Atmospheric Sciences

COMPASS (Combined OCE MPO ATM Seminar Series) FRIDAY

FALL 2017

(11:00 in the Auditorium, unless stated otherwise)

Aug 25 --- NO SEMINAR

Sep 01 --- NO SEMINAR

Sep 08 --- NO SEMINAR

Sep 15 --- NO SEMINAR

Sep 22 --- Student Seminars, Room MSC 343:

Sanchit Mehta (AMP)

"A Comparison of Sea Spume Production Between Fresh and Salt Water in High Wind Conditions"

Under high wind conditions, sea spray (particularly larger particles or sea spume) plays a significant role in the exchange of heat and momentum across the air-sea interface. It is thus critical for the development of tropical cyclones and other extreme marine boundary layer events that occur on a wide range of spatial-temporal scales over different bodies of water (oceans, rivers and lakes). While considerable differences are known to exist in spray formation via bubble production between saltwater and freshwater, herein is described the first laboratory experiment (conducted in the SUSTAIN facility at University of Miami) quantifying both fresh and saline spume droplet production in hurricane wind conditions (U10 around 36-54 m/s), particles with radii 80 µm to 1400 µm were observed. Spume number concentration estimates were found to increase with wind speeds across the board, with comparatively higher magnitudes (1.5-2.5 x) observed for saltwater up to 500 µm radii. Vertical profiles for number and mass concentration were observed to vary linearly with the scaled height, with comparatively steeper slopes for fresh water and more gradual slopes for salt water. While significant differences were observed in the profiles near the surface (concentrations were almost twice as large for salt water than fresh water at higher speeds), the profiles tend to converge as the wind forcing increases. The radius dependency observed for the difference in mass concentration between two media showed bimodality, with a minimum near 700-900 µm particle radii. An exponential decrease was observed in the differences in number concentration between salt and fresh water showed an exponential decrease from 100 µm through 500 µm radii with very little differences observed beyond.

Sep 29 --- Diane Palko (MPO, 1-Hour Student Seminar)

"Northwestern Atlantic Sea Level in a High Resolution Global Coupled Climate Model"

Sea level rise is one of the most often cited consequences of climate change and is presently impacting cities along the east coast of the United States. Typical general circulation models have 1.0° resolution in the ocean, and are unable to resolve features like the Gulf Stream, which may have large effects on regional sea level. As technology has improved we are now capable of running fully coupled climate models with a resolution of 10 km in the ocean. With this enhanced resolution, models can recreate modes of climate variability that are present in observations but missing from climate models with a coarser resolution. Our main goal is to utilize high-resolution (HR, 0.1° in the ocean) coupled model runs of CCSM4 to analyze regional sea surface height (SSH). We characterize the natural variability of SSH along the US coastline using HR runs of CCSM4 with fixed external forcing (i.e. constant present day values of greenhouse gases). The variability of coastal SSH from these runs is compared to reanalysis and unforced, low-resolution (LR, 1.0°) runs of CCSM4. The connection of coastal SSH to basin-wide climate patterns is examined by correlating SSH at coastal locations with SSH, sea surface temperature (SST), and surface kinetic energy at all other locations in the north Atlantic. The HR natural variability model runs are also used to analyze a coupled ocean-atmosphere climate mode in the Gulf of Mexico, which links patterns of precipitation variability to SST and SSH and has potential implications on tidal flooding in the region. We also use the HR models to evaluate the SSH response to fresh water forcing with two hosing experiments (0.1 Sv and 1.0 Sv). A preliminary comparison is made between these HR hosing runs and a typical LR hosing experiment, however these experiments are still ongoing.

