James D. Spinhirne

Micro pulse lidar

An eye safe, compact, solid-state lidar for profiling atmospheric cloud and aerosol scattering is described. The transmitter of the micro pulse lidar is a diode pumped microjoule pulse energy, high-repetition-rate Nd:YLF laser. Eye safety is obtained through beam expansion. The receiver uses a photon counting solid-state Geiger mode avalanche photodiode detector. Data acquisition is by a single card multichannel scaler. Daytime background induced quantum noise is controlled by a narrow receiver field-of-view (FOV) and a narrow bandwidth temperature controlled interference filter. Dynamic range of the signal is limited by optical geometric signal compression. Signal simulations and initial atmospheric measurements indicate that systems built on the micro pulse lidar concept are capable of detecting and profiling all significant cloud and aerosol scattering through the troposphere and into the stratosphere. The intended applications are scientific studies and environmental monitoring. which require full-time unattended measurements of the cloud and aerosol height structure. >

On the formation and persistence of subvisible cirrus clouds near the tropical tropopause

We have used a detailed cirrus cloud model to evaluate the physical processes responsible for the formation and persistence of subvisible cirrus near the tropical tropopause and the apparent absence of these clouds at midlatitudes. We find that two distinct formation mechanisms are viable. Energetic tropical cumulonimbus clouds transport large amounts of ice water to the upper troposphere and generate extensive cirrus outflow anvils. Ice crystals with radii larger than 10 – 20 μm should precipitate out of these anvils within a few hours, leaving behind an optically thin layer of small ice crystals (τvis ≃ 0.01 – 0.2, depending upon the initial ice crystal size distribution). Given the long lifetimes of the clouds, wind shear is probably responsible for the observed cloud thickness ≤1 km. Ice crystals can also be generated in situ by slow, synoptic scale uplift of a humid layer. Given the very low temperatures at the tropical tropopause (≃−85°C), synoptic-scale uplift can generate the moderate ice supersaturations (less than 10%) required for homogeneous freezing of sulfuric acid aerosols. In addition, simulations suggest that relatively large ice crystal number densities should be generated (more than 0.5 cm−3). The numerous crystals cannot grow larger than about 10–20 μm given the available vapor, and their low fall velocities will allow them to remain in the narrow supersaturated region for at least a day. The absorption of infrared radiation in the thin cirrus results in heating rates on the order of a few K per day. If this energy drives local parcel temperature change, the cirrus will dissipate within several hours. However, if the absorbed radiative energy drives lifting of the cloud layer, the vertical wind speed will be about 0.2 cm-s−1, and the cloud may persist for days with very little change in optical or microphysical properties. The fact that these clouds form most frequently over the tropical western Pacific is probably related (through the nucleation physics) to the very low tropopause temperatures in this region. Simulations using midlatitude tropopause temperatures near −65°C suggest that at the higher temperatures, fewer ice crystals nucleate, resulting in more rapid crystal growth and cloud dissipation by precipitation. Hence, the lifetime of thin cirrus formed near the midlatitude tropopause should be limited to a few hours after the synoptic-scale system that initiated cloud formation has passed.

Thin and subvisual tropopause tropical cirrus: Observations and radiative impacts

Abstract In situ microphysical, remote sensing, and satellite observations of thin and subvisible cirrus have been used to establish their frequency of occurrence, determine their mean optical depths and radiative forcings, and to analyze their association with deep convection. A spatially thin layer of cirrus, with both base and top above 15 km, was observed in the central Pacific Tropics 29% of the time, with a mean thickness of 0.47 km, using a nadir-pointing Nd:YAG lidar operating at 1.064 μm during the Central Equatorial Pacific Experiment (CEPEX). In situ microphysical data collected in the mid-1970s and mid-1980s by a WB-57 and Learjet near Kwajalein, Marshall Islands, are revisited to determine typical ice crystal sizes and shapes that occur in this cloud type. Three observed vertical profiles, obtained from ascents/descents through cloud, are used with a δ-four-stream radiative transfer model to calculate observed heating rates of up to 1.0 K day−1, principally in the infrared, and cloud radiativ...

Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part II: Marine Stratocumulus Observations

A multispectral scanning radiometer has been used to obtain measurements of the reflection function of marine stratocumulus clouds at 0.75, 1.65 and 2.16 pm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE), conducted off the coast of southern California during July 1987. Multispectral images of the reflection function were used to derived the optical thickness and effective particle radius of stratiform cloud layers on four days. In addition to the radiation measurements, in situ microphysical measurements were obtained from the University of Washington Convair C- 13 I A aircraft. In this paper we compare remote sensing results with in situ observations, which show a good spatial correlation for both optical thickness and effective radius. These comparisons further show systematic differences between remote sensing and in situ values, with a tendency for remote sensing to overestimate the effective radius by -2-3 pm, independent of particle radius. The optical thickness, in contrast, is somewhat overestimated for small optical thicknesses and underestimated for large optical thicknesses. An introduction of enhanced gaseous absorption at a wavelength of 2.16 pm successfully explains some of these observed discrepancies. Marginal probability density functions of optical thickness, liquid water path and effective radius have been derived from our remote sensing results. The joint probability density function of liquid water path and effective radius shows that the effective radius increases as the liquid water path increases for optically thin clouds, in contrast to optically thick clouds for which the effective radius decreases with increasing liquid water path.

