Tammy M. Weckwerth

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...

Tropospheric water vapor, convection, and climate

[1] Recent progress is reviewed in the understanding of convective interaction with water vapor and changes associated with water vapor in warmer climates. Progress includes new observing techniques (including isotopic methods) that are helping to illuminate moisture‐convection interaction, better observed humidity trends, new modeling approaches, and clearer expectations as to the hydrological consequences of increased specific humidity in a warmer climate. A theory appears to be in place to predict humidity in the free troposphere if winds are known at large scales, providing a crucial link between small‐scale behavior and large‐scale mass and energy constraints. This, along with observations, supports the anticipated water vapor feedback on climate, though key uncertainties remain connected to atmospheric dynamics and the hydrological consequences of a moister atmosphere. More work is called for to understand how circulations on all scales are governed and what role water vapor plays. Suggestions are given for future research.

Boundary Layer Clear-Air Radar Echoes: Origin of Echoes and Accuracy of Derived Winds

Abstract Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D. Operational and research meteorologists are using these Doppler velocities to derive winds. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. This paper uses dual-wavelength and dual-polarization radars to examine the cause of these echoes and the use of Doppler velocities from the clear-air return to estimate winds. The origin of these echoes has been an ongoing controversy in radar meteorology. These echoes have been attributed to refractive-index gradient (Bragg scattering) and insects and birds (particulate scattering). These echoes are most commonly observed over land from spring through autumn. Seldom do they occur over large bodies of water. Widespread clear-air echoes have also been observed in winter when temperatures are above 10°C. Radar reflectivity comparisons of clear-air echoes in Florida and Colorado wer...

Thermodynamic Variability within the Convective Boundary Layer Due to Horizontal Convective Rolls

Abstract Data from the Convection and Precipitation/Electrification (CaPE) Experiment conducted during the summer of 1991 are used to examine and quantify the horizontal variability of temperature and moisture within the convective boundary layer (CBL). Potential temperature variations were only about 0.5 K, while variations in water vapor mixing ratio values of 1.5–2.5 g kg−1 were observed throughout the CBL. Using radar, aircraft, and sounding data, it is shown that horizontal convective rolls are the likely cause of these variabilities. The enhanced moisture occurred within the roll updraft regions, thus rolls were transporting moist air from the surface upward. The observed cloud-base heights, obtained from cloud photogrammetry, were produced from the highest moisture values within the roll updraft regions. Since the roll ascending branches contained moisture values that were most representative of the observed cloud-base heights, it is likely that measurements from within the roll updrafts would prov...

Horizontal Convective Rolls: Determining the Environmental Conditions Supporting their Existence and Characteristics

Abstract Data from the Convection and Precipitation/Electrification (CaPE) project, as well as results from numerical simulations, are used to study horizontal convective rolls. The environmental conditions necessary for sustaining rolls and for influencing the aspect ratio, ratio of roll wavelength to convective boundary layer (CBL) depth, and orientation are examined. Observations and numerical model simulations both suggest that a moderate surface sensible heat flux and some vertical wind shear are necessary for roll existence. Unlike some previous studies, however, it is shown that rolls occurred within very low CBL shear conditions (∼2 × 10−3 s−1). In addition, the low-level (i.e., ∼200 m) shear seems to be more important than the shear through the depth of the CBL in roll sustenance. The aspect ratio is shown to be proportional to the CBL instability, measured in terms of the Monin–Obukhov length. The roll orientation is similar to the wind direction at 10 m AGL, the CBL wind direction, the inversio...

The Convective and Orographically Induced Precipitation Study:A Research and Development Project of the World Weather Research Program for Improving Quantitative Precipitation Forecasting in Low-mountain Regions

Abstract The international field campaign called the Convective and Orographically-induced Precipitation Study (COPS) took place from June to August 2007 in southwestern Germany/eastern France. The overarching goal of COPS is to advance the quality of forecasts of orographically-induced convective precipitation by four-dimensional observations and modeling of its life cycle. COPS was endorsed as one of the Research and Development Projects of the World Weather Research Program (WWRP), and combines the efforts of institutions and scientists from eight countries. A strong collaboration between instrument principal investigators and experts on mesoscale modeling has been established within COPS. In order to study the relative importance of large-scale and small-scale forcing leading to convection initiation in low mountains, COPS is coordinated with a one-year General Observations Period in central Europe, the WWRP Forecast Demonstration Project MAP D-PHASE, and the first summertime European THORPEX Regional...

