Showing papers by "Cooperative Institute for Research in the Atmosphere published in 2003"

Impact of Irrigation on Midsummer Surface Fluxes and Temperature under Dry Synoptic Conditions: A Regional Atmospheric Model Study of the U.S. High Plains

Abstract: The impact of irrigation on the surface energy budget in the U.S. high plains is investigated. Four 15-day simulations were conducted: one using a 1997 satellite-derived estimate of farmland acreage under irrigation in Nebraska (control run), two using the Olson Global Ecosystem (OGE) vegetation dataset (OGE wet run and OGE dry run), and the fourth with the Kuchler vegetation dataset (natural vegetation run) as lower boundary conditions in the Colorado State University Regional Atmospheric Modeling System (RAMS). In the control and OGE wet simulations, the topsoil in the irrigated locations, up to a depth of 0.2 m, was saturated at 0000 UTC each day for the duration of the experiment (1‐15 July 1997). In the other two runs, the soil was allowed to dry out, except when replenished naturally by rainfall. Identical observed atmospheric conditions were used along the lateral boundary in all four cases. The area-averaged model-derived quantities for the grid centered over Nebraska indicate significant differences in the surface energy fluxes between the control (irrigated) and the ‘‘dry’’ simulations. For example, a 36% increase in the surface latent heat flux and a 2.6 8C elevation in dewpoint temperature between the control run and the OGE dry run is shown. Surface sensible heat flux of the control run was 15% less and the near-ground temperature was 1.28C less compared to the OGE dry run. The differences between the control run and the natural vegetation run were similar but amplified compared to the control run‐OGE dry run comparisons. Results of statistical analyses of long-term (1921‐2000) surface temperature data from two sites representing locations of extensive irrigated and nonirrigated land uses appear to support model results presented herein of an irrigationrelated cooling in surface temperature. Growing season monthly mean and monthly mean maximum temperature data for the irrigated site indicate a steady decreasing trend in contrast to an increasing trend at the nonirrigated site.

Relationship between Low-Level Jet Properties and Turbulence Kinetic Energy in the Nocturnal Stable Boundary Layer

Abstract: In the nighttime stable boundary layer (SBL), shear and turbulence are generated in the layer between the maximum of the low-level jet (LLJ) and the earth's surface. Here, it is investigated whether gross properties of the LLJ—its height and speed—could be used to diagnose turbulence intensities in this subjet layer. Data on the height and speed of the LLJ maximum were available at high vertical and temporal resolution using the high-resolution Doppler lidar (HRDL). These data were used to estimate a subjet layer shear, which was computed as the ratio of the speed to the height of the jet maximum, and a jet Richardson number RiJ, averaged at 15-min intervals for 10 nights when HRDL LLJ data were available for this study. The shear and RiJ values were compared with turbulence kinetic energy (TKE) values measured near the top of the 60-m tower at the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) main site. TKE values were small for RiJ greater than 0.4, but as RiJ decreased to less ...

Nonhydrostatic, Three-Dimensional Perturbations to Balanced, Hurricane-Like Vortices. Part II: Symmetric Response and Nonlinear Simulations

Abstract: This paper is the second part of a study on the dynamics of nonhydrostatic perturbations to dry, balanced, atmospheric vortices modeled after tropical cyclones. In Part I, the stability and evolution of asymmetric perturbations were presented. This part is devoted to the stability and evolution of symmetric perturbations—particularly those that are induced by the wave–mean flow interactions of asymmetric perturbations with the symmetric basic-state vortex. The linear model shows that the vortices considered in Part I are stable to symmetric perturbations. Furthermore, the model can be used to derive the steady, symmetric response to stationary symmetric forcing, similar to the results from quasi-balanced dynamics as originally presented by Eliassen. The secondary circulations that develop act to oppose the effects of the forcing, but also to warm the core and intensify the vortex. The model is also used to simulate the response to impulsive symmetric forcings, that is, symmetric perturbations. Mu...

Shear-Flow Instability in the Stable Nocturnal Boundary Layer as Observed by Doppler Lidar during CASES-99

Abstract: This study investigates a shear-flow instability observed in the stably stratified nighttime boundary layer on 6 October 1999 during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) in south-central Kansas. A scanning Doppler lidar captured the spatial structure and evolution of the instability, and high-rate in situ sensors mounted on a nearby 60-m tower provided stability and turbulence data with excellent vertical resolution. Data from these instruments are analyzed and linear stability analysis (LSA) is employed to carefully characterize the wave field, its interaction with the mean flow, and its role in turbulence generation. The event persisted for about 30 min and was confined within the shear zone between the surface and a low-level jet (LLJ) maximum. Eigenvalues corresponding to the fastest growing mode of the LSA showed good agreement with the basic wave parameters determined from the lidar data. Good qualitative agreement was also obtained between the eigenfunction of the f...

