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For the first time, geometrical, optical and radiative properties of cirrus clouds, measured with a Polarized Micro Pulse Lidar (P-MPL) in Barcelona, have been analysed. For this purpose, the identification of cirrus scenes and the application of the two-way transmittance method have been applied to P-MPL products, published in the website of Micro Pulse Lidar NETwork (MPLNET), along with radiosondes from 2019-2021. The radiative properties of cirrus scenes have been determined through the use of an ensemble scattering model for cirrus clouds and their radiative effects have been calculated with GAME (Global Atmospheric ModEl) model. The results show that the highest occurrence of cirrus is in spring, the cirrus being in 81% of the cases either visible or opaque. Most cirrus have a COD between 0.1-0.2 (49%), LR around 30 sr (53%), linear cloud depolarization ratio between 0.3-0.5 (50%) and IWC lower than 0.01g/m3 (80%). Also, the net radiative effects of 4 selected cirrus case studies were found to be positive at both daytime and nighttime., Peer Reviewed, Postprint (published version)

During spring 2020, the COVID-19 pandemic caused massive reductions in emissions from industry and ground and airborne transportation. To explore the resulting atmospheric composition changes, we conducted the BLUESKY campaign with two research aircraft and measured trace gases, aerosols, and cloud properties from the boundary layer to the lower stratosphere. From 16 May to 9 June 2020, we performed 20 flights in the early COVID-19 lockdown phase over Europe and the Atlantic Ocean. We found up to 50% reductions in boundary layer nitrogen dioxide concentrations in urban areas from GOME-2B satellite data, along with carbon monoxide reductions in the pollution hot spots. We measured 20%-70% reductions in total reactive nitrogen, carbon monoxide, and fine mode aerosol concentration in profiles over German cities compared to a 10-yr dataset from passenger aircraft. The total aerosol mass was significantly reduced below 5 km altitude, and the organic aerosol fraction also aloft, indicative of decreased organic precursor gas emissions. The reduced aerosol optical thickness caused a perceptible shift in sky color toward the blue part of the spectrum (hence BLUESKY) and increased shortwave radiation at the surface. We find that the 80% decline in air traffic led to substantial reductions in nitrogen oxides at cruise altitudes, in contrail cover, and in resulting radiative forcing. The light extinction and depolarization by cirrus were also reduced in regions with substantially decreased air traffic. General circulation-chemistry model simulations indicate good agreement with the measurements when applying a reduced emission scenario. The comprehensive BLUESKY dataset documents the major impact of anthropogenic emissions on the atmospheric composition., Aircraft Noise and Climate Effects

Prominent cirrus cloud banding occurred episodically within a northern cirrus canopy of Typhoon Talim (2017) during its recurvature. The generation mechanisms of the cirrus bands and low-static-stability layers that support the bands are investigated using a numerical simulation with the Advanced Research version of the Weather Research and Forecasting Model. Inspection of model output reveals that thin layers of near-neutral to weakly unstable static stability are persistently present in the upper and lower parts of the upper-level outflow, and shallow convection aligned along the vertical shear vector is prevalent in these layers. The cirrus banding occurs as the lowered outflow from the weakening storm ascends slantwise over a midlatitude baroclinic zone, and updrafts of the preexisting shallow convection in the upper part of the outflow layer become saturated. It is shown that the strong outflow resulting from violation of gradient-wind balance in the core region, by itself, creates the low-static-stability layers. Analyses of potential temperature and static stability budgets show that the low-static-stability layers are created mainly by the differential radial advection of radial thermal gradients on the vertical edges of the outflow. The radial thermal gradients occur in response to the outward air parcel acceleration in the core region and deceleration in the outer region, which, by inducing compensating vertical mass transport into and out of the outflow, act to tilt the isentropes within the shear layers. The effects of environmental flow and cloud-radiative forcing on the cirrus banding are also addressed.

Prominent cirrus cloud banding occurred episodically within a northern cirrus canopy of Typhoon Talim (2017) during its recurvature. The generation mechanisms of the cirrus bands and low-static-stability layers that support the bands are investigated using a numerical simulation with the Advanced Research version of the Weather Research and Forecasting Model. Inspection of model output reveals that thin layers of near-neutral to weakly unstable static stability are persistently present in the upper and lower parts of the upper-level outflow, and shallow convection aligned along the vertical shear vector is prevalent in these layers. The cirrus banding occurs as the lowered outflow from the weakening storm ascends slantwise over a midlatitude baroclinic zone, and updrafts of the preexisting shallow convection in the upper part of the outflow layer become saturated. It is shown that the strong outflow resulting from violation of gradient-wind balance in the core region, by itself, creates the low-static-stability layers. Analyses of potential temperature and static stability budgets show that the low-static-stability layers are created mainly by the differential radial advection of radial thermal gradients on the vertical edges of the outflow. The radial thermal gradients occur in response to the outward air parcel acceleration in the core region and deceleration in the outer region, which, by inducing compensating vertical mass transport into and out of the outflow, act to tilt the isentropes within the shear layers. The effects of environmental flow and cloud-radiative forcing on the cirrus banding are also addressed.

