COMPASS Friday - Archive

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FALL 2019
Fridays at 11:00 am, RSMAS Auditorium




Sep 13: NO SEMINAR (Auditorium in use for RSMAS Staff Retreat)

Sep 20: NO SEMINAR (RSMAS Faculty Meeting at COMPASS time)

Sep 27: Mingming Shao
Department of Ocean Sciences, RSMAS
(one-hour AMP student seminar)

Air-Sea Interaction and Submesoscale Processes near Fronts

Both theoretical and numerical studies have shown that submesoscale fronts (SF) play a critical role in the downscale energy transfer. Due to their short temporal scales (hours to days) and fine spatial features(0.1-10 km), direct observations are still challenging and rare. Thus the dynamics of air-sea interaction and the upper ocean processes near SF are largely unknown. To address this, a study of the air-sea response to SFs was conducted during the LAgrangian Submesoscal ExpeRiment (LASER), which focused on surface material dispersion in the northern Gulf of Mexico. This presentation focuses on  ship-based measurements, including air-sea flux masts, an X-band marine radar, a moving vessel CTD profiler. It mainly contains three parts: The first part documents the general variation of surface winds and fluxes across SFs. Cross-frontal wind variance was depended on the airflow relative to in-water thermal gradient. Also, SFs were found to locally enhance the heat flux across the air-sea interface compared to bulk values. The second part focuses on how the atmospheric surface layer (ASL) turbulence responds to SF. The ASL shifted from slightly unstable to  stable conditions across SFs. The quadrant analysis showed that the ejection and sweep events happened more frequently in the stable condition, which led to the difference of transport efficiencies between momentum and scalar fluxes. In addition, the wavelet analysis was used to detect the size of coherent eddies. The last part focuses on the phenomena of internal wave generation associated with frontal features. The high frequency internal waves with a spacing of ~150 m were observed to  propagate towards the SF at 0.30 m/s. A second-order KdV model based on the environmental parameters agrees with the observed HIW speed. Additionally, vertical profiles of potential vorticity indicated that the dynamic conditions were conducive to inertial and symmetric instabilities. Given the prevalence of SF, these findings suggest that SFs may have a cumulative impact on air-sea interactions and energy transfers in the upper ocean, which merits further exploration.

Oct 04: Joshua Wadler
Department of Ocean Sciences, RSMAS
(one-hour MPO student seminar)

The Relationship Between Downdraft Structure and
Tropical Cyclone Boundary Layer Thermodynamic Variability:
Implications for Intensity Change

A recent paradigm to explain tropical cyclone (TC) intensity states that when a storm is exposed to environmental wind shear, downdrafts underneath quasi-stationary convection transport low equivalent potential temperature (θe) air into the boundary layer (BL). While this paradigm is supported by composites from observational data, few case studies have looked at how this process is related to downdraft structure and variability. We present two observational case studies, Hurricane Earl (2010) and Michael (2018), that relate downdraft structure and storm-relative location to BL thermodynamic variability and evolution. In Hurricane Earl, strong convective downdrafts (>2 ms–1) in the inner core were associated with maxima in BL θe, while weaker convective downdrafts were associated with minima in BL θe. In addition, non-convective downdrafts away from the eyewall region were associated with minima in θe. For every downdraft, the variability in θe was due to variability in specific humidity. Implications for each downdraft are discussed through the potential for BL recovery from the air-sea enthalpy fluxes and turbulent mixing. In Hurricane Michael, the impact of downdrafts was related to the broader-scale storm structure, including its interaction with oceanic thermal gradients and nearby dry environmental air. An ideal interaction between the sea surface temperature (SST) gradient and inner-core convective distribution was revealed, with higher SSTs in the direction of storm motion and left of shear. After Michael became a major hurricane, downdrafts in the downshear region outside of ~100 km radius contained BL θe values greater than 360 K, potentially because mid-level (~4–8 km) outflow from the eyewall region transported high entropy air away from the eyewall region. The results from the observational studies highlight the significance of variability in downdraft structure. The general conclusions from the observations are supported by preliminary results from idealized numerical modeling simulations. Altogether, this study demonstrates the need for high-density observations in both the atmosphere and ocean to fully capture storm structure and evolution.

