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Le jeudi 7 juin 2018 à 11h

LATMOS site Guyancourt
Amphithéâtre Gérard Mégie, Observatoire Versailles St Quentin

Probabilistic Quantitative Precipitation Estimates with Ground- and Space-based Remote Sensing

Pierre-Emmanuel Kirstetter

National Severe Storm Laboratory, University of Oklahoma (USA)

Progress in precipitation estimation is critical to advance weather and water budget studies and prediction of natural hazards caused by extreme rainfall events from local to global scale. An interdisciplinary challenge in remote sensing, meteorology and hydrology is the impact, representation, and use of uncertainty. Understanding of hydrometeorological processes and applications require more than just one deterministic "best estimate" to adequately cope with the intermittent, highly skewed distribution that characterizes precipitation. Yet the uncertainty structure of quantitative precipitation estimation (QPE) from ground-based radar networks like NEXRAD and satellite-based active and passive sensors of the Global Precipitation Measurement (GPM) mission is largely unknown at fine spatiotemporal scales near the sensor measurement scale (e.g. 1-km/5-min for ground-based radars, 5-km/instantaneous for space-based radars). We propose to advance the use of uncertainty as an integral part of QPE for ground-based and space-borne sensors. Probability distributions of precipitation rates are computed instead of deterministic values using models quantifying the relation between the sensor measurement and the corresponding "true" precipitation. This approach preserves the fine space/time sampling properties of the sensor and integrates sources of error in QPE. It provides a framework to diagnose uncertainty when instruments sample raining scenes or processes challenging the assumptions of the QPE algorithms. Precipitation probability maps compare favorably to deterministic QPE and improve precipitation estimation. Probabilistic QPE is shown to mitigate systematic biases from deterministic retrievals, quantify uncertainty, and advance the monitoring of precipitation extremes. It provides the basis radar and satellite precipitation ensembles needed for multisensor merging of precipitation, early warning and mitigation of hydrometeorological hazards, and hydrological modeling. Perspectives for improved understanding and parameterizations of precipitation processes, estimation of precipitation at multiple scales, hydrological prediction and risk monitoring will be presented.

Vendredi 9 mars 2018 à 11h

Salle de réunion 411, tour 45-46, 4ème étage

Ozone under (Laser) Light : new insights into an old molecule

Christof Janssen

LERMA, Paris

Ozone is a key molecule in atmospheric chemistry. The measurement of this reactive molecule poses a challenge for the scientific community. We will present new precision measurements of the ozone cross sections at 325 nm that resuscitate an old debate about the consistency of currently recommended UV data with IR intensities. Implications of the accuracy of ozone measurements in the Huggins bands are discussed.

We will also report on new measurements of the isotope fractionation in the vis light decomposition of ozone.

Lundi 20 novembre 2017 à 11h

Salle de réunion 411, tour 45-46, 4ème étage

On the Nexus between Carbon Cycle and Air Quality: Exploring Multiple Constraints on Anthropogenic Combustion and Fires

Ave Arellano

University of Arizona

It is imperative that we provide more accurate and consistent analysis of anthropogenic pollution emissions at scales that is relevant to air quality, energy, and environmental policy. Here, we present several proof-of-concept studies that explore observational constraints from ground, aircraft, and satellite-derived measurements of atmospheric composition on bulk characteristics of anthropogenic combustion in megacities and fire regions. We focus on jointly analyzing co-emitted combustion products such as CO2, NO2, CO, SO2, and aerosols from GOSAT, OCO-2, OMI, MOPITT, IASI, GOME, and MODIS retrievals, in conjunction with USEPA AQS and NASA field campaigns. Each of these constituents exhibit distinct atmospheric signatures that depend on fuel type, combustion technology, process, practices and regulatory policies. Our results show that distinguishable patterns and relationships between the increases in concentrations across the megacity or large fire events due to emissions of these constituents enable us to: a) identify trends in combustion activity and efficiency, and b) reconcile discrepancies between state- to country-based emission inventories and modeled concentrations of these constituents. For example, the trends in enhancement ratios of these species reveal combustion emission pathways for China and United States that are not captured by current emission inventories and chemical reanalysis. Analysis of their joint distributions has considerable potential utility in current and future integrated constituent data assimilation and inverse modeling activities like in CAMS for monitoring, verifying, and reporting emissions, particularly for regions with few observations and limited information on local combustion processes. Our targeted evaluation of the global forecast and analysis of CAMS CO and CO2 during KORUS-AQ field campaign in May 2016 suggests that CAMS is able to capture the constrast in combustion efficiency between Chinese outflow and Seoul. Analyses of MOPITT and IASI XCO as well as GOSAT XCO2 column retrievals in CAMS better capture the variability in Seoul but not in the Chinese outflow suggesting insufficient constraints over this region. This work also motivates the need for continuous and preferably collocated satellite measurements of atmospheric composition (including O3, CH4) and studies related to improving the applicability and integration of these observations with ground- and aircraft-based measurements.

Mardi 28 novembre 2017 à 11h

Salle de réunion 411, tour 45-46, 4ème étage

Evidence for continued ozone layer reduction

William Ball

ETH Zürich and PMOD, Davos

Stratospheric ozone protects life from harmful ultraviolet radiation. The Montreal Protocol, enacted to prevent ozone depletion from human activity, halted declines in column-integrated ozone in the 1990s; the upper stratosphere is recovering, but lower stratospheric trends are unknown. I will present clear evidence that lower stratospheric ozone continues to decline, preventing total ozone recovery. This result contradicts expectations and, with no clear understanding of why this trend has emerged, implies that the problem of ozone depletion is not yet fully solved. The decline may impact future estimates of ozone layer recovery, the stratospheric response to climate change, radiative climate forcing, ozone exchange between the stratosphere and troposphere, and surface levels of ultraviolet radiation.

Vendredi 10 novembre 2017 à 11h

Salle de réunion 411, tour 45-46, 4ème étage

On the role of stratospheric ozone feedbacks in the climate system

Gabriel Chiodo

Dep. of Applied Physics and Applied Mathematics
Columbia University, New York

Ozone chemistry and its feedbacks in the climate system are commonly neglected in state-of-the-art climate models involved in future climate projections; the impact of this simplification is still unclear. By carrying out model simulations from the Community Earth System Model (CESM), I quantify the effect of coupling the stratospheric ozone chemistry onto the model's sensitivity to solar and anthropogenic greenhouse gases (GHG). I will show that interactive stratospheric ozone reduces the model response to GHG and solar forcing, albeit through two different mechanisms. Accordingly, stratospheric ozone responses yield an important and yet undocumented negative feedback in the climate system.