Vertical Optical Depth Research Articles

Cirrus crystal fall velocity estimates using the Match method with ground-based lidars: first investigation through a case study

Abstract. Cirrus ice particle sedimentation velocity (vs) is one of the critical variables for the parameterization of cirrus properties in a global climate model (GCM). In this study a methodology to estimate cirrus properties, such as crystal mean fall speed, through successive lidar measurements is evaluated. This "Match" technique has been applied on cirrus cloud observations and then tested with measurements from two ground-based lidars located in the Mediterranean area. These systems, with similar instrumental characteristics, are installed at the Observatory of Haute Provence (OHP, 43.9° N, 5.7° E) in France and at Rome Tor Vergata (RTV, 41.8° N, 12.6° E) in Italy. At a distance of approximately 600 km, the two lidar stations have provided systematic measurements for several years and are along a typical direction of an air path. A test case of an upper tropospheric cirrus, observed over both sites during the night between 13 and 14 March 2008, has been selected and the feasibility of the Match-cirrus approach investigated through this case. The analysis through lidar principal parameters (vertical location, geometrical thickness and optical depth) reveals a case of a thin sub-visible cirrus (SVC) located around the tropopause. A first range of values for vs (1.4–1.9 cm s−1, consistent with simple-shaped small crystals) has been retrieved with a simplified approach (adiabatic transport and "frozen" microphysical conditions inside the cirrus). The backward trajectory analysis suggests a type of cirrus formed by large-scale transport processes (adiabatic cooling of moist air masses coming from the subtropical area around Mexico gulf), which is characterized by a long atmospheric lifetime and horizontal extension of several hundred km. The analysis of this case study reveals that many uncertainties reduce the confidence of the retrieved estimates of the crystal fall velocity. However, this paper allows for assessing the technique feasibility by identifying the main critical issues for future similar investigations. This study shows that such approach is feasible; however, the methodology should be improved and some directions have been suggested for future campaigns.

Radiative Transfer in Accretion Disks Under Irradiation of the Central Star

Radiative transfer in a geometrically thin accretion disk, which is irradiated by a central star, is examined under a non-gray frequency-dependent treatment. The emergent intensity as well as other radiative quantities were analytically obtained under the Eddington approximation. In the case where the irradiation heating dominates the viscous one, the emergent spectra generally have two components. One is a multi-color blackbody part, which is reprocessed light of the irradiation flux, with a lower peak-frequency than that of the central star. The other is a single blackbody part, which is scattered light, with the same peak-frequency as that of the central star. When the vertical optical depth of the disk is small, the emergent intensity decreases in the poleward direction, but increases in the edgeward direction (limb-brightning), due to a scattering effect of the irradiated light. The overall spectra were calculated for a passive disk around a white dwarf and a neutron star. They exhibit multi-color blackbody spectra modified by the scattering light in the high-frequency region.

Exposing Global Cloud Biases in the Community Atmosphere Model (CAM) Using Satellite Observations and Their Corresponding Instrument Simulators

Abstract Satellite observations and their corresponding instrument simulators are used to document global cloud biases in the Community Atmosphere Model (CAM) versions 4 and 5. The model–observation comparisons show that, despite having nearly identical cloud radiative forcing, CAM5 has a much more realistic representation of cloud properties than CAM4. In particular, CAM5 exhibits substantial improvement in three long-standing climate model cloud biases: 1) the underestimation of total cloud, 2) the overestimation of optically thick cloud, and 3) the underestimation of midlevel cloud. While the increased total cloud and decreased optically thick cloud in CAM5 result from improved physical process representation, the increased midlevel cloud in CAM5 results from the addition of radiatively active snow. Despite these improvements, both CAM versions have cloud deficiencies. Of particular concern, both models exhibit large but differing biases in the subtropical marine boundary layer cloud regimes that are known to explain intermodel differences in cloud feedbacks and climate sensitivity. More generally, this study demonstrates that simulator-facilitated evaluation of cloud properties, such as amount by vertical level and optical depth, can robustly expose large and at times radiatively compensating climate model cloud biases.