Oct 11: Sanchit Mehta
Department of Ocean Sciences, RSMAS
(one-hour AMP student seminar)

Air-Sea Interaction Dynamics Under Hurricane Wind Conditions

The physical nature of the near surface boundary layer remains less known, especially under high winds due to the development of an intermediate substrate layer of large spray droplets known as spume. The size-dependent vertical distribution of spume particles in high wind conditions is necessary to understand their effect on air-sea fluxes of heat and momentum. Given spume's role in mediating air-sea fluxes, the predominant focus of present literature has been within the marine environment, and any potential differences between sea- and freshwater have been neglected. Direct measurements of spray generation and momentum fluxes under hurricane wind conditions remain scarce till date due to challenges in making robust field measurements. Thus the laboratory remains the primary means for directly observing these processes. However, absence of any present standardization for the water type used in laboratory experiments, and for any potential effects that different water masses have on the spume generation process, adds uncertainty in our ability to model these processes. To address this gap, we have conducted a series of laboratory experiment at the Air-Sea Interaction Saltwater Tank facility (ASIST), directly comparing spume concentrations and surface drag coefficient behavior above fresh- and seawater under hurricane winds. Droplets in the air above the intensely breaking wind waves were optically observed, and their distribution as function of wind speed, height, and droplet radius was compared between the two water types. Drag coefficients were calculated using the eddy co-variance method. Our results show substantially higher concentrations of seawater spume as compared to freshwater across all particle sizes and wind speeds. The seawater particles' vertical distribution was concentrated near the surface, whereas the freshwater droplets were more uniformly distributed. Drag coefficient values for seawater were found less than those for freshwater at all wind speeds, suggesting modulation of momentum fluxes in the near surface layer due to the presence of spray droplets. These findings point to an unanticipated role of physiochemical properties in the spume generation mechanism which may impact spray-mediated flux parameterization and momentum fluxes over water bodies of different salinities.

Oct 18: NO SEMINAR (Fall Recess)


Anne Barkley (ATM)
African Biomass Burning is a Substantial Source of Phosphorus Deposition
to the Amazon, Tropical Atlantic Ocean, and Southern Ocean

The deposition of phosphorus (P) from African dust is believed to play an important role in bolstering primary productivity in the Amazon Basin and Tropical Atlantic Ocean (TAO), leading to sequestration of carbon dioxide. However, there are few measurements of African dust in South America that can robustly test this hypothesis and even fewer measurements of soluble P, which is readily available for stimulating primary production in the ocean. To test this hypothesis, we measured total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a TAO coastal site located at the northeastern edge of the Amazon. Our measurements confirm that in boreal spring when African dust transport is greatest, dust supplies the majority of P, of which 5% is soluble. In boreal fall, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of soluble P which we show is associated with the transport of biomass burning (BB) from southern Africa. Integrating our results into a chemical transport model, we show that African BB supplies up to half of the P deposited annually to the Amazon from transported African aerosol. This is the first observational study that links P-rich BB aerosols from Africa to enhanced P deposition in the Amazon. Contrary to current thought, we also show that African BB is a more important source of soluble P than dust to the TAO and oceans in the southern hemisphere and may be more important for marine productivity, particularly in boreal summer and fall.

Kexin Song (MPO)
Accuracy Assessment of Summertime ERA5 and Passive Microwave
Sea Ice Concentration Products in The Central Arctic

Since the late 20th century, Arctic sea ice has been declining at a rate of 12.85% per decade based on measurements taken from satellites. Sea ice concentration (SIC), the key parameter for monitoring sea ice, can be retrieved from passive microwave (PM) radiometers using different retrieval algorithms. However, the consistency and accuracy of PM SICs are typically low during summer even in packed-ice regions. In this research, three PM SIC products from 2002 to 2018 in the Central Arctic were compared with each other and with SIC from the ERA5 reanalysis dataset, which includes assimilated satellite data. The PM SIC are derived using the NASA-Team2 (NT2) algorithm, the Arctic Radiation and Turbulence Interaction Study Sea Ice (ASI) algorithm, and the Ocean and Sea Ice Satellite Application Facility (OSISAF) hybrid algorithm. In cloud-free cases where the derived SIC was <75%, the accuracies of the various SIC estimations were evaluated using higher resolution images from the Moderate Resolution Imaging Spectroradiometer (MODIS) of sea ice reflectance. The results showed that 1) all PM SIC products tend to underestimate SIC values but ASI-SIC and NT2-SIC provide higher accuracy; 2) the ERA5 reanalysis does not show enhanced performance compared to its remote sensing input; and 3) the discrepancy between PM SICs and MODIS retrievals is associated with the existence of melt ponds on the surface of the sea-ice.