Global monitoring of clouds and aerosols using a network of micropulse lidar systems

Long-term global radiation programs, such as AERONET and BSRN, have shown success in monitoring column averaged cloud and aerosol optical properties. Little attention has been focused on global measurements of vertically resolved optical properties. Lidar systems are the preferred instrument for such measurements. However, global usage of lidar systems has not been achieved because of limits imposed by older systems that were large, expensive, and logistically difficult to use in the field. Small, eye-safe, and autonomous lidar systems are now currently available and overcome problems associated with older systems. The first such lidar to be developed is the Micro-pulse lidar System (MPL). The MPL has proven to be useful in the field because it can be automated, runs continuously (day and night), is eye-safe, can easily be transported and set up, and has a small field-of-view which removes multiple scattering concerns. We have developed successful protocols to operate and calibrate MPL systems. We have also developed a data analysis algorithm that produces data products such as cloud and aerosol layer heights, optical depths, extinction profiles, and the extinction-backscatter ratio. The algorithm minimizes the use of a priori assumptions and also produces error bars for all data products. Here we present an overview of our MPL protocols and data analysis techniques. We also discuss the ongoing construction of a global MPL network in conjunction with the AERONET program. Finally, we present some early results from the MPL network.

Lidar observations of vertically organized convection in the planetary boundary layer over the ocean

Abstract Observations of a convective planetary boundary layer (PBL) were made with an airborne, downward-looking lidar system over the Atlantic Ocean during a cold air outbreak. The lidar data revealed well-organized, regularly spaced cellular convection with dominant spacial scales between two and four times the height of the boundary layer. It is demonstrated that the lidar can accurately measure the structure of the PBL with high vertical and horizontal resolution. Parameters important for PBL modeling such as entrainment zone thickness, entrainment rate, PBL height and relative heat flux can be inferred from the lidar data. It is suggested that wind shear at the PBL top may influence both entrainment and convective cell size.

The 27–28 October 1986 FIRE IFO Cirrus Case Study: Spectral Properties of Cirrus Clouds in the 8–12 μm Window

Abstract Lidar and high spectral resolution infrared radiance observations taken on board the ER-2 on 28 October 1986 are used to study the radiative properties of cirrus cloud in the 8–12 μm window region. Measurements from the High-spectral resolution Interferometer Sounder (HIS) indicate that the spectral variation of the equivalent blackbody temperature across the window can be greater than 5°C for a given cirrus cloud. This difference is attributed to the presence of small particles. A method for detecting cirrus clouds using 8 μm, 11 μm, and 12 μm bands is presented. The 8 μm band is centered on a weak water-vapor absorption line while the 11 μm and 12 μm bands are between absorption lines. The brightness temperature difference between the 8 and 11 μm bands is negative for clear regions, while for ice clouds it is positive. Differences in the 11 and 12 μm channels are positive, whether viewing a cirrus cloud or a clear region. Inclusion of the 8 μm channel therefore removes the ambiguity associated ...

Vertical Distribution of Aerosol Extinction Cross Section and Inference of Aerosol Imaginary Index in the Troposphere by Lidar Technique

Abstract Vertical profiles of aerosol extinction and backscatter in the troposphere are obtained from multizenith angle lidar measurements. A direct slant path solution was found to be not possible due to horizontal inhomogeneity of the atmosphere. Regression analysis with respect to zenith angle for a layer integration of the angle-dependent lidar equation was thus employed to determine the optical thickness and aerosol extinction-to-backscatter ratio for defined atmospheric layers, and subsequently, cross-section profiles could be evaluated. Measurements were made with an elastic backscatter ruby lidar system with calibration by a standard target procedure. The results from 20 measurement cases are presented. For layer-aerosol optical thicknesses >0.04, useful results were obtained, and corroboration by solar radiometer aerosol optical depth data was found. The mean mixed-layer aerosol extinction-to-backscatter ratio for the measurements was 19.5 sr with a standard deviation of 8.3 sr. With the use of a...

The 27–28 October 1986 FIRE IFO Cirrus Case Study: Cloud Microstructure

Abstract The temperature and windfield structure and hydrometeor composition of cirrus clouds sampled by the NCAR King Air and Sabreliner aircraft on 28 October 1986 near Madison, Wisconsin are described as part of a case study that examines cirrus cloud radiative and microphysical properties. Two cloud layers were sampled from top to base. The upper layer was found at altitudes between 8.5 and 11.5 km and the lower between 6.0 and 8.5 km. Vertical velocities calculated from the increase in ice mass flux with height were typical of synoptic scale lifting. Stronger vertical velocities were measured in convective cells at the top of the lower layer. The total ice particle concentration was dominated by particles <200 μm. Mean particle size and ice water content increased with decreasing altitude. The largest particles, exceeding 1000 μm in the upper layer and 1500 μm in the lower layer, probably resulted from aggregation, even at cold temperatures. Cloud emissivity and optical depth were calculated from the...

The 5–6 December 1991 FIRE IFO II Jet Stream Cirrus Case Study: Possible Influences of Volcanic Aerosols

Abstract In presenting an overview of the cirrus clouds comprehensively studied by ground-based and airborne sensors from Coffeyville, Kansas, during the 5–6 December 1992 Project FIRE IFO II case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur-based particles that became effective cloud-forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded within the upper-level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol-enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations incl...