Divisional Activities: Research
With the outcome of ATD's field project support comes reams of data that our Research Data Program organizes and makes available to the scientific community via our Research Data and Deployment Archive. ATD scientists also play an intergral part in the study of this data and provide ongoing analysis in collaboration with the international scientific community. This document outlines the progress made in various projects throughout the year.
Strategic Initiatives
Water Cycle and Related Research
International H2O Project (IHOP_2002) and Related Research
ATD scientists are utilizing IHOP data in a variety of research activities focused on the IHOP goal of understanding water vapor fields as a way to improve quantitative predictions of precipitation. These research activities include:
- Use of S-Pol and mobile Doppler radar, radiosonde, TAOS, mobile mesonet, fixed surface station, and airborne water vapor DIAL data from IHOP to understand details and evolution of horizontal and vertical distributions of water vapor in the quiescent boundary layer;
- Use of serial radiosonde ascents from many sites to assess the representativeness and evolution of soundings which are typically used to forecast daily convective potential;
- Use of IHOP data to investigate mechanisms that can initialize and maintain nocturnal precipitation systems;
- Comparison of radar refractivity data from IHOP with mobile mesonet, fixed surface station, TAOS, and NASA scanning Raman lidar measurements from the same region to produce a thorough evaluation of the utility of surface-layer refractivity retrievals to be used as an indicator of boundary-layer moisture distributions (collaboration with F. Fabry (McGill University), funding from water cycle, FAA, and USWRP);
- Use of multiple airborne and ground-based water vapor sensors from IHOP to map the moisture field prior to convection initiation, including examination of moisture variability along boundaries, and combining water vapor measurements with kinematic measurements from the mobile and fixed Doppler radars (collaboration with C. Flamant (CNRS, France), funding from water cycle and USWRP); and
- Use of reference radiosonde and other data from IHOP to explore the impact of instrumental differences on temporal and spatial inhomogeneity of U.S. radiosonde humidity data.
Evaluation of performance of dropsonde humidity sensor:
The dropsonde humidity data are under-utilized due to lack of knowledge
on the quality of dropsonde humidity data. The dry bias in dropsonde
humidity
data found by previous studies were preliminarily evaluated by comparing
with
collocated radiosonde data during IHOP. The comparison shows good agreements
except inside moist layers. The performance of the dropsonde humidity sensor
within clouds was evaluated using DYCOMS-II data. The maximum RH inside clouds
does not show 100% all the time, but is within the sensor accuracy range
(95-100%). The dropsonde humidity data were compared with that collected
by a dew-point hygrometer on C130 aircraft. The dropsonde humidity sensor
experienced large time-lag errors when it descended from a very dry environment
above clouds into clouds. Mean estimated time-constant of the sensor is 5
s at 15C, which is much larger than 0.1 s at 20C given by the manufacture.
The dropsonde humidity sensor still reported near-saturation RH after it
exited
clouds because of water on the sensor. The preliminary study suggests that
the time it takes for evaporation of water on the sensor depends on cloud
thickness and cloud liquid water path. The alternative sensor heating for
twin humidity
sensor (not currently implemented) might help speed up evaporation of
the water.
Comparisons of seven years (1996-2002) of radiosonde data at two neighboring stations:
Sixty-three pairs of soundings were launched within half hours at Norman (Vaisala RS80-H used) and ARM-B6 (Vaisala RS90 used) sites during IHOP. Two sites are 25 km apart. By visually examining all of them individually, 52 pairs of soundings were selected for comparisons of temperature and humidity data from RS80-H and RS90. The comparisons show that the RS90 data are consistently drier than RS80-H by ~5% at ~9 km, and are warmer by ~0.5C. In order to understand the consistent and significant differences, we expanded the comparison to 7-years (1996-2002) of radiosonde data collected at these two stations. The comparisons show unexplained significant and consistent drier RS90 than RS80-H in the upper troposphere, and warmer temperature (~0.5C) in the middle and upper troposphere at ARM-B6 than at Norman even when Vaisala RS80-H sondes were launched at both sites.
