Climatic Impacts and Transport of Martian Atmospheric Dust from Assimilation of Spacecraft Observations

Mars' dust cycle and the radiative-dynamical impacts of atmospheric dust were investigated by assimilating Mars Climate Sounder (MCS) temperature and dust observations into a martian global climate model. Dust plays a key role in Mars' climate by interacting with solar and infrared radiation, thereby modifying atmospheric dynamics and winds, which themselves lift and transport dust. The 2018 Global Dust Storm (GDS) provided an opportunity to investigate how dust affects martian surface temperatures, polar dynamics, and the dust cycle. The 2018 GDS was found to warm the martian surface by 0.9 K. The effects were highly spatially heterogeneous, with net warming from enhanced backscattering of surface infrared emission at low thermal inertia regions, while elsewhere blocked incident solar radiation caused net cooling. Comparisons with the 2001 GDS and free-running simulations show that GDS geographical structure is key in determining the surface temperature impact. Martian dust lifting and deposition were shown to have a consistent interannual pattern, except during planetary-scale dust storms. Dust lifting patterns correspond to regular dynamical features including baroclinic waves, low-level jets, and CO2 sublimation flow. Regional dust storms affect southern seasonal cap-edge dust lifting by enhancing the meridional circulation, causing increased sublimation. The 2018 GDS increased dust lifting over high-topography regions like Tharsis, but inhibited northern wave-related dust lifting. Southern high-latitude winds were found to be highly sensitive to the precise thermal structure in MCS temperature observations, with direct impacts on dust lifting. The 2018 GDS was found to alter the elliptical structure of Mars' polar vortices via the GDS' effect on stationary topographic planetary waves, proving the link between these waves and vortex morphology. Enhanced GDS heating also significantly accelerated the destruction of the southern vortex. These results show the asymmetrical effects of an equinoctial GDS on the polar vortices, which govern tracer transport into polar regions

Assimilation of Mars Climate Sounder Dust Observations: Challenges and Ways Forward

