50 research outputs found
Scale dependence of cirrus heterogeneity effects. Part II: MODIS NIR and SWIR channels
In a context of global climate change, the understanding of the radiative
role of clouds is crucial. On average, ice clouds such as cirrus have a
significant positive radiative effect, but under some conditions the effect
may be negative. However, many uncertainties remain regarding the role of ice
clouds on Earth's radiative budget and in a changing climate. Global
satellite observations are particularly well suited to monitoring clouds,
retrieving their characteristics and inferring their radiative impact. To
retrieve ice cloud properties (optical thickness and ice crystal effective
size), current operational algorithms assume that each pixel of the observed
scene is plane-parallel and homogeneous, and that there is no radiative
connection between neighboring pixels. Yet these retrieval assumptions are
far from accurate, as real radiative transfer is 3-D. This leads to the
plane-parallel and homogeneous bias (PPHB) plus the independent pixel
approximation bias (IPAB), which impacts both the estimation of
top-of-the-atmosphere (TOA) radiation and the retrievals. An important factor
that determines the impact of these assumptions is the sensor spatial
resolution. High-spatial-resolution pixels can better represent cloud
variability (low PPHB), but the radiative path through the cloud can involve
many pixels (high IPAB). In contrast, low-spatial-resolution pixels poorly
represent the cloud variability (high PPHB), but the radiation is better
contained within the pixel field of view (low IPAB). In addition, the solar
and viewing geometry (as well as cloud optical properties) can modulate the
magnitude of the PPHB and IPAB. In this, Part II of our study, we simulate
TOA 0.86 and 2.13 µm solar reflectances over a cirrus uncinus
scene produced by the 3DCLOUD model. Then, 3-D
radiative transfer simulations are performed with the 3DMCPOL
code at spatial resolutions ranging from 50 m to 10 km, for 12
viewing geometries and nine solar geometries. It is found that, for simulated
nadir observations taken at resolution higher than 2.5 km, horizontal
radiation transport (HRT) dominates biases between 3-D and 1-D reflectance
calculations, but these biases are mitigated by the side illumination and
shadowing effects for off-zenith solar geometries. At resolutions coarser
than 2.5 km, PPHB dominates. For off-nadir observations at resolutions
higher than 2.5 km, the effect that we call THEAB (tilted and homogeneous
extinction approximation bias) due to the oblique line of sight passing
through many cloud columns contributes to a large increase of the
reflectances, but 3-D radiative effects such as shadowing and side
illumination for oblique Sun are also important. At resolutions coarser than
2.5 km, the PPHB is again the dominant effect. The magnitude and resolution
dependence of PPHB and IPAB is very different for visible, near-infrared and
shortwave infrared channels compared with the thermal infrared channels
discussed in Part I of this study. The contrast of 3-D radiative effects
between solar and thermal infrared channels may be a significant issue for
retrieval techniques that simultaneously use radiative measurements across a
wide range of solar reflectance and infrared wavelengths.</p
Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach
Nitrous oxide (N2O) -- a product of microbial nitrogen metabolism -- is a
compelling exoplanet biosignature gas with distinctive spectral features in the
near- and mid-infrared, and only minor abiotic sources on Earth. Previous
investigations of N2O as a biosignature have examined scenarios using Earthlike
N2O mixing ratios or surface fluxes, or those inferred from Earth's geologic
record. However, biological fluxes of N2O could be substantially higher, due to
a lack of metal catalysts or if the last step of the denitrification metabolism
that yields N2 from N2O had never evolved. Here, we use a global biogeochemical
model coupled with photochemical and spectral models to systematically quantify
the limits of plausible N2O abundances and spectral detectability for Earth
analogs orbiting main-sequence (FGKM) stars. We examine N2O buildup over a
range of oxygen conditions (1%-100% present atmospheric level) and N2O fluxes
(0.01-100 teramole per year; Tmol = 10^12 mole) that are compatible with
Earth's history. We find that N2O fluxes of 10 [100] Tmol yr would lead
to maximum N2O abundances of ~5 [50] ppm for Earth-Sun analogs, 90 [1600] ppm
for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e.