Biogeochemistry Initiative
Biogeosciences Initiative (BGSI): Research and Instrument Developments
Airborne CO2 and CO Measurements (This section also found on Divisional Activities: Technology)
As part of the Biogeosciences Strategic Initiative, the NCAR Airborne Community Trace Gas Measurement Group (ATD/ACD) designed and constructed an instrument that measures CO2 mixing ratios or fluxes. Precise and accurate airborne measurements of its mixing ratio is a key tool in understanding regional scale land-atmosphere-ocean carbon exchange, which can reveal much about the health of the regional environment.
Vertical flux measurements using the eddy correlation method yield information on dynamic exchange at interfaces, including for example, the surface - boundary layer and boundary layer - free tropospheric interfaces. The highly modified instrument is based on a commercial broadband infrared absorption instrument. The new design represents a novel approach to data acquisition and processing which are expected to improve instrument noise specifications to meet or exceed present state-of-the-art capabilities.
Initial flight tests were conducted as part of the IDEAS-III test program in September, 2003. Preliminary assessments give some hope that airborne flux measurements may be possible with small or no artifact produced from interaction between the sensor components and aircraft motion. The instrument hardware and processing software will be refined in FY04 to provide further precision improvements for both mixing ratio and flux measurement modes of operation.
Lower tropospheric carbon monoxide is a useful though non-specific combustion tracer. CO mixing ratio measurements are quite powerful air mass indicators when combined with other tracer measurements such as ozone, carbon dioxide, and/or hydrocarbons. A new design was developed to modify our commercial vacuum ultraviolet resonance fluorescence carbon monoxide instrument for more improved reliabity and ease of field deployment. The existing instrument has been successfully deployed on several airborne missions. Most recently the instrument underwent a successful laboratory intercalibration exercise in April, 2003, as part of the CRYSTAL-FACE experiment. Collaborators included Dr. Teresa Campos (NCAR), Dr. Max Loewenstein, Dr. Jimena Lopez, and Dr. Hans-Jurg Jost (all of NASA-Ames). A proposal was submitted during the summer of 2003 by Dr. Ian Faloona (UC-Davis) to collaboratively explore the potential for improvement of the sensor's time response beyond its present 0.5- to 1-Hz capability.
Miniaturized gas modules were designed and constructed to allow significant weight and size reduction of support and calibration gas installations used in both CO2 and CO instruments. This approach will also allow standard gas calibration under controlled laboratory conditions.
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The RAF Oxygen Analyzer (ROXAN) was adapted and tested for airborne oxygen measurements during the IDEAS II campaign. The cause of a persistent motion-sensitivity was identified and resolved, resulting in a precision comparable to or better than existing ground-based techniques. This instrument is currently being repackaged by ATD-BGSI for future laboratory and field studies relating to the global carbon cycle.
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WLEF Tall-tower O2 Measurements
A modified commercial fuel-cell O2 analyzer has been operating semi-continuously at a tall tower research site in Northern Wisconsin since June of 2000, in collaboration with NOAA, USFS, and Penn State scientists. In the past year, several field visits were made to repair and upgrade the instrumentation. Results are currently being prepared for publications on the measurement technique, and on applications in plant physiology, forest ecology, industrial emission verification, and continental boundary-layer mixing.
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NCAR CO2 and O2 Calibration Facility
In collaboration with NOAA CMDL and Scripps Institution of Oceanography, ATD-BGSI is working to establish internal calibration scales for CO2 and O2 that will be used to support a wide range of NCAR studies. This facility will include a suite of 6 primary reference cylinders with an expected lifetime greater than 20 years and capabilities for filling, spiking, and calibrating secondary cylinders.
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CO2 Budget and Regional Airborne Study, North America (COBRA-NA 2003)
ATD-BGSI scientists participated in this study during June, investigating regional CO2 fluxes across North America. The UND Citation was based at RAF for instrument integration and staging during the month-long campaign. The NCAR Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) was used to collect over 400 discrete samples which are being analyzed for CO2, CH4, N2O, H2, and SF6 concentrations, and 13C in CO2, 18O in CO2, 13C in CH4, O2/N2, and Ar/N2 ratios. Collaborators in this study include Harvard University, NOAA CMDL, and University of North Dakota.
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Niwot Ridge CO2 Studies
As part of the 2002 Niwot Ridge Pilot Experiment, ISFF data were analyzed
and synthesized with observations from other groups. In collaboration
with CU, the HYDRA CO2 measurement system is being redeployed for an
undersnow CO2 experiment this winter.
