Thin Cloud Layers Research Articles

The algorithm of microphysical-parameter profiles of aerosol and small cloud droplets based on the dual-wavelength lidar data

Abstract. This study proposed an inversion method for atmospheric-aerosol or cloud microphysical parameters based on dual-wavelength lidar data. The matching characteristics between aerosol and cloud particle size distributions and gamma distributions were studied using aircraft observation data. The feasibility of the retrieval of the particle effective radius from lidar ratios and backscatter ratios was simulated and studied. A method for inverting the effective radius and number concentration of atmospheric aerosols or small cloud droplets using the backscatter ratio was proposed, and the error sources and applicability of the algorithm were analyzed. This algorithm was suitable for the inversion of uniformly mixed and single-property aerosol layers or small cloud droplets. Compared with the previous study, this algorithm could quickly obtain the microphysical parameters of atmospheric particles and has good robustness. For aerosol particles, the inversion range that this algorithm can achieve is 0.3–1.7 µm. For cloud droplets, it is 1.0–10 µm. An atmospheric-observation experiment was conducted using the multi-wavelength lidar developed by Xi'an University of Technology, and a thin cloud layer was captured. The microphysical parameters of aerosol and clouds during this process were retrieved. The results clearly demonstrate the growth of the effective radius and number concentration.

Investigation of the effective transmittance of Mach-Zehnder interferometer in the multi-longitudinal-mode high-spectral-resolution lidar

Effective transmittance is an important parameter of Mach-Zehnder interferometer in the retrieval of aerosol optical properties using the multi-longitudinal-mode high-spectral-resolution lidar, which is a function of the complex degree of coherence. In this paper, we retrieve the parameter of the complex degree of coherence for the multi-longitudinal-mode laser from the theoretical analysis, optical simulation and experimental measurements, which are 0.726, 0.678 and 0.453 using the Mach-Zehnder interferometer with the optimal optical path difference of two times of laser optical cavity length (∼1000 mm) and under the light illumination of the high power Nd:YAG laser with the mode number of 101 and mode interval of 300 MHz under the condition of laser linewidth of 1 cm−1 (30 GHz). The preliminary measurement results of aerosol optical properties using the constructing multi-longitudinal-mode high-spectral-resolution lidar show that the difference of the complex degree of coherence retrieved from the presented methods results in the relative error of 40% in the aerosol backscattering coefficient with thin cloud layer and 90% with the presence of dense aerosol layer.

Biases in the thermodynamic structure over the Southern Ocean in ERA5 and their radiative implications

AbstractThe thermodynamic structure of the lower troposphere in the 37 standard levels ERA5 reanalysis has been evaluated against 2,186 high‐resolution upper air soundings collected over the Southern Ocean (SO). The reanalysis, which incorporated these soundings, was found to be skilled in depicting the general synoptic meteorology and thermodynamic structure as defined by the cluster analysis of Truong et al. (2020) Journal of Geophysical Research: Atmospheres, 125, e2020JD033214. Using dew‐point depression as a proxy for cloud, however, we found a significant reduction in the number of inferred cloud layers, which is inherited from a bias in the specific humidity in the ERA5 reanalysis, most notably over the high latitudes of the SO, where a multilayer cloud structure is frequently observed. The reanalysis was also found to have thinner inferred cloud geometric layer and shallower cloud top heights. Further analysis showed that the reanalysis displays a greater percentage of soundings having no inversion with this bias being more pronounced at high latitudes that tends to be associated with the colder sea surface temperature. While the statistics of the main inversion height are largely consistent, the average inversion strength in the ERA5 reanalysis is found to be weaker than the observations. We anticipate the 137‐level ERA5 reanalysis simulation yields a smoothed vertical structure, from which the 37 standard levels ERA5 reanalysis is linearly interpolated. An examination of the sensitivity of the radiative transfer to cloud macrophysics suggests that the correct representation of thin multiple cloud layers can help reduce the amount of downward shortwave surface radiation over the SO.

