Air Advection Research Articles

Local and non‐local controls on a persistent cold‐air pool in the Arve River Valley

AbstractA numerical model is used to simulate a persistent cold‐air pool (PCAP) event that occurred in the section of the Arve River Valley around Passy in the French Alps. During this period, an upper‐level ridge from the Atlantic moved over Europe, allowing a PCAP to form and persist over time. The impact of the upper‐level ridge on the PCAP and on the dynamics within the valley section is quantified by examining the mass and heat budgets of the valley atmosphere. During the persistent stage, the magnitude of the flow through the tributary valleys is enhanced by the large‐scale flow. Also, the direction of the flow through one of the tributaries is found to be determined by the height of the PCAP with respect to that of the tributary above the valley floor. The tributary flows, together with subsiding motions at the valley top, control by and large the night‐time valley‐scale circulation and the thermal structure of the upper part of the PCAP, whereas thermally driven valley flows control its lower part. When the upper‐level ridge passes over the Arve River Valley, warm air advection through the tributaries continuously erodes the upper part of the PCAP during night‐time, thereby reducing its depth, while down‐valley flows export the air mass out of the valley. As the ridge moves away from the valley, the near‐surface air is found to be trapped within the valley. This trapping results from the advection of warm air in the upper part of the PCAP by the large‐scale flow channelled through one of the tributaries. This reduces the thermally induced pressure difference in the down‐valley direction, thereby suppressing the near‐surface down‐valley flow. The study therefore highlights the interplay between the large‐scale flow, the tributary flows and the thermal structure of the PCAP.

How Does the Winter Jet Stream Affect Surface Temperature, Heat Flux, and Sea Ice in the North Atlantic?

AbstractThe role of the atmospheric jet stream in driving patterns of surface heat flux, changes in sea surface temperature, and sea ice fraction is explored for the winter North Atlantic. Seasonal time-scale ensemble hindcasts from the Met Office Hadley Centre are analyzed for each winter from 1980 to 2014, which for each year includes 40 ensemble members initialized at the start of November. The spread between ensemble members that develops during a season is interpreted to represent the ocean response to stochastic atmospheric variability. The seasonal coupling between the winter atmosphere and the ocean over much of the North Atlantic reveals anomalies in surface heat loss driving anomalies in the tendency of sea surface temperature. The atmospheric jet, defined either by its speed or latitude, strongly controls the winter pattern of air–sea latent and sensible heat flux anomalies, and subsequent sea surface temperature anomalies. On time scales of several months, the effect of jet speed is more pronounced than that of jet latitude on the surface ocean response, although the effect of jet latitude is important in altering the extent of the ocean subtropical and subpolar gyres. A strong jet or high jet latitude increases sea ice fraction over the Labrador Sea due to the enhanced transport of cold air from west Greenland, while sea ice fraction decreases along the east side of Greenland due either to warm air advection or a strong northerly wind along the east Greenland coast blowing surface ice away from the Fram Strait.

Air advection induced differences between canopy and surface heat islands

The relationship between canopy urban heat islands (CUHIs) and surface urban heat islands (SUHIs) at four times daily temporal resolution remains unclear. The CUHI–SUHI relationship is investigated using four observations per day without temporal averaging in Shanghai, Beijing, Birmingham, and Taipei, with 201 of 2232 CUHI–SUHI pairs exhibiting significant UHI differences in their spatial distributions and intensities. These 201 UHI difference cases are determined by the correlation coefficients between air and surface temperature <0.2. The SPAtial EFficiency (SPAEF) multiple-component performance metric is applied to compare the spatiotemporal patterns of SUHIs derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data and CUHIs acquired from either meteorological observations or numerical simulations. The results indicate that 81.09% of the UHI differences occurred during the daytime, and were caused by local air advection related to wind speed ≥2 m/s and land surface conditions in the study areas. We conclude that joint analysis of CUHIs and SUHIs should be conducted to characterize urban thermal environments and that current urban planning procedures should integrate these UHI differences to develop effective mitigation strategies and adaptation measures.