Introduction: Atmospheric dust is ubiquitous on Mars, and as a result of its absorption and scattering of radiation, is the key driver of the martian circulation. Accurately representing the complex spatial and temporal distribution of dust is therefore crucial for understanding Mars’ atmospheric dynamics. In particular, the vertical representation of the dust distribution in Mars’ atmosphere has been shown to have a significant effect on results from modelling and assimilation [1,2,3]. With the goal of more accurately representing this distribution, the assimilation of dust vertical information is a valuable technique which is being increasingly explored [4,5]. However, it brings with it its own challenges and methodological questions to be explored. Model and assimilation details: We use the LMD-UK Mars Global Circulation Model (MGCM), which solves the meteorological primitive equations of fluid dynamics, radiative and other parameterised physics to calculate the state of the martian atmosphere [6,7]. The UK version of the MGCM possesses a spectral dynamical core and semi-Lagrangian advection scheme [8], and is a collaboration between the Laboratoire de Météorologie Dynamique, The Open University, the University of Oxford, and the Instituto de Astrofisica de Andalucia. The model was run using a range of spectral and vertical resolutions, the latter spaced logarithmically. The assimilation scheme used was a modified version of the Analysis Correction scheme developed at the Met Office [9], adapted for use on Mars [10]. This method has the advantage of being computationally in-expensive, and its use of repeated insertion, weighted over a time window of about six hours, helps counter the issue of relaxation of the atmospheric state – an especially significant problem given the low thermal inertia of Mars’ atmosphere. Retrievals: The retrievals used in this study are from the Mars Climate Sounder (MCS) instrument aboard the Mars Reconnaissance Orbiter (MRO) [11], which now has amassed over five full martian years’ worth of data. For this study, the assimilated MCS variables were temperature and dust profiles. Temperature profiles extend from the surface to approximately 100 km, and dust profiles from as low as 10 km above the surface up to a maximum height of approximately 50 km. Retrieval of dust profiles allows MCS to observe the complex vertical dust structure in the atmosphere. The retrieval version used is 5.2, a re-processing using updated 2D geometry [12]. This results in improved retrievals, especially in the polar regions. While not used in this study, the NOMAD instrument aboard ExoMars TGO will soon provide another high-volume source of dust profiles alongside MCS [13], and should return observations with an even higher vertical resolution. Discussion: The assimilation of MCS dust profiles poses unique technical challenges, but presents the opportunity of representing Mars’ vertical dust distribution with unprecedented spatial and temporal accuracy within a GCM. Some outstanding questions for further experimentation and discussion include: What are the optimal spatial and, in particular, vertical model resolutions for assimilation of this data? Can dust profile assimilation aid in forecasting? Previous indirect assimilation of vertical dust via its MCS temperature signature has yielded a forecast time of 10 sols [5]; how dependent is this on the assimilation scheme and the choice of assimilating variables? How should we approach the bimodal nature of MCS local times? Should we give higher weighting to nightside dust observations, which tend to have better vertical coverage due to reduced scattering? And how much can we validly infer from the high day-night variability seen in MCS dust profiles? What are the best heuristics for filtering spurious opacities which could disrupt the assimilation, for example due to CO2 ice or surface reflectance [16])? What are the optimal ways of dealing with spatial and temporal gaps in the dataset? How can we best represent the dust distribution beyond the range of MCS, especially in the lowest 5-10 km of the atmosphere? What are the advantages and disadvantages of directly assimilating the dust field vs indirectly up-dating the dust field via its temperature signature, as seen in Fig. 1? Dust profile assimilation has been used to track individual dust storm events [4]; what can this tell us about storm formation and evolution, and can it be used for storm forecasting? How can we best constrain and validate the column optical depths of MCS dust profiles? Some ways forward regarding these questions will be explored, including comparative reanalyses and validation against different orbital datasets. Comparisons against MCS and other retrievals (such as NOMAD) should provide insight into the advantages of various in-model representations of features such as the dust distribution as well as the possible advantages or disadvantages of pruning the assimilated dataset. Meanwhile, alternate orbital or even ground-based sources of column opacity (such as Mars Express and MSL) could help better con-strain the distribution of dust not seen by MCS and offer clues how best to proceed in periods when MCS data is missing or limited. Some results of intercomparisons will be presented with the aim of fostering a more general discussion on MCS assimilation techniques. References: [1] Lewis, S. R. et al., Icarus 192 (2), 327-347, 2007. [2] Rogberg, P. et al., QJRMS 136, 1614-1635, 2010. [3] Greybush, S. J. et al., JGR. 117, E11008, 2012. [4] Ruan, T., DPhil Thesis, 2015. [5] Navarro T. et al., Earth and Space Sci., 2017. [6] Forget, F. et al., JGR 104, 24155-24175, 1999. [7] Madeleine, J.-B. et al., JGR (Planets) 116, E11010, 2011. [8] Newman, C. E. et al., JGR 107, 5123, 2002. [9] Lorenc, A. C. et al., QJRMS 117, 59-89, 1991. [10] Lewis, S. R. et al., Icarus 192, 327-347, 2007. [11] McCleese, D. J. et al., J. Geophys. Res. 115, E12016, 2010. [12] Kleinböhl, A. et al., J. Quant. Spectrosc. Radiat. Transfer 187, 511-522, 2017. [13] Patel, M. R. et al., Appl. Opt. 56 (10), 2771-2782, 2017. [14] Navarro, T. et al., Geophys. Res. Lett. 41, 6620-6626, 2014. [15] Streeter, P. M. et al., 6th Intl. Workshop on the Mars Atmosphere, 2017. [16] Kleinböhl, A. et al., Icarus 261, 118-121, 2015

Martian Polar Vortex Dynamics and the 2018 Global Dust Storm

Mars’ polar vortices play key roles in the planet’s general circulation and atmospheric transport. The 2018 Global Dust Storm had a strong impact on dynamics at both poles, with implications for tracer transport into and around the polar regions

