardless of transport time over water or the time of day when the flow left the land, provided only that the flow is offshore. This similarity is forced by the (roughly) constant water temperature and the (roughly) constant temperature of the free troposphere over the continent. However, the processes leading to the similar profiles are quite different depending on the time of day when the flow crosses the coast. Air leaving the coast at night already has a stable profile, whereas air leaving the coast at midday or afternoon has a deep mixed layer. In the latter case, the stable layer formation over the water is of interest. Using observations of surface fluxes, profiles, and winds on the NOAA Research Vessel Ronald H. Brown from the 2004 International Consortium for Atmospheric Research in Transport and Transformation (ICARTT)/New England Air Quality Study, we show that the formation of the stable layer, which involves cooling a roughly 50‐ to 100‐m‐deep layer by 5–15 K, occurs within 10 km and a half hour after leaving the coast. The internal boundary layer near shore is deeper than predicted by standard relationships. Historical data are explored and also show deeper internal boundary layers than predicted. We also describe one exceptional case where a 200‐m‐deep neutral layer was observed and discuss the degree of isolation of the stable boundary layer and its duration.

Evaluating Formulations of Stable Boundary Layer Height

Abstract Stable boundary layer height h is determined from eddy correlation measurements of the vertical profiles of the buoyancy flux and turbulence energy from a tower over grassland in autumn, a tower over rangeland with variable snow cover during winter, and aircraft data in the stable marine boundary layer generated by warm air advection over a cool ocean surface in summer. A well-defined h within the tower layer at the grass site (lowest 50 m) and the snow site (lowest 30 m) was definable only about 20% of the time. In the remaining stable periods, the buoyancy flux and turbulence energy either (a) remained constant with height, indicating a deep boundary layer, (b) increased with height, or (c) varied erratically with height. Approximately one-half of the tower profiles did not fit the traditional concepts of a boundary layer. The well-defined cases of h are compared with various formulations for the equilibrium depth of the stably stratified boundary layer based on the Richardson number or surface fluxes. The diagnostic models for h have limited success in explaining both the variance and mean magnitude of h at all three sites. The surface bulk Richardson number and gradient Richardson number approaches perform best for the combined data. For the surface bulk Richardson number method, the required critical value varies systematically between sites. The surface bulk Richardson number approach is modified to include a critical value that depends on the surface Rossby number, which incorporates the influence of surface roughness and wind speed on boundary layer depth.

Tropospheric Sulfate Aerosol Formation Via Ion-Ion Recombination

ABSTRACT We propose a source of aerosols in the lower atmosphere associated with the creation, growth, and recombination of ubiquitous cosmogenically generated ions. This particle source should be favorable in the relatively clean, stable marine boundary layer, providing a uniform, continuous fine particle generator in the presence of dimethylsulfide emissions. Through this mechanism, new sulfate aerosols can be formed at sulfuric acid vapor partial pressures well below the supersaturations required for homogeneous binary nucleation of sulfuric acid/water solutions, which is consistent with numerous observations of new particle formation under sub-saturated conditions. The evolving aerosols in turn control the acid vapor concentration and thus modulate the sizes of the precursor ions and the rate of new particle formation. A simple model representing this nonlinear coupled system predicts that the physical and chemical processes connecting ions, vapors, and aerosols effectively constrain the particle population to a relatively narrow range of values. This self-limiting behavior may explain in part the apparent stability of the marine sulfate aerosol, with mean concentrations of the order of several hundred per cubic centimeter.

Modeling the Canadian southern Atlantic region oxidants: A study of a Canadian EMEFS‐1 hyperintensive period

The roles and mechanisms of transport and chemical transformation affecting surface ozone in the Canadian southern Atlantic region (SAR) are investigated using a three‐dimensional Eulerian comprehensive modeling system. The investigation is focused a regional ozone episode over the eastern North America during the first week of August 1988. The model performance is evaluated with available observations during this period. The model is shown to reproduce the general features of this regional episode, with better performance for the sites with clear local photochemical activities than for those strongly influenced by complex coastal flow. It is shown from the reconstructed time history of various processes along the backward trajectories originating from a site in Nova Scotia that the elevated ground‐level ozone in the SAR during the study period was associated with transport at low levels, under a strong southwesterly flow. The high ozone brought to the site was mostly produced within the stable marine boundary layer from precursors picked up earlier over the emission area on the east coast of the United States. A second transport situation was also simulated and shown to be associated with transport aloft that was never mixed to the surface. This study also shows that differential advection, due to stable stratification and vertical shear of horizontal wind, can deform surface‐based plumes to produce elevated layers of pollutants over the Gulf of Maine.

Likelihood estimation of tropospheric duct parameters from horizontal propagation measurements

A Bayesian estimation technique using an embedded electromagnetic parabolic equation model is implemented to solve the inverse problem of determining the atmospheric refractivity from measurements of beyond‐line‐of‐sight radio‐frequency propagation factors. The inverted refractivity structure is associated with the trapping layer that can be formed by the capping inversion of the stable marine boundary layer. Why this inverse problem is of interest is established by describing the refractivity structure associated with the stable marine boundary layer in the context of its effect on horizontal propagation and then describing the difficulty of obtaining representative estimates of the tropospheric refractivity structure using existing sensing methods. The implementation is then described. A three‐parameter model of the marine boundary layer is used; two of the three parameters are inverted, while the third is assumed to be a random variable. The inversion method is applied to radio data from the Variability Of Coastal Atmospheric Refractivity Experiment (VOCAR). A matrix of propagation factors corresponding to the two parameters to be inverted is calculated for the path geometry and the frequencies of VOCAR. Parameter likelihoods are determined for the entire parameter space, and mean estimates of refractivity parameters are computed from the distributions. The parameter estimates are compared with values obtained from radiosondes launched during VOCAR. The remotely sensed refractivity parameters for multiple‐frequency sensing show good agreement with directly sensed observations.