Airborne mineral dust influences cloud occurrence and optical properties, which may provide a pathway for recent and future changes in dust concentration to alter the temperature at Earth's surface. However, despite prior suggestions that dust-cloud interactions are an important control on the Earth's radiation balance, we find global mean cloud radiative effects to be insensitive to widespread dust changes. Here we simulate uniformly applied shifts in dust amount in a present-day atmosphere using a version of the CAM5 atmosphere model (within CESM v1.2.2) modified to incorporate laboratory-based ice nucleation parameterizations in stratiform clouds. Increasing and decreasing dustiness from current levels to paleoclimate extremes caused effective radiative forcings through clouds of +0.02 +/- 0.01 and -0.05 +/- 0.02 W/m(2), respectively, with ranges of -0.26 to +0.13 W/m(2) and -0.21 to +0.39 W/m(2) from sensitivity tests. Our simulations suggest that these forcings are limited by several factors. Longwave and shortwave impacts largely cancel, particularly in mixed-phase clouds, while in warm and cirrus clouds opposite responses between regions further reduce each global forcing. Additionally, changes in dustiness cause opposite forcings through aerosol indirect effects in mixed-phase clouds as in cirrus, while in warm clouds indirect effects are weak at nearly all locations. Nevertheless, regional forcings and global impacts on longwave and shortwave radiation were found to be nonnegligible, suggesting that cloud-mediated dust effects have significance in simulations of present and future climate.

Parametrizations of Arctic cirrus particle size distributions are presented depending on the cloud origin, either in situ or liquid. The data set used originate from balloon‐borne measurements above Kiruna during winter. The observed particle size distributions are represented by gamma functions. The gamma coefficients exhibit large differences with regard to cloud origin. The functions for Arctic cirrus confirm established parametrizations for continental cirrus sorted by two particle size modes but differ from others depending only on temperature. We suppose that the agreement between the parametrizations of the geographically different cirrus is because in situ and liquid origin cirrus also distinguish by particle size modes. Since cloud sorting by their origin is based on physical processes which are independent of geographical region, we further hypothesize that these cloud‐type‐based parametrizations might be generally valid for use in global models and satellite retrievals, given the distribution of the cloud types is known., Validerad;2019;Nivå 2;2019-12-09 (johcin);Artikeln har tidigare förekommit som manuskript i avhandling.

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 34 (2017): 355-373, doi:10.1175/JTECH-D-15-0226.1., Passive longwave infrared radiometric satellite–based retrievals of sea surface temperature (SST) at instrument nadir are investigated for cold bias caused by unscreened optically thin cirrus (OTC) clouds [cloud optical depth (COD) ≤ 0.3]. Level 2 nonlinear SST (NLSST) retrievals over tropical oceans (30°S–30°N) from Moderate Resolution Imaging Spectroradiometer (MODIS) radiances collected aboard the NASA Aqua satellite (Aqua-MODIS) are collocated with cloud profiles from the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument. OTC clouds are present in approximately 25% of tropical quality-assured (QA) Aqua-MODIS Level 2 data, representing over 99% of all contaminating cirrus found. Cold-biased NLSST (MODIS, AVHRR, and VIIRS) and triple-window (AVHRR and VIIRS only) SST retrievals are modeled based on operational algorithms using radiative transfer model simulations conducted with a hypothetical 1.5-km-thick OTC cloud placed incrementally from 10.0 to 18.0 km above mean sea level for cloud optical depths between 0.0 and 0.3. Corresponding cold bias estimates for each sensor are estimated using relative Aqua-MODIS cloud contamination frequencies as a function of cloud-top height and COD (assuming they are consistent across each platform) integrated within each corresponding modeled cold bias matrix. NLSST relative OTC cold biases, for any single observation, range from 0.33° to 0.55°C for the three sensors, with an absolute (bulk mean) bias between 0.09° and 0.14°C. Triple-window retrievals are more resilient, ranging from 0.08° to 0.14°C relative and from 0.02° to 0.04°C absolute. Cold biases are constant across the Pacific and Indian Oceans. Absolute bias is lower over the Atlantic but relative bias is higher, indicating that this issue persists globally., Authors JWM, NJS, and JZ acknowledge the support of NASA Project NNX14AJ13G andNSF Project IIA-1355466.Author JZ also acknowledges the support of ONR N00014-16-1-2040 (Grant 11843919). Author JWM further recognizes the Naval Research Enterprise Internship Program (NREIP). Support for his NREIP fellowship came from NASA Interagency Agreement NNG15JA17P on behalf of theMicro-Pulse LidarNetwork (E. J. Welton). Authors JRC, JAC and DLW acknowledge the support of Office of Naval Research Code 322 (PE0602435). Author JRC also acknowledges the support of NASA Interagency Agreement RPO201522 on behalf of the CALIPSO Science Team (C. R. Trepte)., 2017-08-06

On 21 September 2012 the entire sky was covered by contrails over the Gulf of Lyon (NW of the Mediterranean basin). These clouds were well recorded by ground observers as well as by Meteosat imagery. The atmospheric characteristics at the levels where these anthropic clouds formed are analyzed by performing a WRF simulation in the area where Meteosat recorded contrail clouds. According to the vertical profiles of temperature and the relative humidity respect to the ice (RHI), the environmental condition favors that the water vapor exhaust emitted by the aircraft engines reaches the deposition point and form crystal clouds, which spread out because the temperature remained below 230 K and the RHI was higher than 70% during the whole episode., Peer Reviewed, Postprint (published version)

Utilizing experienced gained from analysis of TODWL and P3DWL data , we have been analyzing the shipboard HRDL data taken during DYNAMO with a focus on occasions when rain is in the field of view. In particular, investigating if the HRDL data can be used to discriminate between the air motions and the hydrometeor motions occurring simultaneously within the illuminated volumes. A second main objective of the past years' research was to use the HRDL data and the W band radar data taken aboard the USS Revelle to investigate vertical motions within physically thick (optically thin) cirrus layers. Much of the first year's effort has been in acquiring HRDL data, W-band radar data, shipboard flux measurements and observer notes for cases studies with and without rain. This has been followed by a total reprocessing of the raw HRDL data in order to understand what can be obtained from it regarding our main objectives. The HRDL data is recorded using auto-correlations of 6 lags. A main conclusion from our analyses and reprocessing were that 6 lags are insufficient to allow us to discriminate rain from air motions under most conditions. In addition , we also found that using just 10 data points from the HRDL data to obtain 30 meters of range resolution risks a high bias in the LOS wind estimation., The original document contains color images.

