Kwang-Mog Lee

Role of the quasi-biennial oscillation in the transport of aerosols from the tropical stratospheric reservoir to midlatitudes

The temporal evolution of the stratospheric aerosol distribution in the tropical stratospheric reservoir after the eruption of Mount Pinatubo was observed from 1992 to 1995 by the HALOE instrument on the UARS satellite. Since the spatial gradient of aerosol loading is large at the boundaries of the tropical stratospheric reservoir due to the volcanic aerosols, the effect of the meridional circulation on the distribution is seen clearly. The mechanism for dispersal of aerosol in the lower stratosphere from the tropics into midlatitudes strongly depends on the phase of the equatorial zonal wind. The time-latitude crosssections of the normalized distribution of aerosol on isentropic surfaces are used to observe the equatorial variation as well as change in meridional dispersal during the quasibiennial period. Observed tropical stratospheric winds are used with a simple analytical dynamical model to examine transport processes of tracers from the tropics during several phases of the quasibiennial oscillation (QBO) from 1992 to 1995. The Lagrangian meridional circulation in the tropics is consistent with the vertical and meridional velocities correlated with the QBO in the zonal wind. We find that vertical motion plays a crucial role in vertical and subsequent meridional transport. The pattern of meridional divergence derived from the vertical velocity is closely related to the observed HALOE aerosol distributions and their temporal development in the equatorial region. The westerly (easterly) shear phase of the QBO is associated with sinking (rising) motions at the equator and subsequent poleward (equatorward) transport in the lower stratosphere.

On the secondary meridional circulation associated with the quasi-biennial oscillation

Abstract Concentrations and distributions of stratospheric aerosol, hydrogen fluoride and ozone from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) are used to investigate features associated with transport by the secondary meridional circulation induced by the quasi-biennial oscillation (QBO). The points of maxima in the divergence and convergence of the QBO-induced meridional velocity at the equator are identified from the meridional gradients of the tracers. Such points can be identified from the tracer fields in the westerly shear zones but not in the easterly shear zones. The temporal variation of tracer concentration at the equator is determined mainly by vertical advection, which is significantly larger during the westerly shear phase of the QBO than during the easterly shear phase, since the QBO-induced equatorial sinking motion amplifies the vertical gradient. Thus, the vertical advection associated with the secondary circulation has a stronger influence on the equatorial tracer variation during the westerly shear phase than during the easterly shear phase.

Intercomparison of ILAS and HALOE ozone at high latitudes

Atmospheric O 3 mixing ratios measured by ILAS and HALOE are compared for periods from March 25-31, 1997 in the NH and from November 20-24, 1996 and December 13-14, 1996 in the SH. Both instruments observe consistent vertical and horizontal structure in the highly variable polar vortex regions. In the ozone peak region, ILAS ozone values are smaller compared to HALOE values in both hemispheres, but the differences are less than 13% and 20% in 40-20 km altitude range, respectively, for the NH and the SH, which are both close to the sum of uncertainties of the two instruments. When validation is complete, ILAS data will be useful for the study of the chemistry and dynamics at high latitudes.

An analysis of HALOE observations in summer high latitudes using airmass trajectory and photochemical model calculations

Stratospheric ozone, HCl, NO, and NO2 observed by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) in late summer at high latitudes show nonuniform distributions in their volume mixing ratios along a latitude circle, while the mixing ratios of the long-lived tracers, such as CH4 and HF, are observed to be quite uniform. These anomalous distributions of the reactive gases are confined in approximately 20–35 km. It is found that in the summer high latitudes, the isosurfaces of long-lived tracers nearly coincide with isentropes. On the basis of meridional distributions of these species, it appears that the HALOE-observed features are the results of quasi-isentropic transport across latitude lines with a timescale shorter than the chemical relaxation times of those HALOE observed ozone and other gases. A spectral analysis of the United Kingdom Meteorological Office meridional wind data indicates the existence of episodically amplified wave 1 that is quasi-stationary in July and then slowly westward moving after mid-August 1992. A quasi-stationary wave 2 is also found to be important in late August. The isentropic trajectory calculations show that these planetary scale waves seem to pull air masses out of the polar region as far as 10°–15° latitude equatorward in a week. Midlatitude air is also pushed into the high summer latitude region. A photochemical box model is used to simulate the chemical evolution of HALOE-observed species along air parcel trajectories which are initialized at HALOE positions. Reasonable agreement is found in comparisons between coincident HALOE measurements and the model results in later days. The characteristic structures in HALOE-observed ozone and other gases in the low and middle stratosphere indicate that the chemical relaxation times taken for these species to relax to their new local mixing ratios under new sunlit conditions are comparable with or longer than the timescale of the meridional transport induced by waves in late summer.