A Sensitivity Study of Arctic Air‐Mass Transformation Using Large Eddy Simulation

AbstractArctic air mass transformation is linked to the evolution of low‐level mixed‐phase clouds. These clouds can alter the structure of the boundary layer and modify the surface energy budget. In this study, we use three‐dimensional large eddy simulation and a bulk sea ice model to examine the lifecycle of clouds formed during wintertime advection of moist and warm air over sea ice, following a Lagrangian perspective. We investigate the stages of cloud formation, evolution, and decay. The results show that radiative cooling at the surface gives rise to fog formation which subsequently rises and transforms into a mixed‐phase cloud. In our baseline simulation, the cloud persists for about 5 days and increases the surface temperature by on average 17 °C. Sensitivity tests show that the lifetime of the cloud is sensitive to changes in the vapor supply at cloud top. This flux is mainly impacted by changes in the divergence rate; an imposed convergence decreases the lifetime to 2 days while an imposed large‐scale divergence increases the lifetime to more than 6 days. The largest difference in cloud radiative properties is found in the experiment with increased ice crystal number concentrations. In this case, the lifetime of the cloud is similar compared to baseline but the amount of liquid water is clearly depleted throughout the whole cloud sequence and the surface temperature is on average 6 °C cooler. The cloud condensation nuclei concentration has a weaker effect on the radiative properties and lifetime of the cloud.

Regional Thermodynamic Characteristics Distinguishing Long- and Short-Duration Freezing Rain Events over North America

Abstract Freezing rain is an especially hazardous winter weather phenomenon that remains particularly challenging to forecast. Here, we identify the salient thermodynamic characteristics distinguishing long-duration (six or more hours) freezing rain events from short-duration (2–4 h) events in three regions of the United States and Canada from 1979 to 2016. In the northeastern United States and southeastern Canada, strong surface cold-air advection is not common during freezing rain events. Colder onset temperatures at the surface and in the near-surface cold layer support longer-duration events there, allowing heating mechanisms (e.g., the release of latent heat of fusion when rain freezes at the surface) to act for longer periods before the surface reaches 0°C and precipitation transitions to rain. In the south-central United States, cold air at the surface is replenished via continuous cold-air advection, reducing the necessity of cold onset surface temperatures for event persistence. Instead, longer-duration events are associated with warmer and deeper &amp;gt;0°C warm layers aloft and stronger advection of warm and moist air into this layer, delaying its erosion via cooling mechanisms such as melting. Finally, in the southeastern United States, colder and especially drier onset conditions in the cold layer are associated with longer-duration events, with evaporative cooling crucial to maintaining the subfreezing surface temperatures necessary for freezing rain. Through an improved understanding of the regional conditions supporting freezing rain event persistence, we hope to provide useful information to forecasters in their attempt to predict these potentially damaging events.

The role of atmospheric conditions in CO2 and radon emissions from an abandoned water well

Boreholes and wells are complex boundary features at the earth-atmosphere interface, connecting the subsurface hydrosphere, lithosphere, and biosphere to the atmosphere above it. It is important to understand and quantify the air exchange rate of these features and, consequently their contribution as sources for greenhouse gas (GHG) emissions to the atmosphere. Here, we investigate the effect of atmospheric conditions, namely atmospheric pressure and temperature, on air, CO2, and radon transport across the borehole-ambient atmosphere interface and inside a 110-m deep by 1-m diameter borehole in northern Israel. Sensors to measure temperature, relative humidity, CO2, and radon were placed throughout a cased borehole. A standard meteorological station was located above the borehole. Data were logged at a high 0.5-min resolution for 9 months. Results show that climatic driving forces initiated 2 different advective air transport mechanisms. (1) Diurnal and semidiurnal atmospheric pressure cycles controlled daily air transport events (barometric pumping); and (2) There was a correlation between borehole-atmosphere temperature differences and transport on a seasonal scale (thermal-induced convection). Barometric pumping was identified as yielding higher fluxes of vadose zone gases than thermal-induced convection. Air velocities inside the borehole and CO2 emissions to the atmosphere were quantified, fluctuating from zero up to ~6 m/min and ~5 g-CO2/min, respectively. This research revealed the mechanisms involved in the process throughout the year and the potential contribution role played by boreholes to GHG emissions.