Cirrus cloud forecasting is of particular importance to various Department of Defense programs. This thesis takes a case study approach to study Air Force Weather Agency Mesoscale Model 5 (AFWA MM5) skill in forecasting cirrus clouds, which are not represented explicitly by the model (ice water mixing ratio is used as a surrogate.) Two cases are selected for study. For each case, an initial forecast time of interest is determined which serves as the beginning point for the case study. GOES data and 3-hourly MM5 data are then obtained at 3- hourly intervals to coincide with model forecast time steps between the initial time through the 30-hour forecast. A standard analysis is performed on all data to determine general atmospheric structure for each case at each 3- hourly point. Following this, the model's relative humidity with respect to ice, explicit ice water content, vertical velocity, and other fields are considered to determine if the model possesses the proper dynamical factors for cirrus formation. Finally, model coverage of ice cloud is compared to the ABL cloud mask results to determine how well the model s ice cloud forecasts verify against each 3-hourly observed ice water field taken from the GOES data. Results indicate that the MM5 underforecasts cirrus coverage, and that the 90% relative humidity field with respect to ice may be a better approximation of observed cirrus coverage than the ice water field., http://archive.org/details/developmentndevo109452919, Approved for public release; distribution is unlimited.

The objective of this basic research effort is to provide the theoretical foundations for modeling/simulating the interaction of aerosols and ice at high altitudes (near and above the tropopause). The AF, together with the ABL SPO (through the MDA), anticipate that aerosols/ice will impact the efficacy of the ABL and the ability to predict possible impacts is of great value. The PI and his colleagues will interrogate a number of conceptual schemes which incorporate cloud microphysics (with 6 classes of moisture species together with actual ice concentrations). Particular attention will be paid to ice nucleation scenarios., The original document contains color images.

To understand the radiative impact of tropical thin cirrus clouds, the frequency of occurrence and optical depths of these clouds have been derived. "Thin" cirrus clouds are defined here as being those that are not detected by the operational Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask, corresponding to an optical depth value of approximately 0.3 or smaller, but that are detectable in terms of the cirrus reflectance product based on the MODIS 1.375-micron channel. With such a definition, thin cirrus clouds were present in more than 40% of the pixels flagged as "clear sky" by the operational MODIS cloud mask algorithm. It is shown that these thin cirrus clouds are frequently observed in deep convective regions in the western Pacific. Thin cirrus optical depths were derived from the cirrus reflectance product. Regions of significant cloud fraction and large optical depths were observed in the Northern Hemisphere during the boreal spring and summer and moved southward during the boreal autumn and winter. The radiative effects of tropical thin cirrus clouds were studied on the basis of the retrieved cirrus optical depths, the atmospheric profiles derived from the Atmospheric Infrared Sounder (AIRS) observations, and a radiative transfer model in conjunction with a parameterization of ice cloud spectral optical properties. To understand how these clouds regulate the radiation field in the atmosphere, the instantaneous net fluxes at the top of the atmosphere (TOA) and at the surface were calculated. The present study shows positive and negative net forcings at the TOA and at the surface, respectively. The positive (negative) net forcing at the TOA (surface) is due to the dominance of longwave (shortwave) forcing. Both the TOA and surface forcings are in a range of 0-20 W/sq m, depending on the optical depths of thin cirrus clouds., Published in Journal of the Atmospheric Sciences, v66 p3721-3731, Dec 2009. Sponsored in part by NSF.

The AF anticipates fielding the ABL in the near future and needs to account for all the atmospheric conditions, such as cloud crystals, which would impede its successful operation. The objective of this additional work basic research effort is to provide additional characterization of the optical properties of cirrus clouds (particularly the ice crystals which form within these clouds at night) together with an improved prediction of the deleterious reflections these crystals provide of laser beams which propagate through them. The PI and his team will analyse the data collected from several new experimental campaigns and then improve/update their laser beam propagation simulation code in light of this analysis.

From a mass-weighted perspective, cirrus clouds exert an enormous influence on the radiative energy budget of the earth?s climate system. Owing to their location in the cold upper troposphere, cirrus can significantly reduce the outgoing longwave radiation while, at the same time, remaining relatively transmissive to solar energy. Thus, cirrus clouds are the only cloud genre that can exert a direct radiative warming influence on the climate system (Ackerman et al. 1988). It is not surprising, therefore, that general circulation models (GCMs) are especially sensitive to the presence of cirrus in the model atmosphere. Lohmann and Roeckner (1995), for instance, show that the climate sensitivity can vary by as much as 40% due to the properties of cirrus varying between transparent and opaque limits. Lohmann and Roeckner (1995) also identify a key feedback by cirrus that is often overlooked; on longer time scales cloud heating in the upper troposphere can act to maintain and modulate the general circulation of the atmosphere through accelerating the subtropical and polar jet streams. Understanding these mechanisms and representing them in models is complicated by the fact that cirrus properties vary over an enormous dynamic range compared to most other clouds.

