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[1] The chemical, physical, and optical properties of sub- and supermicrometer aerosols over the equatorial Indian Ocean were measured on board the R/V Revelle during the fall 2011 Dynamics of the Madden-Julian Oscillation field campaign. During this time, both the retreating of the Asian monsoon and two Madden-Julian Oscillation (MJO) events were observed. The R/V Revelle was on station (0.1°N and 80.5°E) to measure atmospheric and oceanic conditions between 4 October and 30 October 2011 (Leg 2) and 11 November and 4 December 2011 (Leg 3). Throughout the campaign, background marine atmospheric conditions were usually observed. As the Asian monsoon season retreated over the boreal fall and the general wind direction changed from southerly to northerly transporting, respectively, clean marine and polluted continental air masses, the average submicrometer aerosol mass nearly doubled from Leg 2 to Leg 3 and the aerosol appeared to be more influenced by continental sources. The effect of MJO-associated convection anomalies on aerosols in the remote marine boundary layer (MBL) was measured during November when a complete MJO convection wave moved over the equatorial Indian Ocean and during October when a partial MJO event was observed. MJO-associated convection strongly affected the local aerosol as increased vertical mixing introduced new particles into the MBL, rainout cleared the atmosphere of submicrometer aerosol particles, and high winds enhanced the concentration of sea salt aerosol particles in the local atmosphere. Four stages of MJO-affected aerosol population changes in the remote Indian Ocean are defined.

[1] The New England Air Quality Study (NEAQS) was a regional portion of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) planned by groups in North America and Europe to develop a better understanding of the factors that shape air quality in their respective regions and the remote North Atlantic. The NOAA research vessel Ronald H. Brown was only one of a number of platforms given the task of monitoring the emissions of aerosol and ozone precursors and the atmosphere in which they reside. Two remote and one in situ sensor were used to measure wind profiles. A radar wind profiler (RWP) permanently deployed on the ship and corrected in real time for ship motion provided continuous hourly profiles at 60- and 100-m vertical resolutions. A high-resolution Doppler lidar (HRDL) was also operated during the experiment and provided continuous low-level wind profiles. Rawinsondes were launched 4–6 times daily and provided a detailed profile of winds. Initial results show that the RWP, HRDL, and rawinsonde data compare very well. The ability of HRDL to monitor low-level winds below the minimum range gate of the RWP, while the RWP wind data extend to a much greater height than can be reached by HRDL, make the two systems complementary. Single merged profiles were generated using the RWP and HRDL data, which in turn were used to calculate trajectories to help better understand the transport of pollutants within the Gulf of Maine.

[1] In this paper, we examine observations of shallow, stable boundary layers in the cool waters of the Gulf of Maine between Cape Cod, Massachusetts, and Nova Scotia, obtained in the 2004 New England Air Quality Study (NEAQS-04), which was part of the International Consortium for Atmospheric Research into Transport and Transformation (ICARTT). The observations described herein were made from the NOAA Research Vessel Ronald H. Brown. The ship was instrumented for measurements of meteorological, gas-phase and aerosol atmospheric chemistry variables. Meteorological instrumentation included a Doppler lidar, a radar wind profiler, rawinsonde equipment, and a surface flux package. In this study, we focus on direct comparisons of the NEAQS-04 flux observations with the COARE bulk flux algorithm to investigate possible coastal influences on air-sea interactions. We found significant suppression of the transfer coefficients for momentum, sensible heat, and latent heat; the suppression was correlated with lighter winds, more stable surface layers, S-SE wind direction, and lower boundary layer heights. Analysis of the details shows the suppression is not a measurement, stability correction, or surface wave effect. The correlation with boundary layer height is consistent with an interpretation that our measurements at 18-m height do not realize the full surface flux in shallow boundary layers. We also find that a bulk Richardson number threshold of 0.1 gives a better estimate of boundary layer height than 0.25 or 0.5. Mean ozone deposition velocity is estimated as 0.44 mm s−1, corresponding to a boundary removal timescale of about 1 day.

[1] 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.

[1] This study investigates the impact of surface heterogeneity on the timing of the morning transitions observed during the 1999 Southern Oxidants Study Nashville/Middle Tennessee Intensive (SOS99N). The end of the morning transition occurs at a measurement site located in the Cumberland River Valley approximately 8 km northeast of downtown Nashville, on average, 1.25 h earlier than at three rural sites surrounding Nashville. This study shows that the early morning behavior of the boundary layers observed at the river valley site is dominated by local terrain features, whereas a previous study showed that the afternoon boundary layers observed at this site are more strongly influenced by its urban characteristics. Profiles of backscatter intensity recorded by monostatic sodars and Doppler wind profilers are analyzed to provide the evolution of boundary layer depth on days when a nocturnal inversion formed. These measurements indicate that the early morning growth rate of the convective boundary layer was faster, on average, at the river valley site in Nashville than at a rural, non-river valley site where similar measurements were made (125 m h−1 versus 73 m h−1 from 0600 to 0800 h CST). A comparison of temperature profiles measured with radio acoustic sounding systems indicates that low-level stability resulting from the nocturnal boundary layer had a stronger influence than surface energetics on the observed variations in boundary layer depth and morning transition end times. A single case study is shown to demonstrate the potential impact of the morning transition on gas phase chemistry measurements collected at a rural site. Case study and diurnally averaged time series show how surface-layer humidity can be an effective tracer for diagnosing the end of the morning transition.