E-DATA: A Comprehensive Field Campaign to Investigate Evaporation Enhanced by Advection in the Hyper-Arid Altiplano

In the endorheic basins of the Altiplano, water is crucial for sustaining unique ecological habitats. Here, the wetlands act as highly localized evaporative environments, and little is known about the processes that control evaporation. Understanding evaporation in the Altiplano is challenging because these environments are immersed in a complex topography surrounded by desert and are affected by atmospheric circulations at various spatial scales. Also, these environments may be subject to evaporation enhancement events as the result of dry air advection. To better characterize evaporation processes in the Altiplano, the novel Evaporation caused by Dry Air Transport over the Atacama Desert (E-DATA) field campaign was designed and tested at the Salar del Huasco, Chile. The E-DATA combines surface and airborne measurements to understand the evaporation dynamics over heterogeneous surfaces, with the main emphasis on the open water evaporation. The weather and research forecasting model was used for planning the instruments installation strategy to understand how large-scale air flow affects evaporation. Instrumentation deployed included: meteorological stations, eddy covariance systems, scintillometers, radiosondes and an unmanned aerial vehicle, and fiber-optic distributed temperature sensing. Additional water quality and CO2 fluxes measurements were carried out to identify the link between meteorological conditions and the biochemical dynamics of Salar del Huasco. Our first results show that, in the study site, evaporation is driven by processes occurring at multiple spatial and temporal scales and that, even in the case of available water and energy, evaporation is triggered by mechanical turbulence induced by wind.

The influence of DACCIWA radiosonde data on the quality of ECMWF analyses and forecasts over southern West Africa

AbstractDuring the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) field campaign ∼900 radiosondes were launched from 12 stations in southern West Africa from 15 June to 31 July 2016. Subsequently, data‐denial experiments were conducted using the Integrated Forecasting System of the European Centre for Medium‐range Weather Forecasts (ECMWF) to assess the radiosondes' impact on the quality of analyses and forecasts. As observational reference, satellite‐based estimates of rainfall and outgoing long‐wave radiation (OLR) as well as the radiosonde measurements themselves are used. With regard to the analyses, the additional observations show positive impacts on winds throughout the troposphere and lower stratosphere, while large lower‐tropospheric cold and dry biases are hardly reduced. Nonetheless, downstream, that is farther inland from the radiosonde stations, we find a significant increase (decrease) in low‐level night‐time temperatures (monsoon winds) when incorporating the DACCIWA observations, suggesting a possible linkage via weaker cold air advection from the Gulf of Guinea. The associated lower relative humidity leads to reduced cloud cover in the DACCIWA analysis. Closer to the coast and over Benin and Togo, DACCIWA observations increase low‐level specific humidity and precipitable water, possibly due to changes in advection and vertical mixing. During daytime, differences between the two analyses are generally smaller at low levels. With regard to the forecasts, the impact of the additional observations is lost after a day or less. Moderate improvements occur in low‐level wind and temperature but also in rainfall over the downstream Sahel, while impacts on OLR are ambiguous. The changes in precipitation appear to also affect high‐level cloud cover and the tropical easterly jet. The overall rather small observation impact suggests that model and data assimilation deficits are the main limiting factors for better forecasts in West Africa. The new observations and physical understanding from DACCIWA can hopefully contribute to reducing these issues.