The Carbon in the Mountains Experiment (CME) is a BGSI proposal to NSF Biocomplexity
involving ATD, CGD, MMM, and CU researchers. This study will involve the deployment
of 9 independent CO2 systems at the Niwot Ridge site and interpretation of
their measurements using a high-resolution
data-assimilation model to examine carbon exchange in complex environments.
In FY03, several robust, inexpensive CO2 analyzers were tested and a prototype
CO2 measurements system is now being assembled. This work will benefit from
advances in communications, power, meteorological observations, and network
deployments being made in the Intelligent Sensor Array Initiative.
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ATD scientists have assembled and tested a prototype of the TRAM
system,
which features sensor packages which move along a cable supported by
towers. This approach allows the spatial variation of a variety of atmospheric
quantities at small scales, such as those within a forest canopy, to
be sampled. Improvements to the system are planned based upon the test
results.
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Advanced Observing Systems Research
Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CrystalFACE)
NASA’s CRYSTAL-FACE project, which is a collaborative research effort with UND, CSU, U. Illinois, CalTech, U. Colorado and Oregon State U, is a comprehensive study with modeling, field observations and analyses to improve the scientific understanding of tropical cirrus cloud systems and their roles in regional and global climates. ATD’s participation in the NASA CRYSTAL-FACE project emphasized two areas of research:
1) Gas phase measurements of H2O and CO to document the origin of air and mixing processes affecting cirrus clouds. ATD’s water vapor and trace gas instruments were mounted on the CIRPAS Twin Otter aircraft, and measurements were made in the boundary layer that feeds the cirrus cloud environment through deep convection. The results are providing insight about air mass origin, mixing processes, and anthropogenic effects on cirrus properties.
2) Cirrus ice formation and measurements of ice nucleating particles in the cirrus environment. ATD personnel participated in the installation and operation of ice nuclei instrumentation in the UND Citation aircraft to study the role of aerosol particles in the formation of cirrus cloud crystals. One particularly interesting result was the observation of mid-tropospheric dust layers that were tracked by satellite from northern Africa to southern Florida. On a few flights, the aircraft encountered these dust layers in Florida and found concentrations of ice nuclei much higher (~100X) than non-dust regions and non-dust flights. The significance of these observations relates to sources of cloud-active aerosol, long-range transport, and indirect effects of aerosols on clouds on climate.
Convection and Precipitation Processes
Inhibition of Snowfall by Pollution Aerosols (ISPA)
RTF scientists are continuing to analyze data from the Inhibition of Snowfall by Pollution Aerosols (ISPA) experiments carried out at Steamboat Springs during the winters of 2001 and 2002 in collaboration with R. Borys and D. Lowenthal of the DRI Storm Peak Laboratory. Previous work on the relationship between ice crystal fall speed and riming index is being extended using a combination of in-situ measurements made at the Storm Peak Lab on top of Mt Werner, and wind profiler measurements at the same level from the ISS site at the base of the mountain. This work used MM5 model analysis of airflow over the mountain to determine the contribution of vertical winds to fall speed measurements. In addition, techniques are being explored to estimate ice crystal size distributions based on Doppler spectral observations.
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Kinematic and Thermodynamic Controls of Deep Convection
David Parsons of ATD, in collaboration with Drs. Jean-Luc Redelsperger and Francoise Guichard of METEO France/CNRM, have published their work during the past year in the Journal of Atmospheric Science on the dependence of convective activity over the tropical Pacific on variations in tropospheric humidity. The primary finding is that lower and middle level humidity levels place a strong constraint on the depth and intensity of deep convection through the entrainment of dry air into cumulus convection. This work has implications for parameterization of deep convection in large-scale models and Parsons will be collaborating with James Hack of CGD on testing and possibly improving the treatment of convection in NCAR's community climate model.
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Tropical Rainfall Measuring Mission (TRMM)
Tropical storms play a major role in our climate through their role in the earth’s heat and moisture budget. They produce a major fraction of the Earth’s rainfall. Current attempts to predict tropical convection must understand in detail how precipitation forms in these clouds, yet there are relatively few measurements on which to base this understanding. ATD scientists, in cooperation with MMM and university scientists, are studying the formation of precipitation in tropical clouds, in cooperation with the NASA Tropical Rainfall Measuring Mission (TRMM) program. Unexpectedly, they are finding more small ice particles at warm temperatures than anticipated by current scientific thinking on tropical clouds. These results are presently in the process of being published.