Oct 06 --- Student Seminars:

Kayleen McMonigal (MPO)

"Seasonal Variability of the South Indian Ocean Subtropical Gyre Circulation"

The circulation of the south Indian Ocean subtropical gyre has been investigated using data from three hydrographic crossings. These crossings span 22 years and were conducted in different seasons. Here, we investigate whether previously reported changes to the gyre circulation between these hydrographic crossings can be explained by the seasonal variability of the upper 2000m gyre circulation. 34 months of in situ data of the Agulhas Current transport constrain the circulation at the western boundary. The interior gyre geostrophic velocity relative to 2000m is mapped using a combination of altimeter and Argo float profile data designed to suppress unresolved mesoscale variability. These data sources are combined to investigate the cumulative transport of the gyre and the strength of the gyre on a seasonal basis. A seasonal cycle is found, with a maximum gyre strength in austral summer and a seasonal amplitude of less than 10 Sv. The seasonal cycle cannot explain a previously reported gyre strength increase of 17 Sv between 1987 and 2002. Neither the phasing nor the amplitude of the seasonal cycle match that of the Agulhas Current, as determined by a 23 year transport proxy. This suggests that another component of the basin wide flow must be compensating the Agulhas and interior flows on seasonal time scales to maintain mass balance.

Joshua Wadler (MPO)

"Convective Downdrafts and Boundary Layer Recovery in Hurricane Earl (2010) Before and During Rapid Intensification"

Mariana Bernardi Bif (OCE)

"Controls on the Fate of Dissolved Organic Carbon Resulting from Net Community Production"

Dissolved organic carbon (DOC) comprises the largest marine reservoir of reduced and reactive carbon at 662 PgC, comparable in size to atmospheric CO2. Most of the fresh DOC released by primary production in the euphotic zone is the "energy fuel" for heterotrophic bacteria, and is rapidly converted back into CO2. The remaining fraction takes longer to be remineralized, resulting in positive net community production (NCP), and ends up exported to the deep ocean via the biological carbon pump. The unsolved processes controlling the production of resistant DOC, and whether this fraction is variable, was the motivation for this work. We simulated upwelling systems of different intensities by combining natural waters, and incubated with microbial communities from the Florida Strait. Main results show that initial availability of inorganic nutrients modify the microbial community structure and magnitude of NCP. A larger fraction of DOC resulting from NCP accumulated in high nutrient incubations (strong upwelling simulation) during autotroph growth. However, this portion was further remineralized by bacteria when the experiment was extended in the dark. In contrast, PO43- limitation in the weak upwelling decreased NCP, producing a small fraction of DOC that was resistant to bacterial remineralization. Upwellings of different intensities, thus, affect the quality of dissolved organic matter, thereby affecting DOC remineralization and residence time in the ocean.

Oct 13 --- NO SEMINAR

Oct 20 --- Student Seminars:

Samantha Ballard (AMP)

"Improving Coastal Wind and Wave Retrievals Using Synthetic Aperture Radar"

An accurate knowledge of wind and wave fields is important for understanding the dynamic coastal environment. Coastal wind data can be difficult to obtain from atmospheric models due to poor resolution near the coast. Satellite-based synthetic aperture radar (SAR) can resolve wind and wave fields quite well, but the well-established SAR wind algorithms for the open oceans are not reliable in coastal areas, where the relationship between wind, waves, and radar image intensities is more complicated. The objective of this work is to develop an improved SAR wind algorithm for coastal areas. During the Coastal Land Air Sea Interaction (CLASI) field experiment in June, 2016, X-band COSMO-SkyMed and C-band Sentinel-1 SAR images were acquired over Monterey Bay, California. An image pair from June 11, 2016 shows a strong image intensity gradient 2.5 kilometers off the coast for which no corresponding wind gradient exists in a Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) model result. Comprehensive investigations were initiated to determine if this feature can be attributed to an oceanic front, an atmospheric front, or something different, and what the correct wind field for the scenario should look like. We discuss how this data interpretation process can be automated and what can be done to avoid incorrect wind retrievals for areas where SAR image intensity variations are dominated by non-atmospheric features.