Kurt Hansen (ATM)
Identifying Subseasonal Variability Relevant to Atlantic Tropical Cyclone Activity

The purpose of this study is to identify phenomena that influence subseasonal tropical cyclone (TC) activity in the Atlantic basin. First Atlantic accumulated cyclone energy (ACE), our measure of TC activity, is related to combined phases of the Madden Julian Oscillation (MJO) and El Niño Southern Oscillation (ENSO). In general, MJO phases 1 and 2 and cool ENSO events see more TC activity, consistent with previous works. However, the influence of MJO on TC activity becomes greater when the ENSO state is cooler. There is also a shift in favorable MJO phase with ENSO state; for strong La Niñas MJO phases 4 and 5 are most likely to have above average TC activity while MJO phases 1 and 2 see near normal activity. To investigate other potential factors that influence subseasonal TC activity two methods are developed, ACE by year (ABY) and seasonal and climatology removed (SNCR). These methods pull out subseasonal signals of environmental conditions in association with a variable of interest. Vorticity, sea surface temperatures, relative humidity and genesis potential (a combined metric of several variables) all show little signal in association with subseasonal Atlantic TC activity. The most important predictor that signals enhanced TC activity is low vertical wind shear anomalies in the main development region of the Atlantic in conjunction with high shear anomalies in the subtropical Atlantic. The MJO is partially responsible for this shear pattern associated with increased activity however another factor is likely influencing TC activity.


Marybeth Arcodia (ATM)
Tropical Indo-Pacific Variability Influences U.S. Precipitation
on Subseasonal Timescales

North American atmospheric conditions can be modulated by low frequency tropical influences via large scale remote connections, known as "teleconnections". The interannual variability, including that associated with the El Niño Southern Oscillation (ENSO), can modify the seasonal background flow (e.g., El Niño and La Niña basic states) affecting the distribution, strength, and propagation of the intraseasonal oscillation and the extratropical teleconnection patterns. The combined effects of the ENSO and the Madden-Julian Oscillation (MJO) signals result in both spatial and temporal interference and modulation of North American precipitation. The results to be presented show that when an MJO envelope is active and in a particular phase, the extratropical response from the MJO can considerably enhance or mask the interannual ENSO signal in the United States. Analyses of specific MJO events during an El Niño or La Niña episode reveal significant contributions to extreme precipitation events via constructive and destructive interference of the MJO and ENSO signals.

Amie Dobracki (ATM)
Measuring the Loss of African Biomass Burning Aerosol
over the Southeast Atlantic Ocean

Smoke particles from burning fields and forests are the largest source of organic aerosol in the atmosphere, affecting air quality, cloud properties, and the Earth's radiative balance. At low altitudes lifetimes of these particles are typically limited by wet and dry deposition, but large fire plumes often enter the free troposphere, where they can last much longer. Photochemical destruction becomes important at these time scales, but rates are difficult to measure and have remained poorly known. Here we use an extensive plume over the Atlantic Ocean from thousands of fires in southern Africa to determine a lifetime of approximately 6.5 days for particulate organic material, using a new age-aware module in a chemical transport model that performs well in this challenging environment, correlating with a chemically-derived aerosol aging indicator. This loss of organics leaves constituents such as sulfates and soot intact, changing not only particle size but their ability to act as cloud condensation nuclei and optical properties such as single scatter albedo. While strictly valid only in a tropical plume, this lifetime finding should help to guide incorporation of organic aerosol lifetimes into chemical transport models, which have typically ignored this issue due to lack of data.