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Scanning Radar Wind Field measurements
The ISS group is currently running a series of experiments that involve operating the MAPR wind profiler in scanning mode. MAPR, normally a vertically pointing radar, has been temporarily mounted on a pedestal enabling it to scan in the azimuthal and elevation directions. MAPR has multiple receiving antennas which allows it to measure winds transversely to the main antenna beam. Typical scanning weather radars, such as the Nexrad radars or S-Pol, cannot directly measure transverse winds in this manner. Scanning radars can scan over an extended area, however they only measure winds along the beam of the radar. MAPR can measure both the Doppler shift along the beam and the transverse beam winds. By applying scanning mode to MAPR, the radar can directly measure the full wind field over an extended area without relying on dual Doppler or sophisticated tracing techniques.
An
example of preliminary observations made by the system is shown in the
figure to the left.
The scanning capability of MAPR is limited by the wide beam of the radar, however the system is an important testbed for wind measurement techniques.
This work is being carried out in collaboration with Dr G. Zhang of RAP and Dr TY. Yu of the University of Oklahoma.
Boundary Layer Turbulence and Fluxes
The turbulence data collected during the HATS (Horizontal Array Turbulence Study) field project has been used by ATD scientist Tom Horst to quantify the attenuation of scalar flux measurements caused by spatial displacement of a scalar sensor from the sonic anemometer (used to measure vertical velocity). The attenuation is found to depend on the dimensionless product of the spatial displacement and the wavenumber at the peak of the scalar flux cospectrum. The functional form depends on both atmospheric stability and on the angle of the sensor displacement relative to the wind direction.
Bi-Static Radar Wind
ATD successfully implemented a set of Matlab programs for unfolding winds estimated by a bi-static receiver and then recalculated vector wind in a 2D plane using a pair of S-Pol and bi-static receivers. The software seems to function well for most of the IMPROVE I data sets except when wind pattern is tangential to the receiver configuration. Nonetheless, the software is efficient and is the only analysis tool that is capable of directly reading a netCDF file format and performing interactive analysis in the radar coordinate space. The research can be extended to quantify accuracy of the retrieved 2D wind and also to suggest an optimum configuration for bi-staic radar deployment.
Radar Remote Sensing of Supercooled Large Drops and Icing
Recent upgrades to the Particle ID (PID) algorithm, which utilizes dual-polarization weather radar data, include improving the distinction of ice crystals from super-cooled liquid water (SLW) and expanding the SLW category to include a full range of sizes from cloud droplets to large drizzle drops. The SLW category in the past consisted only of the smallest drops and the larger drops were classified as drizzle or rain, which was confusing when it occurred above the freezing level. Previous improvements to the PID focused on the larger particles such as graupel and heavy rain. Optimization efforts have shifted to the smaller, more difficult to distinguish particles.
The
improvement in SLW detection follows Vivekanandan et al. (1999) and was
implemented by Scott Ellis and Vivekanandan. Preliminary tests on IMPROVEII
data show encouraging results. On November 11, 2001 the University
of Washington Convair, research aircraft experienced icing while penetrating
a cloud consisting mostly of SLW. Simultaneous observations with the
NCAR S-Pol radar were performed allowing comparison with the aircraft.
The figure to the right shows an RHI scan taken during the time when
SLW was observed. It can be seen that the PID identifies the cloud as
mostly SLW, in qualitative agreement with aircraft in situ observations.
A more definitive study is in progress to compare in situ liquid water
content and droplet size observation with the regions classified as SLW
using polarization radar observations.
Hurricane/Tornado Research
Wen-Chau Lee and Michael Bell continue collaboration with NOAA Hurricane Research Division, National Hurricane Center, Naval Research Laboratory, and National Taiwan University to improve the operational Ground-Based Velocity Track Display (GBVTD) algorithm. A version of the GBVTD algorithm was operational at NHC during the 2002 hurricane season. This activity has been published in a Weather and Forecasting article (in press, link forthcoming). A simpler version of the code was running at the Central Weather Bureau in Taiwan during the 2002 and 2003 typhoon season. The primary research effort has been on improving the automatic hurricane center finding algorithm that takes into account both spatial and temporal continuity of key characteristics of hurricanes such as the maximum tangential wind and radius of maximum wind. The professors and graduate students at National Taiwan University and National Central University have been heavily involved in analyzing landfalling typhoons using GBVTD algorithm.