Conceptual model of diurnal cycle of low-level stratiform clouds over southern West Africa

Abstract. The DACCIWA (Dynamics Aerosol Chemistry Cloud Interactions in West Africa) project and the associated ground-based field experiment, which took place during summer 2016, provided a comprehensive dataset on the low-level stratiform clouds (LLSCs), which develop almost every night over southern West Africa. The LLSCs, inaccurately represented in climate and weather forecasts, form in the monsoon flow during the night and break up during the following morning or afternoon, affecting considerably the radiation budget. Several published studies give an overview of the measurements during the campaign, analyse the dynamical features in which the LLSCs develop, and quantify the processes involved in the LLSC formation. Based on the main results of these studies and new analyses, we propose in this paper a conceptual model of the diurnal cycle of the LLSCs over southern West Africa. Four main phases compose the diurnal cycle of the LLSC. The stable and the jet phases are the two steps during which the relative humidity increases, due to cooling of the air, until the air is saturated and the LLSCs form. Horizontal advection of cold air from the Guinean coast by the maritime inflow and the nocturnal low-level jet (NLLJ) represents 50 % of the local total cooling. The remaining half is mainly due to divergence of net radiation and turbulence flux. The third step of the LLSC diurnal cycle is the stratus phase, which starts during the night and lasts until the onset of surface-buoyancy-driven turbulence on the following day. During the stratus phase, interactions between the LLSCs and the NLLJ lead to a modification of the wind speed vertical profile in the cloud layer, and a mixing of the sub-cloud layer by shear-driven turbulence below the NLLJ core. The breakup of the LLSC occurs during the convective phase and follows three different scenarios which depend on the intensity of the turbulence observed during the night in the sub-cloud layer. The breakup time has a considerable impact on the energy balance of the Earth's surface and, consequently, on the depth of the convective boundary layer, which could vary by a factor of 2 from day-to-day.

Characterizing and Predicting Marine Fog Offshore Newfoundland and Labrador

Abstract As several review papers have concluded, marine fog is imperfectly characterized, and quantitative visibility forecasts are difficult to produce accurately. Some unique measurements have been made offshore Newfoundland and Labrador of the climatology of occurrence and the microphysical characteristics of marine, or open-ocean, fog. Based on measurements made at an offshore installation over 21 years, the percent of time with visibilities less than 0.5 n mi or approximately 1 km (1 n mi ≈ 1.85 km) reaches 45% in July, with a low of about 5% during the winter. The occurrence of fog is mainly due to warm air advection, with the highest frequency occurring with wind directions from over the warm Gulf Stream, and with air temperatures about 2°C warmer than the sea surface temperature. There is no diurnal variation in the frequency of occurrence of fog. The microphysical properties of the fog have been documented in the summer time frame, with over 550 h of in situ measurements made offshore with fog liquid water content greater than 0.005 g m−3. The fog droplet number concentration spectra peaks near 6 μm, with a secondary peak near 25–40 μm, which typically contains most of the liquid water content. The median droplet concentration is approximately 70–100 cm−3. The microphysical spectra have been used to develop a new NWP visibility parameterization scheme, and this scheme is compared with other parameterizations currently in use.

The Impact of an Intense Cyclone on Short‐Term Sea Ice Loss in a Fully Coupled Atmosphere‐Ocean‐Ice Model

AbstractArctic cyclones may be associated with periods of locally enhanced sea ice loss during the summer, and some studies have found that an intense cyclone in August 2012 resulted in a rapid sea ice retreat. This study uses a coupled atmosphere‐ocean‐ice model (Navy‐ESPC) to explore the relationship between the 2012 cyclone and short‐term sea ice melting. There are two mechanisms of cyclone‐induced melting in Navy‐ESPC: turbulent mixing of a warm layer located at 15‐ to 35‐m depth increases bottom melting and warm air advection by the strong surface winds increases surface melting. Although the rate of sea ice melt is substantially increased in association with the cyclone, this effect is confined to a relatively small region and only lasts for a few days. There is no clear signature of the cyclone on the overall Arctic sea ice extent in Navy‐ESPC.