Surface Warming During the 2018/MY 34 Mars Global Dust Storm

Surface warming during the 2018 Global Dust Storm is revealed from assimilating Mars Climate Sounder observations into a GCM. Net warming occurred at low thermal inertia regions, due to inhibited night-time cooling; elsewhere, net cooling occurred

An In Vivo Evaluation of the Effect of Repeated Administration and Clearance of Targeted Contrast Agents on Molecular Imaging Signal Enhancement

Competitive inhibition diminishes ligand adhesion as receptor sites become occupied with competing ligands. It is unknown if this effect occurs in ultrasound molecular imaging studies where endothelial binding sites become occupied with adherent bubbles or bubble fragments. The goal of this pilot study was to assess the effect that repeated administration and clearance of targeted agents has on successive adhesion. Two groups of animals were imaged with 3-D ultrasonic molecular imaging. Injections and imaging were performed on Group 1 at time 0 and 60 minutes. Group 2 received injections of microbubbles at 0, 15, 30, 45 and 60 minutes with imaging at 0 and 60 minutes. At 60 minutes, Group 1 targeting relative to baseline was not significantly different from Group 2 (1.06±0.27 vs. 1.08±0.34, p=0.93). Data suggest that multiple injections of targeted microbubbles do not block sufficient binding sites to bias molecular imaging data in serial studies

Surface warming during the 2018/Mars Year 34 Global Dust Storm

The impact of Mars’ 2018 Global Dust Storm (GDS) on surface and near‐surface air temperatures was investigated using an assimilation of Mars Climate Sounder (MCS) observations. Rather than simply resulting in cooling everywhere from solar absorption (average surface radiative flux fell 26 Wm‐2), the globally‐averaged result was a 0.9 K surface warming. These diurnally‐averaged surface temperature changes had a novel, highly non‐uniform spatial structure, with up to 16 K cooling/19 K warming. Net warming occurred in low thermal inertia (TI) regions, where rapid night‐time radiative cooling was compensated by increased longwave emission and scattering. This caused strong nightside warming, outweighing dayside cooling. The reduced surface‐air temperature gradient closely coupled surface and air temperatures, even causing local dayside air warming. Results show good agreement with MCS surface temperature retrievals. Comparisons with the 2001 GDS and free‐running simulations show that GDS spatial structure is crucial in determining global surface temperature effects