Ryder Fox (ATM)
The Impact of Ice Microphysics on Flooding Due to Landfalling Hurricanes

At $24 billion in wind and water damage, the long-lasting and slow-moving Hurricane Florence (2018) became the ninth-most-destructive hurricane to impact the United States. Despite making landfall as a Category 2, Florence's persistent rainbands flooded the coastal state, breaking records with over 30 inches in some areas. Numerical weather models have shown dependencies of rainfall totals to ice parameterizations, but overall microphysics in numerical modeling is still poorly understood. Recently, the advent of dual-polarization radar has meant access to improved ice-species observations with which to validate model microphysics. However, since that advancement, few landfalling hurricanes have occurred in the US. Moreover, few studies have analyzed the effects of microphysical schemes on flooding associated with landfalling hurricanes. This study will employ the Weather Research and Forecasting model to simulate Hurricane Florence's landfall using various double-moment microphysics and planetary boundary layer schemes. In particular, the study will seek relationships between frozen hydrometeors and rainfall totals, using dual-polarization ground radar for verification.

James Hlywiak (MPO)
Sensitivities of the Decay of the Near-Surface Tropical Cyclone Wind Field
to Inland Surface Roughness and Soil Moisture

Given favorable atmospheric conditions, deep layers of warm ocean waters below the surface allow for the development and maintenance of tropical cyclones (TCs) due to large latent and sensible heat fluxes into the storm. After landfall, dramatic increases in frictional drag at the surface and decreases in surface moisture transports into the TC inner core lead to rapid weakening of the storm. However, past observational and numerical modelling studies suggest that the rate of decay of a landfalling TC is sensitive to land surface characteristics. Here, results from high-resolution, idealized simulations of landfalling TCs, performed using the Weather, Research, and Forecasting (WRF) model coupled to ocean and land surface models, show that both increasing surface roughness lengths and decreasing soil moisture contents cause larger reductions in TC intensity and diminished wind fields at the surface. Furthermore, extreme wind values near the radius of maximum winds decay more quickly than weaker, outer-core winds. Additionally, winds to the left of the storm motion decay more quickly than to the right of motion, increasing the wind field asymmetry. The results have implications for improving our understanding of landfalling cyclone structural decay and in identifying TC-relative regions that feature the most substantial changes in the wind field.

Sara Purdue (ATM)
Aircraft Observations of Southeast Atlantic Cloud and Aerosol Properties Along 5°E

Marine stratocumulus clouds are an important part of the earth's climate, producing a net cooling effect in the global radiation budget. In regions with large, semi-permanent stratocumulus decks, their formation and modulation are controlled by many interconnected dynamic and microphysical processes. In areas such as the southeast Atlantic, large quantities of biomass burning aerosols can further complicate the basic processes through the absorption of solar radiation, thereby altering the thermodynamic profiles of the cloud decks (otherwise known as the semi-direct effect). The ORACLES campaign collected data in the southeast Atlantic in 2016 to 2018, providing valuable in-situ aircraft data from the region with a focus on the impact of biomass burning aerosol on stratocumulus clouds. During the 2017 and 2018 deployment, around one-half of the research flights were devoted to repeated sampling along the same routine flight path, which approximately followed the boundary layer flow northward along the 5°E transect from the heart of the stratocumulus deck to the equator. This work uses data collected in August 2017 and October 2018 to analyze how aerosols, meteorology, and environmental conditions interact and impact stratocumulus clouds along the routine flight path along 5°E.