Humidity‐dependent optical properties of fine particles during the Big Bend Regional Aerosol and Visibility Observational Study

Abstract: [1] The Environmental Protection Agency (EPA) and National Park Service (NPS) initiated a comprehensive field experiment called The Big Bend Regional Aerosol and Visibility Observational Study (BRAVO) to investigate the source of visibility-reducing aerosols at Big Bend National Park, Texas. The study was carried out over a period of 4 months starting in the first week of July 1999. One objective of the study was to gain insight into the atmospheric light-scattering properties of ambient aerosols, especially as they relate to their hygroscopicity. This paper will report on comparisons between measured and modeled estimations of dry and ambient scattering and comparisons between measured and modeled ratios of wet and dry scattering, f(RH), as a function of relative humidity (RH). Two equilibrium models, exercised in combination with Mie scattering theory, were used to predict atmospheric aerosol water content and associated increase in atmospheric scattering. Modeled and measured deliquescence and crystallization points were also compared. Measured and modeled deliquescence were always within 10% RH while crystallization RHs were always within a few percentage points. The analysis suggests that on most days some water is retained by the aerosol at low RHs (20–30%) and in most cases the hygroscopic growth of only inorganic salts accounted for all the observed increase in scattering as a function of RH.

Improved detection of airborne volcanic ash using multispectral infrared satellite data

Abstract: [1] A technique for improved detection of airborne volcanic ash has been developed that uses three infrared (IR) bands from meteorological satellites. The three IR bands are centered near 3.9, 10.7, and 12.0 μm wavelength. The technique is based on the sum of two brightness temperature differences (BTDs), scaled to maximize the brightness and contrast of volcanic ash in the output image. The physical effects attributed to the observed BTDs that help distinguish the volcanic ash from various meteorological cloud types are (1) differential absorption by volcanic ash or sulfur dioxide at 3.9 μm, 10.7 μm, and 12.0 μm and (2) strong solar reflectance by ash at 3.9 μm, which varies diurnally. On the basis of two examples using data from the Geostationary Operational Environmental Satellite (GOES) the three-band IR technique is shown to provide better discrimination of volcanic ash from meteorological clouds than is possible using existing two-band methods. This conclusion is supported by comparisons of brightness count profiles and estimation of false ash detection rate statistics. The best results from the three-band IR technique are obtained during daylight hours over any surface, and at night when the ash cloud is over the ocean or other large body of water. The three-band IR technique is one of the tools currently being employed operationally at the Washington Volcanic Ash Advisory Center.

Mapping and monitoring of the snow cover fraction over North America

Abstract: [1] Automated snow maps over North America have been produced at the National Environmental Satellite Data and Information Service (NESDIS) of the National Oceanic and Atmospheric Administration (NOAA) since 1999. The developed snow-mapping system is based on observations in the visible, middle infrared, infrared, and microwave spectral bands from operational geostationary and polar orbiting meteorological satellites and generates daily maps of snow cover at a spatial resolution of 4 km. Recently, the existing snow-mapping technique was extended to derive the fractional snow cover. To obtain snow fraction, we use measurements of the Imager instrument on board Geostationary Operational Environmental Satellite (GOES). The algorithm treats every cloud-clear image pixel as a “mixed scene” consisting of a combination of snow-covered and snow-free land surface. To determine the portion of the pixel that is covered with snow, we employ a linear mixture approach, which relies on the Imager measurements in the visible spectral band. The estimated accuracy of subpixel snow fraction retrievals is about 10%. In this paper, we present a description of the snow cover and snow fraction mapping algorithms. Application of the developed algorithms over North America for three winter seasons from 1999–2000 to 2001–2002 has shown that the spatial distribution of the fractional snow cover over areas affected by seasonal snow closely corresponds to the distribution of the forest cover. The fraction of snow in the middle of the winter season generally varied from 100% over croplands, grasslands, and other nonforested areas to 20–30% over dense boreal forests. The snow fraction over dense boreal forests exhibited a slight intraseason variability; however, no obvious correlation of these changes with snowfalls was noticed. Over areas with no or sparse tree vegetation cover (croplands, grasslands), snow fraction showed a noticeable correlation with snow depth for snow depths up to 35–40 cm.

