COMPASS Wednesday - Archive

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COMPASS Wednesday

FALL 2018
Wednesdays at 3:00 pm, SLAB 103 (unless stated otherwise)




Sep 12: Dr. Lisa Beal
Department of Ocean Sciences, RSMAS

Shallow Cross-Equatorial Gyres of the Northern Indian Ocean
Driven by Seasonally Reversing Monsoon Winds
Lisa M. Beal and Pierre L'Hegaret

Typically an ocean gyre – defined as the large-scale, wind-driven, horizontal motion of the ocean closed by a western boundary current – is depicted by time-mean currents, or by steady streamlines [Stommel, 1948]. But in the northern Indian Ocean the circulation is fundamentally unsteady, reversing seasonally, so how is the gyre circulation depicted?

We use real and simulated surface drifter trajectories to reveal three cross-equatorial gyres in the northern Indian Ocean that are driven by the seasonally reversing monsoon winds. Simulated trajectories are com-puted by advecting hundreds of thousands of particles through a monthly drifter climatology. Gyres are de-picted using probabilistic pathlines, defined as the most probable trajectories that, at some point in their life-time, pass through the Somali Current.

We find that monsoon gyres connect across seasons and across years in unexpected ways, for instance, linking the Somali Current of the southwest monsoon with the Northeast Monsoon Current of the northeast monsoon via the inter-monsoon Wyrtki jet. A significant component of the monsoon gyres is cross-equatorial flow in the interior, driven by Ekman currents. This flow carries southward heat transport as the upper arm of the shallow overturning cell [Schott, 2005]. A striking result of our analysis is that the Somali Current, and as-sociated cool, upwelled waters, does not connect with the Arabian basin at the surface and this may have consequences for the evolution of mixed layer depth and productivity in the interior. Large differences be-tween surface (Ekman) and subsurface (geostrophic) connectivity may explain why the oxygen minimum zone is shifted eastward of the regions of high productivity in the Arabian basin.


Sep 26: Dr. Kazuaki Yasunaga
University of Toyama

Space-Time Spectral Analysis of Moist Static Energy Budget Equation

The budget of column-integrated moist static energy (MSE) is examined in wavenumber-frequency transforms of longitude-time sections over the tropical belt. Cross spectra with satellite-derived precipitation (TRMM-3B42) are used to emphasize precipitation-coherent signals in reanalysis (ECMWF-Interim, ERAI) estimates of each term in the budget equation.

Results reveal different budget balances in convectively coupled equatorial waves (CCEWs) as well as in the Madden-Julian oscillation (MJO) and tropical depression (TD)-type disturbances. The real component (expressing amplification or damping of amplitude) for horizontal advection is modest for most wave types, but substantially damps the MJO. Its imaginary component is hugely positive (it acts to advance phase) in TD-type disturbances, and is positive for MJO and Equatorial Rossby (ERn1) wave disturbances (almost negligible for the other CCEWs). The real component of vertical advection is negatively correlated (damping effect) with precipitation with a magnitude of approximately 10% of total latent heat release for all disturbances except for TD-type disturbance. This effect is overestimated by a factor of 2 or more if advection is computed using the time-zonal mean MSE, suggesting that nonlinear correlations between ascent and humidity would be positive (amplification effect).

ERAI-estimated radiative heating has a positive real part, reinforcing precipitation-correlated MSE excursions. The magnitude is up to 14% of latent heating for the MJO, and much less for other waves. ERAI-estimated surface flux has a small effect, but acts to amplify MJO and ERn1 waves. The imaginary component of budget residuals is large and systematically positive, suggesting that the reanalysis model's physical MSE sources would not act to propagate the precipitation-associated MSE anomalies properly.

Oct 03: Dr. Roland Romeiser
Department of Ocean Sciences, RSMAS
(Candidate for Promotion)

Phase-Resolving Ocean Wave Field Remote Sensing
by Spotlight-Mode SAR

A detailed two-dimensional picture of the ocean wave field from above is of interest for many applications, in particular in coastal regions, where wave refraction patterns can provide information on the underlying current field and bathymetry. It has been known since the mid-1960s that spaceborne synthetic aperture radars (SARs) can resolve ocean wave patterns. However, the retrieval of quantitative wave information from SAR images is difficult because the relation between image intensity variations and surface slopes can be highly nonlinear, the clarity of the signatures of interest is degraded by speckle noise, and the interpretation of a single image as a snapshot in time causes directional ambiguities. Traditional wave retrieval techniques use a first-guess wave spectrum from an external source (e.g. a numerical wave prediction model) as input, which is fed into a SAR imaging model and modified in an iterative manner until satisfactory agreement between simulated and observed SAR image spectra is obtained. The optimized input wave spectrum used at this stage is then provided as best estimate of the true ocean wave spectrum.