Haley Royer (ATM)
Playa Dust Mineralogy and Its Impact on Atmospheric Reactivity
and the Production of Reactive Halogens
Haley M. Royer1, Dhruv Mitroo1, Patricia Blackwelder1, Sarah Hayes2, Savannah Haas3,
Kerri Pratt3, Thomas E. Gill4, and Cassandra J. Gaston1
1University of Miami, 2U.S. Geological Survey, 3University of Michigan, 4University of Texas at El Paso

Tropospheric ozone is a trace gas that can affect plant health, human health, and the climate. The chlorine radical (Cl*) is a potent atmospheric gas that enhances tropospheric ozone formation and atmospheric reactivity. Nitryl chloride (ClNO2) – a source of Cl* – forms when gaseous dinitrogen pentoxide (N2O5) reacts with Cl-containing aerosol. ClNO2 formation is expected to occur primarily in urban coastal regions due to the presence of Cl in sea spray, but our lab recently showed that a reaction between saline lakebed (playa) dust and N2O5 can produce ClNO2 inland as well. ClNO2 production is currently predicted using only total aerosol Cl content and relative humidity (RH), but our recent work shows that these variables cannot reliably predict ClNO2 production. To better understand this anomaly, I utilized four analytical techniques to determine saline playa dust chemical and mineral composition. My results reveal several factors aside from Cl content and RH that affect ClNO2 chemistry, such as highly hygroscopic minerals that can initiate ClNO2 chemistry at low RH. I also detected organic matter and clay minerals that diminish ClNO2 production even when Cl and RH conditions are favorable. Finally, I determined through comparisons between various analytical techniques that ClNO2 chemistry occurs on the surface rather than in the bulk of a particle, which is an important finding considering that current air quality models use bulk data to predict ClNO2 concentrations. My findings suggest that current parameterizations are not effectively predicting ClNO2 yields used to inform air quality and chemistry-climate models.

Andrew Smith (MPO)
Entrained Bubble Populations and Their Influence
on Turbulent Kinetic Energy and Dissipation
Beneath Breaking and Non-Breaking Waves

The development and collapse of the air-sea interface as a defined two-phase boundary of evolving surface waves entrains air and other gases and results in bubbles and bubble plumes. These bubbles facilitate gas, heat, and momentum exchange and have been previously reported to alter the turbulent kinetic energy (TKE) spectrum, as well as the water-side TKE budget. In July 2018, an extensive series of air-sea gas transfer experiments was conducted in the SUSTAIN wind-wave tank laboratory; an imaging system and 3D acoustic Doppler current profiler captured the bubble populations of 30-1500 microns and sub-surface velocity measurements in a variety of wind and wave conditions. Wind conditions ranged from U10 of 10.6- 50 ms–1, and both monochromatic and spectral mechanically generated waves were produced for at least one of three water temperatures between 20 and 32°C. Individual wave crests observed by conductive wave wires were analyzed using a breaking onset likelihood technique. Preliminary results show that bubble populations become more widely distributed in the observed radii at higher wind speeds and the peak radius shifts from moderate to smaller bubbles in monochromatic waves. Conversely, an opposite trend is seen for the spectrum waves. Due to steepness-related instability of the wave crests, breaking is found to be most likely for the 20°C monochromatic waves, and kinetic energy profiles are largest in magnitude beneath these conditions. TKE spectra show a steepening of the slope is seen at high frequencies in breaking conditions. The results have implications for improving the parameterizations of air-sea interaction physics in numerical models and understanding how wave breaking alters the energy balance in the near-surface water  especially in hurricane conditions.

Bosong Zhang (ATM)
Radiative Feedbacks Associated with the MJO

Radiative kernels derived from CloudSat CALIPSO measurements are used to diagnose radiative feedbacks induced by the Madden-Julian oscillation (MJO). Over the Indo-Pacific warm pool, positive cloud and water vapor feedbacks are coincident with the convective envelope of the MJO during its active phases, whereas the lapse rate feedback shows faster eastward propagation than the convective envelope. During phase 2/3, when the convective envelope is over the Indian Ocean, water vapor exhibits a vertically coherent response, with the largest anomalies and strongest feedback in the midtroposphere. Though spatial structures of the feedbacks vary, the most prominent difference lies in the magnitude. Cloud changes induce the largest radiative perturbations associated with the MJO. It is also found that the strength of the cloud feedback per unit of precipitation is greater for strong MJO events, suggesting that the strength of individual MJO events is largely dictated by the magnitude of cloud radiative heating of the atmosphere. In addition, stronger radiative heating due to water vapor and clouds helps the MJO survive the barrier effect of the Maritime Continent, leading to farther eastward propagation. These results offer process-oriented metrics that could help to improve model simulations and predictions of the MJO in the future.


