NCAR/ATD - ASR99 - ATD Research Activities

ATD Research Activities

Footprint for Measurement of Atmosphere-Surface Exchange Fluxes -- T. Horst (ATD/SSSF)

The flux footprint relates the vertical flux measured at some height above the surface to the upwind spatial distribution of atmosphere-surface exchange fluxes. A quantitative description of the flux footprint is required both for the design of field experiments and to interpret micrometeorological flux measurements. Tom Horst (ATD/SSSF) has published theoretical estimates of the eddy-covariance flux footprint that are based on an analytic model for vertical diffusion within the atmospheric surface layer. Currently, the flux footprint is being used to estimate the dependence on atmospheric stability of the depth of the internal boundary layer that develops downwind of a step change in the surface flux and of the so-called "blending height" at which fluxes above a heterogeneous surface are spatially uniform.

Thermodynamical and Radiative Impact of the Correction of Sounding Humidity Bias in the Tropics -- F. Guichard, D. Parson, E. Miller (all ATD/SSSF)

The bias discovered in the measurement of water vapor in widely deployed radiosonde systems has significant meteorological implications. A scheme correcting for this dry bias has been applied to data taken during the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE).

The impact of the bias on the general stability of the tropical atmosphere to deep convection is quite large. The uncorrected data set leads to the erroneous conclusion that it is extremely difficult to trigger deep convection. When the correction is taken into account, the atmosphere over the tropical western Pacific becomes typically unstable to deep convection, with convective instability is similar to that measured from aircraft.

The precipitable water increase also leads to significant changes in the radiative budget. For clear sky conditions, it is found that, on average, the net surface radiative flux increases by 4 W m-2 and the outgoing long wave flux decreases by more than 2 W m-2, due to the humidity correction.

Middle Latitude-Tropical Exchanges and the Stability of the Tropical Atmosphere -- D. Parsons (ATD/SSSF)

Collaborative research using TOGA COARE data has found that extremely dry air from middle latitude waves frequently intrudes from the upper-troposphere of the middle latitudes into the equatorial troposphere over the western Pacific (Yoneyama and Parsons 1998). A subsequent study has examined the behavior of the equatorial atmosphere following the arrival of one of these middle latitude air masses (Parsons et al. 2000). From the magnitude of the drying and from the frequency of these events, we propose that dry intrusions must be a major contributor to the tropospheric moisture budget over the region during COARE, making it difficult for the atmosphere to reach a radiative-convective equilibrium. Intrusions, instead, can help to recharge the tropical atmosphere by decreasing convective activity and, thus, driving the atmosphere toward unusually large values of convective available potential energy. In contrast to the hypothesis of a pure radiative-convective equilibrium it was found that convective inhibition can be large enough to partly suppress convection following dry intrusions and that the diurnal variation in rainfall is partly due to modulations in convective inhibition. The modulations in convective inhibition are, in turn, caused by diurnal variations in the vertical profiles of radiation, in surface fluxes and perhaps in large-scale subsidence, leading to a minimum in convective inhibition and a maximum in rainfall during the late afternoon. In contrast, studies of this type of convection have generally emphasized diurnal variations in the surface fluxes and often ignored convective inhibition and diurnal variations in atmospheric radiative heating.

Data Assimilation -- D. Parsons (ATD/SSSF)

Two different data assimilation paths were explored in collaborations between ATD and outside investigators. The first study examined mesoscale data assimilation strategies for a strong convective event (Guo et al. 1999). In this study, we performed a series of real-data assimilation experiments using the MM5 four-dimensional variational data assimilation (4DVAR) system with a full physics adjoint. With a grid size of 20 km and 15 vertical layers, the MM5-4DVAR system successfully assimilated wind profiler, hourly rainfall, surface dew point, and ground-based GPS precipitable water vapor data. The MM5-4DVAR system was able to reproduce the observed rainfall in terms of precipitation pattern and amount, and substantially reduced the model errors when verified against independent observations. Additional data assimilation experiments were conducted to assess the relative importance of different types of mesoscale observations on the results of assimilation. In terms of the assimilation model's ability to recover the vertical structure of moisture and in reproducing the rainfall pattern and amount, the wind profiler data had the maximum impact.