Changes in Global Monsoon Circulations Since 1950

Abstract: We examined changes in several independent intensity indices of four major tropical monsoonal circulations for the period 1950-1998. These intensity indices included observed land surface precipitation and observed ocean surface pressure in the monsoon regions as well as upper- level divergence calculated at several standard levels from the NCAR/NCEP reanalysis. These values were averaged seasonally over appropriate regions of southeastern Asian, western Africa, eastern Africa and the Australia/Maritime continent and adjacent ocean areas. As a consistency check we also examined two secondary indices: mean sea level pressure trends and low level convergence both from the NCEP reanalysis. We find that in each of the four regions examined, a consistent picture emerges indicating signi- ficantly diminished monsoonal circulations over the period of record, evidence of diminished spatial maxima in the global hydrological cycle since 1950. Trends since 1979, the period of strongest reported surface warming, do not indicate any change in monsoon circulations. When strong ENSO years are removed from each of the time series the trends still show a general, significant reduc- tion of monsoon intensity indicating that ENSO variability is not the direct cause for the observed weakening. Most previously reported model simulations of the effects of rising CO2 show an increase in monsoonal activity with rising global surface temperature. We find no support in these data for an increasing hydrological cycle or increasing extremes as hypothesized by greenhouse warming scenarios.

Satellite Data Assimilation in Numerical Weather Prediction Models. Part I: Forward Radiative Transfer and Jacobian Modeling in Cloudy Atmospheres

Abstract: Satellite data assimilation requires rapid and accurate radiative transfer and radiance gradient models. For a vertically stratified scattering and emitting atmosphere, the vector discrete-ordinate radiative transfer model (VDISORT) was developed to derive all Stokes radiance components at the top of the atmosphere. This study further enhances the VDISORT to compute the radiance gradients or Jacobians. The band matrix used in the VDISORT is simplified and confined along the diagonal direction so that the Jacobians relative to atmospheric and surface parameters are directly derived from its analytic solutions. The radiances and Jacobians at various wavelengths from the VDISORT are compared against those from other techniques that have been benchmarked before. It is shown that the present method is accurate and computationally efficient. In the VDISORT, both emissivity vector and reflectivity matrix are integrated as part of the radiance and Jacobian calculations. In this study, only the emissivity models at microwave frequencies are tested and implemented for VDISORT applications. Over oceans, a full polarimetric emissivity model is utilized. The cutoff wavenumber separating the large-scale waves from the small-scale waves is derived from an ocean wave spectrum model. Over land, a microwave emissivity model previously developed is used to compute various emissivity spectra.

On the Drop-Size Dependence of Organic Acid and Formaldehyde Concentrations in Fog

Abstract: Concentration differences between small (r 85 μm) were observed for formic acid, acetic acid and formaldehyde in fog droplets collected in California's Central Valley The concentration ratios (large/small droplets) of these compounds were investigated by a stepwise model approach Assuming thermodynamic equilibrium (KH eff) results in an overestimate of the concentration ratios Considering the time dependence of gas phase diffusion and interfacial mass transport, it appears that the lifetime of fog droplets might be sufficiently long to enable phase equilibrium for formaldehyde and acetic acid, but not for formic acid (at pH ≈ 7) Oxidation by the OH radical has no effect on formaldehyde concentrations but reduces formic acid concentrations uniformly in all drop size classes The corresponding reaction for acetic acid is less efficient so that only in large droplets, where replenishment is slowed because the uptake rate of acid from the gas phase is slower, is the acid concentration reduced leading to a smaller concentration ratio Formaldehyde concentrations in fog can be higher than predicted by Henry's Law due to the formation of hydroxymethanesulfonate Its formation is dependent on the sulfur(IV) concentration At high pH values the uptake rate for sulfur(IV) is drop-size dependent However, the observed concentration ratios for formaldehyde cannot be fully explained by the adduct formation Finally, it is estimated that mixing effects, ie, the combination of individual droplets into a bulk sample, have a minor influence (<15%) on the measured heterogeneities

Diffuse fraction of UV radiation under partly cloudy skies as defined by the Automated Surface Observation System (ASOS)

