Bart Geerts

An Overview of the International H2O Project (IHOP_2002) and Some Preliminary Highlights

The International H2O Project (IHOP_2002) is one of the largest North American meteorological field experiments in history. From 13 May to 25 June 2002, over 250 researchers and technical staff from the United States, Germany, France, and Canada converged on the Southern Great Plains to measure water vapor and other atmospheric variables. The principal objective of IHOP_2002 is to obtain an improved characterization of the time-varying three-dimensional water vapor field and evaluate its utility in improving the understanding and prediction of convective processes. The motivation for this objective is the combination of extremely low forecast skill for warm-season rainfall and the relatively large loss of life and property from flash floods and other warm-season weather hazards. Many prior studies on convective storm forecasting have shown that water vapor is a key atmospheric variable that is insufficiently measured. Toward this goal, IHOP_2002 brought together many of the existing operational and new st...

Rain in shallow cumulus over the ocean: the RICO Campaign

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...

Influence of Land Cover and Soil Moisture on the Horizontal Distribution of Sensible and Latent Heat Fluxes in Southeast Kansas during IHOP_2002 and CASES-97

Abstract Analyses of daytime fair-weather aircraft and surface-flux tower data from the May–June 2002 International H2O Project (IHOP_2002) and the April–May 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) are used to document the role of vegetation, soil moisture, and terrain in determining the horizontal variability of latent heat LE and sensible heat H along a 46-km flight track in southeast Kansas. Combining the two field experiments clearly reveals the strong influence of vegetation cover, with H maxima over sparse/dormant vegetation, and H minima over green vegetation; and, to a lesser extent, LE maxima over green vegetation, and LE minima over sparse/dormant vegetation. If the small number of cases is producing the correct trend, other effects of vegetation and the impact of soil moisture emerge through examining the slope ΔxyLE/ΔxyH for the best-fit straight line for plots of time-averaged LE as a function of time-averaged H over the area. Based on the surface energy balance, H + LE ...

Hurricane Georges's landfall in the Dominican Republic : Detailed airborne doppler radar imagery

Abstract Current understanding of landfalling tropical cyclones is limited, especially with regard to convective–scale pro–cesses. On 22 September 1998 Hurricane Georges made landfall on the island of Hispaniola, leaving behind a trail of death and devastation, largely the result of excessive rainfall, not storm surge or wind. Detailed airborne measurements were taken as part of the Third Convection and Moisture Experiment. Of particular interest are the ER–2 nadir X–band Doppler radar data, which provide a first–time, high–resolution view of the precipitation and airflow changes as a hurricane interacts with mountainous terrain. The circulation of Hurricane Georges obviously declined during landfall, evident in the rapid increase in minimum sea level pressure, the subsidence of the eyewall anvil, and the decrease in average ice concentrations in the eyewall. The eye, as seen in satellite imagery, disappeared as deep convection erupted within the eye. The main convective event within the eye, with upper–l...

Finescale Vertical Structure of a Cold Front as Revealed by an Airborne Doppler Radar

Abstract In the afternoon of 24 May 2002, a well-defined and frontogenetic cold front moved through the Texas panhandle. Detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary. This paper primarily uses reflectivity and Doppler velocity data from an airborne 95-GHz radar, as well as flight-level thermodynamic data, to describe the vertical structure of the cold front as it intersected with the dryline. The prefrontal convective boundary layer was weakly capped, weakly sheared, and about 2.5 times deeper than the cold-frontal density current. The radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail (∼40 m). The echo structure and dual-Doppler-inferred airflow in the vertical plane reveal typical features such as a nose, a head, a rear-inflow current, and a broad current of rising prefrontal air that feeds the accelerating front-to-rear current over the head. The 2D cross-fro...