Modified technique for processing multiangle lidar data measured in clear and moderately polluted atmospheres

We present a modified technique for processing multiangle lidar data that is applicable for relatively clear atmospheres, where the utilization of the conventional Kano–Hamilton method meets significant issues. Our retrieval algorithm allows computing the two-way transmission and the corresponding extinction-coefficient profile in any slope direction searched during scanning. These parameters are obtained from the backscatter term of the Kano–Hamilton solution and the corresponding square-range-corrected signal; the second component of the solution, related with the vertical optical depth, is completely excluded from consideration. The inversion technique was used to process experimental data obtained with the Missoula Fire Sciences Laboratory lidar. Simulated and real experimental data are presented that illustrate the essentials of the data-processing technique and possible variants of the extinction-coefficient profile retrieval.

The atmospheric transparency measured with a LIDAR system at the Telescope Array experiment

An atmospheric transparency was measured using a LIDAR with a pulsed UV laser (355nm) at the observation site of Telescope Array in Utah, USA. The measurement at night for two years in $2007\sim 2009$ revealed that the extinction coefficient by aerosol at the ground level is $0.033^{+0.016}_{-0.012} \rm km^{-1}$ and the vertical aerosol optical depth at 5km above the ground is $0.035^{+0.019}_{-0.013}$. A model of the altitudinal aerosol distribution was built based on these measurements for the analysis of atmospheric attenuation of the fluorescence light generated by ultra high energy cosmic rays.

Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission II: Aerosol extinction profiles in the 600–1420 cm −1 spectral range

We have analyzed the continuum emission of limb spectra acquired by the Cassini/CIRS infrared spectrometer in order to derive information on haze extinction in the 3–0.02 mbar range (∼150–350 km). We focused on the 600–1420 cm −1 spectral range and studied nine different limb observations acquired during the Cassini nominal mission at 55°S, 20°S, 5°N, 30°N, 40°N, 45°N, 55°N, 70°N and 80°N. By means of an inversion algorithm solving the radiative transfer equation, we derived the vertical profiles of haze extinction coefficients from 17 spectral ranges of 20-cm −1 wide at each of the nine latitudes. At a given latitude, all extinction vertical profiles retrieved from various spectral intervals between 600 and 1120 cm −1 display similar vertical slopes implying similar spectral characteristics of the material at all altitudes. We calculated a mean vertical extinction profile for each latitude and derived the ratio of the haze scale height ( H haze ) to the pressure scale height ( H gas ) as a function of altitude. We inferred H haze / H gas values varying from 0.8 to 2.4. The aerosol scale height varies with altitude and also with latitude. Overall, the haze extinction does not show strong latitudinal variations but, at 1 mbar, an increase by a factor of 1.5 is observed at the north pole compared to high southern latitudes. The vertical optical depths at 0.5 and 1.7 mbar increase from 55°S to 5°N, remain constant between 5°N and 30°N and display little variation at higher latitudes, except the presence of a slight local maximum at 45°N. The spectral dependence of the haze vertical optical depth is uniform with latitude and displays three main spectral features centered at 630 cm −1, 745 cm −1 and 1390 cm −1, the latter showing a wide tail extending down to ∼1000 cm −1. From 600 to 750 cm −1, the optical depth increases by a factor of 3 in contrast with the absorbance of laboratory tholins, which is generally constant. We derived the mass mixing ratio profiles of haze at the nine latitudes. Below the 0.4-mbar level all mass mixing ratio profiles increase with height. Above this pressure level, the profiles at 40°N, 45°N, 55°N, at the edge of the polar vortex, display a decrease-with-height whereas the other profiles increase. The global increase with height of the haze mass mixing ratio suggest a source at high altitudes and a sink at low altitudes. An enrichment of haze is observed at 0.1 mbar around the equator, which could be due to a more efficient photochemistry because of the strongest insolation there or an accumulation of haze due to a balance between sedimentation and upward vertical drag.

Extending differential optical absorption spectroscopy for limb measurements in the UV