Wen-Chau Lee and Michael Bell continue collaboration with Howie Bluestein
(U. of Oklahoma) on millimeter wave
mobile radar tornado data analysis. The analysis showed a wavenumber two (elliptical
shaped) circulation in a
weak tornado. The results are documented in a Monthly Weather Review article
(in press, link forthcoming).
Vertical Transport and MiXing (VTMX) Data Analysis
The research of VTMX data sets during FY03 has focused on understanding the link between mixing and orographic flows within the stable nocturnal boundary layer. To this end ATD scientists have been collaborating with NOAA/ETL and DOE/Pacific Northwest National Laboratories on understanding how nocturnal flow evolves across the Great Salt Lake Basin and how it influences local stability (paper recently submitted). Dr. James Pinto, an ATD scientist, has also led a team at NCAR in the analysis of sodar, lidar, TAOS tether balloon and sounding data to determine the vertical extent and duration of mixing events associated with the nocturnal jet (paper to be submitted in Dec 03). A deep, pulsing nocturnal down-valley flow develops driven during the night by a combination of local pressure gradients and channeling of the larger-scale flow above the basin. This analysis demonstrates the importance of a local reduction in stability driven by increasing vertical shear associated with this drainage flow which results in Kelvin-Helmholtz waves that generate turbulence and mixing in the stable boundary layer over portions of the basin.
Profiler Measurements of Boundary-Layer Fine-Scale Structure
ATD's advanced wind profiler, the Multiple Antenna Profiler Radar (MAPR), was operated in FDI (Frequency Domain Interferometry) RIM (Range Imaging) mode during the IHOP experiment. This mode improves the range resolution of the radar from 100 meters to about 20 meters enabling very fine scale observations of the boundary layer to be made.
Recent
refinements to our analysis procedures improved the quality of the fine
scale reflectivity images produced by this technique. A significant advance
this year was the implementation of RIM wind measurements at the same
fine scale as the reflectivity imaging. An example of a RIM wind profile
in comparison with a radiosonde wind profile is shown in the figure to
the left. As can be seen, the RIM profiles show very similar fine scale
fluctuations as the radiosonde profiles. This work is being carried out
in collaboration with Dr T-Y. Yu of the University of Oklahoma and Dr.
S. Frasier of the University of Massachusetts.
Tropospheric Chemistry Advances
The Analytical Photonics & Optoelectronics Laboratory (APOL) group has been involved over the past several years in efforts to advance our understanding of tropospheric chemistry. This effort has continued this past year in the newly formed joint ATD/ACD group. Specifically, highly accurate and sensitive measurements of CH2O, acquired during the 2001 NASA-funded TRACE-P campaign, were analyzed and published in a recent special issue of the Journal of Geophysical Research.
Comparisons of CH2O measurements with those from box model calculations in all atmospheric regimes are an important component for further assessing our understanding of atmospheric processes. Unfortunately, measurement-model comparisons, even for remote background conditions where CH4 oxidation is the primary CH2O precursor, have sometimes exhibited both positive and negative deviations. These discrepancies clearly point to gaps in our understanding of CH2O production and destruction pathways, and hence in tropospheric oxidation processes. One such regime is uptake of CH2O in clouds and on marine aerosols. Although such uptake has been predicted and has been modeled, many complicating factors tend to obfuscate the observation of such uptake. In some cases where large CH2O uptake has been suspected, questions regarding measurement accuracy have been raised.
The newly published study by the APOL group report for the first time clear evidence of CH2O uptake in clouds. In one case, measurement-model comparisons revealed a peak uptake of 85% upon entering a cloud where the complicating effects of pollution were not present. This same study also revealed large CH2O uptake in the lower marine troposphere in the presence of haze. These observations are important for furthering our understanding of radical chemistry and transport over the oceans and how such chemistry may be altered by the presence of clouds and marine aerosols.