Abstract: [1] A major limitation in predicting the ultraviolet-B (UVB) irradiance on organisms is the difficulty in determining the UVB under partly cloudy sky conditions. This study developed models for estimating the diffuse fraction of ultraviolet-A (UVA) and UVB radiation under partly cloudy skies based on National Weather Service (NWS) cloud cover measurements and Department of Agriculture (USDA) UVB Radiation Monitoring and Research Network UV radiation measurements made from 1997 through 1999 at nine locations in the United States. The UVB diffuse fraction and the difference between clear and partly cloudy sky diffuse fraction were empirically modeled as a function of solar zenith angle and cloud fraction with a resulting mean bias error (MBE) of 0.019 and 0.014, respectively, and a root mean squared error (RMSE) of 0.069 and 0.079, respectively. A semiempirical model was also evaluated where the diffuse fraction was treated as the summation of clear and overcast sky diffuse fractions, respectively, weighted by the probability of the Sun's direct beam being obstructed or not for a given cloud cover fraction. This model MBE was less than 0.01 of both the UVA and UVB wave bands, while the RMSE was 0.035 within the UVB wave band and −0.030 in the UVA wave band. Model errors were greatest for low solar zenith angles and high cloud fractions in the UVB. The cloud fraction measurements used in these models only include clouds up to 3.6 km. Consequently, they should only be used when the cloud information has the same characteristics.

Sensitivity of Model-Generated Daytime Surface Heat Fluxes over Snow to Land-Cover Changes

Abstract: Snow cover can significantly suppress daytime temperatures by increasing the surface albedo and limiting the surface temperature to 0°C. The strength of this effect is dependent upon how well the snow can cover, or mask, the underlying surface. In regions where tall vegetation protrudes through a shallow layer of snow, the temperature-reducing effects of the snow will be suppressed since the protruding vegetation will absorb solar radiation and emit an upward turbulent heat flux. This means that an atmospheric model must have a reasonable representation of the land cover, as well as be able to correctly calculate snow depth, if an accurate simulation of surface heat fluxes, air temperatures, and boundary layer structure is to be made. If too much vegetation protrudes through the snow, then the surface sensible heat flux will be too large and the air temperatures will be too high. In this study four simulations are run with the Regional Atmospheric Modeling System (RAMS 4.30) for a snow event that...

Interannual changes of active fire detectability in North America from long-term records of the advanced very high resolution radiometer

Abstract: [1] This paper addresses practical issues related to the processing of 1-km National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) data for producing a consistent, long-term time series of active fire locations over the Continental United States and Canada The effects of the interannual changes in measured background temperatures, caused by the orbital drift of the afternoon NOAA satellites and by environmental factors, are investigated Background temperature changes are analyzed using a time series of monthly mean clear-sky brightness temperatures from the NOAA National Environmental Satellite, Data, and Information Service (NESDIS) Pathfinder Atmosphere (PATMOS) data set at a 1° × 1° resolution Examples of target areas over four predominant land cover types, as defined in the International Geosphere-Biosphere Programme (IGBP) global 1 km data set, are presented The results indicate that over forests (defined as >60% tree canopy cover) the contrast between nonburning background and fire pixels is nearly always sufficient for successful fire detection Over nonforested areas, however, the low saturation temperature of the mid-IR channel on the NOAA 7 to NOAA 14 satellites often sets a physical limit to the separation of valid fire pixels and false ones Moreover, the severity of this effect changes over the years with the changing background temperatures The results suggest that because of the potential spurious trends in the number of fires, nonforested areas be excluded from the multiyear analysis However, a detailed assessment of the emissions from nonforest fires is needed to quantify the effect of this on continental-scale emission estimates