We have recently developed a new SAR-based wave remote sensing technique that overcomes most of the traditional problems by utilizing spotlight-mode images from the satellite systems TerraSAR-X and COSMO-SkyMed in combination with advanced data processing. Spotlight-mode images are acquired with particularly long synthetic aperture times of several seconds, which enable spatial resolutions better than 1 m, but initially lead to a noticeable blurring of moving wave signatures, comparable to the effect of a long exposure time in photography. It is possible to reprocess the same data into a series of images with a shorter integration time, representing time steps during the SAR overpass. This image series reveals motions like a short video, and a dispersion shell filter can be applied to separate physically meaningful ocean wave signatures from contributions with different motion characteristics and noise. The filtered image spectrum is free of contributions unrelated to the waves and it shows the correct propagation direction of each component, making its inversion into an ocean wave spectrum much more straightforward and eliminating the need for an external first-guess spectrum. Furthermore, our processing technique is phase preserving, permitting an accurate reconstruction of the original moving wave patterns in the space-time domain.

In this seminar presentation, we will show how the proposed wave remote sensing technique works and discuss example results as well as perspectives for further development.

Oct 10: Dr. John van Leer
Department of Ocean Sciences, RSMAS

Building Our Better Future Faster

Building Our Better Future Faster is the title of a book I am writing. My seminar will emphasize the positive future we can have, using well known techniques that are realistically scalable over the next three decades. The underlying methods have been quantified by the DRAWDOWM Project, based upon existing climate, economic and sustainability literature. A revenue neutral Carbon Fee and Dividend policy will accelerate all of these solutions. In addition, we will need an economic policy for the preservation of carbon resources for use as feedstocks by our descendants, rather than their immediate combustion. A path leading to the implementation of these policies is suggested.

Oct 17: Dr. Francisco J. Beron-Vera
Department of Atmospheric Sciences, RSMAS
(Candidate for Promotion)

Enduring Lagrangian Aspects of the Malvinas Current as Revealed by
Deterministic and Probabilistic Nonlinear Dynamics Tools

Deterministic and probabilistic tools from nonlinear dynamics are used to assess enduring Lagrangian aspects of the Malvinas Current. The deterministic tools are applied on a multi-year record of velocities derived from satellite altimetry data, revealing a quasi-steady cross-stream transport barrier. This is composed of shearless-parabolic Lagrangian coherent structures (LCS), which, extracted over sliding time windows along the multi-year altimetry-derived velocity record, lie in near coincidental position. The probabilistic tools are applied on a large collection of historical satellite-tracked drifter trajectories, revealing weakly communicating flow regions on either side of the altimetry-derived barrier. Shearless-parabolic LCS are detected for the first time from altimetry data, and their significance is supported on satellite-derived ocean color data, which reveal shapes that quite closely resemble the peculiar V shapes, dubbed "chevrons,'' that have recently confirmed the presence of similar LCS in the atmosphere of Jupiter. The significance of the quasi-steady cross-stream transport barrier nature of the Malvinas Current is independently supported on drifter data.

Oct 24: Jiawei Bao
University of New South Wales, Australia

Extreme Precipitation Scaling and Its Links with Convective Organization

Extreme precipitation and its induced hazards pose huge threats to economy, agriculture, infrastructure and human lives. It's important to understand how extreme precipitation changes with warming. Over the past decades, numerous observational studies have worked on it by applying a simple "binning method". Based on this method, many interesting results have been reported including super Clausius-Clapeyron scaling of sub-daily extreme precipitation in the Netherlands, and negative rates in the tropics. The first part of the talk will focus on this "binning method" and explain that the results from this method are not reliable to apply to climate change. Many extreme precipitation events are closely related to organized convection systems. Thus, the behavior of this convective organization with warming may affect extremes. In the second part, I will present results from a suite of idealized simulations showing that extreme precipitation scaling is strongly associated with the change in the degree of convective organization. In the last part, I will show the impact of self-aggregation on updraft and extreme precipitation, and explain different roles that aggregation plays in instantaneous vs. daily-accumulated precipitation extremes.

Oct 31: Anis Elyouncha
Chalmers University of Technology, Gothenburg, Sweden

Ocean Surface Current Retrieval from Space-Borne Along-Track
Interferometric SAR: Challenges and Results from the Baltic Sea

Accurate measurements of ocean surface currents are of interest for a variety of applications, such as marine navigation, pollution monitoring, coastal hazards management, and ocean energy exploitation. In-situ measurements are sparse, and the deployment and maintenance of the instruments is costly. Satellite remote sensing with large spatial coverage offers a good complement to the in-situ observations. Synthetic Aperture Radar (SAR) has become a vital tool for ocean remote sensing owing mainly to its high spatial resolution and its independence of sunlight and weather conditions. SAR is particularly useful in coastal and shelf seas, where small-scale processes dominate the upper ocean dynamics and where other satellite sensors have limited capabilities. In this presentation, an overview of the along-track interferometric SAR (ATI-SAR) technique will be given. This includes the basic principles and interferometric processing from a complex SAR image into surface radial velocity. The interferometric phase provided by the ATI-SAR technique is directly related to the sea surface velocity. In practice, however, imperfections in the ATI-SAR system and the complexity of the sea surface dynamics make ocean current retrieval challenging. The main challenging problems are the absolute phase calibration and the removal of ocean wave contributions. This presentation will focus on these two problems and the proposed solutions. Finally, examples using acquisitions from the X-band system TanDEM-X over the Baltic Sea will be shown to illustrate the obtained results.