Heather Hunter (AMP)

"Application of Artificial Neural Networks and Deep Learning to SAR Automatic Target Recognition"

Automatic target recognition (ATR) in synthetic aperture radar (SAR) is concerned with the automated detection and classification of objects of interest, or targets. For oceanographic applications, targets may be ships, icebergs, or pollution, while for land-based applications, targets may be ground and air vehicles or vegetation. To classify a target, the standard SAR ATR system is composed of 3 stages: detection and discrimination of regions of interest, low-level classification or feature extraction, and high-level classification. The high-level classification stage is often performed with clustering techniques, tree-based classifiers, or K-nearest neighbor classifiers, and more recently, artificial neural networks (ANNs). In this study, ANNs and convolutional neural networks (CNNs) are implemented for the classification stage and their results compared. An ANN is a type of supervised machine learning algorithm that learns to classify objects in an image given labeled training data in the form of feature vectors. By contrast, a convolutional neural network (CNN) classifies objects by learning features directly from an input image, without the need for pre-engineered feature vectors. In this study, the features selected for training the ANN include first-order statistics, wavelet coefficients, and Gabor features. Experimental results show that the CNN has a greater classification accuracy than the ANN. This illustrates the potential for CNNs to be used as the classifier for a SAR ATR system, rather than the conventional ANN, and motivates further research into the use of other deep learning algorithms for SAR ATR.

Lisa Nyman (AMP)

"Using Doppler Marine Radar to Detect Surface Currents and Ocean Features"

Oceanographic features such as fronts, small-scale eddies, and internal waves are analyzed more easily when they are represented spatially on a map, not just based on point measurements. During the FLEAT DRI field experiment off the coast of Palau in May 2016, members of the University of Miami remote sensing team operated Helmholtz-Zentrum Geesthacht’s X-band, VV-polarization, Doppler Marine Radar (DMR) onboard the R/V Oceanus. The DMR records real and imaginary parts of the received signals, which can be converted to amplitudes and phases. The difference between the phase of the wave as it returns to the radar and the phase of the immediately subsequent pulse is analyzed and can be used to extract radial current information through the knowledge of the Doppler shift. The data from Palau shows that the DMR registers ship motion, which is the dominant velocity component on a moving ship. During the Inner Shelf DRI field experiment off the California coast in September and October 2017, the DMR was used again to detect ocean features such as internal waves and tidal bores. An algorithm is being developed to process this Doppler information from the rotating antenna on the moving ship in such a way that a temporally-averaged two-dimensional vector velocity field is obtained with a spatial resolution of 370 m × 370 m.  Although still under development, this process seems to be a promising new way to measure two-dimensional surface current fields from a ship.

Oct 27 --- Student Seminars, Room MSC 343:

Luo Bingkun (MPO)

"Improving Satellite Retrieved Infrared Sea Surface Temperatures in Aerosol Contaminated Regions"

Infrared satellite observations of sea surface temperature (SST) have become essential for many applications in meteorology, climatology, and oceanography. Tropospheric aerosol concentrations increase infrared signal attenuation and prevent the retrieval of accurate satellite SST. We compare satellite-derived skin SST with measurements from the Marine- Atmospheric Emitted Radiance Interferometer (M-AERI) deployed on ships during the Aerosols and Ocean Science Expeditions (AEROSE) and with quality-controlled subsurface drifter temperatures. After match-up with in-situ SST and filtering of cloud contaminated data, the results indicate that SST retrieved from MODIS (Moderate Resolution Imaging Spectroradiometer) aboard the Terra and Aqua satellites have negative (cool) biases compared to shipboard radiometric measurements. There is also a pronounced negative bias in the Saharan outflow area that can introduce SST errors >1 K at aerosol optical depths > 0.5. In this study, we present a new method to derive night-time Saharan Dust Index (SDI) algorithms based on simulated brightness temperatures at infrared wavelengths of 3.9, 8.7, 10.8 and 12.0 μm, derived using RTTOV. We derived correction coefficients for Aqua MODIS measurements by regression of the SST errors against the SDI. The biases and standard deviations are reduced by 0.25K and 0.19K after the SDI correction. The goal of this study is to understand better the characteristics and physical mechanisms of aerosol effects on satellite retrieved infrared SST, as well as to derive empirical formulae for improved accuracies in aerosol-contaminated regions.