Abstract. Methods of UV/VIS absorption spectroscopy to determine the constituents in the Earth's atmosphere from measurements of scattered light are often based on the Beer-Lambert law, like e.g. Differential Optical Absorption Spectroscopy (DOAS). While the Beer-Lambert law is strictly valid for a single light path only, the relation between the optical depth and the concentration of any absorber can be approximated as linear also for scattered light observations at a single wavelength if the absorption is weak. If the light path distribution is approximated not to vary with wavelength, also linearity between the optical depth and the product of the cross-section and the concentration of an absorber can be assumed. These assumptions are widely made for DOAS applications for scattered light observations. For medium and strong absorption of scattered light (e.g. along very long light-paths like in limb geometry) the relation between the optical depth and the concentration of an absorber is no longer linear. In addition, for broad wavelength intervals the differences in the travelled light-paths at different wavelengths become important, especially in the UV, where the probability for scattering increases strongly with decreasing wavelength. However, the DOAS method can be extended to cases with medium to strong absorptions and for broader wavelength intervals by the so called air mass factor modified (or extended) DOAS and the weighting function modified DOAS. These approaches take into account the wavelength dependency of the slant column densities (SCDs), but also require a priori knowledge for the air mass factor or the weighting function from radiative transfer modelling. We describe an approach that considers the fitting results obtained from DOAS, the SCDs, as a function of wavelength and vertical optical depth and expands this function into a Taylor series of both quantities. The Taylor coefficients are then applied as additional fitting parameters in the DOAS analysis. Thus the variability of the SCD in the fit window is determined by the retrieval itself. This new approach provides a description of the SCD the exactness of which depends on the order of the Taylor expansion, and is independent from any assumptions or a priori knowledge of the considered absorbers. In case studies of simulated and measured spectra in the UV range (332–357 nm), we demonstrate the improvement by this approach for the retrieval of vertical profiles of BrO from the SCIAMACHY limb observations. The results for BrO obtained from the simulated spectra are closer to the true profiles, when applying the new method for the SCDs of ozone, than when the standard DOAS approach is used. For the measured spectra the agreement with validation measurements is also improved significantly, especially for cases with strong ozone absorption. While the focus of this article is on the improvement of the BrO profile retrieval from the SCIAMACHY limb measurements, the novel approach may be applied to a wide range of DOAS retrievals.

Evolution of the stratospheric aerosol enhancement following the eruptions of Okmok and Kasatochi: Odin‐OSIRIS measurements

Limb‐scattered sunlight measurements made by the OSIRIS instrument on the Odin spacecraft during the months following the eruptions of Mount Okmok and Kasatochi volcanos in 2008 are used to retrieve vertical profiles of the stratospheric aerosol extinction. These measurements reveal the evolution of the formation, transport, and decay of the stratospheric aerosol cloud that forms as a result of the volcanic eruptions. The maximum vertical optical depth of the stratospheric aerosol, measured by OSIRIS at 750 nm, occurs at midlatitude to high latitude 4 to 6 weeks after the Kasatochi eruption and is approximately a factor of 2 above the background level. Significant meridional transport in the lower stratosphere is evident in the observations, with measurable enhancement extending up to 21 km altitude and reaching tropical latitudes. Seven months following the eruption of Kasatochi, the stratospheric aerosol optical depth is very near the background level measured before the eruptions.

라이다 시스템을 이용한 ice-crystal cloud의 광학적 특성 관측 및 복사 전달 모델을 통한 복사강제력 산출

Ice-crystal clouds observation was conducted using a GIST/ADEMRC Multi-wavelength Raman lidar system in order to measure vertical profile and optical depth at Gwangju (<TEX>$35^{\circ}$</TEX>10'N, <TEX>$126^{\circ}$</TEX>53'E), Korea in December 2002, and March and April 2003. Ice-crystal clouds at high altitude can be distinguished from atmospheric aerosols by high depolarization ratio and high altitude. Ice-crystal clouds were observed at 5~12 km altitudes with a high depolarization ratio from 0.2 to 0.5. Optical depth of ice-crystal clouds had varied from 0.14 to 1.81. The radiative effect of observed ice-crystal cloud on climate system was estimated to be negative net flux in short wavelength (0.25~<TEX>$4.0{\mu}m$</TEX>) and positive net flux in short+long wavelength (0.25~<TEX>$100{\mu}m$</TEX>) at top of the atmosphere. Net flux by ice-crys tal cloud per unit optical depth was comparable to that of Asian dust.