Intercontinental transport, chemical transformations, and baroclinic systems

Abstract: [1] The adjoint sensitivity technique is used to diagnose the transport pathway and chemical transformations of a pollutant plume from Asia to Hawaii in a three-dimensional chemical transport model (HANK) driven by winds from the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Modeling System (MM5). The simulation takes place within the timeframe of the Mauna Loa Observatory (MLO) Photochemistry Experiment (MLOPEX) 2c (15 April to 15 May 1992). Unlike trajectories, the adjoint technique takes into account all model processes including diffusion and convection in accounting for the transport of the pollutant plume and clearly defines the footprint of the plume. The transport of the pollutant plume is primarily analyzed from an isentropic perspective. The plume is lofted upward out of the Asian boundary layer in a very narrow band immediately above the surface cold front in the warm conveyor belt of a developing synoptic storm. Once lofted into the free troposphere, the plume travels almost isentropically to Hawaii, with very little additional mixing. The chemical and physical transformations within the plume are consistent with measurements on Hawaii as part of MLOPEX 2c: in particular, the initial loss of water and HNO3, the conversion of NOy from PAN to NOx to HNO3, the slow mixing rate in the free troposphere and the small ratio of HNO3 to NOx are consistent with the measurements. The chemical signature of pollutant plumes at MLO is investigated. The plumes have much higher concentrations of hydrocarbons than average and low concentrations of HNO3. The concentrations of PAN, NOx, H2O, and H2O2 were not found to distinguish rapid transport from Asia.

The scalable modeling system: directive-based code parallelization for distributed and shared memory computers

Abstract: A directive-based parallelization tool called the Scalable Modeling System (SMS) is described. The user inserts directives in the form of comments into existing Fortran code. SMS translates the code and directives into a parallel version that runs efficiently on shared and distributed memory high-performance computing platforms including the SGI Origin, IBM SP2, Cray T3E, Sun, and Alpha and Intel clusters. Twenty directives are available to support operations including array re-declarations, inter-process communications, loop translations, and parallel I/O operations. SMS also provides tools to support incremental parallelization and debugging that significantly reduces code parallelization time from months to weeks of effort. SMS is intended for applications using regular structured grids that are solved using finite difference approximation or spectral methods. It has been used to parallelize 10 atmospheric and oceanic models, but the tool is sufficiently general that it can be applied to other structured grids codes. Recent performance comparisons demonstrate that the Eta, Hybrid Coordinate Ocean model and Regional Ocean Modeling System model, parallelized using SMS, perform as well or better than their OpenMP or Message Passing Interface counterparts.

A multichannel temporally adaptive system for continuous cloud classification from satellite imagery

Abstract: A two-channel temporal updating system is presented, which accounts for feature changes in the visible and infrared satellite images The system uses two probabilistic neural network classifiers and a context-based predictor to perform continuous cloud classification during the day and night Test results for 27 h of continuous classification and updating are presented on a sequence of Geostationary Operational Environmental Satellite 8 images Further test results of the system on two new sets of data with 1-2 weeks time difference are also presented that show the potential of this system as an operational continuous cloud classification system

Validation of two‐channel VIRS retrievals of aerosol optical thickness over ocean and quantitative evaluation of the impact from potential subpixel cloud contamination and surface wind effect

Abstract: [1] TRMM/CERES-VIRS Single Satellite Footprint (SSF) data and AERONET Sun/sky radiometer observations from 1998 have been combined to validate SSF aerosol optical thickness (τ) retrievals over ocean along with a quantitative evaluation of the effects of potential subpixel cloud contamination and surface wind on the satellite τ retrievals. Potential subpixel cloud contamination is verified in Visible/Infrared Scanner (VIRS) SSF aerosol retrievals and constitutes a major source of systematic and random errors of the retrieval algorithm as determined from comparisons with AERONET observations. A positive correlation between the surface wind speed (which determines the roughness of the ocean surface) and the SSF τ has been observed for large surface wind speed. The validation results imply this correlation represents the real relationship between the surface wind and the wind-driven aerosols rather than the disturbing effect of the surface reflectance associated with the rough ocean surface. After the potential subpixel cloud contamination is minimized and the effects of large surface wind are removed in the τ match-ups, the positive biases in the SSF τ (compared to AERONET τ) for mean conditions have been reduced from 0.05 to 0.02 in VIRS channel 1 (0.63 μm) and 0.05 to 0.03 in channel 2 (1.61 μm). Random errors have also been reduced from 0.09 to 0.06 at 0.63 μm, and from 0.06 to 0.05 at 1.61 μm. The validation results support the application of the SSF aerosol data in radiation and climate studies as well as supply useful guidance for the adjustment and improvement of the aerosol retrieval algorithm.