Impacts of the 2018 Global Dust Storm on Martian Polar Dynamics

Introduction: Mars’ winter atmos-phere is characterized by a polar vortex of low temperatures around the winter pole, circumscribed by a strong westerly jet [e.g. 1]. These vortices are a key part of the atmospheric circulation and impact heavily on dust and volatile transport. In particular, they have a complex and asymmetrical (north/south) relationship with atmospheric dust loading [1]. Re-gional and global dust events have been shown to cause rapid vortex displace-ment [2,3] in the northern vortex, while the southern vortex appears more robust. Suspended atmospheric dust aerosol is a crucial active component of Mars’ at-mosphere, with significant radiative-dynamical effects through its scattering and absorption of radiation [5]. The exact nature of these effects depends on a vari-ety of factors: aerosol optical depth is important, as are the specific radiative properties of the aerosol particles [6,7], and the vertical distribution of the dust itself [8]. Mars Global Dust Storms (GDS) are spectacular, planet-spanning events which dramatically increase atmospheric dust loading. The 2018 GDS was ob-served through its lifecycle by the Mars Climate Sounder (MCS) instrument aboard the Mars Reconnaissance Orbiter [9]; using data assimilation [10] to inte-grate MCS retrievals [11] with the LMD-UK Mars Global Circulation Model (MGCM) [12] therefore offers an oppor-tunity to examine the effects of the GDS on the polar vortices, and the interplay between the factors described above. The reanalysis contains the MGCM’s best possible representation of the GDS geo-graphical, temporal, and in particular ver-tical structure. Model and assimilation scheme: We use the LMD-UK Mars Global Circula-tion Model (MGCM), which solves the meteorological primitive equations of fluid dynamics, radiative and other pa-rameterised physics to calculate the state of the martian atmosphere [3,8]. The UK version of the MGCM possesses a spec-tral dynamical core and semi-Lagrangian advection scheme [13], and is a collabo-ration between the Laboratoire de Météorologie Dynamique, The Open University, the University of Oxford, and the Instituto de Astrofisica de Andalucia. The model was run at spectral spatial res-olution T42 and a vertical resolution of 50 levels, the latter spaced non-linearly. The assimilation scheme used was a mod-ified version of the Analysis Correction scheme developed at the Met Office, adapted for use on Mars [6]. Retrievals used: The retrievals used in this study are from the Mars Climate Sounder (MCS) instrument aboard the Mars Reconnaissance Orbiter (MRO) [4], which now has amassed over five full martian years’ worth of data. For this study, the assimilated MCS variables were temperatures, derived column dust optical depth (CDOD), and dust profiles. Temperature profiles extend from the surface to approximately 100 km, and dust profiles from as low as 10 km above the surface up to a maximum height of approximately 50 km. The retrieval ver-sion used is 5.2, a re-processing using updated 2D geometry [7]. This results in improved retrievals, especially in the po-lar regions. Results: The 2018 GDS had large and asymmetric impacts on dynamics at both poles. This will be presented via changes in zonal winds and polar vorticity at both poles relative to a clear martian year, MY 30. The GDS provided a natural laborato-ry for testing the effects of equinoctial high dust loading on polar dynamics, al-lowing investigation of both how the po-lar atmosphere behaves in a clear year and under the case of extreme dust load-ing at this time of year. We present re-sults on the effects of the GDS on both southern and northern polar dynamics, with implications for tracer transport. Discussion: The 2018 GDS dataset al-lows the opportunity for investigation of the polar dynamical effects of that specif-ic event, the first fully observed by MCS. The polar vortices and associated zonal jets act as a barrier for cross-vortex tracer transport; their weakening can therefore allow dust to be transported onto the sea-sonal CO2 ice caps. Understanding how these barriers work is therefore important for understanding the evolution of Mars’ past climate: the Mars’ ice caps contain a record of past dust deposition [e.g. 8]. Upcoming retrievals from the ExoMars 2016 Trace Gas Orbiter and its NOMAD spectrometer suite [9] will allow for fur-ther investigation of tracer transport and an opportunity to both cross-validate and jointly assimilate NOMAD and MCS data, including over a range of martian local times, which will enable investigation of the diurnal cycles of tracer transport and atmospheric dynamics at the poles. Acknowledgements: PMS acknowl-edges support from the UK Science and Technology Facilities Council under STFC grant ST/N50421X/1 and The Open University in the form of a PhD student-ship. SRL, MRP and JAH also acknowledge the support of the UK Space Agency and STFC under grants ST/R001405/1, ST/S00145X/1 and ST/P001262/1 and STFC under ST/P000657/1. The authors are particu-larly grateful for ongoing collaborations with Dan McCleese, David Kass and the MCS team (NASA-JPL) and with Peter Read (Oxford) and François Forget and colleagues (LMD/CNRS Paris). References: [1] Waugh, D. W. et al (2016) J. Geophys. Res. Planets, 121, 1770-1785. [2] Guzewich, S. D. et al (2016) Icarus, 278, 100-118. [3] Mitch-ell, D. M. et al (2015) Q.J.R. Meteorol. Soc., 141, 550-562. [4] McCleese D. J. et al (2010) J. Geophys. Res., 115(E12016). [5] Gierasch P. J. and Goody R. M. (1972) J. Atmos. Sci., 29(2), 400-402. [6] Turco R. P. et al (1984) Scientific American, 251(2), 33-43. [7] Madeleine J.-B. et al (2011) JGR (Planets), 116 (E11010). [8] Tanaka, K. L. (2000), Icarus, 144(2), 254-266. [9] Patel, M. R. et al (2017), Appl. Opt., 56(10), 2771-2782