Samantha Kramer (MPO)

"Saharan Dust: Lidar and Dust Mass Observations Compared to MERRA2 Reanalysis”

Saharan dust is the dominant aerosol in summertime southern Florida, peaking climatologically in July. Daily filter-based dust concentrations, at times size-resolved, and micropulse lidar-derived dust vertical structure from June-September of 2014-2016, Miami, Florida, are compared to the dust representation in NASA’s Modern-Era Retrospective analysis for Research and Application-Version 2 (MERRA2). Observed surface dust loadings reaching 80 µgm-3 occur episodically, with some dust present at almost all times, indicating a consistent aerosol presence within the marine boundary layer. MERRA2, through its assimilation of satellite and surface-based aerosol optical depth, adequately captures day-to-day Saharan dust variability. The daily MERRA2 near-surface dust mass concentrations are highly correlated with the in-situ measurements, but overestimated. The MERRA2 dust mass is overestimated for diameters between 2.0 to 6.0 µm, and underestimated for particles with diameters between 6.0 to 20 µm and less than 2.0 µm compared to size-resolved measurements. This is thought to reflect MERRA2’s parameterized size-distribution which is based on data gathered near dust sources, while the Miami dust observations are made on dusty air masses that have undergone deposition. MERRA2 surface-layer sea salt mass concentrations are also overestimated, despite realistic wind and relative humidity within the reanalysis. MERRA2 places most dust below 5 km altitude agreeable with lidar observations, but does not produce a layered vertical structure as seen in observations. A better understanding of the MERRA2 dust representation, which is used to initialize dust transport forecasts, will increase confidence in reanalysis-derived dust impact studies and subsequent forecasts.

Rebecca Evans (MPO)

"The Response of Tropical Cyclones to Diurnal Heating As Seen in a Linear Dynamical Model."

The diurnal cycle of the cirrus canopy of tropical cyclones (TCs) is a continuing area of research. It has been shown through observations that this diurnal cycle may be fundamental for changing storm intensity and structure as heat anomalies propagate outwards overnight (Dunion et al., 2014). Recent work on this subject has used high resolution 3D models to explore this outward propagating wave-like diurnal signal at cloud top height (O’Neill et al., 2017). Navarro et al. (2017) have used a method analogous with the periodic Sawyer-Eliassen equations to simulate the dynamics by which the diurnal response is achieved. In the work presented here, we use more realistic (non-harmonic) diurnal forcings in a non-hydrostatic, linear model with arbitrary time dependence, known as 3DVPAS. We explore the transition between balanced and radiating responses, and compare the results to the harmonic cases from Navarro et al. (2017). This study will test the forcing on axisymmetric TCs of different sizes and strengths and with different stratifications.

Nov 03 --- Student Seminars, Room MSC 343:

Andrew Smith (MPO)

"Dissipative Heating and the TKE Budget: Field Measurements and Implications for Air-Sea Exchanges"

Heat and momentum exchange are key physical processes underlying the energy budget at the air-sea interface. Dissipation of shear-induced turbulence results in momentum transfer to surface waves and dissipative heating, which can contribute significantly to the dynamics in the boundary layer atmosphere and upper ocean at larger scales.  (e.g. tropical cyclones, mixing, contributions to internal ocean waves, etc.). Here we present an examination of oft-neglected dissipative heating in the context of the turbulent kinetic energy budget, and its implications for air-sea exchanges. Prior studies have largely focused on the magnitude of dissipative heating in the hurricane environment (Bister and Emanuel, 1998; Businger and Businger, 2001; Zhang et al. 2011), or its parameterization in numerical hurricane models (Zhang D. and Altschuler, 1999; Jin et al. 2007) rather than connecting the physical process directly to the budget or exchanges of energy at the air-sea interface. In this study, we using use high-frequency (20 Hz) ship and radar data from the LAgrangian Submesoscale ExpeRiment (LASER) conducted in the Gulf of Mexico from January-February 2016 to compute the dissipative heating., evaluate the balance of TKE production with dissipation, and determine air-sea transfer coefficients as functions of stability and wind-speed in the low-wind (< 20 ms-1) non-hurricane environment. Surface density fronts (order 100 – 1000 m) identified from these data additionally allow us to investigate whether dissipation is enhanced near the fronts (e.g. D’Asaro et al. 2011), and the impact of changes in seawater properties on air-sea exchanges. Preliminary results have shown the ratio between the dissipative heating and thermodynamic (sensible and latent heat) fluxes to be between 40-80 percent in near-neutral stability conditions, where TKE shear and buoyancy production terms nearly balance to promote the dissipation while working to maintain the ambient stratification necessary to prevent a fully-homogenized boundary layer. This work will be combined with a forthcoming high-wind study in SUSTAIN to quantify the TKE budget in those conditions and examine the balance of sea-spray evaporation and dissipative heating.