Variational assimilation of radar reflectivities in a cirrus model. II: Optimal initialization and model bias estimation

Abstract: In this paper the microphysical model of cirrus clouds introduced in a related study is used to assimilate radar data. A variational approach is developed, based on a method of optimal estimation which makes use of the gradient information provided by the adjoint of the model to minimize a quadratic cost function. Multiple experiments to probe different solutions to the assimilation problem are conducted using both synthetic reflectivities and real observations. Various model and environment parameters are tested as control variables. Results indicate that specific humidity is a viable control variable at cloud levels. Even though this field is only indirectly related to the radar signal, information about the ambient humidity in which the cloud formed may be extracted from the radar reflectivities by using the cloud model in conjunction with the observations. The adjustment of the specific-humidity profile brings the model solution closer to the observed values by increasing the crystal number concentration. Systematic errors in the prognostic variables are also estimated as part of the optimization process by including a model bias term in the cost function. This is equivalent to relaxing the assumption of ‘perfect’ model, and allows for better assimilation results when both model bias evaluation and optimal initialization are performed. Copyright © 2003 Royal Meteorological Society

Diagnosis of a Polar Low Warm Core Utilizing the Advanced Microwave Sounding Unit

Abstract: Data from the Advanced Microwave Sounding Unit (AMSU) are used to examine a polar low that occurred in the Labrador Sea on 17–18 March 2000. During its 40-h lifetime, the polar low was observed three times by AMSU, which captured the formation and subsequent intensification of the storm. The AMSU-A channel-5 (53.6 GHz) brightness temperature field clearly identifies the warm core structure of the polar low, with storm center measurements 2–3 K higher than the background environment. Analysis of these data over time can provide a straightforward and real-time method for tracking storm motion and estimating surface wind speed. The impact of cloud, surface, and moisture variability on the measurements at 53.6 GHz is examined. Although they cannot account for the magnitude of warming, the analysis of additional AMSU frequencies illustrates not only how nonatmospheric temperature effects can subtly influence the structure of the channel-5 brightness temperature field but also how they can provide insi...

Variational assimilation of radar reflectivities in a cirrus model. I: Model description and adjoint sensitivity studies

Abstract: This paper presents an ice microphysics model to be used in variational assimilation of cloud-radar data. The model predicts the vertical and temporal evolution of the parameters of a modified gamma size distribution describing an ice-cloud crystal population, given an initial atmospheric state. Microphysical variables are mapped onto radar reflectivities using an explicit radar forward model. Evolution equations take into account microphysical processes relevant to ice-crystal growth, such as vapour-diffusion growth, aggregation, and gravitational sedimentation. The thermodynamic and dynamic state is specified from a numerical forecast or a radiosonde sounding and is assumed constant over the model integration time. Due to this assumption, the model provides no feedback to the environmental state and thus cannot be used for long-term cloud forecasts. However, when the model is integrated over a short time interval, and the atmospheric conditions are close to water saturation at cloud levels, the model is shown to compare well with observations. An adjoint of a linearized version of the cloud model is derived and applied to investigate model sensitivities to input variables and model parameters. Results show a large sensitivity of model outputs to temperature and selected parameters related to the crystal fall-velocity parametrization. Copyright © 2003 Royal Meteorological Society

Hygroscopic organic aerosols during BRAVO

Abstract: The hygroscopic properties of the organic fraction of aerosols are poorly understood. The ability of organic aerosols to absorb water as a function of relative humidity (RH) was examined using data collected during the 1999 Big Bend Regional Aerosol and Visibility Observational Study (BRAVO). (On average, organics accounted for 22% of fine particulate matter with an aerodynamic diameter less than 2.5 microm (PM2.5) mass). Hourly RH exceeded 80% only 3.5% of the time and averaged 44%. BRAVO aerosol chemical composition and dry particle size distributions were used to estimate PM2.5 light scattering (Bsp) at low and high ambient RH. Liquid water growth associated with inorganic species was sufficient to account for measured Bsp for RH between 70 and 95%.

Analysis of two independent methods for retrieving liquid water profiles in spring and summer Arctic boundary clouds