The second study examined methods to improve the landfall prediction of extratropical storm systems (Browning et al. 2000). The study used the British Meteorological Office and European Center for Medium Range Forecast models. The basic principal of the study was to improve the model initial conditions over the data sparse north Atlantic Ocean region through using two different methods to modify of the potential vorticity (PV) fields. The first approach used satellite data and conceptual models to infer position errors in the initial PV analysis, while the second used singular vector analysis to estimate the model sensitivity to errors in the initial conditions. In both approaches, standard inversion techniques were applied to recover the wind field from the new PV field for the initialization of the models. Modest improvements were noted from these two approaches suggesting the need for a systematic follow-up study by a forecast center. Such a study is underway at the British Meteorological Office.

Lidars in Flat Terrain (LIFT) -- S. Cohn (ATD/SSSF)

Three lidars were deployed at the LIFT experiment in 1996, a backscatter lidar (SABL), a Doppler lidar (HRDL), and an ozone DIAL. In addition, two ISS and three Flux-PAM stations were deployed for a companion field study, Flatland Observatory Project II. Stephen Cohn (SSSF), along with Wayne Angevine (NOAA Aeronomy Laboratory) are examining the ability of different instruments used in this study to measure the convective boundary layer height and entrainment zone thickness. Other boundary layer properties are being studied with the LIFT dataset in collaboration with Robert Banta and Christoff Senff (NOAA/NCAR Joint Optical Remote Sensing Group), including vertical velocity statistics, boundary-layer turbulence, and shallow nocturnal low-level jets. In addition, coincident profiler and lidar data are being used by Cohn and Kent Goodrich (Univ. of Colorado) to evaluate new wind profiler signal processing techniques.

Hydrometeor Particle Type Identification

See Highlights under ATD - Radar Echo Classification Algorithm.

Precipitation Estimation -- J. Vivekanandan (ATD/RSF)

S-Pol polarimetric variables and video disdrometer data from the PRECIP-98 experiment in Florida were used to propose a method to retrieve the distribution of raindrop sizes. This technique is expected to provide improved rainfall estimates. The technique will be tested on S-Pol data collected in Brazil during TRMM/LBA and in the Alps during MAP. Traditional techniques utilize reflectivity and differential reflectivity measurements to retrieve drop size distributions. Detailed analysis of the video disdrometer data indicated a strong correlation between median droplet size and width of the droplet size spectrum. The proposed technique utilizes this relationship, together with the radar reflectivity and differential radar reflectivity to obtain improved estimates of the drop size distribution. The retrieved drop size distribution is then used to calculate rain rate.

In another study Vivekanandan and Zhang are studying the affect that raindrop canting angles may have on radar estimates of rainfall. The canting angle of the falling raindrops can affect polarimetric variables such as differential reflectivity and differential phase that are used in estimating rain rates and drop size distributions. It is generally assumed that raindrops fall with their major axis horizontal. Based on correlation measurements of co- and cross-polarized radar signals Vivekanandan and Zhang estimate the mean canting angle is zero and the standard deviation is 10 degrees for Colorado convective storms. They are in the process of expanding this study to other locations including both convective and stratiform precipitation.

PEM Tropics B -- S. Oncley (ATD/SSSF)

Steven Oncley (ATD/SSSF) was supported by the NASA Global Tropospheric Experiment (GTE) to participate in their second Pacific Exploratory Mission to the Tropics. Measurements by the NASA P-3 and DC-8 aircraft primarily focused on the oxidizing potential of the troposphere in the tropics and the origins of sulfur species which affect marine aerosols. Oncley is determining the fluxes of several chemical species related to this study at the ocean surface and through the top of the planetary boundary layer. As part of this effort, he is working with Lee Thornhill at NASA/Langley Research Center to improve measurements from their Turbulent Air Motion Measurement System (TAMMS). He also is working with Donald Blake, Isobel Simpson, and Nicole Blake of the University of California, Irvine, to develop techniques for retrieving fluxes from cannister samples.