The Use of Millimeter Doppler Radar Echoes to Estimate Vertical Air Velocities in the Fair-Weather Convective Boundary Layer

Abstract Vertical velocity characteristics of the optically clear convective boundary layer (CBL) are examined by means of profiling airborne radar data collected in the central Great Plains during the International H2O Project, May–June 2002 (IHOP 2002). Clear-air echoes are sufficiently strong for the radar, a 95-GHz cloud radar, to detect most of the CBL at a resolution of ∼30 m. Vertical radar transects across the CBL are remarkably dominated by well-defined plumes of higher reflectivity. These echo plumes occupy most of the depth of the CBL in the developing and mature stages of the CBL. Gust probe data indicate that the plumes tend to correspond with ascending motion. Evidence exists in the literature, and arises from this study, that the clear-air scatterers are mostly small insects. The close-range Doppler radar velocities, some 100 m above and below the aircraft, are compared to gust probe vertical velocities after both are corrected for aircraft motion. It is found that the radar vertical veloci...

The dryline on 22 May 2002 during IHOP_2002: Convective-scale measurements at the profiling site

Abstract A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer...

Mesoscale Convective Systems in the Southeast United States during 1994–95: A Survey

A preliminary survey of mesoscale convective systems (MCSs) in the southeastern United States is presented. MCSs are identified and characterized by means of high-resolution, digital, composite radar reflectivity data. Surveys of this kind are needed to give forecasters better guidance in their real-time assessment of MCS evolution, severe weather potential, and quantitative precipitation. The average lifetime and maximum length of the nearly 400 MCSs included in this survey are 9 h and 350 km, respectively. MCSs are more common in the summer months, when small and short-lived MCSs dominate. In winter larger and longer-lived systems occur more frequently. Because cold-season MCSs, which are about half as numerous as warm-season MCSs, are larger in size and duration, the MCS probability at any location is about constant throughout the year. In summer MCSs occur more commonly in the afternoon, approximately in phase with thunderstorm activity, but the amplitude of the diurnal cycle is small compared to that of observed thunderstorms. Some characteristic echo patterns are discussed.

Turbulent Mixing Processes in Atmospheric Bores and Solitary Waves Deduced from Profiling Systems and Numerical Simulation

Abstract Families of solitary waves (“solitons”) associated with two atmospheric bores on the same day were observed by an unprecedented number of ground-based and airborne profiling systems during the International H2O Project (IHOP). In addition, a very high-resolution numerical weather prediction model initialized with real data was used with success to simulate one of the bores and the evolving soliton. The predicted wave amplitude, phase speed, wavelength, and structure compared well to these extraordinarily detailed observations. The observations suggest that during the active phase (when turbulent mixing was active, which was prior to bore collapse), the bores and waves vigorously mixed dry air from above a nocturnal boundary layer down to the surface. Refractivity computed from near-surface radar observations showed pronounced decreases due to sudden drying during the passage of the bores in this phase, but refractivity increases appeared during the period of bore collapse. During both phases, the...

Simulating the IHOP_2002 Fair-Weather CBL with the WRF-ARW–Noah Modeling System. Part II: Structures from a Few Kilometers to 100 km across

Abstract Fair-weather data along the May–June 2002 International H2O Project (IHOP_2002) eastern track and the nearby Argonne Boundary Layer Experiments (ABLE) facility in southeast Kansas are compared to numerical simulations to gain insight into how the surface influences convective boundary layer (CBL) structure, and to evaluate the success of the modeling system in replicating the observed behavior. Simulations are conducted for 4 days, using the Advanced Research version of the Weather Research and Forecasting (WRF) model coupled to the Noah land surface model (LSM), initialized using the High-Resolution Land Data Assimilation System (HRLDAS). Because the observations focus on phenomena less than 60 km in scale, the model is run with 1-km grid spacing, offering a critical look at high-resolution model behavior in an environment uncomplicated by precipitation. The model replicates the type of CBL structure on scales from a few kilometers to ∼100 km, but some features at the kilometer scales depend on ...