Chelsi Lopez (OCE)
High Seasonal Variability of Total Organic Carbon in the Deep Northeastern Pacific

Organic carbon, a product of ocean primary production, plays a vital role in the sequestration and export of carbon. Commonly understood, but not well observed, is the fate of that material upon export. Here we consider the seasonal variability of total organic carbon (TOC) in the deep northeastern Pacific Ocean, as a product of high primary production in this eutrophic area. Samples were collected from throughout the water column seasonally in 2017 and 2018 along the Line P transect. Variance in concentration (i.e., high heterogeneity) at >1000 meters during winter, spring, and summer coinciding with variance in fluorescence in the upper layer suggests that biogenic particles sinking from the upper ocean are seasonally delivering particulate and dissolved organic carbon to depth, uniquely observable in the TOC fraction. High abundances of gelatinous zooplankton in spring 2018 enhanced the export of organic carbon to the bathypelagic zone during the seasonal bloom. It is this carbon that supports the deep microbial heterotrophic community, thus sequestering of CO2 into the deep via the biological pump.

Heather Hunter (AMP)
Object Detection in TerraSAR-X Images Using Transfer Learning from Simulated Data

Owing to their ability to automatically learn effective features in complex data, deep learning (DL) algorithms are an increasingly popular tool for object detection and classification in satellite remote sensing. Though powerful, DL algorithms require large datasets on the order of tens of thousands of images in order to generalize well across a variety of tasks, from detection of ground vehicles to detection of ships. In the domain of synthetic aperture radar (SAR), the greatest hurdle toward a good, DL-based object detector is the lack of labelled examples for training. Benchmark datasets do exist for select applications; however, these do not provide the breadth of context that a generalized detector in SAR requires. Additionally, owing to the complexity of the SAR imaging process, obtaining images representing every possible SAR-to-target geometry is impractical. In this presentation I will show results of a convolutional neural network (CNN) based object detection algorithm trained on simulated TerraSAR-X images of aircraft, and I will show the extent to which transfer learning from simulated images improves the accuracy of detection on real images. I will show how the activations of different feature extraction layers in the CNN respond to simulated versus real TerraSAR-X images, and I will discuss the transferability of these features between the two image domains.

Nov 29: NO SEMINAR (Thanksgiving Recess)

Dec 06: Mampi Sarkar
Department of Atmospheric Sciences, RSMAS
(one-hour ATM student seminar)

Observational Analysis of Stratocumulus-to-Cumulus Transition
in the North Pacific Ocean

Three genuine stratocumulus-to-cumulus transitions sampled during the Cloud System Evolution over the Trades (CSET) campaign are documented. The focus is on Lagrangian evolution of in-situ precipitation thought to exceed radar / lidar retrieved values because of Mie scattering. Two of the three initial stratocumulus cases are pristine (cloud droplet number concentrations (Nd) of ~22 cm–3 ) but occupied boundary layers of different depths, while the third is polluted (Nd ~225 cm–3). Hourly satellite-derived cloud fraction along Lagrangian trajectories indicate that more quickly deepening boundary layers tend to transition faster, into more intense but more occasional precipitation. These transitions begin either in the morning or late afternoon, suggesting that preceding night processes can precondition or delay the inevitable transition. The precipitation shifts towards larger drop-sizes throughout the transition as the boundary layers deepen, with aerosol concentrations only diminishing in two of the three cases. Ultra-clean (Nd < 1 cm–3) cumulus clouds evolved from pristine stratocumulus clouds with unusually high precipitation rates occupying a shallow, well-mixed boundary layer. Results from a simple one-dimensional evaporation model and from radar / lidar retrievals suggest sub-cloud evaporation likely increases throughout the transition. This, coupled with larger drop-sizes capable of lowering the latent cooling profile, facilitates the transition to more surface-driven convection. The co-association between boundary layer depth and precipitation does not provide definitive conclusions on the isolated effect of precipitation on the pace of the transition. Differences between the initial conditions of the three examples provide opportunities for further modeling studies. Measures are suggested to account for Mie scattering by large raindrops for better radar / lidar precipitation retrievals.