Газопылевые структуры в окрестности рукавов спиральных галактик

A two-dimensional model of the flow of the gas-dust interstellar medium in the vicinity of the spiral arm of the galaxy is constructed. The flow in a vertical plane transverse to the disk plane is considered. The effects of nonadiabatic flow (volumetric heating and cooling of the gas by radiation) are taken into account. The balance of heating and cooling ensures the coexistence of two phases, cold parsec-sized clouds of atomic hydrogen and warm intercloud gas. The consideration includes polydisperse dust, represented by three fractions of particles of different sizes and masses. Dust particles have finite inertia, their movements do not exactly repeat the movement of gas. Turbulence in the disk and in the spiral arm is also taken into account. Models are considered that use different combinations of the location of turbulence sources in the disk and/or in the arm. The main results obtained by the methods of computer hydrodynamic modeling are as follows. Clouds undergo significant transformations as they pass through the spiral arm. A significant part of the clouds is absorbed into a thin dense cloud layer, which extends in a spiral arm along the equatorial plane in the vicinity of the center of the arm and has a size of approximately half the width of the arm. A smaller part of the clouds passes without destruction or with partial destruction through the sleeve, experiencing strong deformations along the way. The small-scale cloud component is partially restored under the action of turbulence, which perturbs the extended cloud layer inside the arm and partially destroys it into separate fragments. A wedge-shaped galactic shock wave is formed on the rear side of the arm with respect to the incoming gas flow, attached to the rear edge of the extended cloud layer. A flow limited by a shock wave has the character of a jet that performs quasi-periodic transverse oscillations. The reason for the oscillations, apparently, is the instability of the shear flow, since tangential discontinuities are formed inside the jet along the flow and at a small angle to the shock fronts. Dust particles are dragged by turbulent eddies and carried to heights of 150–200 pc above the disk plane, which naturally explains the existence of chaotic filamentous dust structures extending above the galactic disk to heights of several hundred parsecs. The grains of dust are distributed differently inside the vortices. Dust grains with sizes of 0.01-0.1 μm cluster more easily than larger dust grains with a radius of 1 μm. Turbulence serves as a mechanism to effectively trap dust particles on the front side of the spiral arm. Modeling shows that dust lanes are more pronounced on the front side of the arm.

A prolonged and widespread thin mid-level liquid cloud layer as observed by ground-based lidars, radiosonde and space-borne instruments

A ~ 50-h-lasting mid-level thin liquid cloud layer was present over south and central China on December 13–15, 2018. Its characteristics were revealed by the observations from a polarization lidar and a water vapor Raman lidar at a mid-latitude site together with two geostationary satellites and the space-borne lidar CALIOP, as well as the conventional radiosondes. This layer was optically thick with cloud top at ~3.5-km altitude and a thickness of ~0.7 km. It resided on a warm-front-related inversion layer with the cloud top extending slightly above the inversion top and having a temperature of ~ − 4 to 1 °C. We found that the relative humidity over water peaked (~82–98%) near the liquid cloud center where the inversion layer had the temperature minima (−7 to −5 °C). This suggests that the moisture condensation into liquid droplets proceeded around the temperature minima of the inversion layer. The drizzle-sized water drop virga (no ice) occurred beneath the thin liquid cloud layer only during the first 7 h of its presence over our lidar site. The warm cloud top temperature (−3 °C) and strong wind shear at the cloud altitudes might be responsible for its formation. The Raman lidar observed a discernible increase in the subcloud water vapor mixing ratio due to the evaporation of the water drop virga. Satellite data revealed that the thin cloud layer covered a huge area over southern and central China. We found that its cloud top altitude increased overall with increasing latitude. This is consistent with the well-known warm-front cloud structure. It is shown through examining ECMWF reanalysis data that in the presence of the thin liquid cloud layer, a tropical cyclone located in the Bengal bay was uplifting continuously a large amount of moisture from the Indian Ocean. The resultant mid-level warm moist air masses were transported to China by the Indian monsoon, and then dispersed to the entire southern and central China under the influence of the southern branch of the Westerlies. Our findings provide a new insight into the formation and persistence of thin mid-level supercooled liquid cloud layers.