We simulate emission and transmission spectra for intermediate and maximum N2O
concentrations that are relevant to current and future space-based telescopes.
We calculate the detectability of N2O spectral features for high-flux scenarios
for TRAPPIST-1e with JWST. We review potential false positives, including
chemodenitrification and abiotic production via stellar activity, and identify
key spectral and contextual discriminants to confirm or refute the biogenicity
of the observed N2O.Comment: 22 pages, 17 figures; ApJ, 937, 10
CAMEMBERT: A Mini-Neptunes GCM Intercomparison, Protocol Version 1.0. A CUISINES Model Intercomparison Project
With an increased focus on the observing and modelling of mini-Neptunes,
there comes a need to better understand the tools we use to model their
atmospheres. In this paper, we present the protocol for the CAMEMBERT
(Comparing Atmospheric Models of Extrasolar Mini-neptunes Building and
Envisioning Retrievals and Transits) project, an intercomparison of general
circulation models (GCMs) used by the exoplanetary science community to
simulate the atmospheres of mini-Neptunes. We focus on two targets well studied
both observationally and theoretically with planned JWST Cycle 1 observations:
the warm GJ~1214b and the cooler K2-18b. For each target, we consider a
temperature-forced case, a clear sky dual-grey radiative transfer case, and a
clear sky multi band radiative transfer case, covering a range of complexities
and configurations where we know differences exist between GCMs in the
literature. This paper presents all the details necessary to participate in the
intercomparison, with the intention of presenting the results in future papers.
Currently, there are eight GCMs participating (ExoCAM, Exo-FMS, FMS PCM,
Generic PCM, MITgcm, RM-GCM, THOR, and the UM), and membership in the project
remains open. Those interested in participating are invited to contact the
authors.Comment: Accepted to PS
No phosphine in the atmosphere of Venus
The detection of phosphine (PH₃) has been recently reported in the atmosphere of Venus employing mm-wave radio observations (Greaves et at. 2020). We here demonstrate that the observed PH₃ feature with JCMT can be fully explained employing plausible mesospheric SO₂ abundances (~100 ppbv as per the SO₂ profile given in their figure 9), while the identification of PH₃ in the ALMA data should be considered invalid due to severe baseline calibration issues. We demonstrate this by independently calibrating and analyzing the ALMA data using different interferometric analysis tools, in which we observe no PH₃ in all cases. Furthermore, for any PH₃ signature to be produced in either ALMA or JCMT spectra, PH₃ needs to present at altitudes above 70 km, in stark disagreement with their photochemical network. We ultimately conclude that this detection of PH₃ in the atmosphere of Venus is not supported by our analysis of the data
Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System
The TRAPPIST-1 system, consisting of an ultra-cool host star having seven
known Earth-size planets will be a prime target for atmospheric
characterization with JWST. However, the detectability of atmospheric molecular
species may be severely impacted by the presence of clouds and/or hazes. In
this work, we perform 3-D General Circulation Model (GCM) simulations with the
LMD Generic model supplemented by 1-D photochemistry simulations at the
terminator with the Atmos model to simulate several possible atmospheres for
TRAPPIST-1e, 1f and 1g: 1) modern Earth, 2) Archean Earth, and 3) CO2-rich
atmospheres. JWST synthetic transit spectra were computed using the GSFC
Planetary Spectrum Generator (PSG). We find that TRAPPIST-1e, 1f and 1g
atmospheres, with clouds and/or hazes, could be detected using JWST's NIRSpec
prism from the CO2 absorption line at 4.3 um in less than 15 transits at 3
sigma or less than 35 transits at 5 sigma. However, our analysis suggests that
other gases would require hundreds (or thousands) of transits to be detectable.
We also find that H2O, mostly confined in the lower atmosphere, is very
challenging to detect for these planets or similar systems if the planets'
atmospheres are not in a moist greenhouse state. This result demonstrates that
the use of GCMs, self-consistently taking into account the effect of clouds and
sub-saturation, is crucial to evaluate the detectability of atmospheric
molecules of interest as well as for interpreting future detections in a more
global (and thus robust and relevant) approach.Comment: 36 pages, 19 figure