Future high-altitude laser systems may be affected by cirrus clouds. Laser transmission models were applied to measured and retrieved cirrus properties to determine cirrus impact on power incident on a target or receiver. A major goal was to see how well radiosondes and geostationary satellite imagery could specify the required properties. Based on the use of ground-based radar and lidar measurements as a reference, errors in cirrus-top and cirrus-base height estimates from radiosonde observations were 20%- 25% of geostationary satellite retrieval errors. Radiosondes had a perfect cirrus detection rate as compared with 80% for satellite detection. Ice water path and effective particle size were obtained with a published radar-lidar retrieval algorithm and a documented satellite algorithm. Radar-lidar particle size and ice water path were 1.5 and 3 times the satellite retrievals, respectively. Radar-lidar-based laser extinction coefficients were 55% greater than satellite values. Measured radar-lidar cirrus thickness was consistently greater than satellite-retrieved thickness, but radar-lidar microphysical retrieval required detection by both sensors at each range gate, which limited the retrievals' vertical extent. Greater radar-lidar extinction and greater satellite-based cirrus thickness yielded comparable optical depths for the two independent retrievals. Laser extinction-transmission models applied to radiosonde-retrieved cirrus heights and satellite-retrieved microphysical properties revealed a significant power loss by all models as the laser beam transits the cirrus layer. This suggests that cirrus location is more important than microphysics in high-altitude laser test support. Geostationary satellite imagery may be insufficient in cirrus detection and retrieval accuracy. Humidity-sensitive radiosondes are a potential proxy for ground-based remote sensors in cirrus detection and altitude determination., Pub. in Jnl. of Applied Meteorology and Climatology, v47 p1322-1336, 2007.

SD/CP/01A, info:eu-repo/semantics/published

A new parameterisation is described that predicts the temperature perturbations due to sub-grid scale orographic gravity waves in the atmosphere of the 19 level HadAM3 version of the United Kingdom Met Office Unified Model. The explicit calculation of the wave phase allows the sign of the temperature perturbation to be predicted. The scheme is used to create orographic clouds, including cirrus, that were previously absent in model simulations. A novel approach to the validation of this parameterisation makes use of both satellite observations of a case study, and a simulation in which the Unified Model is nudged towards ERA-40 assimilated winds, temperatures and humidities. It is demonstrated that this approach offers a feasible way of introducing large scale orographic cirrus clouds into GCMs., Published in the Journal of Clim Dyn, v28 p581-597, 2007.

Cirrus clouds are known to impact certain Air Force missions and systems. Mission planners need accurate cirrus predictions in advance of system deployment. This report describes a project to evaluate the ability of operational mesoscale numerical weather prediction models to predict cirrus cloud characteristics. The Air Force Weather Agency (AFWA) MM5 and the National Centers for Environmental Prediction (NCEP) North American Mesoscale (NAM) prognostic models, along with the AFWA Diagnostic Cloud Forecast (DCF) diagnostic algorithm, were considered in the study. Geostationary Operational Environmental Satellite (GOES) imagery channel data were processed to detect ice cloud picture elements (pixels) and to retrieve cloud top height and ice water path as a reference. The presence/absence of cloud ice in the model grid cell was compared with the presence of ice cloud in within-grid-cell pixels. Results showed that the comparison statistics vary significantly by region and to a lesser degree by season. Study results have been summarized for use in providing guidance supporting high altitude laser system tests., The original document contains color images. All DTIC reproductions will be in black and white.

This study describes the collection of field measurements of cirrus clouds during an eleven-month period over Hanscom AFB, MA, and how those measurements were used to estimate laser transmittance through the cirrus. Using ground-based radar and lidar measurements as a reference, it is shown that radiosonde observations can better detect the presence and geometric attributes of cirrus layers than can geostationary satellites. In comparing satellite retrievals of ice particle size and ice water content with radar/lidar retrievals, the satellite-retrieved effective particle size and ice water content were less than the radar/lidar retrievals. The radar/lidar geometric thickness retrieval of the cirrus is less due to the vertical extent being limited to the higher base and lower top detected by the radar-lidar tandem. Cirrus properties from the case with the smallest optical depth were used in three laser extinction models. Results from all three indicated that a significant power loss occurs as the laser beam propagates through the cloud layer. It seems that knowing cirrus location is more important than knowing the optical properties in laser operations support., The original document contains color images. All DTIC reproductions will be in black and white.

0, info:eu-repo/semantics/nonPublished

Space-time variations of tropical upper tropospheric water vapor and cirrus clouds associated with the intraseasonal oscillation (ISO) are investigated using data from the Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS). Composite moisture and meteorological fields based on five ISO events selected in two boreal winters (1991–1993) are analyzed using 20–80 day band-pass-filtered data. At 215 and 146 hPa, wet anomalies with frequent appearance of cirrus clouds exist over the convective system and move eastward from the Indian Ocean to the central Pacific, suggesting a direct effect of convective activity up to this level. At 100 hPa, however, the moisture field seems to be indirectly affected by convective activity through the dynamical response to the convective heating. Dry anomalies are observed over the Indian Ocean around the developing stage and over the eastern Pacific around the mature-to-decaying stage of the ISO. Cirrus clouds are frequently found over the cold region located to the east of the convective system. These structures around the tropopause level are closely related to the eastward moving Kelvin and Rossby wave responses to the convective heating with the equatorial cold anomaly and with the subtropical anticyclonic gyres. Between the two gyres the easterly wind blowing through the equatorial cold region may cause dehydration through cirrus formation when the convective system develops over the Indian Ocean and the western Pacific. As the northern gyre intensifies, tropical dry air is transported to the subtropical Pacific and eventually to the equatorial eastern Pacific. It is suggested that the temperature and flow variations due to the coupled Kelvin-Rossby wave structure play an important role in dehydrating air in the tropical and subtropical tropopause region.

Laser transmission model development. We developed a number of 2D radiative transfer models based on the successive-order-of-scattering approach for the computation of airborne high-energy laser transmission and backscattering through homogeneous and inhomogeneous thin cirrus clouds in both plane-parallel and spherical geometries. These models took into account the relative positions of aircraft, target, and cirrus clouds in the atmospheres. We investigated the sensitivity of laser transmission and backscattering on variation of the cloud optical depth, particle size, and cloud-base height, as well as the positions of aircraft and target. We also evaluated the effects of the earth's sphericity on laser transmission and backscattering. A paper describing our detailed research findings has been published in Applied Optics (Ou, et al. 2002). Our present research involves the theoretical formulation of a three-dimensional direct and first-order transmission-backscattering model for thin cirrus.