Dual-Wavelength Polarimetric Lidar Observations of the Volcanic Ash Cloud Produced during the 2016 Etna Eruption

Lidar observations are very useful to analyse dispersed volcanic clouds in the troposphere mainly because of their high range resolution, providing morphological as well as microphysical (size and mass) properties. In this work, we analyse the volcanic cloud of 18 May 2016 at Mt. Etna, in Italy, retrieved by polarimetric dual-wavelength Lidar measurements. We use the AMPLE (Aerosol Multi-Wavelength Polarization Lidar Experiment) system, located in Catania, about 25 km from the Etna summit craters, pointing at a thin volcanic cloud layer, clearly visible and dispersed from the summit craters at the altitude between 2 and 4 km and 6 and 7 km above the sea level. Both the backscattering and linear depolarization profiles at 355 nm (UV, ultraviolet) and 532 nm (VIS, visible) wavelengths, respectively, were obtained using different angles at 20°, 30°, 40° and 90°. The proposed approach inverts the Lidar measurements with a physically based inversion methodology named Volcanic Ash Lidar Retrieval (VALR), based on Maximum-Likelihood (ML). VALRML can provide estimates of volcanic ash mean size and mass concentration at a resolution of few tens of meters. We also compared those results with two methods: Single-variate Regression (SR) and Multi-variate Regression (MR). SR uses the backscattering coefficient or backscattering and depolarization coefficients of one wavelength (UV or VIS in our cases). The MR method uses the backscattering coefficient of both wavelengths (UV and VIS). In absence of in situ airborne validation data, the discrepancy among the different retrieval techniques is estimated with respect to the VALR ML algorithm. The VALR ML analysis provides ash concentrations between about 0.1 μg/m3 and 1 mg/m3 and particle mean sizes of 0.1 μm and 6 μm, respectively. Results show that, for the SR method differences are less than <10%, using the backscattering coefficient only and backscattering and depolarization coefficients. Moreover, we find differences of 20–30% respect to VALR ML, considering well-known parametric retrieval methods. VALR algorithms show how a physics-based inversion approaches can effectively exploit the spectral-polarimetric Lidar AMPLE capability.

ATmospheric LIDar (ATLID): Pre-Launch Testing and Calibration of the European Space Agency Instrument That Will Measure Aerosols and Thin Clouds in the Atmosphere

ATLID (ATmospheric LIDar) is the atmospheric backscatter Light Detection and Ranging (LIDAR) instrument on board of the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) mission, the sixth Earth Explorer Mission of the European Space Agency (ESA) Living Planet Programme. ATLID’s purpose is to provide vertical profiles of optically thin cloud and aerosol layers, as well as the altitude of cloud boundaries, with a resolution of 100 m for altitudes of 0 to 20 km, and a resolution of 500 m for 20 km to 40 km. In order to achieve this objective ATLID emits short duration laser pulses in the ultraviolet, at a repetition rate of 51 Hz, while pointing in a near nadir direction along track of the satellite trajectory. The atmospheric backscatter signal is then collected by its 620 mm aperture telescope, filtered through the optics of the instrument focal plane assembly, in order to separate and measure the atmospheric Mie and Rayleigh scattering signals. With the completion of the full instrument assembly in 2019, ATLID has been subjected to an ambient performance test campaign, followed by a successful environmental qualification test campaign, including performance calibration and characterization in thermal vacuum conditions. In this paper the design and operational principle of ATLID is recalled and the major performance test results are presented, addressing the main key receiver and emitter characteristics. Finally, the estimated instrument, in-orbit, flight predictions are presented; these indicate compliance of the ALTID instrument performance against its specification and that it will meet its mission science objectives for the EarthCARE mission, to be launched in 2023.