Abstract: (1) A large number of all-liquid, nondrizzling stratus clouds (163 hours of measurements) were observed with a dual-channel microwave radiometer and a colocated 35-GHz cloud radar during the spring and summer months of the Surface Heat Budget of the Arctic Ocean (SHEBA) project. An algorithm developed by Frisch et al. (1995, 1998) to derive the liquid water content (LWC) is applied to these measurements assuming constant cloud drop number density and cloud drop size distribution breadth with height. A second algorithm developed by Lohnert et al. (2001) is specifically adapted for SHEBA clouds using a priori information from a large eddy simulation (LES) model initialized with summertime SHEBA radiosondes; about 50 soundings during nondrizzling, low-level, all- liquid water clouds are used. Using model-derived drop size distributions, a relationship between simulated radar reflectivity (Z) and model LWC is derived as well as an a priori LWC profile. Once the theoretical error covariance matrix of the Z-LWC relation is derived and the covariance matrix of the LWC profile is calculated, an optimal estimation method is applied to the SHEBA data. The Frisch et al. and Lohnert et al. methods are also applied to the LES model output, resulting in overall root-mean-square differences on the order of 30 to 60%. Both methods are sensitive to the assumed accuracies of the microwave-radiometer-derived LWP. When applied to LES model output, the Frisch et al. method shows a LWC overestimation in the lower parts of the cloud. These systematic errors are induced by the assumption of constant cloud number concentration with height. INDEX TERMS: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 3307 Meteorology and Atmospheric Dynamics: Boundary layer processes; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; KEYWORDS: comparison of Arctic cloud liquid water profiles, sensor synergy, active and passive ground-based remote sensing, optimal estimation, LES model with explicit microphysics, cloud liquid water algorithms

Investigating anomalous absorption using surface measurements

Abstract: [1] Flux measurements from the 415 nm band of the multifilter rotating shadowband radiometer and liquid water path from the microwave radiometer were used to derive effective radii in warm boundary layer clouds at the US Department of Energy Atmospheric Measurement Program Southern Great Plains site Surface fluxes computed using the effective radii retrieved using 415 nm measurements showed no bias when compared with observed broadband fluxes It is therefore inferred that there is no excess absorption at solar and near-infrared wavelengths in the presence of warm boundary layer clouds

Remote Sensing Observations of Effects of Mountain Blocking on Travelling Gravity-Shear Waves and Associated Clouds

Abstract: Trapped Kelvin–Helmholtz (K–H) waves and vortices were monitored as they were generated immediately upwind of a mountain and driven into the barrier by a low-level jet. A stratus cloud visually revealed the embedded, propagating, gravity-shear waves. Interactions of the waves with the mountain were deciphered using remote sensing measurements of the structure, motions, and microphysics within the cloud and conceptual models based on existing theories. The observations show that the mountain acted as a dam to the flow that was primed for, but did not spontaneously induce, the waves. In response to the blocking, the waves spatially developed a pattern of formation, amplification, and breakdown between the upstream flow and the barrier, and altered the associated clouds in the process. Notably, radar signatures of velocity variance depicted organized, intertwined ribbons of relatively large vorticity within the wave layer. These provided measured evidence of the vortex sheet and streamwise vortex tubes predicted by advanced K–Hinstability theory, the three-dimensional version of Scorer's `stripe', the layer of rotational fluid between opposed flows that led to the wave generation. A theory of resonant interaction of wave trains, but with blocking imposed, appears to explain the internal structure of the pile-up of the flow and wave amplification approaching the barrier. Evolution of the supporting atmospheric thermal structure and introduction of a boundary-layer flow reversal follow a current model of blocking, although some features may have developed more directly from wave-driven mixing. The remote sensors also measured the influence of the waves on the cloud liquid water, including a cumulative enlargement of droplets as they were carried through a series of waves.

GPS validation of AIRS water vapor

Abstract: The Atmospheric Infrared Sounder (AIRS) aboard the Aqua spacecraft launched in April 2002 is being used to provide vertical profiles of moisture and column-integrated precipitable water vapor (IPW.) NOAA’s Forecast Systems Laboratory operates a network of over 200 geodetic-quality Global Positioning System (GPS) receivers at fixed surface locations across the United States to determine IPW from the excess zenith-scaled signal delays induced by the presence of water vapor in the troposphere. The GPS IPW observations are accurate, precise to about 1 mm, and can be made under all meteorological conditions encountered to date. They are characterized by a small (20 km) footprint and are delivered every 30 minutes with less than 20 minutes latency. These data are shown to be an ideal resource with which to validate AIRS IPW retrievals and to provide a reliable constraint for the verification of AIRS vertical water vapor profiles using radiosonde observations. This paper describes the principle of measurement of GPS zenith delay, the identification of the residual zenith delay attributable to moisture, and the conversion of the moist delay to IPW. Standards for hardware and software for producing validation-quality observations are outlined. The extraction of a database of collocated AIRS and GPS IPW and radiosonde moisture profiles is described. Plans for statistical and case study analyses are discussed, and preliminary intercomparison results are presented.