Wildfire Research -- L. Radke (ATD)

In collaboration with NASA Ames and the US Forest Service, Larry Radke (ATD) and Terry Clark (NCAR/MMM) participated in the Wildfire Experiment (WIFE) in 1998 and the Boreal Frost Fire Experiment in 1999. This research focused on extreme fire behavior such as rapid intensification of fire-line dynamics, horizontal fire vortices, crown fires and erratic spread rates and is being used to validate NCAR's Atmosphere-Fire Model. High speed imagery by NCAR's Thermacam, which was deployed on the C-130 and a US Forest Service Piper Navajo aircraft, allowed observations of fires at temporal scales of seconds and studies of fire behavior over periods of tens of minutes.

MAP -- T. Weckwerth, J. Vivekanandan, J. Wilson, and W.-C. Lee (all ATD/RSF)

Weckwerth and her RSF colleagues participated in MAP as project managers for the S-Pol and ELDORA radars. An excellent data set was obtained in orographic rain situations. RSF scientists will concentrate on a) verification of precipitation estimates and hydrometeor particle type estimates by S-Pol, b) evaluation and utilization of water vapor measurements by a French lidar that was on board a French aircraft and c) detailed wind analysis along the south slope of the Alps during orographic rainfall events. RSF scientists are establishing a variety of collaborations with foreign and U.S colleagues to analyze these and other data collected in MAP.

Convective Storm Research -- J. Wilson, T. Weckwerth, P. Hildebrand, W.-C. Lee (all ATD/RSF)

Jim Wilson, in collaboration with Rit Carbone of MMM and Tom Keenan of the Australian Bureau of Meteorology submitted manuscripts describing the evolution of large thunderstorms (17-20 km high) which occur regularly over the Tiwi Islands off the north coast of Australia. These tropical thunderstorms, called Hector, occur over flat terrain. By late morning small showers and thunderstorms form along the lee coast sea breeze front. By early to mid-afternoon these storms grow to a stage where a gust front of sufficient strength develops allowing storms to move away from the sea-breeze front. As the gust front moves westward it intersects the east-west orientated sea breeze front. Hector then forms where the gust front intersects existing cumulus clouds along the sea breeze front. When gust fronts collide particularly large Hectors form.

Jim Wilson in collaboration with Roger Wakimoto of UCLA prepared an invited manuscript for the Bulletin of the AMS entitled The Discovery of the Downburst - T.T. Fujita Contribution. This paper and planned presentation is part of a symposium and special issue of the Bulletin in honor of the late Ted Fujita.

Currently Weckwerth is assembling a manuscript discussing convection initiation by rolls. She has shown that significant variations in moisture occur on small scales that are not well measured by radiosondes. When soundings are launched outside of roll updraft regions (where clouds and thunderstorms will form), they are not representative of the environmental stability and cannot accurately be used to forecast thunderstorm initiation. Using aircraft data collected during the Convection and Precipitation/Electrification (CaPE) project, maximum boundary layer moisture measurements were found in the roll updraft branches. Using these moisture values to modify the soundings and re-calculate the stability parameters produced an accurate forecast of thunderstorm potential.

Wen-Chau Lee continued his severe storm research with UCLA and NOAA HRD. The 3-D structure of a mid-latitude squall line has been deduced using a newly developed 3-D variational dual-Doppler analysis technique. They were able to analyze data for any elevation angle using this new technique. The circulation near the storm top and anvil region over the aircraft has been revealed in this analysis.

Peter Hildebrand using high resolution data collected by ELDORA during VORTEX (1995), is preparing a manuscript that describes hail growth and hail recirculation in a multi-cellular storm.

Boundary-Layer Research -- T. Horst (ATD/SSSF), T. Weckwerth, J. Wilson, V. Wulfmeyer (all ATD/RSF)

Tammy Weckwerth, Tom Horst and Jim Wilson published a paper describing the evolution of horizontal convective rolls. They found that rolls were the first form of boundary layer convection observed by radar every morning during the Small Cumulus and Microphysical Study (SCMS) in Florida during the summer of 1995. Additionally rolls occur with negligible wind speeds when a sensible heat flux threshold was achieved.