Nov 07: Dr. Paquita Zuidema
Department of Atmospheric Sciences, RSMAS

What Can We Learn from Field Campaigns Focused on Biomass-Burning
Aerosols and the Marine Stratocumulus Deck of the Southeast Atlantic?

Seasonal biomass burning in southern Africa during the southern hemisphere spring produces sunlight-absorbing smoke particles, that are lofted into the mid-troposphere and transported westward over the South-East Atlantic. Here, the smoke interacts with one of the three semi-permanent subtropical stratocumulus cloud decks in the world, either through cloud adjustments to the aerosol-induced solar heating or through cloud-aerosol microphysical interactions. The characterization of the smoke and clouds and the representation of their interactions remains highly uncertain, reflecting a scarcity of observational knowledge. Recent field campaigns just ending in October, 2018, sponsored by NASA, DOE, the UK and France, are beginning to fill this observational gap. This presentation provides an overview of the deployments, highlighting new knowledge as we know it on aerosol absorptive and cloud-nucleating properties, their vertical distribution relative to clouds, the locations and degree of aerosol mixing into clouds, and cloud changes in response to such mixing. New findings based on the in-situ and surface-based assessments include that smoke is mixed into the cloudy boundary layer much more than originally thought, after the smoke has moved around the Atlantic basin for several weeks. The aged smoke possesses a surprisingly-low single-scattering albedo, which will increase its ability to absorb sunlight. Precipitation is efficient at removing smoke from the cloudy boundary layer as it advects equatorward.

Nov 14 (Auditorium): Dr. Yair Cohen
California Institute of Technology

A Unified EDMF Parameterization for Boundary Layer Turbulence,
Shallow and Deep Convection
Y. Cohen, C.M. Kaul, A. Jaruga, I. Lopez, K.G. Pressel, T. Schneider, and J. Teixeira

Eddy-Diffusivity Mass-Flux (EDMF) parameterizations decompose turbulent flows into coherent updrafts, modeled by mass flux closures, and a turbulent environment, modeled by an eddy diffusivity closure. We have recently extended EDMF parameterizations to be prognostic, so they have explicit subgrid-scale memory, and to allow for features such as a variable updraft fraction, which are essential as we approach the grey zone for deep convection (Tan et al., 2018 JAMES). The extended EDMF parameterization is de-signed to unify parameterizations of boundary layer turbulence, shallow and deep convection in a single con-sistent framework. The framework also includes a consistent coupling to microphysical processes, which are coupled to dynamical equations for mean values, variances, and covariances of thermodynamic varia-bles. We test and optimize the parameterization with LES simulations in a variety of conditions, ranging from stratocumulus-topped boundary layers to deep convection. Specifically, we investigate how mixing lengths and entrainment / detrainment rates can be formulated based on similarity arguments, and how the functional form that is not determined by similarity arguments can be learned from LES output. We demonstrate that the extended EDMF parameterization can capture essential aspects of diverse cloud and boundary layer re-gimes, including convective life cycles. Further, we describe first steps of a machine learning approach to optimize unclosed parameters and parametric functions in the EDMF closure. A Bayesian inversion is per-formed using a Markov Chain Monte Carlo method to determine the posterior distribution of closure parame-ters and coefficients that arise in the EDMF scheme.

Nov 21: NO SEMINAR (Thanksgiving Break)


Dec 05: Dr. Hilary Close
Department of Ocean Sciences, RSMAS

Distinguishing the Roles of Microbial and Metazoan Heterotrophy
in the Ocean Carbon Cycle

The downward flux of organic matter in the ocean is a major carbon cycle component, contributing both to the sequestration of atmospheric carbon and to the diets of animals in the deep ocean. Microbial and meta-zoan heterotrophy are the major agents acting on organic matter to reduce its downward flux through the wa-ter column; both the intensity of heterotrophy at the ocean's surface and the vertical distribution of heterotro-phy beneath the surface help to control the ocean carbon budget. I will describe my recent work using natu-ral variations in the stable isotopes of organic compounds to expose the varying net effects of microbial and metazoan heterotrophy on a range of particle sizes involved in vertical carbon flux. Using multiple simultane-ous parameters from compound-specific isotope analysis of amino acids, I will introduce a new classification system for the trophic history of detrital organic matter. Under this multivariate framework, I distinguish sea-sonal variability in the prevalence of zooplankton- versus microbially-altered detritus in the oligotrophic Pacif-ic, and I identify evidence for the transfer of microbially-altered small particles into the higher food web at mesopelagic depths. Additionally, I find that changes in the net trophic reworking of all particles across depth in the euphotic zone can affect the interpretation of bulk nitrogen isotope ratios of marine organic matter.