Asian dust impacts on heterogeneous ice formation at Wuhan based on polarization lidar measurements

We analyzed 456 thin mid-level cloud layers based on two-year polarization lidar measurements in Wuhan (30.53°N, 114.37°E), China. Dust separation algorithm based on lidar measurements was implemented to identify clouds under influences of dust and non-dust components. Through comparisons of ice-containing cloud fractions under dust and dust-free conditions, significant impacts of Asian dust on heterogeneous ice formation are found. 20% more ice-containing cloud fractions occurred under influence of Asian dust at temperature range from −25 to −10 °C. More than 90% of dust-related heterogeneous ice formation were associated with polluted dust. In order to investigate the efficiency of non-dust components to serve as ice nuclei (IN), results at two continental sites (Wuhan, East Asia and Leipzig, Central Europe) with very different aerosol loadings were compared and discussed. Weak and similar ice nucleation characteristics under dust-free conditions of different loadings of sulfate, black carbon and organic carbon, may suggest such components are all less effective INs.

Falling Mixed-Phase Ice Virga and their Liquid Parent Cloud Layers as Observed by Ground-Based Lidars

Falling mixed-phase virga from a thin supercooled liquid layer cloud base were observed on 20 occasions at altitudes of 2.3–9.4 km with ground-based lidars at Wuhan (30.5 °N, 114.4 °E), China. Polarization lidar profile (3.75-m) analysis reveals some ubiquitous features of both falling mixed-phase virga and their liquid parent cloud layers. Each liquid parent cloud had a well-defined base height where the backscatter ratio R was ~7.0 and the R profile had a clear inflection point. At an altitude of ~34 m above the base height, the depolarization ratio reached its minimum value (~0.04), indicating a liquid-only level therein. The thin parent cloud layers tended to form on the top of a broad preexisting aerosol/liquid water layer. The falling virga below the base height showed firstly a significant depolarization ratio increase, suggesting that most supercooled liquid drops in the virga were rapidly frozen into ice crystals (via contact freezing). After reaching a local maximum value of the depolarization ratio, both the values of the backscatter ratio and depolarization ratio for the virga exhibited an overall decrease with decreasing height, indicating sublimated ice crystals. The diameters of the ice crystals in the virga were estimated based on an ice particle sublimation model along with the lidar and radiosonde observations. It was found that the ice crystal particles in these virga cases tended to have smaller mean diameters and narrower size distributions with increasing altitude. The mean diameter value is 350 ± 111 µm at altitudes of 4–8.5 km.

The diurnal cycle of the clouds extending above the tropical tropopause observed by spaceborne lidar

Abstract. The presence of clouds above the tropopause over tropical convection centers has so far been documented by spaceborne instruments that are either sun-synchronous or insensitive to thin cloud layers. Here we document, for the first time through direct observation by spaceborne lidar, how the tropical cloud fraction evolves above the tropopause throughout the day. After confirming previous studies that found such clouds most frequently above convection centers, we show that stratospheric clouds and their vertical extent above the tropopause follow a diurnal rhythm linked to convective activity. The diurnal cycle of the stratospheric clouds displays two maxima: one in the early night (19:00–20:00 LT) and a later one (00:00–01:00 LT). Stratospheric clouds extend up to 0.5–1 km above the tropopause during nighttime, when they are the most frequent. The frequency and the vertical extent of stratospheric clouds is very limited during daytime, and when present they are found very close to the tropopause. Results are similar over the major convection centers (Africa, South America and the Warm Pool), with more clouds above land in DJF (December–January–February) and less above the ocean and in JJA (June–July–August).