Sudden changes in arctic atmospheric aerosol concentrations during summer and autumn

The International Arctic Ocean Expedition of 1991 measured number concentrations of condensation nuclei and cloud condensation nuclei from 1 August to 6 October between latitudes 70°N and 90°N. Changes in concentration of more than a factor of 2 in 1 h or less were frequent in spite of the long distances from significant sources and often appeared in groups separated by times of the order of 1 h. These observations provide a unique opportunity to study interacting meteorological mechanisms in the stable marine boundary layer. It is suggested that most of these changes were due to two factors: (1) large vertical concentration gradients caused mainly by interaction of aerosol and clouds and (2) intermittent localised mixing into the shallow surface mixed layer, caused by atmospheric wave motions or roll vortices. Quasi-periodic changes of smaller amplitude were present for about 35% of the total time with mean periods in the range 60–90 min. About 1/6 of the sudden large changes, usually involving an isolated decrease and recovery in accumulation mode particle number concentrations, is attributed to mixing caused by shear-induced Kelvin-Helmholtz breaking waves. Regular sequences of maxima and minima were about as common but involved 2/3 of all such changes because of the number of events in each sequence. Satellite images, radiosonde wind profiles and the typical mean periods suggested that roll vortices were common in the high Arctic. The associated mixing changes seemed to account adequately for the alternating maxima and minima in aerosol number concentrations. The special conditions of atmospheric stability and high frequencies of low cloud cover in the high Arctic combined with the absence of strong local sources of particles have allowed us to identify processes of mixing that often drastically modified aerosol size distributions and concentrations near the surface which are likely to have a far more general application. These factors have to be added to studies of chemical and physical processes influencing the life cycle of the aerosol and air transport, in order to understand the sources, sinks and to define the radiative forcing by the atmospheric aerosol in the Arctic and elsewhere. DOI: 10.1034/j.1600-0889.1996.t01-1-00009.x

Sudden changes in arctic atmospheric aerosol concentrations during summer and autumn

The International Arctic Ocean Expedition of 1991 measured number concentrations of condensation nuclei and cloud condensation nuclei from 1 August to 6 October between latitudes 70°N and 90°N. Changes in concentration of more than a factor of 2 in 1 h or less were frequent in spite of the long distances from significant sources and often appeared in groups separated by times of the order of 1 h. These observations provide a unique opportunity to study interacting meteorological mechanisms in the stable marine boundary layer. It is suggested that most of these changes were due to two factors: (1) large vertical concentration gradients caused mainly by interaction of aerosol and clouds and (2) intermittent localised mixing into the shallow surface mixed layer, caused by atmospheric wave motions or roll vortices. Quasi-periodic changes of smaller amplitude were present for about 35% of the total time with mean periods in the range 60–90 min. About 1/6 of the sudden large changes, usually involving an isolated decrease and recovery in accumulation mode particle number concentrations, is attributed to mixing caused by shear-induced Kelvin-Helmholtz breaking waves. Regular sequences of maxima and minima were about as common but involved 2/3 of all such changes because of the number of events in each sequence. Satellite images, radiosonde wind profiles and the typical mean periods suggested that roll vortices were common in the high Arctic. The associated mixing changes seemed to account adequately for the alternating maxima and minima in aerosol number concentrations. The special conditions of atmospheric stability and high frequencies of low cloud cover in the high Arctic combined with the absence of strong local sources of particles have allowed us to identify processes of mixing that often drastically modified aerosol size distributions and concentrations near the surface which are likely to have a far more general application. These factors have to be added to studies of chemical and physical processes influencing the life cycle of the aerosol and air transport, in order to understand the sources, sinks and to define the radiative forcing by the atmospheric aerosol in the Arctic and elsewhere. DOI: 10.1034/j.1600-0889.1996.t01-1-00009.x

Observations of Height-dependent Pressure-Perturbation Structure of a Strong Mesoscale Gravity Wave

Airborne observations using a downward-looking, dual-frequency, near-infrared, differential absorption lidar system provide the first measurements of the height-dependent pressure-perturbation field associated with a strong mesoscale gravity wave. A pressure-perturbation amplitude of 3.5 mb was measured within the lowest 1.6 km of the atmosphere over a 52-km flight line. Corresponding vertical displacements of 250-500 m were inferred from lidar-observed displacement of aerosol layers. Accounting for probable wave orientation, a horizontal wavelength of about 40 km was estimated. Satellite observations reveal wave structure of a comparable scale in concurrent cirrus cloud fields over an extended area. Smaller-scale waves were also observed. Local meteorological soundings are analyzed to confirm the existence of a suitable wave duct. Potential wave-generation mechanisms are examined and discussed. The large pressure-perturbation wave is attributed to rapid amplification or possible wave breaking of a gravity wave as it propagated offshore and interacted with a very stable marine boundary layer capped by a strong shear layer.