John R. Roadcap

Effect of Compressible Flow on Perceived Temperature Fluctuations Measured by Moving Sensor

Equations are developed for the effect of velocity fluctuations on the perceived temperature fluctuations detected by a moving temperature sensor. The impact of these fluctuations on perceived fluctuations of index of refraction are computed. The magnitude of this effect is estimated using known ranges of atmospheric fluctuations in temperature and velocity. Using a method to estimate velocity fluctuations from temperature fluctuations and concurrently measured atmospheric data, balloon derived fluctuation data is used to estimate what would be perceived by a faster moving sensor. We conclude that normally observed fluctuations can have a significant impact on the perceived optical properties. NOMENCLATURE a = Constant b = Constant Br(x) = Spatial Covariance (units of r squared) Cx2 = Structure constant of the quantity x (units are the units of x^m273) CP = Specific heat at constant pressure (J/kg/K) Dx(r) = Structure function of quantity x (units are the units of x squared) d = Distance (m)

In Situ Measurement of Waves and Turbulence in the T-REX Campaign

The Air Force Research Laboratory participated in the NSF Terrain Rotor Experiment (TREX) from 20 March through 6 April 2006, which included 3 intensive observation periods (IOPs) of the 15 IOPs of the two month T-REX campaign. AFRL focused on the higher altitude turbulence associated with mountain waves. The AFRL flew thermosondes to measure optical turbulence up to 30km. They include a radiosonde to measure meteorological data. Standard radiosondes were also launched to sense atmospheric data including atmospheric wave signatures. These instruments were launched from the windward side of the Sierra Nevada Mountains in order to try to examine the atmosphere at high altitudes. Fifteen thermosondes launches and 10 radiosonde launches were successful. Many of the launches showed evidence of mountain waves, some accompanied by high levels of optical turbulence

A preliminary comparison of daylit and night Cn2 profiles measured by thermosonde

Abstract : The refractive index structure constant C(2/n) is needed to characterize optical wave propagation in a refractive turbulent scattering medium. A limited number of in situ measurements of C(2/n) made during day and night conditions from the surface to 10 km above sea level are compared in three different atmospheric boundary layer environments: dry convective, moist convective, and marine inversion. C(2/n) on average appears to be higher through the convective boundary layer depth during the day compared to night for the same air mass type and location but is generally lower than night values within the stable marine inversion layer. Calculations of path scintillation effects for slant paths in the lower atmosphere at near-infrared wavelengths are also compared for day and night conditions associated with the different air mass types.

A new Green's function Monte Carlo algorithm for the solution of the two-dimensional nonlinear Poisson–Boltzmann equation: Application to the modeling of the communication breakdown problem in space vehicles during re-entry

Abstract The objective of this paper is the exposition of a recently-developed, novel Greens function Monte Carlo (GFMC) algorithm for the solution of nonlinear partial differential equations and its application to the modeling of the plasma sheath region around a cylindrical conducting object, carrying a potential and moving at low speeds through an otherwise neutral medium. The plasma sheath is modeled in equilibrium through the GFMC solution of the nonlinear Poisson–Boltzmann (NPB) equation. The traditional Monte Carlo based approaches for the solution of nonlinear equations are iterative in nature, involving branching stochastic processes which are used to calculate linear functionals of the solution of nonlinear integral equations. Over the last several years, one of the authors of this paper, K. Chatterjee has been developing a philosophically-different approach, where the linearization of the equation of interest is not required and hence there is no need for iteration and the simulation of branching processes. Instead, an approximate expression for the Greens function is obtained using perturbation theory, which is used to formulate the random walk equations within the problem sub-domains where the random walker makes its walks. However, as a trade-off, the dimensions of these sub-domains have to be restricted by the limitations imposed by perturbation theory. The greatest advantage of this approach is the ease and simplicity of parallelization stemming from the lack of the need for iteration, as a result of which the parallelization procedure is identical to the parallelization procedure for the GFMC solution of a linear problem. The application area of interest is in the modeling of the communication breakdown problem during a space vehicles re-entry into the atmosphere. However, additional application areas are being explored in the modeling of electromagnetic propagation through the atmosphere/ionosphere in UHF/GPS applications.

Comparison of CO2 Doppler lidar and GPS rawinsonde wind velocity measurements

A comparison of CO2 Doppler lidar and GPS rawinsonde measurements of horizontal wind velocity was conducted during May 2000 at Hanscom AFB, Massachusetts. Seven days of side-by-side measurements using both lidar and GPS sondes were achieved comparing wind velocity as a function of altitude up to 6 km. The horizontal wind velocity was determined by the CO2 Doppler lidar using the Velocity Azimuth Display (VAD) method. Horizontal winds were also determined simultaneously using a differential GPS-tracked rawinsonde which provides GPS position coordinates once per second. Both lidar VAD wind speed Root Mean Squared Difference (RMS) and lidar vs. GPS sonde RMS were calculated and compared as a function of altitude, time, and stability regime. On average, significant increases in both the lidar VAD RMS and lidar vs. GPS RMS were observed during unstable conditions compared to stable conditions. Analyses of lidar VAD RMS show the smallest typical values average near 0.5 m/s over a single profile.