Extinction coefficients and properties of Pinatubo aerosol determined from Halogen Occultation Experiment (HALOE) data

Relative extinction coefficients as a function of wavelength are determined for stratospheric aerosols from the Mount Pinatubo eruptions in June of 1991, using the Halogen Occultation Experiment (HALOE) data at latitudes 20°–55° in the northern hemisphere from November 1991 to February 1992. Extinction coefficients at each of the eight ELALOE channels are obtained from the ratio of two transmittance profiles of consecutive occultation measurements separated by 25° longitude, one loaded with aerosols larger than the other during the early stage of aerosol dispersion after the eruptions. These coefficients are compared to theoretical Mie calculation values. Composition and a single mode particle size distribution are derived as a function of altitude. The retrievals indicate that the weight percentage of H2SO4 for 45 occultation cases is larger than the equilibrium value by about 5 wt %, while the size distribution parameters are within the range of those measured in situ at Laramie, Wyoming.

Optical constants for Asian dust in midinfrared region

Asian dust aerosols are composed of quartz, plagioclase, K-feldspar, calcite, and phyllosilicates. The optical constants for mixtures of these minerals are important not only to understand the effects of Asian dust on climate but also to retrieve the properties of Asian dust. In this work, the optical constants for labradorite and orthoclase, representative minerals of plagioclase and K-feldspar, respectively, are determined for the spectral range of 500–2000 cm−1 using bidirectional reflectance data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer. Assuming an internal mixture of component minerals, the effective refractive indices for Asian dust are calculated using Bruggemans rule. The results are proposed as optical constants for Asian dust and differ from those for other dust aerosols, such as the refractive indices for “Saharan dust” derived from aerosol samples collected at Barbados. The imaginary refractive index for Asian dust is larger, indicating more absorptive, than the index for Saharan dust in the range of 1000–1300 cm−1. Using the optical constants derived in this study, the brightness temperatures of satellite measurements are simulated for typical loading scenarios of Asian dust aerosols. The simulated brightness temperatures exhibit a notable decrease with wave number in the region of 800–1000 cm−1. The results also corroborate the spectral features of the Atmospheric Infrared Sounder (AIRS) measurements for an Asian dust storm. AIRS brightness temperatures near 1223 cm−1, lower than the maximum near 830 cm−1, can also be simulated using the derived optical constants for Asian dust.

Line-by-line radiative transfer model for infrared spectrum of AERI

Infrared radiance spectra measured in space or on the ground have been used for many applications, such as the retrieval of atmospheric temperature and humidity profiles. The Korean Meteorological Administration (KMA) recently installed an Atmospheric Emitted Radiance Interferometer (AERI) system at the Korea Global Atmosphere Watch Center (36°32′N, 125°19′E) in Anmyondo to measure the downward radiance spectra on the ground. For further utilization of such interferometeric radiance measurements, an accurate line-by-line radiative transfer model is required. This study introduces a line-by-line radiative transfer model developed at Kyungpook National University (KNU_LBL) and presents comparisons of spectra simulated using the KNU_LBL model and measured by the AERI system, that is installed inside a secure container. When compared with the Atmospheric and Environmental Research (AER) radiative transfer codes, the KNU_LBL model provides nearly identical spectra for various model atmospheres. The simulated spectra are also in good agreement with the AERI spectra for clear sky conditions, and a further improvement is made when taking into account of the emissions and absorption by CO2 and H2O for the light path inside the container, even though the path is short.

Properties of Stratospheric Aerosol Estimated from HALOE Data

Extinction coefficients for stratospheric aerosols at 8 HALOE wavelengths are determined by comparing transmittances data for two adjacent solar occultation measurements, where one limb path is loaded with aerosols but the other path is free of aerosols. These extinction coefficients are used to infer the aerosol properties such as composition and size distribution parameters. Mie theory has been used to calculate the extinction coefficients, and a nonlinear least square method is applied to determine the aerosol properties. Sixteen cases are selected for the retrieval in southern hemisphere at latitudes from 21 to 48 degrees S for the period of 29 Mar - 31 May 1992. Retrieved size width ranges from 1.1 to 1.5 and radius ranges from 0.25 to 0.45 micrometer. These size parameters are within the ranges of in situ measurements at Laramie, Wyoming. Retrieved weight % of H2SO4 is larger than the equilibrium value by about 5 to approximately 10 weight %, similar to the results for northern hemisphere at latitudes 20 to 55 degree N for the period from Nov 1991 to Feb 1992.