Synoptic and meteorological conditions during extreme snow cover ablation events in the Great Lakes Basin

AbstractSnow cover ablation in the Great Lakes basin is a common and hydrologically important process during the cold season, contributing to a majority of the basin's runoff, and less frequent, extreme ablation events are highly impactful due to an increased flooding risk and warrant specific investigation. A brief climatology of extreme ablation events is presented, where extreme is considered within the top 5% of the distribution. Using synoptic classification techniques, individual weather patterns associated with extreme snow ablation in the Great Lakes basin are isolated. A single pattern deemed the most influential in generating extreme ablation events, southerly flow‐1, is examined in detail, and three case studies are presented to determine the meteorological conditions and surface energy fluxes responsible for ablation. Over 75% of extreme events are associated with southerly flow patterns that predominantly ablate snow with sensible heat fluxes, while rain‐on‐snow patterns induce the remaining extreme events from 1980–2009. Type southerly flow‐1 is responsible for 45% of the extreme events and is characterized by strong southerly advection of warm air into the basin, where sensible heat fluxes of 45–125 Wm−2 are responsible for the majority of energy transfer into the snowpack. When compared with an average ablation event, an extreme ablation event for southerly flow‐1 exhibits air temperatures, dew point temperatures, and wind speeds that are 3.8°C, 3.0°C, and 1.2 ms−1 warmer and faster than an average event, indicating a greater potential for larger ablation.

Observational aspects of tropical mesoscale convective systems over southeast India

To enhance the knowledge of various physical mechanisms related to the evolution of Tropical Mesoscale Convective Systems (MCSs), detailed analysis has been performed using suite of observations (weather radar, electric field mill, surface weather station, flux tower, microwave radiometer and wind profilers) available at Gadanki (13.5°N/79.2°E), located over southeast India. Analysis suggests that these systems developed in warm, moist environment associated with large scale low level convergence. Significant variations in cloud to ground (CG) lightning activity indicate the storm electrification. Deep (shallow) vertical extents with high (low) reflectivity and cloud liquid water; dominant upward (downward) motion reveals variant distribution in convective (stratiform) portions. Existence of both +CG and –CG flashes in convective regions, dominant –CG in stratiform regions explains the relation between lightning polarity and rain and cloud type. Sharp changes in surface meteorological variables and variations in surface fluxes are noticed in connection to cold pool of the system. Increase (decrease) in temperature, moisture and equivalent potential temperature (θe) within the boundary layer in convective (stratiform) regions associated with latent heat warming (cooling) of air parcel are apparent. Presence of updrafts and downdrafts in convective region and dominant downdrafts in stratiform regions are evident from vertical velocity measurements. Isentropic upgliding (downgliding) illustrate the existence of isentropic ascents (descent) of air parcels in the storm vicinity. Veering (backing) of wind due to warm (cold) and moist (dry) air advections demonstrated the formation of θe ridge in storm environment. Blend of observations provided considerable insight of electrical, microphysical, thermodynamic, dynamic and kinematic features of MCS.

The Role of Atmospheric Processes Associated with a Flash-Flood Event Over Northwestern Turkey

This study investigates the atmospheric mechanisms triggering flash-flood event in Thrace Basin of Turkey on November 27, 2018. Underestimation of this extreme precipitation amounts by NWP global and regional models (i.e. ECMWF, ALARO, WRF) and other meteorological difficulties (i.e. complex topography, land-sea interactions) in weather forecasting disabled disaster risk reduction before the event occurred. Detailed synoptic, thermodynamic, in-situ, and remote sensing analyzing results showed that significant amount of moisture during the afternoon times of the day was transferred to the atmosphere (from ground to 300-hPa) as a consequence of the excessive heating of sea surface temperatures (SSTs) of the Aegean Sea (16.5 °C in Ayvacik-Gulpinar place, 0.9 °C above its long-term normals). Strong southwesterly wind speeds associated with slow meridional movement of mid-latitude cyclone from its origin to the Eastern Mediterranean (EM) enabled transferring of relatively warm moist air to the land areas of Thrace Region (> 300 km fetch distance). Strong updraft and instability conditions under developed a supercell resulted with lightning (totally 63 cloud-to-ground and 59 intra cloud) and heavy rainfall especially Suloglu, Kofcaz, and Edirne settlements with the 12-hour total amounts 160.0, 123.0, and 97.4 mm (rainfall return period ~ 100 years), respectively. Flash-flood event caused numerous injuries and the death of a person and damaged, automobiles, houses, crops, and infrastructure of the Edirne and its neighboring settlements. From Showalter, K, Total of Totals, SWEAT, and CAPE instability indices; SWEAT is most appropriate to represent high possibility of occurrence of severe thunderstorms over the Edirne province owing to low-level moisture, warm air advection and low and mid-level wind speed terms in its equation.