Seasonal evolution of the tropospheric aerosol vertical profile in the central Mediterranean and role of desert dust

The seasonal evolution of the aerosol vertical distribution in the Central Mediterranean is studied using measurements made in the period 1999–2008 at Lampedusa with an aerosol Lidar and a multi filter rotating shadowband radiometer (MFRSR). Measurements show that the aerosol vertical distribution is largely influenced by Saharan dust, which produces a strong annual cycle both in aerosol vertical extension and optical depth. Dust layers are present in the profile in 38% of the cases throughout the year, and in 57% in summer. The dust top altitude peaks in late spring, up to 9 km. The monthly average optical depth at 500 nm for dust cases shows a main peak in July (0.38), and values exceeding 0.2 throughout March–September. Conversely, non‐dust cases show a very limited seasonality, both in vertical distribution and aerosol optical depth. The monthly average optical depth for non‐dust cases is smaller than 0.17 throughout the year. During winter, the vertical distribution and optical depth are very similar for both dust and non‐dust cases. The seasonal average extinction coefficient profiles for dust and non‐dust cases show remarkable differences in spring and summer, when values of the extinction coefficient exceed 0.5 × 10−4 m−1 throughout the altitude range 0–4.5 km for dust cases, and 0–1 km altitude for non‐dust cases, respectively. Estimates of the Lidar Ratio are derived by combining Lidar and MFRSR measurements. The average Lidar Ratio at 532 nm is about 30 sr.

Solar infrared occultation observations by SPICAM experiment on Mars-Express: Simultaneous measurements of the vertical distributions of H 2O, CO 2 and aerosol

The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1–1.7 μm with a spectral resolution of ∼3.5 cm −1. We report results from 24 orbits obtained during MY28 at L s 130°–160°, and the latitude range of 40°–55° N. For these orbits the atmospheric density from 1.43 μm CO 2 band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H 2O ice allows to retrieve particle size ( r eff ∼ 0.5 – 1 μm ) and number density (∼1 cm −3 at 15–30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°–50° E, where high-altitude clouds at 50–60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius r eff = 0.1 – 0.3 μm , number density N ∼ 10 cm − 3 ) distributed with variance ν eff = 0.1 – 0.2 μm . The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO 2 band in the altitude range of 10–90 km, and H 2O mixing ratio is determined at 15–50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H 2O mixing ratio indicates ∼5 K warmer atmosphere at 25–45 km than predicted by models.

Effect of clouds on the calculated vertical distribution of shortwave absorption in the tropics

High vertical resolution profiles of cloud properties were obtained from cloud radars operated by the Atmospheric Radiation Measurement (ARM) program on the islands of Nauru and Manus in the Tropical Western Pacific (TWP). Broadband flux calculations using a correlated k‐distribution model were performed to estimate the effect of clouds on the total column and vertical distribution of shortwave absorption at these tropical sites. Sensitivity studies were performed to examine the role of precipitable water vapor, cloud vertical location, optical depth, and particle size on the shortwave (SW) column absorption. On average, observed clouds had little impact on the calculated total SW column absorption at the two sites, but a significant impact on the vertical distribution of SW absorption. Differences in the column amount, vertical profiles, and diurnal cycle of SW absorption at the two sites were due primarily to differences in cirrus cloud frequency.

A Ring of Warm Dust in the HD 32297 Debris Disk

We report the detection of a ring of warm dust in the edge-on disk surrounding HD 32297 with the Gemini-N/MICHELLE mid-infrared imager. Our N'-band image shows elongated structure consistent with the orientation of the scattered-light disk. The Fnu(11.2 um) = 49.9+/-2.1 mJy flux is significantly above the 28.2+/-0.6 mJy photosphere. Subtraction of the stellar point spread function reveals a bilobed structure with peaks 0.5-0.6 from the star. An analysis of the stellar component of the SED suggests a spectral type later than A0, in contrast to commonly cited literature values. We fit three-dimensional, single-size grain models of an optically thin dust ring to our image and the SED using a Markov chain Monte Carlo algorithm in a Bayesian framework. The best-fit effective grain sizes are submicron, suggesting the same dust population is responsible for the bulk of the scattered light. The inner boundary of the warm dust is located 0.5-0.7 (~65 AU) from the star, which is approximately cospatial with the outer boundary of the scattered-light asymmetry inward of 0.5. The addition of a separate component of larger, cooler grains that provide a portion of the 60 um flux improves both the fidelity of the model fit and consistency with the slopes of the scattered-light brightness profiles. Previous indirect estimates of the stellar age (~30 Myr) indicate the dust is composed of debris. The peak vertical optical depths in our models (~0.3-1 x 1e-2) imply that grain-grain collisions likely play a significant role in dust dynamics and evolution. Submicron grains can survive radiation pressure blow-out if they are icy and porous. Similarly, the inferred warm temperatures (130-200 K) suggest that ice sublimation may play a role in truncating the inner disk.