ATLID, ESA atmospheric backscatter LIDAR for the ESA EarthCARE mission

The ATmospheric LIDAR (Light Detection and Ranging), ATLID, is part of the payload of the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) mission, the sixth Earth Explorer Mission of the European Space Agency (ESA) Living Planet Programme (http://esamultimedia.esa.int/docs/SP_1279_1_EarthCARE.pdf). The EarthCARE payload consists of four instruments that will, in a synergetic manner, retrieve vertical profiles of clouds and aerosols, and the characteristics of the radiative and micro-physical properties, to determine flux gradients within the atmosphere and top of atmosphere radiance and flux. ATLID’s task is to provide vertical profiles of optically thin cloud and aerosol layers, as well as the altitude of cloud boundaries. With that purpose ATLID emits < 35 ns duration laser pulses with 40 mJ energy in the UV, at a repetition rate of 51 Hz, while pointing in a near nadir direction along track of the satellite trajectory. The backscatter signal is collected by a 620 mm aperture telescope and is then filtered and redirected through the optics of the instrument focal plane assembly, in such a way that the atmospheric Mie and Rayleigh scattering contributions are separated and independently measured. After the manufacturing, qualification and delivery of all ATLID units, the optical and electrical integration has been conducted in parallel to assemble the Optical Flight Model (OFM) and the Electrical Flight Model (EFM). These two models, precursor to the instrument integration, allowed the early execution of the first performance and functional tests. Following these initial verification activities, and with the latest integration of the flight laser cooling system, the instrument assembly approaches its final flight configuration, paving the way for the ambient performance and environmental test campaigns at full instrument level.

Dynamic Backscattering Halo of Pulsed Laser Beams in Thin Cloud Layers

We consider the effect of the dynamic backscattering halo of pencil beams of light in thin layers of scattering media. Numerical modeling, which immediately demonstrates this effect, is performed. A simple analytical theory that provides a numerical criterion for the effect visual detection, is proposed.

Remote Sensing of Lower Tropospheric Aerosols and Clouds over Islamabad Region Using a Self-Engineered Mie Scattering Lidar

Atmospheric changes in the lower troposphere have been remotely studied using a self-engineered Mie scattering lidar with special emphasis on aerosols and clouds profiling over Islamabad region in Pakistan. The lidar is based on a Nd:YAG laser operating at 1064 nm, with maximal energy of 350 mJ at 20-Hz repetition rate and 5-ns pulse length. A silicon avalanche photodiode (Si-APD, C30950E) module is used as a detector. A higher resolution of the lidar revealed time evolution of thermal transport phenomena in the convective boundary layer. Regions of incessant wind speed, temperature, and particulates concentration have been detected through band-like structures at altitudes above 900 m. Strong backscattering (β) and extinction (α) due to a partially invisible thin cloud layer falling in the field-of-view of the lidar beyond 4 km have been identified.

First Simultaneous Rocket and Radar Detections of Rare Low Summer Mesospheric Clouds

AbstractOn 30 June 2016 a layer of dust, possibly meteoric smoke particles (MSPs), was observed with a rocket borne probe at 69.29°N, 16.02°E and altitudes of ~74 km where patchy thin cloud layers, detected with the Middle Atmosphere Alomar Radar System, were present. The rocket traversed a layer with a net positive dust charge density of ~107 unit charges per cubic meters and a number density of neutral dust particles with sizes ≥4 nm of ~108 m−3. The positive charge density may require that elements that lower the photoelectric work function coat MSPs. The presence of this relatively large dust is consistent with smaller MSPs being swept out of the low mesospheric cloud region during the summer, while larger MSPs remain where their fall velocities equals the circulation updraught velocities. Large MSPs initially embedded in icy particles that subsequently sublimate may also fall until their fall velocities match the updraught velocities.

Large-Eddy Simulation of the Stratocumulus-Capped Boundary Layer with Explicit Filtering and Reconstruction Turbulence Modeling

