Aerosols & Clouds
Clouds play an important role in weather and climate on local to global scales. Clouds reflect sunlight and trap heat, affecting the Earth’s heat budget. The release of latent heat energy through the formation of clouds and precipitation is an important heat source for the atmosphere, affecting the large-scale circulations of the atmosphere. Tiny airborne particles of dust, soot, mold, and bacteria, collectively referred to as aerosols, are critical to these processes as well, either directly by reflecting sunlight or indirectly by modulating the formation of the liquid drops and ice particles in clouds.S cientists actively seek to advance our knowledge of clouds and aerosols, seeking new information on their composition, distribution, and development. Scientists conceive and use a variety of satellite-based, aircraft-based and ground-based instruments and measurement networks to make observations and develop computer models of aerosols, clouds, and their combined impacts on weather and climate.
Precipitation and Convective Systems
Diem and Brown (2003) found that anthropogenic activities in the arid Phoenix area appear to have positively affected summer precipitation totals in downwind areas, particularly the Lower Verde Basin. Shepherd (2005) extended this work by using a 108-year data rain gauge record dating back to the 1890s to establish that the Phoenix anomaly has only appeared in the post-urban period of 1950-2003. Their results showed statistically significant increases in summer monsoon season rainfall of 11-14% in the Lower Verde Basin whereas other stations, including high terrain sites had not experienced significant increases from the pre-urban to post-urban period. Shepherd (2005) also used satellite rainfall estimates to identify the anomaly even during the severe drought period of 2003 (figure 1-left). Shepherd's satellite analysis also revealed that summer monsoon storms that form in the mountains east of Phoenix often propagate westward towards the city. Shepherd hypothesized that convection-induced outflow boundaries east of Phoenix interacted with urban dynamic circulations and possibly moisture from irrigation to produce a preferred convective region during the monsoon period. Relevant References: Diem, J.E., and D.P. Brown, 2003: Anthropogenic impacts on summer precipitation in central Arizona, U.S.A. The Professional Geographer, 55(3), 343-355. Shepherd, J. M., H. Pierce, and A. J. Negri, 2002: Rainfall modification by major urban areas: Observations from spaceborne rain radar on the TRMM satellite. J. of Appl. Meteor., 41, 689-701. Shepherd, J.M., and S.J. Burian, 2003: Detection of urban-induced rainfall anomalies in a major coastal city. Earth Interactions, 7, 1-17. Shepherd, J.M., L. Taylor, and C. Garza, 2004: A dynamic multi-criteria technique for siting NASA-Clark Atlanta rain gauge network. J. of Atm. and Oceanic Technology, 21, 1346-1363. Shepherd, J.M., 2005: A Review of Current Investigations of Urban-Induced Rainfall and Recommendations for the Future, Earth Interactions (in press)
ER-2 Doppler Radar (EDOP)
The ER-2 Doppler Radar (EDOP) aboard the high-altitude ER-2 aircraft is a dual-beam 9.6 GHz radar to measure reflectivity and wind structure in precipitation systems. Beginning in 1993 EDOP has obtained reflectivity and Doppler wind measurements from a variety of mesoscale precipitation systems and hurricanes, including a classic squall line over the Gulf of Mexico and a number of major hurricanes. These data sets have provided valuable information on the structure of precipitation systems and data sets for the testing of spaceborne rain estimation algorithms. Figure 1 shows an example of EDOP reflectivity and Doppler velocity observations taken from Hurricane Georges in the Caribbean during 2001. This section shows the storm as it was passing over the Dominican Republic while being ripped apart by tall mountains on the island. Extremely strong convection is noted over the mountains that produced huge amounts of rainfall. During a number of the EDOP flights, EDOP flew in conjunction with radiometers covering frequencies from the visible to high-frequency microwaves. The combined radar/radiometer data sets from these flights have been used to develop rain estimation algorithms for the Tropical Rainfall Measuring Mission (TRMM). EDOP has also been used for TRMM validation.