Breanna Zavadoff (MPO)

"Anticyclonic Rossby Wave Breaking over the North Atlantic During Boreal Summer: Climatology and Impacts"

Anticyclonic Rossby wave breaking (ARWB) events are characterized by the rapid and irreversible deformation of PV contours on isentropic surfaces, manifesting themselves as tongues of high-PV stratospheric air extending from the extratropics into the tropics. Previous studies have highlighted the connection between Rossby wave breaking and the modulation of localized atmospheric phenomena such as the NAO and tropical cyclogenesis. However, to the author’s knowledge, Nno study,  hashowever, has focused on the evolution of the synoptic scale environment throughout the lifecycle of ARWB events over the entirety of the North Atlantic as well as the basin-wide environmental changes that occur related to development and decay of these events.
This study uses 54 years (1960-2013) of NCEP-NCAR Reanalysis data to identify and develop a comprehensive spatio-temporal climatological analysis of ARWB on the 350 K isentropic surface over the North Atlantic. This data is further utilized to investigate the synoptic environments surrounding these events from both basin-wide and high-PV streamer centroid-relative perspectives. Preliminary findings suggest that the high-PV streamers associated with ARWB events introduce dry stable air from the extratropics into the tropical environment, subsequently inhibiting convection there. Results from this study may provide aid in short to medium range forecasting of North Atlantic tropical convection, which could have applications extending into the field of tropical cyclogenesis forecasting as well.

Shun-Nan Wu (MPO)

Nov 10 --- Student Seminars, Room MSC 343:

Gedun Chen (MAC)

"Processes Responsible for the Linear Shape of the Cu Concentration Profiles"

Copper (Cu) plays a dual role in biological growth that it helps the electron transport and iron transport in phytoplankton while being toxic when “free” Cu ion concentration is too high. The vertical distribution of Cu exhibits a unique linear shape which is very different from trace metals like Cd or Zn that correlate with macronutrient (P and Si, respectively) distributions. Although several mechanisms have been proposed to explain the linear shape of Cu vertical profile, the major processes responsible for such shape are still in debate. To investigate the underlying mechanism for the linear distribution of Cu, a model based on the mass balance between regeneration and particle scavenging has been used to reproduce the Cu profile. The model focuses on the circulation and regeneration of nutrients and has been successful in reproducing global P and Si distributions with few parameters and simple assumptions. However, this model results in lower Cu concentrations than observed values in the deep and bottom waters in Pacific, which indicates that regeneration itself cannot account for the deep and bottom enrichment of Cu. Such result shows that there must be an extra Cu source from the bottom which leads to the linear increase of Cu in the water column.