Dust Dynamics, Surface Brightness Profiles, and Thermal Spectra of Debris Disks: The Case of AU Microscopii

AU Microscopii is a 12 Myr old M dwarf that harbors an optically thin, edge-on disk of dust. The scattered light surface brightness falls with projected distance b from the star as b-α; within b = 43 AU, α ≈ 1-2, while outside 43 AU, α ≈ 4-5. We devise a theory to explain this profile. At a stellocentric distance r = rBR = 43 AU, we posit a ring of parent bodies on circular orbits: the ``birth ring,'' wherein micron-sized grains are born from the collisional attrition of parent bodies. The ``inner disk'' at r rBR comprises grains just large enough to remain bound to the star, on orbits rendered highly eccentric by stellar wind and radiation pressure. How the vertical optical depth τ⊥ scales with r depends on the fraction of grains that migrate inward by CPR drag without suffering a collision. If this fraction is large, the inner disk and birth ring share the same optical depth, and τ⊥ r-5/2 in the outer disk. By contrast, under collision-dominated conditions, the inner disk is empty, and τ⊥ r-3/2 outside. These scaling relations, which we derive analytically and confirm numerically, are robust against uncertainties in the grain size distribution. By simultaneously modeling the surface brightness and thermal spectrum, we break model degeneracies to establish that the AU Mic system is collision dominated and that its narrow birth ring contains a lunar mass of decimeter-sized bodies. The inner disk is devoid of micron-sized grains; the surface brightness at b 43 AU arises from light forward scattered by the birth ring. Inside b = 43 AU, the disk's V - H color should not vary with b; outside, the disk must become bluer as ever smaller grains are probed.

Techniques for measuring atmospheric aerosols at the high resolution fly’s eye experiment

We describe several techniques developed by the high resolution fly’s eye experiment for measuring aerosol vertical optical depth, aerosol horizontal attenuation length, and aerosol phase function. The techniques are based on measurements of side-scattered light generated by a steerable ultraviolet laser and collected by an optical detector designed to measure fluorescence light from cosmic-ray air showers. We also present a technique to cross-check the aerosol optical depth measurement using air showers observed in stereo. These methods can be used by future air fluorescence experiments.

Climatology of the subvisual cirrus clouds as seen by OSIRIS on Odin

The InfraRed Imager (IRI) subsection of the OSIRIS instrument onboard the Odin spacecraft collects limb images of the 1.53 μm sunlight that is scattered from the upper troposphere and lower stratosphere. Due to the enhancement provided by limb geometry, the IRI is capable of detecting cloud layers with vertical equivalent optical depths as low as 10 −5. These measurements can be used to map subvisual cirrus clouds present in the upper troposphere. The dusk–dawn orbit of the Odin spacecraft provides sunlit coverage of the equatorial region throughout the year. This work presents the subvisual cirrus cloud climatology that has been measured by the InfraRed Imager. The global seasonal distribution of sub-visible clouds detected by OSIRIS is similar to that measured previously by other instruments, but is obtained on a shorter timescale due to the sampling characteristics of the limb scatter data.

Total Ozone Mapping Spectrometer (TOMS) observations of increases in Asian aerosol in winter from 1979 to 2000

Emission inventories indicate that the largest increases in SO2 emissions have occurred in Asia during the last 20 years. By inference, the largest increases in aerosol, produced primarily by the conversion of SO2 to sulfate, should have occurred in Asia during the same time period. Decadal changes in regional aerosol optical depths are calculated by analyzing Total Ozone Mapping Spectrometer (TOMS) vertical aerosol optical depths (converted to 550 nm) from 1979 to 2000 on a 1° by 1° global grid. The anthropogenic component of the TOMS aerosol record is maximized by examining the seasonal cycles of desert dust and boreal fire smoke and identifying the months of the year for which the desert dust and boreal fire smoke are least conspicuous. Gobi and Taklimakan desert dust in Asia is prevalent in the TOMS record during spring, and eastern Siberian smoke from boreal forest fires is prevalent during summer. Aerosol trends are calculated on a regional basis during winter (November–February) to maximize the anthropogenic component of the aerosol record. Large increases in aerosol optical depths between 1979 and 2000 are present over the China coastal plain and the Ganges River basin in India. Aerosol increased by 17% per decade during winter over the China coastal plain, while SO2 emissions over the same geographical region increased by 35% per decade.