Abstract Large-eddy simulation (LES) has been an essential tool in the development of theory and parameterizations for clouds, but when applied to stratocumulus clouds under sharp temperature inversions, many LES models produce an unrealistically thin cloud layer and a decoupled boundary layer structure. Here, explicit filtering and reconstruction are used for simulation of stratocumulus clouds observed during the first research flight (RF01) of the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMS II). The dynamic reconstruction model (DRM) is used within an explicit filtering and reconstruction framework, partitioning subfilter-scale motions into resolvable subfilter scales (RSFSs) and unresolvable subgrid scales (SGSs). The former are reconstructed, and the latter are modeled. Differing from traditional turbulence models, the reconstructed RSFS stress/fluxes can produce backscatter of turbulence kinetic energy (TKE) and, importantly, turbulence potential energy (TPE). The modeled backscatter reduces entrainment at the cloud top and, meanwhile, strengthens resolved turbulence through preserving TKE and TPE, resulting in a realistic boundary layer with an adequate amount of cloud water and vertically coupled turbulent eddies. Additional simulations are performed in the terra incognita, when the grid spacing of a simulation becomes comparable to the size of the most energetic eddies. With 20-m vertical and 1-km horizontal grid spacings, simulations using DRM provide a reasonable representation of bulk properties of the stratocumulus-capped boundary layer.

Single‐Column Model Simulations of Subtropical Marine Boundary‐Layer Cloud Transitions Under Weakening Inversions

AbstractResults are presented of the GASS/EUCLIPSE single‐column model intercomparison study on the subtropical marine low‐level cloud transition. A central goal is to establish the performance of state‐of‐the‐art boundary‐layer schemes for weather and climate models for this cloud regime, using large‐eddy simulations of the same scenes as a reference. A novelty is that the comparison covers four different cases instead of one, in order to broaden the covered parameter space. Three cases are situated in the North‐Eastern Pacific, while one reflects conditions in the North‐Eastern Atlantic. A set of variables is considered that reflects key aspects of the transition process, making use of simple metrics to establish the model performance. Using this method, some longstanding problems in low‐level cloud representation are identified. Considerable spread exists among models concerning the cloud amount, its vertical structure, and the associated impact on radiative transfer. The sign and amplitude of these biases differ somewhat per case, depending on how far the transition has progressed. After cloud breakup the ensemble median exhibits the well‐known “too few too bright” problem. The boundary‐layer deepening rate and its state of decoupling are both underestimated, while the representation of the thin capping cloud layer appears complicated by a lack of vertical resolution. Encouragingly, some models are successful in representing the full set of variables, in particular, the vertical structure and diurnal cycle of the cloud layer in transition. An intriguing result is that the median of the model ensemble performs best, inspiring a new approach in subgrid parameterization.

The first close‐up images of Jupiter's polar regions: Results from the Juno mission JunoCam instrument

AbstractDuring Juno's first perijove encounter, the JunoCam instrument acquired the first images of Jupiter's polar regions at 50–70 km spatial scale at low emission angles. Poleward of 64–68° planetocentric latitude, where Jupiter's east‐west banded structure breaks down, several types of discrete features appear on a darker background. Cyclonic oval features are clustered near both poles. Other oval‐shaped features are also present, ranging in size from 2000 km down to JunoCam's resolution limits. The largest and brightest features often have chaotic shapes. Two narrow linear features in the north, associated with an overlying haze feature, traverse tens of degrees of longitude. JunoCam also detected an optically thin cloud or haze layer past the northern nightside terminator estimated to be 58 ± 21 km (approximately three scale heights) above the main cloud deck. JunoCam will acquire polar images on every perijove, allowing us to track the state and evolution of longer‐lived features.

Microphysical Properties of Ice Crystal Precipitation and Surface-Generated Ice Crystals in a High Alpine Environment in Switzerland

AbstractDuring the Cloud and Aerosol Characterization Experiment (CLACE) 2013 field campaign at the High Altitude Research Station Jungfraujoch, Switzerland, optically thin pure ice clouds and ice crystal precipitation were measured using holographic and other in situ particle instruments. For cloud particles, particle images, positions in space, concentrations, and size distributions were obtained, allowing one to extract size distributions classified by ice crystal habit. Small ice crystals occurring under conditions with a vertically thin cloud layer above and a stratocumulus layer approximately 1 km below exhibit similar properties in size and crystal habits as Antarctic/Arctic diamond dust. Also, ice crystal precipitation stemming from midlevel clouds subsequent to the diamond dust event was observed with a larger fraction of ice crystal aggregates when compared with the diamond dust. In another event, particle size distributions could be derived from mostly irregular ice crystals and aggregates, which likely originated from surface processes. These particles show a high spatial and temporal variability, and it is noted that size and habit distributions have only a weak dependence on the particle number concentration. Larger ice crystal aggregates and rosette shapes of some hundred microns in maximum dimension could be sampled as a precipitating cirrostratus cloud passed the site. The individual size distributions for each habit agree well with lognormal distributions. Fitted parameters to the size distributions are presented along with the area-derived ice water content, and the size distributions are compared with other measurements of pure ice clouds made in the Arctic and Antarctic.

