36 research outputs found
Excess science accommodation capabilities and excess performance capabilities assessment for Mars Geoscience and Climatology Orbiter: Extended study
The excess science accommodation and excess performance capabilities of a candidate spacecraft bus for the Mars Geoscience and Climatology Orbiter MGCO mission are assessed. The appendices are included to support the conclusions obtained during this contract extension. The appendices address the mission analysis, the attitude determination and control, the propulsion subsystem, and the spacecraft configuration
Boundary-layer water vapor profiling using differential absorption radar
Remote sensing of water vapor in the presence of clouds and precipitation
constitutes an important observational gap in the global observing system. We
present ground-based measurements using a new radar instrument operating near
the 183 GHz H2O line for profiling water vapor inside of
planetary-boundary-layer clouds, and develop an error model and inversion
algorithm for the profile retrieval. The measurement technique exploits the
strong frequency dependence of the radar beam attenuation, or differential
absorption, on the low-frequency flank of the water line in conjunction with
the radar's ranging capability to acquire range-resolved humidity
information. By comparing the measured differential absorption coefficient
with a millimeter-wave propagation model, we retrieve humidity profiles with
200 m resolution and typical statistical uncertainty of 0.6 g mâ3 out
to around 2 km. This value for humidity uncertainty corresponds to
measurements in the high-SNR (signal-to-noise ratio) limit, and is specific to the frequency band
used. The measured spectral variation of the differential absorption
coefficient shows good agreement with the model, supporting both the
measurement method assumptions and the measurement error model. By performing
the retrieval analysis on statistically independent data sets corresponding
to the same observed scene, we demonstrate the reproducibility of the
measurement. An important trade-off inherent to the measurement method
between retrieved humidity precision and profile resolution is discussed.</p
Observing convective aggregation
Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad a distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a ground-based observational network
The feasibility of water vapor sounding of the cloudy boundary layer using a differential absorption radar technique
The feasibility of differential absorption radar (DAR) for the spaceborne
remote profiling of water vapor within the cloudy boundary layer is assessed
by applying a radar instrument simulator to large eddy simulations (LES).
Frequencies near the 183 GHz water vapor absorption line attenuate too
strongly to penetrate the large vapor concentrations that are ubiquitous in
the boundary layer. However it is shown that lower frequencies between 140
and 170 GHz in the water vapor absorption continuum and on the wings of the
absorption line, which are attenuated less efficiently than those near the
line center, still have sufficient spectral variation of gaseous attenuation
to perform sounding. The high resolution LES allow for assessment of the
potential uncertainty in the method due to natural variability in
thermodynamic and dynamic variables on scales smaller than the instrument
field of view. The (160, 170) GHz frequency pair is suggested to best
maximize signal for vapor profiling while minimizing noise due to undesired
spectral variation in the target extinction properties. Precision in the
derived water vapor is quantified as a function of the range resolution and
the instrument precision. Assuming an observational spatial scale of 500 m
vertical and 750 m full width at half maximum (FWHM) horizontal, measurement
precision better that 1 g mâ3 is achievable for stratocumulus scenes and
3 g mâ3 for cumulus scenes given precision in radar reflectivity of 0.16 dBZ. Expected precision in the column water vapor (CWV) is achievable
between 0.5 and 2 kg mâ2 on these same spatial scales. Sampling
efficiency is quantified as a function of radar sensitivity. Mean biases in
CWV due to natural variability in the target extinction properties do not
exceed 0.25 kg mâ2. Potential biases due to uncertainty in the
temperature and pressure profile are negligible relative to those resulting
from natural variability. Assuming a â35 dBZ minimum detectable signal,
40 %(21.9 %) of stratocumulus(cumulus) atmospheric boundary layer range
bins would be sampled. Simulated surface reflectivities are always greater
than â5 dBZ, which implies the DAR technique could provide near spatially
continuous observation of the CWV in subtropical boundary layers at a
spatial resolution better than 1 km