Hanjing Dai (AMP)

"Physical Processes Influencing Surface Roughness Variability at Submesoscale Fronts"

Oceanic fronts are often highly visible in active remote sensing images due to changes in the surface roughness. Quantifying these roughness changes in different wind, wave, and current for a range of density gradients can help to improve the interpretation of these images. During the Lagrangian Submesoscale Experiment (LASER) in the Gulf of Mexico, our research cruise crossed frontal interfaces up to 23 times. Fronts were located based on temperature, salinity, and X-band radar backscatter intensity and current maps. Direct measurements of winds, waves and currents were collected across these frontal interfaces. The detailed manifestations of the frontal interfaces on the surface roughness were sampled using a Polarimetric Slope Sensing (PSS) optical system. Polarimetric images of the short wave slopes crossing three frontal interfaces were obtained and one has analyzed in terms of the directional wave slope spectra, omnidirectional slope spectra, and the surface mean square slope (mss). The slope spectra dependence on winds, currents and sea-state at frontal interfaces, and the effect of fronts on short scale surface roughness will be discussed first. Moreover, the preliminary results of corresponding time-series of mean square slope before, during and after crossing the frontal interface show that the surface roughness changes dramatically to respond to the existence of a convergent or divergent front. The changes in mean square slope of short gravity waves and capillary gravity ones crossing the frontal interface also highlight the short time-scales of the frontal processes shaping the roughness of sea surface.

Nov 17 --- Student Seminars, Room MSC 343:

Nektaria Ntaganou (MPO)

"Topographic controls in the Gulf of Mexico"

The Loop Current (LC) is a major feature governing the circulation in the Gulf of Mexico (GoM). Incoming flow enters the GoM from the Yucatan Channel, forms the LC, and exits through the Florida Straits, feeding into the Gulf Stream. The variability of the LC is strongly associated with the behavior of the Loop Current Frontal Eddies (LCFEs), cyclonic eddies that follow the path of the LC, forming a belt of cyclonic vorticity around the anticyclonic LC flow. The LCFEs are known to play a role in Loop Current Eddy (LCE) shedding events. Along their path, they come across strong topographic gradients, in the Mississippi Fan area, located in the northwestern GoM, and near the West Florida Shelf slope. Hence, we seek to examine the topographic controls on the propagation and intensification of the LCFEs, using a numerical approach. We employ the Hybrid Coordinate Ocean Model (HYCOM), applied on the GoM (GoM-HYCOM), at 4km resolution and 26 hybrid vertical layers. To address the topographic control, we ran two simulations with modified and realistic bottom topographies along the West Florida Shelf. Preliminary results show that deepening the topography causes intensification of the LCFEs for a short period of time, which slows down relatively quickly. Overall, changes on shelf slope seem to affect the LCFEs, as they follow a different pattern, compared to realistic topography. The results are quantified by employing relative vorticity and eddy kinetic energy.

Tiago Bilo (MPO)

"On the mean Deep Western Boundary Current recirculation in the North Atlantic subtropics: structure and forcing mechanisms"

The Deep Western Boundary Current (DWBC) is the primary component of the lower limb of the Atlantic Meridional Overturning Circulation. As the DWBC flows equatorward it carries North Atlantic Deep Water (NADW) in highly variable and dynamically complex pathways. In the subtropical North Atlantic (~15-30N), part of the flow recirculates in the ocean interior decreasing the net meridional transport along its path. In this work, we investigate the DWBC recirculation patterns and forcing mechanisms at ~15-30N using observations and outputs from an eddy-resolving (OFES) hindcast simulation. We use ARGO-derived absolute geostrophic velocity and the OFES model to study the ocean circulation at 1000-2000 m depth. Both model and observations show that the mean DWBC recirculation is composed of multiple cyclonic cells embedded in a larger scale recirculation. The cells are also characterized by the presence of flow crossing mean potential vorticity (PV) gradients, requiring forcing to maintain them. The OFES outputs indicate that eddy-PV fluxes are responsible for forcing the recirculation at z~900-3000 m and present significant changes in the thickness fluxes with depth while relative vorticity fluxes remain vertically coherent. To better investigate the vertical structure of one of these cells, more than 20 lowered-ADCP quasi-synoptic transects obtained in the last 15 years are used to analyze the mean velocity patterns of the DWBC and its adjacent recirculation at 26.5N. Our analysis reveals a clear distinction between the recirculation of the upper (z~1000-3000 m) and lower NADW flows (z~3000-5000 m), with a net significant recirculation near the western boundary.