Lidar backscatter from cirrus cloud particles at multiple wavelengths (532 nm, 1064 nm, 10591 nm) was measured at Hanscom AFB, MA in August 2001. Range-resolved measurements were made for several hours daily at altitudes from 9 km to 16 km over a temperature range of -30C to -70C. Three days were selected for study -- 9 August, 14 August, and 16 August 2001. Cirrus backscatter measurements were analyzed in combination with scattering theory for ice spheres and long ice cylinders to better understand their behavior. For 532 nm and 1064 nm, the logarithm (base 10) of backscatter magnitudes (/m-sr) ranged from 4.5 to -6.0 within the cloud area while backscatter magnitudes for 10591 nm ranged from -6.5 to -8.5. Agreement existed between the range of measured backscatter wavelength ratio magnitudes and those computed from scattering theory but no consistent information was discerned concerning modal particle sizes. Calculations of cirrus ice water content, exinction-to- backscatter ratio, and optical depth yielded magnitudes consistent with published values for in-situ and remote measurements.

The purpose of this report is to set forth a new set of cirrus cloud models for use with the MODTRAN4 atmospheric transmission model and to provide a wider range of cirrus models with which to investigate future remote sensing techniques that may have to observe through cirrus. The cirrus cloud models were derived by selecting mode particle sizes between and beyond the two existing cirrus cloud models in MODTRAN4: Standard (mode particle radius 64 micrometers) and subvisual (mode 4 micrometers). The new set of models consists of 70 models whose mode particle radius range between 0.1 and 64 micrometers. The format of the models meets the input requirements for MODTRAN4 and consist of files containing the input information on card 2E2. All the model calculations are based on spherical particles.

Since approximately 20% of the globe is covered with cirrus clouds at any given time, it is clear that any airborne or spaceborne system using a laser will intercept cirrus clouds at some point. Cirrus clouds contain a very complex microphysical structure that will affect laser power by scattering and reflecting it away from the intended target, thus reducing efficiency and possibly even making it ineffective. Using two thin cirrus laser transmission models, a single homogeneous cloud layer model and a multiple cloud layer model, laser transmission profiles are generated from a simulated cirrus cloud case created by the RAMS model. Sensitivity studies are performed on the laser transmission model to examine the effects of aerosols and water vapor, ice crystal orientation, multiple scattering contributions, and the differences between the single and multiple layer models. Different parts of the RAMS simulated cloud are examined as well as the development of a particular cloud feature. The two different laser transmission models are compared against each other for a variety of different cirrus cloud conditions within the simulated case. Optical depth is a cloud variable that is fairly well measured using remote sensing techniques and airborne lidar. Average optical depth is examined as a viable parameter to indicate the likely transmission through a cloud.

The normalized wavelength-dependent optical depth of the two cirrus cloud models resident in MODTRAN 4 ('standard' and 'subvisual') is computed along with the scattering and absorption components. The wavelength-dependent asymmetry parameter is also calculated. We find that the optical depths as a function of wavelength are significantly different for the two models, most of which is attributable to particle size effects.

The topics investigated under the present Contract can be summarized as follows: 1. Analysis by nondestructive means of the degree of cleanliness of a metallic or dielectric surface. 2. Determination of the contribution to the greenhouse effect from the ice crystal that are present in the high atmosphere (namely in cirrus clouds). 3. Discrimination of the shape and backscattering properties of atmospheric ice crystals in the millimeter wave range. The results of the above mentioned investigations were exponded in several papers that were already published or are at present in the press. The list of these papers is reported at the end of the present report.

A basic research program was conducted for the Air Force Research Laboratory, consisting of three tasks: (1) develop extensible cloud property retrieval algorithms suitable for expanding existing cloud analysis capabilities to utilize data from new and future environmental satellite sensing systems; (2) develop advanced algorithms for retrieval of cloud properties that are insufficiently resolved to satisfy Air Force requirements using existing techniques; and (3) validate cloud property retrieval algorithms using real data from existing meteorological satellite platforms. Under task 1 a series of algorithms developed under previous AFRL programs were modified to expand capabilities in several areas including improved transmissive cirrus detection and classification, cloud phase retrieval, snow/cloud discrimination, and transition across the terminator. In task 2 new capabilities were developed to retrieve cloud radiative and microphysical properties from multispectral data available from environmental satellites including the newly launched MODIS. Under task 3 the new algorithms were implemented in software and applied to real data acquired from the GOES 8 satellite in support of two field programs conducted or supported by AFRL., Original contains color plates: All DTIC reproductions will be in black and white.

This report addresses the problem of utilizing spatial-hyperspectral imaging capabilities of spaceborne sensors to detect and characterize regions of atmospheric turbulence and cirrus cloud clutter which may impact employment and/or performance of space based laser and airborne high energy laser weapons systems. Our concept for worldwide detection, characterization and mapping of atmospheric turbulence and cirrus clouds involves use of satellite-borne (and possibly airborne) spectral and hyperspectral imagers operated in the UV to MWIR spectral range in virtual triangulation geometry. Spectral and hyperspectral imagery allows altitude sounding of atmospheric clutter from turbulence and cirrus clouds. Triangulation geometry allows precise attitude selection by cross correlation of the backscatter signals. The combination of attitude and Fourier-space background spectral discrimination will provide an attitude resolved measurement of atmospheric clutter from the clear air turbulence and from cirrus clouds, both of which may affect performance of the SBL (Space Based Laser) and the Airborne Laser (ABL) systems., Prepared in collaboration with Arizona Univ., Lunar and Planetary Lab., Tucson, AZ

New instrumentation has been designed developed and deployed for characterizing ice and water droplets in clouds in the atmosphere by in situ aircraft measurement. The principle of operation depends on capture of the particles on a forward facing probe and their processing through evaporation and/or melting to give particle mass and density. Data is both video recorded and also obtained as permanent record as a plastic cast. New analysis tools have also been developed for analysis of the results, displaying particle forms, concentration and spatial distribution in high resolution. Test results have been obtained in hurricane outflow, in arctic clouds and in aircraft contrails. Application lies in determining the optical properties of such clouds from the viewpoint of their influence on laser propagation and transfer of atmospheric radiation; it also lies in characterizing clouds in terms of their potential for enhanced aircraft icing when specific spatial distributions of ice particles and supercooled cloud exist.