The effect of dust settling on the appearance of protoplanetary disks

We analyze how the process of dust settling affects the spectral energy distribution and optical appearance of protoplanetary disks. Using simple analytic estimates on the one hand, and detailed 1+1-D models on the other hand, we show that, while the time scale for settling down to the equator may exceed the life time of the disk, it takes much less time for even small grains of 0.1 μm to settle down to a few pressure scale heights. This is often well below the original location of the disk's photosphere, and the disk therefore becomes effectively “flatter”. If turbulent stirring is included, a steady state solution can be found, which is typically reached after a few 105 years. In this state, the downward settling motion of the dust is balanced by vertical stirring. Dependent on the strength of the turbulence, the shape of the disk in such a steady state can be either fully flaring, or flaring only up to a certain radius and self-shadowed beyond that radius. These geometries are similar to the geometries that were found for disks around Herbig Ae/Be stars in our previous papers (Dullemond [CITE] A&A, 395, 853; Dullemond & Dominik 2004, A&A, 417, 159, henceforth DD04). In those papers, however, the reason for a disk to turn self-shadowed was by loss of optical depth through dust grain growth. Here we show that dust settling can achieve a similar effect without loss of vertical optical depth, although the self-shadowing in this case only affects the outer regions of the disk, while in DD04 the entire disk outside of the puffed-up inner rim was shadowed. In reality it is likely that both grain growth and grain settling act simultaneously. The spectral energy distributions of such self-shadowed – or partly self-shadowed – disks have a relatively weak far-infrared excess (in comparison to flaring disks). We show here that, when dust settling is the cause of self-shadowing, these self-shadowed regions of the disk are also very weak in resolved images of scattered light. A reduction in the brightness was already predicted in DD04, but we find that dust settling is far more efficient than grain growth at dimming the scattered light image of the disk. Settling is also efficient in steepening the spectral energy distribution at mid- to far-infrared wavelengths. From the calculations with compact grains it follows that, after about 106 years, most disks should be self-shadowed. The fact that some older disks are still observed with the characteristics of flaring disks therefore seems somewhat inconsistent with the time scales predicted by the settling model based on compact grains. This suggests that perhaps even the small grains (m) have a porous or fractal structure, slowing down the settling. Alternatively, it could mean that the different geometries of observed disks is merely a reflection of the turbulent state of these disks.

Radiative Transfer in the Refractive Atmospheres of Very Cool White Dwarfs

We consider the problem of radiative transfer in stellar atmospheres where the index of refraction departs from unity and is a function of density and temperature. We present modified Feautrier and Λ-iteration methods to solve the equation of radiative transfer with refraction in a plane-parallel atmosphere. These methods are general and can be used in any problem with one-dimensional geometry where the index of refraction is a monotonically varying function of vertical optical depth. We present an application to very cool white dwarf atmospheres in which the index of refraction departs significantly from unity. We investigate how ray curvature and total internal reflection affect the limb darkening and the pressure-temperature structure of the atmosphere. Refraction results in a much weakened limb-darkening effect. We find that through the constraint of radiative equilibrium, total internal reflection warms the white dwarf atmosphere near the surface (τ ≲ 1). This effect may have a significant impact on studies of very cool white dwarf stars.

LIDAR measurements of the vertical aerosol profile and optical depth during the ACE-Asia 2001 IOP, at Gosan, Jeju Island, Korea.

In order to investigate the characteristic of optical properties of Asian dust particles, the atmospheric aerosol vertical profile was measured with the multi-wavelength LIDAR system, at the Gosan super site (33 degrees 17'N, 126 degrees 10'E) in Jeju Island, Korea, during the ACE-Asia intensive observation period, 11 March-4 May 2001. An air mass backward trajectory analysis, using Hysplit-4, was carried out to track the aerosol plume, with high mass loading, from the Chinese desert regions during the period of Asian dust storm events. Vertical atmospheric aerosol profiles on three major Asian dust storm event days, 22 March and 13 and 26 April 2001, have been analyzed. The LIDAR-derived aerosol optical depth values were compared with those measured by a collocated sunphotometer.