Foehn classification and climatology in Sofia for 1975–2014

Foehn is a warm, dry, and downslope wind blowing in the lee side of a mountain range. It is a well known example of a local atmospheric circulation. The foehn wind is also an extreme weather event, and its forecasting is an important task for the short-range weather forecaster. The foehn in Bulgaria is observed on the northern slopes of the mountains, as a result of warm air advection from the south and southwest. Its occurrence is highest north of the Vitosha and Balkan mountains. In this study, a synoptic classification of the meteorological conditions leading to foehn in the central meteorological station in Sofia for the period 1975–2014 is made. Foehn climatology is prepared, and in addition, an evaluation of the foehn as an extreme weather event by wind gust is presented. For the period 1975–2014, there were 298 days with foehn in Sofia, which resulted from 220 synoptic cases. A manual foehn classification was developed with four major types. Type I is associated with the Mediterranean cyclone with the highest frequency – 52% of the foehn days. Foehn climatology gives average annual number of 7.5 foehn days but with a large variance between decades. The lowest annual number of days (4.5) is registered for the 2005–2014 period, and it was associated with the lowest recorded wind gust (22 m/s).

Tropospheric aerosol layers: features of variations in aerosol parameters when changing the direction of air advection

Tropospheric aerosol layers: features of variations in aerosol parameters when changing the direction of air advection

The 2018 Camp Fire: Meteorological Analysis Using In Situ Observations and Numerical Simulations

The November 2018 Camp Fire quickly became the deadliest and most destructive wildfire in California history. In this case study, we investigate the contribution of meteorological conditions and, in particular, a downslope windstorm that occurred during the 2018 Camp Fire. Dry seasonal conditions prior to ignition led to 100-h fuel moisture contents in the region to reach record low levels. Meteorological observations were primarily made from a number of remote automatic weather stations and a mobile scanning Doppler lidar deployed to the fire on 8 November 2018. Additionally, gridded operational forecast models and high-resolution meteorological simulations were synthesized in the analysis to provide context for the meteorological observations and structure of the downslope windstorm. Results show that this event was associated with mid-level anti-cyclonic Rossby wave breaking likely caused by cold air advection aloft. An inverted surface trough over central California created a pressure gradient which likely enhanced the downslope winds. Sustained surface winds between 3–6 m s−1 were observed with gusts of over 25 m s−1 while winds above the surface were associated with an intermittent low-level jet. The meteorological conditions of the event were well forecasted, and the severity of the fire was not surprising given the fire danger potential for that day. However, use of surface networks alone do not provide adequate observations for understanding downslope windstorm events and their impact on fire spread. Fire management operations may benefit from the use of operational wind profilers to better understand the evolution of downslope windstorms and other fire weather phenomena that are poorly understood and observed.