This report summarizes research and results that pertain to AFOSR agreement no. F49620-96-C-0024. The aim of this research program has been to improve prediction of aircraft contrail and cirrus cloud formation, and to improve understanding of their optical and physical properties. Toward this end, the investigators have used analysis of data from research aircraft, balloon-borne ice crystal replicator sondes, and radiosonde relative humidity measurements, as well as theoretical and numerical modeling techniques. The five categories of research covered in this report are: (1) Improving Vaisala RS80-A radiosonde relative humidity measurements; (2) Vertical distribution of cirrus microphysical and optical properties; (3) Relative humidity conditions for the formation of cirrus and contrails; (4) Variability of cirrus microphysical properties; and (5) Improved theory of contrail formation Research findings in each of these five categories resulted in at least one publication whose full or partial sponsorship by the AFOSR was acknowledged. The journal publications provide an in-depth discussion of the research conducted; this final report summarizes the findings.

Cirrus clouds are one of the least understood components of the global climate system. The Atmospheric Radiation Measurement (ARM) program is attempting to resolve the scientific uncertainties surrounding the effects of clouds on global climatic change. To this end, the ARM program is compiling continuous observations of clouds, including cirrus microphysical properties using a diverse suite of instruments, at several climatically significant locations around the globe. One of the algorithms used by the ARM program to derive the cirrus microphysical properties employs reflectivity measurements from a millimeter wavelength radar, and infrared emission spectra observed by an interferometer. In an effort to validate the results derived from this reflectivity radiance algorithm, the effects of cirrus clouds on the radiation field, calculated using the microphysical properties retrieved by the algorithm, are compared against atmospheric radiative quantities derived from satellite observations and surface radiometers. For 15 cirrus events, the top of the atmosphere albedo calculated from the algorithm's retrievals is compared against the shortwave broadband albedo derived from satellite observations. In an attempt to move toward closure of the column radiation budget, for two case studies, comparisons are made between the calculated and observed surface solar fluxes. Using the results from the two case studies, we attempt to draw conclusions regarding the topic of anomalous absorption of solar radiation in cloudy atmospheres. Problems encountered during this research with the Moderate Resolution Transmittance (MODTRAN) radiative transfer code are also presented.

The Cloud Scene Simulation Model (CSSM) was developed to generate high resolution synthetic cloud backgrounds for a variety of DoD simulation systems. This report describes recent efforts to enhance that model and presents plans for future areas of development. Recent efforts were focused in two areas: cirrus model development and software support. Cirrus model development began with a detailed survey of the technical literature to identify models and data from which we could derive cirrus statistics and characteristics for use in the model. We then decided to implement two cirrus types within the model: precipitating and non-precipitating types. Cirrus databases were identified in the literature survey and several were chosen for analysis. These data were used to tune models within the current non-precipitating model. The CSSM was then tuned through comparison to the cirrus observations and statistically tested against the observations. Our future development plans were focused on the implementation of an enhanced cirrus uncinus model and a climatological preprocessor for the CSSM. The cirrus uncinus model design simulates the vertically developed particle distributions observed in nature. The climatological preprocessor will use climatological cloud and meteorological databases along with statistical information to build typical weather conditions for a user specified location/date/time.

The specific objective of this research has been the development and improvement of theoretical models to simulate the effect of nonsphericity on single scattering properties of cirrus cloud particles in the visible and infrared spectral regions. First, we have shown that using a matrix inversion scheme based on a special LU factorization rather than on the standard Gaussian elimination significantly improves the numerical stability of T-matrix computations for nonabsorbing and weakly absorbing nonspherical particles. Second, we use exact T-matrix computations and the Kirchhoff approximation to show that the delta function transmission peak predicted by the GO approximation for hexagonal ice crystals is an artifact of GO completely ignoring physical optics effects and must be convolved with the Fraunhofer pattern, thereby producing a phase function component with an angular profile similar to the standard diffraction component. Third, we have used the improved T-matrix method to compute the linear depolarization ration for polydispersions of randomly oriented ice spheroids, circular cylinders, and Chebyshev particles with sizes typical of young contrails. We have shown that ice crystals with effective radii as small as several tenths of a micron can already produce 3 exceeding 0.5 at visible wavelengths.

A three-year, multi-faceted effort to develop and apply state-of-the-art techniques for analysis of environmental satellite data is described. Principal accomplishments include development of satellite data analysis and image processing algorithms and software suitable for interpreting environmental satellite sensor measurements; adaptation and application of satellite-based cloud property retrieval algorithms for processing of large multiple-satellite data sets; development and application of improved cloud-phase and cloud optical property retrieval algorithms; investigation of techniques potentially applicable for retrieval of cloud spatial properties from very high resolution remotely sensed data; and data analysis in support of a contrail-formation experiment. Software development and data analysis were performed using the Air Force Interactive Meteorological System (AIMS) located at PL. AIMS is an integrated computer facility that includes multiple direct-broadcast satellite ground stations and associated data processing facilities; an extensive satellite and conventional data archive; a database access system; and a library of environmental-property retrieval algorithms. Several system enhancements were developed for AIMS including a new Geostationary satellite acquisition and analysis capability. All software and data-analysis products developed under this effort are available on AIMS.

