The Water-ice Feature in Near-infrared Disk-scattered Light around HD 142527: Micron-sized Icy Grains Lifted up to the Disk Surface?

We study the 3 μm scattering feature of water ice detected in the outer disk of HD 142527 by performing radiative transfer simulations. We show that an ice mass abundance at the outer disk surface of HD 142527 is much lower than estimated in a previous study. It is even lower than inferred from far-infrared ice observations, implying ice disruption at the disk surface. Next, we demonstrate that a polarization fraction of disk-scattered light varies across the ice-band wavelengths depending on ice grain properties; hence, polarimetric spectra would be another tool for characterizing water-ice properties. Finally, we argue that the observed reddish disk-scattered light is due to grains a few microns in size. To explain the presence of such grains at the disk surface, we need a mechanism that can efficiently oppose dust settling. If we assume turbulent mixing, our estimate requires α ≳ 2 × 10−3, where α is a nondimensional parameter describing the vertical diffusion coefficient of grains. Future observations probing gas kinematics would be helpful to elucidate vertical grain dynamics in the outer disk of HD 142527.

Summertime stratospheric processes at northern mid-latitudes: comparisons between MANTRA balloon measurements and the Canadian Middle Atmosphere Model

In this paper we report on a study conducted using the Middle Atmospheric Nitrogen TRend Assessment (MANTRA) balloon measurements of stratospheric constituents and temperature and the Canadian Middle Atmosphere Model (CMAM). Three different kinds of data are used to assess the inter-consistency of the combined dataset: single profiles of long-lived species from MANTRA 1998, sparse climatologies from the ozonesonde measurements during the four MANTRA campaigns and from HALOE satellite measurements, and the CMAM climatology. In doing so, we evaluate the ability of the model to reproduce the measured fields and to thereby test our ability to describe mid-latitude summertime stratospheric processes. The MANTRA campaigns were conducted at Vanscoy, Saskatchewan, Canada (52° N, 107° W) in late August and early September of 1998, 2000, 2002 and 2004. During late summer at mid-latitudes, the stratosphere is close to photochemical control, providing an ideal scenario for the study reported here. From this analysis we find that: (1) reducing the value for the vertical diffusion coefficient in CMAM to a more physically reasonable value results in the model better reproducing the measured profiles of long-lived species; (2) the existence of compact correlations among the constituents, as expected from independent measurements in the literature and from models, confirms the self-consistency of the MANTRA measurements; and (3) the 1998 measurements show structures in the chemical species profiles that can be associated with transport, adding to the growing evidence that the summertime stratosphere can be much more disturbed than anticipated. The mechanisms responsible for such disturbances need to be understood in order to assess the representativeness of the measurements and to isolate long-term trends.

Summertime stratospheric processes at northern mid-latitudes: comparisons between MANTRA balloon measurements and the Canadian Middle Atmosphere Model

In this paper we report on a study conducted using the Middle Atmospheric Nitrogen TRend Assessment (MANTRA) balloon measurements of stratospheric constituents and temperature and the Canadian Middle Atmosphere Model (CMAM) in order to evaluate the ability of the model to reproduce the measured fields and to thereby test our ability to describe mid-latitude summertime stratospheric processes. The MANTRA measurements used here are vertical profiles of ozone, temperature, N2O, CH4, HNO3, and HCl obtained during four campaigns, involving the launch of both ozonesondes and large balloons from Vanscoy, Saskatchewan, Canada (52° N, 107° W). The campaigns were conducted in August and September 1998, 2000, 2002 and 2004. During late summer at mid-latitudes, the stratosphere is close to photochemical control, providing an ideal scenario for the study reported here. From this analysis we found that: (1) reducing the value for the vertical diffusion coefficient in CMAM to a more physically reasonable value results in the model better reproducing the measured profiles of long-lived species; (2) the existence of compact correlations among the constituents, as expected from independent measurements in the literature and from models, confirms the self-consistency of the MANTRA measurements; and (3) the 1998 ozone measurements show a narrow layer of low ozone centered near 25 km that is consistent with fossil debris from the polar vortex, suggesting that localized springtime ozone anomalies can persist through summer, affecting ozone levels at mid-latitudes.