The potential of clear-sky carbon dioxide satellite retrievals

Abstract. Since the launch of the Greenhouse Gases Observing Satellite (GOSAT) in 2009, retrieval algorithms designed to infer the column-averaged dry-air mole fraction of carbon dioxide (XCO2) from hyperspectral near-infrared observations of reflected sunlight have been greatly improved. They now generally include the scattering effects of clouds and aerosols, as early work found that absorption-only retrievals, which neglected these effects, often incurred unacceptably large errors, even for scenes with optically thin cloud or aerosol layers. However, these “full-physics” retrievals tend to be computationally expensive and may incur biases from trying to deduce the properties of clouds and aerosols when there are none present. Additionally, algorithms are now available that can quickly and effectively identify and remove most scenes in which cloud or aerosol scattering plays a significant role. In this work, we test the hypothesis that non-scattering, or “clear-sky”, retrievals may perform as well as full-physics retrievals for sufficiently clear scenes. Clear-sky retrievals could potentially avoid errors and biases brought about by trying to infer properties of clouds and aerosols when none are present. Clear-sky retrievals are also desirable because they are orders of magnitude faster than full-physics retrievals. Here we use a simplified version of the Atmospheric Carbon Observations from Space (ACOS) XCO2 retrieval algorithm that does not include the scattering and absorption effects of clouds or aerosols. It was found that for simulated Orbiting Carbon Observatory-2 (OCO-2) measurements, the clear-sky retrieval had errors comparable to those of the full-physics retrieval. For real GOSAT data, the clear-sky retrieval had errors 0–20 % larger than the full-physics retrieval over land and errors roughly 20–35 % larger over ocean, depending on filtration level. In general, the clear-sky retrieval had XCO2 root-mean-square errors (RMSEs) of less than 2.0 ppm, relative to Total Carbon Column Observing Network (TCCON) measurements and a suite of CO2 models, when adequately filtered through the use of a custom genetic algorithm filtering system. These results imply that non-scattering XCO2 retrievals are potentially more useful than previous literature suggests, as the filtering methods we employ are able to remove measurements in which scattering can cause significant errors. Additionally, the computational benefits of non-scattering retrievals means they may be useful for certain applications that require large amounts of data but have less stringent error requirements.

Expansion of a Holepunch Cloud by a Gravity Wave Front

Abstract A holepunch cloud is a curious phenomenon where a disturbance in a thin cloud layer initiates an expanding circular hole of clear air. Usually triggered by the passage of aircraft, observations of these holes in clouds date back to the earliest days of aviation, but only recently has a holepunch cloud been simulated within a full-physics numerical model. These computations confirm that ice crystal growth through the Wegener–Bergeron–Findeisen process creates a small cloud-free region whose subsequent outward spread defines the holepunch. The mechanics behind this continued expansion, however, has yet to be definitively identified. In this article, the motion of the cloud edge is explained as a propagating gravity wave front. To support this idea, a hierarchy of three idealizations is analyzed: a full-physics numerical model, a fluid mechanical model with simplified moisture effects, and a conservation law analysis for front motion. The essence of the holepunch cloud is established to be a moist air layer that is unsaturated (clear) and weakly stratified within the hole but saturated (cloudy) and moist neutral outside of it. The cloud edge thus represents a barrier to the outward propagation of gravity waves within the clear air—the result of this collision is a wave front whose velocity determines the growth rate of the hole.