Results of the phase function, single scattering albedos, and extinction coefficients for three representative cirrus cloud ice crystal size distributions are presented for the Airborne Remote Earth Sensing (ARES) System 2-6.4 microns wavelengths. The ice crystal sizes range from 5 to 800 microns with shapes spanning from bullet rosettes, hollow and solid columns, plates, to aggregates. The computations are carried out on the basis of a unified theory for light scattering by ice crystals recently developed by our research group. Effects of incorporating small ice crystals on the scattering and absorption calculations are assessed. An airborne retrieval algorithm to infer cirrus cloud temperature, optical depth, and mean effective sizes is subsequently presented using the ARES 5.1-5.3 and 3.7 microns channels data. This scheme has been applied to the ARES data collected on September 16, 1995, over the Hanscom AFB area. Validation of the retrieved cloud temperature, optical depth, and mean effective size is carried out using the collocated and coincident ground-based 8.6-mm radar data and in-situ size distributions from the 2D probe on board HARP.

This technical report includes five reprints and pre-prints of papers associated with the modeling of cirrus cloud and radiation processes as well as remote sensing of cloud optical and microphysical properties from an airborne spectrometer based on radiative transfer principles. The time-dependent two-dimensional cirrus model includes a second-order turbulence closure scheme, an advanced interactive radiative transfer scheme, and ice microphysics parameterization. This model is used to understand the physical processes governing the formation and evolution of cirrostratus clouds.

We derive and examine the general expression for the scattering asymmetry parameter g. For aggregate particles, the asymmetry parameter is made up of two terms. One term accounts for interference effects of the electromagnetic fields radiating from the individual subsystems. The other term accounts for interaction effects of the electromagnetic fields between these subsystems. Enhanced backscatter is one phenomenon resulting from these interactions. Numerical results demonstrate that interference effects play a dominant role when the separation distance between aggregates is smaller than half the incident wavelength. As the separation distance becomes large, both interference and interaction effects drop off, and the asymmetry parameter approaches that of the individual particle constituents., Prepared collaboration with Ngo Co. and Dalhousie Univ.

The specific objective of this research is the development and improvement of theoretical models to simulate the effect of nonsphericity on single-scattering properties of cirrus cloud particles in the visible and infrared spectral regions. First, we have shown that using a matrix inversion scheme based on a special LU factorization rather than on the standard Gaussian elimination significantly improves the numerical stability of T-matrix computations for nonabsorbing and weakly absorbing nonspherical particles. As a result, the maximum convergent size parameter for particles with small or zero absorption can increase by a factor of several and can exceed 100. Comparisons of T-matrix and geometric optics (GO) computations for large, randomly oriented spheroids and finite circular cylinders show that the range of applicability of the ray tracing approximation depends on the imaginary part of the refractive index and is different for different elements of the scattering matrix. Second, we use exact T-matrix computations and physical considerations based on the Kirchhoff approximation to show that the delta-function transmission peak predicted by the GO approximation for hexagonal ice crystals is an artifact of GO completely ignoring physical optics effects and must be convolved with the Fraunhofer pattern, thereby producing a phase function component with an angular profile similar to the standard diffraction component. We have performed this convolution with a simple procedure which supplements the standard ray tracing code and makes the computation of the phase function and its Legendre expansion both more physically realistic and more accurate.

Scientific progress achieved under the support of this research grant has been reported in a series of conference presentations, student thesis and published papers. Copies of these documents comprise the body of this report.

This interim scientific report describes a multi-faceted effort to support PL/GPD in the development and application of state-of-the-art analysis techniques for remotely sensed data. The resulting tools, techniques and data sets will support the improved analysis of archived data as well as current and future geophysical parameter acquisition and analysis. The principal accomplishments during the reporting period were: development of improved algorithms for retrieval of cirrus emissivity and estimation of cloud altitude; development of improved cloud-phase discrimination algorithms for analysis of GOES imager and AVHRR data; application of existing cloud detection and new cloud-phase and emissivity algorithms to a two-month data set collected from one GOES and two NOAA satellites over the eastern and central portions of the continental U.S. and western Atlantic; reduction and quality assurance processing of approximately one month of raw radiosonde data from five sites; and software tool and technique development for enhanced satellite image processing capabilities on the Air Force Interactive Meteorological System.

A numerical scheme has been developed to identify multilayer cirrus cloud systems using AVHRR data based on the physical properties of the AVHRR channels 2-1 reflectance ratios, the brightness temperature differences between Chs. 4 and 5, and the Ch. 4 brightness temperatures. Subsequently, we have devised an algorithm for the retrieval of cirrus cloud optical depths and ice crystal sizes in multilayer cloud systems based on the theory of radiative transfer and parameterizations. Both the detection and retrieval results have been validated from the cloud conditions, optical depths, and mean effective ice crystal sizes inferred from the collocated and coincident ground-based lidar and radar images, balloon-borne replicator data, aircraft 2D probe data, and NCAR CLASS temperature and humidity sounding data on a case-by-case basis.

Support of Environmental Requirements for Cloud Analysis and Archives (SERCAA) was a four-year, two-phase research and development program. Program goals were to produce a global multispectral, multisensor cloud analysis capability for retrieval of spatial, radiative, microphysical, and environmental cloud properties through analysis of sensor data from multiple operational environmental satellites. Program accomplishments during Phase 1 included development of three customized cloud-detection algorithms for analysis of NOAA/AVHRR, DMSP/OLS, and geostationary sensor data; a new algorithm for discrimination of vertical cloud layers within a 25 km grid cell; and an innovative analysis-integration algorithm for optimally combining the results of the source-specific cloud analysis modules. During Phase 2, retrieval capabilities were expanded to include cloud particle size, emissivity/transmissivity, optical depth, and altitude. Additional work toward characterization of cloud-free bidirectional reflectance provides a basis for enhanced analysis of visible-channel data in the cloud detection process. SERCAA algorithms have been selected for operational implementation at the Air Force Global Weather Central., Original contains color plates: All DTIC and NTIS reproductions will be in black and white.

