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.
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.
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.
Atmospheric effects on normalized stellar irradiance fluctuations for the weak fluctuation regime are examined. Both monochromatic and polychromatic effects are considered. Calculations are performed using a Rytov theory which explicitly includes refractive effects arising from the polychromatic stellar source. The atmospheric turbulence along the path is specified in a spherical shell or `onion- skin' model using a vertical profile specified by either the Clear 1 model or actual thermosonde measurements. Finite optical bandwidth, receiver aperture size, and variable turbulent inner scale are also included. Good agreement is found between ground-based measurements and model results in the weak scintillation regime. It is also shown that refraction effects can significantly alter the wavelength dependence of scintillation as well as produce a leveling off and decrease of scintillation with increasing zenith angle that could be misinterpreted as saturation.
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.
A comparison of isoplanatic angles derived from balloon- borne in-situ measurements of the index of refraction structure constant profiles and remote optical measurements of stellar intensity fluctuations using an isoplanometer is shown. Concurrent data taken over a six day period in the spring of 1986 show reasonably good agreement between the methods considering normal atmospheric variability. Possible reasons for differences between individual measurements are discussed.
The Geophysics Directorate of Phillips Laboratory has recently completed redesign of a heterodyne CO2 differential absorption lidar which can simultaneously measure range resolved radial velocity, aerosol backscatter, and differential absorption. The transportable system utilizes two CO2 transversely excited atmospheric (TEA) lasers which can be discretely tuned to many of the rotational lines compromising the 00 degree 1 to 10 degrees 0 vibrational bands of CO2. These lines span a spectral region from about 9.2 to 10.8 micrometers and allow for the DIAL measurement of some minor atmospheric molecular constituents as well as many anthropogenic organic species which have absorption bands in this spectral region. Transmission and reception is coaxial via a single shared 12 inch telescope and hemispherical scanner. Complete spectral processing of the heterodyne signals provides not only backscatter and differential absorption information but also radial wind velocity. Each TEA laser produces a line dependent pulse energy of 20-80 mJ at up to 150 Hz. Presently, the system is processor limited to a net pulse rate of 140 Hz. Results shown will include time-height cross-sections of cirrus backscatter, comparisons of CO2 DIAL-derived water vapor profiles with simultaneous surface and radiosonde in-situ measurements, and wind velocity profiles in the troposphere.
A comparison of observed and modeled isoplanatic angles using synoptic meteorological data is performed over an annual cycle during 1993 - 1994. Time series of isoplanatic angles were observed during 58 night-time episodes at the Phillips Laboratory (PL) Starfire Optical Range. Isoplanatic angles were also computed for corresponding times by integration of modeled Cn2 vertical profiles derived from the Dewan and Hufnagel models. Both models used adjacent Albuquerque rawinsonde data for model input. Results show significantly smaller model error associated with the Dewan isoplanatic angles than associated with the Hufnagel isoplanatic angles. The Dewan model is also sensitive to tropopause height which is used as part of its scale length determination. Synoptic weather comparisons are accomplished for the winter period of December 1993 - February 1994. The comparisons tend to support the correlation of low isoplanatic angle with disturbed and higher isoplanatic angle with undisturbed synoptic weather patterns, respectively.
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