Analysis of Air Force Weather Agency cloud data for assessing single and multiple scattering through clouds at optical wavelengths

Abstract. The Air Force Weather Agency (AFWA) has a long history of providing global cloud analyses and forecasts. Until recently, their focus has been on determining the cloud amount and cloud type. Satellite-based World-Wide Merged Cloud Analysis (WWMCA) data provided by the AFWA are analyzed to understand and assess their capability to characterize cloud single scattering parameters at optical wavelengths. WWMCA represents the most refined version of AFWA’s cloud depiction and forecast system and includes up to four cloud layers and 38 cloud parameters per file at each hemispheric grid point. Findings on WWMCA’s determination of cloud optical depth (COD), consistency with synoptic-scale cloud fields, and its ability to support radiative transfer calculations are as follows: (1) the WWMCA optical depth is strongly correlated with the theoretical optical depth at 550 nm computed using WWMCA’s cloud microphysical parameters. (2) WWMCA captures the large-scale spatial variation of COD as represented by the Mei-yu/Baiu onset and progression of synoptic cloud fields as well as the time-dependent character of mesoscale features. (3) WWMCA does not provide single scattering albedo and the cloud phase function which are needed to solve the radiative transfer equation. Because WWMCA is based on passive sensor processing, obscured cloud layers are not accounted for in the calculation of COD, which may lead to an underestimation of total COD.

A new Green's function Monte Carlo algorithm for the solution of the three-dimensional nonlinear Poisson–Boltzmann equation: Application to the modeling of plasma sheath layers

Abstract. The objective of this paper is the exposition of a recently-developed Greens function Monte Carlo (GFMC) algorithm for the solution of nonlinear partial differential equations and its application to the modeling of the plasma sheath region around a spherical conducting object, carrying a potential and moving at low speeds through a partially ionized medium. The plasma sheath is modeled in equilibrium through the GFMC solution of the nonlinear Poisson–Boltzmann (NPB) equation. The traditional GFMC-based approaches for the solution of nonlinear equations involve the iterative solution of a series of linear problems. Over the last several years, one of the authors of this paper, K. Chatterjee, has been developing a philosophically different approach, where the linearization of the equation of interest is not required and hence there is no need for iteration. Instead, an approximate expression for the Greens function is obtained using perturbation theory, which is used to formulate the random walk equations within the problem sub-domains, where the random-walker makes its walks. On the other hand, as a trade-off, the dimensions of these sub-domains have to be restricted by the limitations imposed by perturbation theory. The greatest advantage of this approach is the ease and simplicity of parallelization stemming from the lack of the need for iteration, as a result of which the parallelization procedure is identical to the parallelization procedure for the GFMC solution of a linear problem. However, the premise of the algorithm is novel and rigorous mathematical justification has to be established in the future. The application areas of interest include the communication blackout problem during a space vehicles re-entry into the atmosphere and electromagnetic propagation through the atmosphere/ionosphere in UHF/GPS applications.

A Characterization of Cirrus Cloud Properties That Affect Laser Propagation

Abstract 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–lid...

Case study of multiple-wavelength lidar backscatter from aerosols

Range-resolved co-pointing multiple wavelength lidar backscatter from aerosols is analyzed for a summer day in the northeast United States. Lidar backscatter wavelengths are 355 nm, 532 nm, and 1064 nm and were measured at a vertical range gate of 60 meters. The altitude range of lidar measurement is from the surface to 4 km above ground level and the measurement period spanned five hours from late afternoon through several hours after sunset. Vertical profiles of temperature, relative humidity, and wind velocity, and surface visibility, were also measured to characterize the prevailing air mass. Lidar aerosol backscatter was significant through 3 km and diminished rapidly above. Several aerosol models selected on an a priori basis are used to compute backscatter ratios for wavelength pairs using scattering theory. These are compared with the profiles of measured backscatter ratios in an attempt to infer the type of aerosol present in the lower atmosphere and estimate multiple wavelength extinction. Measured backscatter ratios agreed with the ratios for soot, water-soluble, and haze aerosol models at the lowest altitudes with little agreement above 1 km for any model. Extinction estimates derived from lidar backscatter at 300 m were significantly higher than the corresponding values deduced from surface observations.

CO2 doppler lidar measurement of wind velocity and relative backscatter associated with the nocturnal boundary layer

Heterodyne CO2 Doppler lidar measurements of horizontal wind velocity from the surface to 11,000 feet AGL using the Velocity Azimuth Display (VAD) method were made at Holloman AFB, NM from the end of July through mid-August 1998. These data were entered real-time into the space maneuver vehicle descent analysis program to make flight performance predictions needed for test decisions. Daily measurements encompassed the early morning time period associated with the stably-stratified nocturnal boundary layer (NBL). Measurement periods were characterized by growth the decay of wind maxima or jets at different altitudes. Strong vertical shears were often observed in conjunction with these wind maxima. Relative backscatter profiles at the lowest altitudes exhibited periodic oscillations on most mornings. Relative backscatter profiles at the lowest altitudes exhibited periodic oscillations on most mornings. The observed NBL wind profiles were poorly represented by the Ekman model.