Understanding the Role of Tropical Moisture in Atmospheric Rivers

AbstractThere has been a long‐standing debate about whether tropical moisture transport or local moisture convergence plays the dominant role in the development and strength of atmospheric rivers (ARs). In this work, we quantify the contribution of tropical moisture to 29 simulated extreme ARs that affected the U.S. Northwest Coast by “tagging” tropical moisture from the eastern tropical Pacific using water vapor tracers in the Weather Research and Forecast model. From a case study with strong tropical moisture, we find three mechanisms by which tropical moisture can contribute to AR strength: direct contribution of moisture, indirect contributions via thermodynamic feedbacks, and indirect contributions via synoptic‐scale dynamic feedbacks. Focusing on these processes, we divide the 29 simulated ARs into two groups (&gt;25% of direct tropical moisture contribution from the eastern tropical Pacific to inland precipitation as DTM‐ARs and the other ARs with &lt;25% as OT‐ARs) and find that DTM‐ARs are characterized by stronger pre‐cold‐frontal low‐level jets and stronger warm air advection. Notably, the ARs with the greatest 2‐day precipitation are found to be DTM‐ARs but with &lt;50% tropical moisture contributions, indicating a more dominant role of indirect enhancement of local moisture convergence due to tropical moisture than the direct moisture contribution supplying precipitation.

Aeolian transport and deposition of carbonaceous aerosols over the Northwest Pacific Ocean in spring

The present study provides insight into the long-range transport of carbonaceous aerosols over the Northwest Pacific Ocean (NWP) based on marine aerosol samples collected onboard a research vessel in the spring of 2015. Organic carbon (OC) and elemental carbon (EC) concentrations showed maxima in proximity to land, with high levels also observed in advective air masses. The decoupled spatial variations in OC and EC levels in relatively pristine air masses from the Pacific, in which aerosol n-alkanes of marine origin were relatively high in abundance, revealed the influence of heterogeneity in air masses and suggested the release of n-alkanes from local marine environments. This was confirmed by the enhanced particle-bound fraction of marine organic matter-sourced n-alkanes, resulting in non-significant relationships between the gas–particle partition coefficient of n-alkanes and the corresponding sub-cooled liquid vapor pressure in some marine air masses. High OC/EC ratios over the NWP indicated secondary organic aerosol formation, possibly as a result of marine emissions followed by gas-to-particle conversion and/or aerosol aging during long-range transport from East Asia. Similar molecular profiles of n-alkanes were found in air and seawater particles in regions dominated by either marine or continental influence, reflecting the strengths of continental and/or marine input and more importantly, suggesting the occurrence of air–sea exchange through wind-induced marine emissions or atmospheric deposition. Relative to the East China Sea, dry particle deposition after long-range transport acted as a more important source for terrestrial lipids in the open NWP.

Episodes of Warm‐Air Advection Causing Cloud‐Surface Decoupling During the MARCUS

AbstractIt has been known for decades that advection of a cloud‐topped marine boundary layer (CTBL) over warmer sea surface causes the stratification (or decoupling) of the CTBL via the entrainment feedback, a mechanism commonly known as “deepening‐warming” decoupling that is typical in subtropics. This study focuses on the opposite direction of advection, that is, low‐level warm air advection (LLWAA), and its impacts on the decoupling degree of a CTBL. Our hypothesis is that LLWAA stabilizes a CTBL, causing a decoupling of the CTBL. It is tested for three LLWAA episodes observed during the Measurements of Aerosols, Radiation, and CloUds over the Southern Ocean (MARCUS) field campaign between the Hobart (43°S, 147°E), Australia, and several Antarctic coast stations. By synthesizing the shipborne measurements of CTBL structure, Himawari‐8 satellite imagery of cloud fields, and reanalysis of meteorological field, four common characteristics of CTBLs under the LLWAA are found: (1) CTBLs are highly stratified to the extent that penetrations of cumulus into main temperature inversions, which are common for subtropical decoupled CTBLs, do not exist; (2) sea surface temperature is 1–2 K lower than the near‐surface air temperature; (3) clouds manifest stratiform with lifetime as long as several tens of hours; and (4) they locate in warm sectors of middle‐latitude cyclones. Possible mechanisms for the maintenance of decoupled clouds under LLWAA are discussed in terms of dynamic and thermodynamic factors. Lapse rates of the decoupled CTBLs are markedly lower than those commonly used for passive satellite estimation of cloud top heights.