Gregory Koman (MPO)

"Seasonal variability of boundary currents in the North Atlantic subpolar gyre from altimetry"

Alexis Denton (AMP)

"Tracking Arctic Sea Ice Buoys with Radar Reflectors"

In the Arctic Ocean, data collocations between satellite imagery and in-situ, sea ice-tethered measurements are usually done via interpolation between the two datasets, leading to unavoidable uncertainty in the position of an instrument within a satellite footprint. Even in very high resolution imagery, where pixel lengths range from 1 to 20 meters, instruments are not resolvable; beyond that, in the submeter resolution, the chance of imaging drifting instruments is small as the narrow swath widths require target acquisition to be precise. To reduce this location uncertainty, radar reflectors were deployed at various ice-stations in the Chukchi Sea within arrays of in-situ instruments as part of a joint effort with the Korean Polar Institute (KOPRI) and the EU’s ICE-ARC Program. Included in the arrays were high precision (within cm) GPS systems called SATICE. Synthetic Aperture Radar (SAR) imagery was acquired by the Center for Southeastern Tropical Advanced Remote Sensing (CSTARS) at the University of Miami to track the arrays and reflectors. The presence of the reflectors allows for the locations of the arrays to be directly discernable within the imagery, thus enabling more precise coordination of the ground and satellite data sets. A more detailed picture of ice dynamics may be formed by this simultaneous monitoring and matching between remote sensing and in-situ data. In addition, the SATICE systems allow for evaluation and comparison of SAR georeferenced locations of the reflectors across three types of SAR products offering varying degrees of spatial accuracy.

Nov 24 --- THANKSGIVING BREAK

Dec 01 --- Student Seminars, Room MSC 343:

Anne Barkley (ATM)

"The Physiochemical Properties and Associated Nutrients of Transported African Dust to the Amazon Basin"

African dust plays an essential role in fertilizing both oceanic and terrestrial ecosystems by supplying vital biological nutrients such as iron and phosphorus. During Boreal winter, large quantities of African dust are transported across the Atlantic Ocean to the Amazon Basin. It is thought that the Bodélé Depression, part of Paleolake Mega Chad, serves as a major source of this dust, although its importance is debated. The soil in this topographical depression contains a distinctive blend of fluvial and diatomaceous sediments that are thought to supply the Amazon with the nutrients necessary to maintain soil fertility. However, the composition and physical properties of dust transported to the Amazon remain under-explored. Here we present measurements of the size, morphology, and chemical composition of transported dust collected in Cayenne, French Guiana and soil samples collected from the Bodélé Depression using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Inductively coupled plasma mass spectrometry and soluble phosphorus measurements were also performed to investigate the nutrient profiles of filters collected during different air mass transport conditions. In addition to mineral dust, SEM revealed the presence of whole and fragmented freshwater diatoms transported from the Bodélé Depression, or other ephemeral African paleolakes, that were mixed with dust containing iron oxides and micronutrient-rich authigenic clays. Interestingly, transported diatoms were found to the be the largest transported particles with diameters well above 10 μm (up to 70 μm). The low density and high surface-to-volume ratios of diatoms could allow a longer range transport than dust of a comparable size. Therefore, the diatoms could act as a vehicle by which higher micronutrient fluxes could be transported to the Amazon.

Lisa Bucci (ATM)

"Testing the Airborne Doppler Wind Lidar in Tropical Cyclones"

While meteorological observations have improved in both quality and density, there still remain important data-sparse regions in the tropical cyclone (TC).   Instruments flown on the NOAA’s P3 Orion Hurricane Hunter Aircraft provide some in situ inner-core observations of Atlantic TCs.  The Tail Doppler Radar (TDR) provides the most comprehensive coverage of the wind field in TCs.  The TDR has the ability to measure winds in areas of precipitation but outside of these regions and below ~1 km, it either cannot make observations or the data is less reliable.  This leaves regions such as the TC boundary layer, moats between the eyewall and outer rainbands or secondary eyewall, and precipitation-free areas largely unobserved.  