Improving Sensitivity in Ultrasound Molecular Imaging by Tailoring Contrast Agent Size Distribution: In Vivo Studies

Molecular imaging with ultrasound relies on microbubble contrast agents (MCAs) selectively adhering to a ligand-specific target. Prior studies have shown that only small quantities of microbubbles are retained at their target sites, therefore, enhancing contrast sensitivity to low concentrations of microbubbles is essential to improve molecular imaging techniques. In order to assess the effect of MCA diameter on imaging sensitivity, perfusion and molecular imaging studies were performed with microbubbles of varying size distributions. To assess signal improvement and MCA circulation time as a function of size and concentration, blood perfusion was imaged in rat kidneys using nontargeted size-sorted MCAs with a Siemens Sequoia ultrasound system (Siemans, Mountain View, CA) in cadence pulse sequencing (CPS) mode. Molecular imaging sensitivity improvements were studied with size-sorted αvβ3-targeted bubbles in both fibrosarcoma and R3230 rat tumor models. In perfusion imaging studies, video intensity and contrast persistence was ≈8 times and ≈3 times greater respectively, for “sorted 3-micron” MCAs (diameter, 3.3 ± 1.95 μm) when compared to “unsorted” MCAs (diameter, 0.9 ± 0.45 μm) at low concentrations. In targeted experiments, application of sorted 3-micron MCAs resulted in a ≈20 times video intensity increase over unsorted populations. Tailoring size-distributions results in substantial imaging sensitivity improvement over unsorted populations, which is essential in maximizing sensitivity to small numbers of MCAs for molecular imaging

Estimated effects of climate change on flood vulnerability of U.S. bridges

We assessed the potential impacts of increased river flooding from climate change on bridges in the continental United States. Daily precipitation statistics from four climate models and three greenhouse gas (GHG) emissions scenarios (A2, A1B, and B1) were used to capture a range of potential changes in climate. Using changes in maximum daily precipitation, we estimated changes to the peak flow rates for the 100-year return period for 2,097 watersheds. These estimates were then combined with information from the National Bridge Inventory database to estimate changes to bridge scour vulnerability. The results indicate that there may be significant potential risks to bridges in the United States from increased precipitation intensities. Approximately 129,000 bridges were found to be currently deficient. Tens of thousands to more than 100,000 bridges could be vulnerable to increased river flows. Results by region vary considerably. In general, more bridges in eastern areas are vulnerable than those in western areas. The highest GHG emissions scenarios result in the largest number of bridges being at risk. The costs of adapting vulnerable bridges to avoid increased damage associated with climate change vary from approximately $140 to$250 billion through the 21st century. If these costs were spread out evenly over the century, the annual costs would be several billion dollars. The costs of protecting the bridges against climate change risks could be reduced by approximately 30% if existing deficient bridges are improved with riprap.United States. Environmental Protection Agency. Office of Atmospheric Programs (Contract #EP-W-07-072

Toward Ultrasound Molecular Imaging With Phase-Change Contrast Agents: An In Vitro Proof of Principle

Phase-change contrast agents (PCCAs), which normally consist of nano/microscale droplets of liquid perfluorocarbons (PFCs) in an encapsulating shell, can be triggered to undergo a phase transition to the highly-echogenic gaseous state upon the input of sufficient acoustic energy. As a result of the subsequent volumetric expansion, a number of unique applications have emerged that are not possible with traditional ultrasound microbubble contrast agents. Although many studies have explored the therapeutic aspects of the PCCA platform, few have examined the potential of PCCAs for molecular imaging purposes. In this study, we demonstrate a PCCA-based platform for molecular imaging using αvβ3-targeted nanoscale PCCAs composed of low-boiling-point PFCs. In-vitro, nanoscale PCCAs adhered to target cells, could be activated and imaged with a clinical ultrasound system and produced a six-fold increase in image contrast compared to non-targeted control PCCAs and a greater than fifty-fold increase over baseline. Data suggest that low-boiling-point nanoscale PCCAs could enable future ultrasound-based molecular imaging techniques in both the vascular and extravascular space