The Cloud Radar System (CRS)
The Cloud Radar System (CRS) is a 94 GHz Doppler radar (W-band; 3 mm wavelength) developed for both autonomous operation in the NASA ER-2 high-altitude aircraft, and for ground-based operation. CRS is highly sensitive and therefore it is extremely useful for studies of cirrus clouds. CRS was first flown in 2002 during the CRYSTAL-FACE field campaign along with a suite of cloud remote sensors. This was the first time that a millimeter-wave radar has flown from high-altitudes with a top-down view of clouds. The image to the right shows an example of CRS-measured reflectivity and Doppler velocity measured from a weak stratiform rain region. CRS measurements have become a valuable for developing retrieval algorithms for CloudSat and the "A-Train".
The Cloud Physics Lidar (CPL)
The Cloud Physics Lidar, or CPL, is a airborne backscatter lidar designed to operate simultaneously at three wavelengths: 1064, 532, and 355 nm. The CPL flies on high-altitude research aircraft, such as the ER-2 or WB-57. The purpose of the CPL is to provide multiwavelength measurements of cirrus, subvisual cirrus, and aerosols with high temporal and spatial resolution. The CPL utilizes state-of-art technology with a high repetition rate, low pulse energy laser and photon-counting detection. Vertical resolution of the CPL measurements is fixed at 30 m; horizontal resolution can vary but is typically about 200 m. From a fundamental measurement of 180-degree volume backscatter coefficients, various data products are derived, including time-height cross-section images; cloud and aerosol layer boundaries; optical depth for clouds, aerosol, and planetary boundary layer; and extinction profiles.
Micro-Pulse Lidar Network (MPLNET)
The Micro-Pulse Lidar Network (MPLNET) is comprised of ground-based lidar systems, co-located with sun/sky photometer sites in the NASA Aerosol Robotic Network (AERONET). The MPLNET project utilizes the micro-pulse lidar (MPL) system, which is a compact and eye-safe lidar capable of determining the range of aerosols and clouds continuously in an autonomous fashion. The unique capability of this lidar to operate unattended in remote areas makes it an ideal instrument to use for a network. The primary purpose of MPLNET is to acquire long-term observations of aerosol and cloud vertical structure at key sites around the world. These types of observations are required for several NASA satellite validation programs, and are also a high priority of the Intergovernmental Panel on Climate Change (IPCC) and related programs. The combined lidar and sunphotometer measurements are able to produce quantitative aerosol and cloud products, such as optical depth, sky radiance, vertical structure, and extinction profiles. MPLNET results have contributed to studies of dust, biomass, marine, and continental aerosol properties, the effects of soot on cloud formation, aerosol transport processes, and polar clouds and snow. MPLNET data has also been used to validate results from NASA satellite sensors such as MISR and TOMS, and to help construct algorithms used to interpret space-based lidar data. MPLNET sites are used for validation of NASA's GLAS and CALIPSO satellite lidar sensors.