In the 2015 through 2016 hurricane seasons, an airborne Doppler Wind Lidar (DWL) was flown for the first time on the P-3 Orion into TCs of different intensities in the Atlantic and East Pacific.  A DWL frequently measures the motion of aerosols along an laser beam above or below the aircraft.  These measurements are converted to wind profiles available with horizontal spacing of as little as 3 km.  Data comparisons show good agreement between coinciding retrieved DWL wind profiles and other airborne wind observing instruments.  Further analysis of DWL data shows it collects complementary wind observations to the data from the TDR. Pending the sampling of additional TC cases, these data have the potential to allow for a more complete depiction of the 3D wind field. This new application of existing technology could provide insights on how TCs behave, improve our understanding of vertical and asymmetric structures that drive tropical cyclone evolution, and provide new observations for numerical models.

Kurt Hansen (ATM)

“Predictability of Atlantic Tropical Cyclone Activity on Sub-Seasonal Time Scales”

The prediction of tropical cyclone (TC) activity is currently confined to weather forecasts within five days or activity over an entire season. However, we expect that there is predictability and utility in forecasts on sub-seasonal time scales. This study focuses on lead times between 2-4 weeks. The ensembles of the Community Climate System Model (CCSM4) were used to assess various factors affecting TC activity in the Atlantic including vertical wind shear, humidity and sea level pressure. It was found that the variability of Atlantic TC activity in the CCSM4 model corresponded with the state of the El Nino Southern Oscillation (ENSO). Additionally, other variables in CCSM4 hindcasts such as area averaged vertical wind shear, vorticity and precipitation, were also compared to actual accumulated cyclone energy (ACE) for Septembers in select years. Favorable conditions in the model were found to be positively correlated with ACE. This approach has been extended to forecasts in the Sub-seasonal Experiment (SUBX) project in 2017 to investigate the ability of the predictive models to capture TC activity and relevant environmental parameters in real time.

Szandra Peter (MPO)

"Summer Heat Waves and Their Frequency Across Europe"

According to the WMO, extreme temperatures were the cause of 94% of total lives lost due to natural disasters in Europe during the 1970-2012 period.Accurate prediction of heat waves is necessary to prepare for such impacts. By calculating specific threshold values from daily maximum 2m temperatures of the NCEP/NCAR reanalysis data (Kalnay et al. 1996), the frequency of heat waves during summer months (JJA) in Europe for the period of 1982 to 2016 is computed. The differences between the first and the second half of the 34-year period are discussed, and the results are compared to 1.0° resolution CCSM4 forecasts of daily maximum 2m temperature with a 12-month hindcast initialized every June 1st, using 10 ensemble members for each initialization. We strive to assess the ability of CCSM4 forecasts to predict the frequency of hot spells in the summer.

Dec 08 --- Eleanor Middlemas (MPO, 1-Hour Student Seminar)

"Cloud Radiative Feedbacks and ENSO Variability in CESM"

Clouds are an integral part of our climate system due to their interaction with water vapor, circulation, radiation balance, and sea surface temperature (SST), but their intricate feedbacks are also the very reason that understanding role in climate variability remains a challenge. For this seminar, I present results of "cloud-locking" in a climate model, or turning off clouds’ ability from radiatively interacting with their environment. We focus on the impact of cloud-locking on the simulation of El Nino Southern Oscillation (ENSO) variability. We find that clouds maintain the magnitude and frequency at which El Nino events occur through their radiative feedbacks on SST and surface winds. In the eastern tropical Pacific, clouds prevent off-equatorial westerly winds that would otherwise lengthen the period of ENSO. We hypothesize that clouds weaken ENSO events by decreasing the zonal gradient sea surface temperatures responsible for maintaining atmospheric circulation associated with ENSO, namely the Bjerknes feedback and Walker circulation. Clouds’ large impact on ENSO has implications for understanding climate model disagreement in SST variability.