The development of the Micro Pulse Lidar (MPL) provides researchers with a system capable of continuous, eye-safe monitoring of atmospheric properties. The MPL operates with low energy, high pulse repetition frequency radiation in the visible portion of the spectrum. To investigate the interaction between visible radiation and atmospheric constituents, a model using Monte Carlo techniques has been refined to simulate MPL return profiles. An inherent feature of the MPL is its narrow receiver field of view (FOV) which is necessary to limit background noise. The effect of such a FOV and the role multiple scattering effects play in MPL operations are investigated in this study. Cloud base height and the radiative properties of cirrus clouds are important for determining the radiation budget of the planet. Inferred cirrus cloud radiative properties vary with the type of crystals assumed to compose the model clouds. To properly model optically thin clouds, it is important to include a standard background atmosphere composed of Rayleigh and aerosol scatterers. Its inclusion allows one to take advantage of information deduced from both the cloud and above-cloud layer. Information that is unavailable when sampling optically thick clouds. This capability plays a pivotal role in an inversion algorithm that is developed and described. It is shown that the algorithm allows one to infer important cloud optical properties such as volume extinction coefficient, cloud optical depth, and isotropic backscatter to extinction ratio, also known as the lidar ratio.

The objective of this study has been to examine and document the development of mid-level Mesoscale Convective Vortices (MCVs) within Mesoscale Convective Systems (MCSs) and Mesoscale Convective Complexes (MCCs) using the Central Plains Wind Profiler Demonstration Network (WPDN). Nine MCSs from the summer of 1993 were picked for this study based on their formation and lifetime spent over the WPDN. Bartels and Maddox's (1991) climatological study of MCVs for 1981 - 1988 estimated that less than 5% of MCSs exhibit a vortex whose clouds persist long enough after the dissipation of the MCSs high-level obscuring cirrus cloud to become apparent in visible satellite imagery. This low estimate of MCVs in MCSs leads to the question of how many MCSs produce MCVs. Some researchers state that the MCV is an inherent part of the MCC circulation (Velasco and Fritsch, 1987; Menard and Fritsch, 1989).

A radiation model was developed to calculate radiance in a plane parallel, vertically nonhomogeneous, nonscattering atmosphere. The radiance model was developed to analyze data collected by a vertically pointed radiometer with a receiver bandwidth of 9.5-11.5 micrometers. These radiometer readings are used in conjunction with backscatter readings from a vertically aligned polarization diversity lidar system to compute various cirrus cloud properties such as infrared (IR) emittance. The model used the correlated k-distribution method (CKD) to calculate absorption coefficient functions and account for the nonhomogeneity of the atmosphere. Transmittance and radiance results from this CKD model were compared with results from FASCODE3, a popular and highly accurate line-by-line (LBL) radiation model. Several different atmospheric profiles, ranging from subarctic to tropical, were compared to determine the accuracy and efficiency of the CKD model with respect to the LBL model. At the highest resolutions, the CKD model was 15-100 times faster than the LBL model. Despite the increased efficiency, the CKD model transmittance errors from the earth surface to 30 km were less than 0.4% with respect to the LBL model in temperate and arctic profiles. Errors were less than 2% in the tropical profile. The CKD model downwelling radiance errors from the surface to 30 km were less than 2% with respect to the LBL model for tropical and temperate profiles. In arctic conditions, radiance errors were as high as 8.5% below 2 km, but tapered off to 3% at cirrus cloud levels. In temperate atmospheres, the CKD model could be run as much as 300 times faster, while still calculating radiance values to within 0.6% of the LBL model.

We have developed a detection scheme to identify single and multilayer cirrus cloud systems based on the physical properties of the AVHRR Chs, 1-2 reflectances and ratios, the brightness temperature differences between Chs. 4 and 5, and the 4 brightness temperatures. Clear pixels are first separated from cloudy pixels which are then classified into three types: cirrus, cirrus/low cloud, and low cloud. This scheme has been applied to the NOAA satellite data collected over FIRE-II-IFO area, Kansas, during nine overpasses within seven observation dates (November - December 1991). We have validated the detection results against cloudy conditions inferred from the collocated and coincident ground-based lidar and radar images, balloon-borne replicator data, and NCAR-CLASS humidity sounding a case-by-case basis. We show that the satellite detection results are considered with the cloudy conditions inferred from these independent and complementary measurements. We have also modified our retrieval scheme for the determination of cirrus optical depth and ice crystal size in multilayer cirrus cloud systems, case study using FIRE-II-IFO data is reported., Sponsored in part by grants NAG5-1050 and NAG1-1719. Present at the Cloud Impacts on DoD Operations and Systems 1995 Conference (CIDOS-95), Hanscom AFB, MA., 24-26 Oct 95.

Six years of cloud data have been collected globally from the NOAA satellite series. The High Resolution Infrared Radiometer Spectrometer (HIRS) data were used to detect clouds and estimate their optical depths in the 11 micron infrared window. The cloud detection algorithm has been called the CO2 Slicing Algorithm because it uses the 13-15 micron infrared channels where partial CO2 absorption occurs, to detect partially transparent clouds and correctly determine their altitude. This algorithm is designed to be sensitive to upper tropospheric cirrus clouds which are difficult to detect. The frequency of these clouds along with their global distribution and seasonal changes have been reported at past CIDOS conferences., Presented at Cloud Impacts on DOD Operations and Systems Conference, Hanscome AFB, MA, 24-26 Oct 95.