Scanning Raman Lidar (SRL)
Because of its importance in radiative transfer, convection, general circulation, and the hydrological cycle, atmospheric water vapor plays a crucial role in understanding atmospheric processes. For example, since water is the most active infrared molecule in the atmosphere, water vapor response is a major factor in any global warming triggered by increasing carbon dioxide. In addition, atmospheric aerosols also have a significant impact on the earth's climate by scattering and absorbing solar radiation and by altering the physical and radiative properties of clouds. Clouds also play an active role in the atmospheric radiation balance. A Scanning Raman Lidar (SRL) was developed and is used to provide frequent and accurate measurements of water vapor, aerosols and clouds to study these atmospheric processes. For this system, laser scattering by molecules (water vapor and nitrogen) and particles (suspended aerosols and cloud droplets or ice crystals) is detected as a function of altitude. Water vapor mixing ratio, which is the ratio of the mass of water vapor to the mass of dry air, is computed from the ratio of the Raman scattering from water vapor and nitrogen. When combined with measurements of temperature, the lidar water vapor data gives profiles of relative humidity. The lidar water vapor data acquired during field experiments have been used to validate radiative transfer models and study atmospheric features such as fronts, gravity waves, drylines and bores. The water vapor measurements also assess the quality of ground, balloon, and space-based sensors. These water vapor data have been used to determine how advanced statistical techniques (spectral, multifractal, and wavelet analysis) can be used to help understand the nature and causes of atmospheric structure and variability. In addition to measuring water vapor, the Scanning Raman Lidar simultaneously measures both aerosol backscattering, extinction and depolarization. Research is underway to use measurements from the system to quantify cirrus cloud ice water content and warm cloud liquid water content. In the International H2O experiment (IHOP) held in the Oklahoma-Kansas region in 2002, the SRL provided simultaneous measurements of water vapor, aerosol backscatter/extinction/depolarization, cirrus cloud optical depth/ice water content/particle diameter and, as a new research experiment rotational, Raman temperature profiles. Figure 13 shows an example of simultaneous water vapor mixing ratio, relative humidity, cirrus cloud scattering, depolarization, ice water content and particle diameter in an evolving cirrus cloud field. Not shown but also quantified for this case are cirrus cloud optical depth and extinction to backscatter ratio. This case is being used to as a case study for state of the art cirrus cloud modeling activity also occurring in Mesoscale Atmospheric Processes. Itemized captions: a) relative humidity with respect to ice calculated from SRL water vapor and radiosonde temperatures at two hour intervals during the development of the cloud system shown in the other images. Potential temperature profiles from radiosonde measurements are shown indicating a well-mixed region in the upper part of the cirrus cloud that decreases in depth over the measurement period.Significant upper tropospheric humidification is observed due to cirrus precipitation. Ice super saturation is also observed inside the cloud. b) time series of aerosol scattering ratio image of a cloud system involving two layers. The upper layer is a cirrus cloud due to outflow from a thunderstorm system to the north. The lower layer, which shows interesting oscillations, is studied further in the main text. c) Upper: volume depolarization ratio calculated for the cloud event of June 19-20. d) Lower: particle depolarization ratio for the same period. The particle depolarization ratio provides a much stronger indication of cirrus precipitation at 27 and 29 UT. e) ice water content retrievals for the cirrus system shown that uses Raman scattering from ice along with the cloud scattering ratioWangZ2004. f) generalized particle diameter retrievals using the newly developed retrieval WangZ2004 that uses Raman scattering from ice along with the cloud scattering ratio.
Raman Airborne Spectroscopic Lidar (RASL)
Raman lidar systems have proven to be very powerful research tools for study of atmospheric radiation and dynamics by offering accurate measurements of water vapor mixing ratio, aerosol backscatter/extinction and cloud properties. However, most of these measurements have been limited to ground-based platforms (Scanning Raman Lidar) and/or nighttime measurement periods. The Raman Airborne Spectroscopic Lidar (RASL), developed under the first NASA Instrument Incubator Program, offers measurements of water vapor, aerosols and clouds under both day and night conditions. It is the only airborne lidar system that combines the ability to measure water vapor mixing ratio and aerosol extinction either daytime or nighttime. The first time that RASL was turned on, it was run for 24 continuous hours and demonstrated measurement capability that met or exceeded numerical simulation predictions. Simultaneous measurements of water vapor mixing ratio, aerosol and cirrus cloud depolarization and cirrus cloud optical depth and extinction to backscatter ratio from that measurement period in September, 2002 are shown in figure xx. RASL is now being configured for first flight. Itemized captions: a) water vapor mixing ratio during a daytime segment of the measurement period. Convective plumes in the water vapor field are observed at ~1400 UT. b) boundary layer aerosol depolarization during the nighttime showing significant stratification. c) Cirrus cloud scattering ratio measured during the daytime d) Cirrus cloud depolarization measured at night. Cirrus precipitation is seen between 1400 and 1500 UT. A segment of the cloud that is believed to be supercooled water can be seen after 1530 UT. e) Cirrus optical depth and extinction to backscatter ratio (EB) quantified during the daytime f) Cirrus optical depth and extinction to backscatter ratio quantified during the nighttime