V. Wulfmeyer investigated turbulent processes in the convective boundary layer using differential absorption lidar (DIAL) and radar-rass. Efforts were focused on comparing the LES model with experimental data; two JAS publications resulted from this work. Wulfmeyer in collaboration with Don Lenschow and Christopher Senff from NOAA, used Doppler and DIAL data to derive turbulent moments in the convective boundary layer. A paper describing the work has been accepted by JTECH.

Wulfmeyer also collaborated with Graham Feingold of CIRA to investigate microphysical aerosol parameters in the boundary layer. They used a combination of DIAL data and a simple aerosol model. They found an interesting low solubility of the aerosol. The results are presented in a paper in JGR which is in press.

Water-Vapor Measurement -- T. Weckwerth, V. Wulfmeyer (all ATD/RSF)

Weckwerth and Wulfmeyer collaborated with scientists from NCAR and NOAA to publish a summary of the NCAR/NOAA Lower-Tropospheric Water Vapor Workshop held at NCAR in June, 1998. The outcome was recently published in the Bulletin of the American Meteorological Society and summarizes the existing water vapor measuring platforms and the scientists' needs for improved measurements.

Wulfmeyer investigated the future potential of the DIAL technique to map water vapor by means of simulations. He explored the potential of DIAL to perform high resolution, scanning airborne and ground-based measurements. The main result was that a DIAL system with an average power of 10W can be used to map 3-D water vapor fields from ground and with lower resolution from aircraft.

Hurricane Research -- W.-C. Lee (ATD/RSF)

Wen-Chau Lee continued his collaborations with the NOAA Hurricane Research Division and the National Taiwan University on the Ground-Based Velocity Track Display technique (GBVTD) to retrieve the tropical cyclone's primary circulation from a single ground-based coastal Doppler radar. On the operational end, the GBVTD technique has been applied in real time in the NOAA Tropical Prediction Center during the 1999 hurricane season on the WSR-88D level-IV data. These GBVTD-derived wind fields provided additional information to hurricane specialists in making forecasts. The Joint Typhoon Warning Center has expressed interests in implementing the GBVTD technique. An objective algorithm to identify tropical cyclone circulation center, the GBVTD-simplex method, has been developed and tested using historical tropical cyclones. The GBVTD-derived 3-D tangential winds have been used to diagnose the barotropic instability mechanism in tropical cyclones. Initial results indicate that barotropic instability did exist in Typhoon Herb (1986).

References

Guo, Y.-R., Y.-H. Kuo, J. Dudhia, D. Parsons, and C. Rocken, 2000: Four-dimensional variational data assimilation of heterogeneous mesoscale observations for a strong convective case. Mon. Wea. Rev., in press.

Horst, T.W., 1999: The footprint for estimation of atmosphere-surface exchange fluxes by profiling techniques. Bound. Layer Meteor. 90, 171-188.

Guichard. F., D. Parsons, and E. Miller, 2000: Thermodynamical and Radiative Impact of the Correction of Sounding Humidity Bias in the Tropics. J. of Clim., accepted.

Horst, T.W., 199: On frequency response corrections for eddy covariance flux measurements. Bound. Layer Meteor., accepted

Parsons, D.B., Kunio Yoneyama, and Jean-Luc Redelsperger , 2000: The Evolution of the Tropical Western Pacific Atmosphere-Ocean System Following the Arrival of a Dry Intrusion, Quart. J. Roy. Meteor. Soc., 126, 1-32.

Weckwerth, T.M., T.W. Horst, and j.W. Wilson, 1999: An observational study of the evolution of horizontal convective rols. Mon. Wea. Rev., 127, 2160-2179

Yoneyama, K., and D.B. Parsons, 1999: A mechanism for the intrusion of dry air into the tropical western Pacific region. J. Atmos. Sci., 56, 1524-1546.