The Effect of Small-Scale Moisture Variability on Thunderstorm Initiation

Observations during the Convection and Precipitation/Electrification (CaPE) project illustrate that horizontal convective rolls are capable of providing sufficient forcing to initiate free moist convection. Rolls occurred on the majority of days during CaPE but on only some of those days were they able to trigger thunderstorms. This study was undertaken to ascertain the difference between the two types of roll days: the storm days and the nostorm days. All obvious sounding parameters were examined: stability parameters, midlevel moisture, and vertical wind shear. None of them showed a difference between the storm and no-storm days. This is not surprising in light of recent work showing that soundings within rolls are not representative of the environmental stability unless they happen to be launched into roll updraft branches. This is due to the upward transport of warm, moist air in the roll updraft regions atop which cloud streets and sometimes thunderstorms form. Numerous other parameters examined were also fruitless in identifying any difference between the days. These included surface station measurements, cell motion relative to roll updraft locations, surface topography, and roll circulation strength and depth. The only useful predictor was obtained by modifying the soundings using aircraft data as they were flying across the rolls and sampling moisture contained within the roll updraft branches. Using these roll updraft moisture measurements to recalculate sounding stability parameters provided an effective means of predicting thunderstorm formation.

A Review of Convection Initiation and Motivation for IHOP_2002

Abstract The International H2O Project (IHOP_2002) included four complementary research components: quantitative precipitation forecasting, convection initiation, atmospheric boundary layer processes, and instrumentation. This special issue introductory paper will review the current state of knowledge on surface-forced convection initiation and then describe some of the outstanding issues in convection initiation that partially motivated IHOP_2002. Subsequent papers in this special issue will illustrate the value of combining varied and complementary datasets to study convection initiation in order to address the outstanding issues discussed in this paper and new questions that arose from IHOP_2002 observations. The review will focus on convection initiation by boundaries that are prevalent in the U.S. southern Great Plains. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or convergence in Doppler velo...

An observational study of the evolution of horizontal convective rolls

Abstract A comprehensive observational dataset encompassing the entire temporal evolution of horizontal convective rolls was obtained for the first time. Florida, Illinois, and Kansas measurements from preroll conditions through the development of well-defined rolls to their dissipation were utilized to determine the factors influencing roll evolution. When the buoyancy flux reached a critical value of 35–50 W m−2, the first form of boundary layer convection resolved by radar was rolls. It was noted that two-dimensional convective rolls can evolve in a convective boundary layer in the absence of significant wind speed and shear. In fact, the value of wind speed or shear in itself did not seem to determine when or if rolls would form, although it did influence roll evolution. Well-defined, two-dimensional rolls only occurred while −zi/L, where zi is the convective boundary layer depth and L is the Monin–Obukhov length, was less than ∼25, which is consistent with previous studies. As −zi/L increased through...

The Initiation and Organization of Convective Cells atop a Cold-Air Outflow Boundary

Abstract On 13 July 1986 a cold-air outflow from thunderstorms over Illinois and Missouri propagated through the MIST (Microburst and Severe Thunderstorm) network over northern Alabama. The study of this outflow is important since the gust front was solely responsible for the initiation of numerous convective cells. Previous studies have documented the initiation of convection due to colliding gust fronts. In addition, there was a pronounced mesoscale organization of the cells atop the outflow boundary. This was most likely due to a combination of Kelvin–Helmholtz (K–H) and internal gravity (IG) wave activity. In contrast to previous cases, the K–H wave crests were oriented nearly perpendicular to the gust front within the analysis area. The resulting intersections between the circulations associated with the K–H waves and the gust front produced favorable locations for the initiation of convection. Subsequently, the convective cells remained along the updraft side of the K–H wave circulations as they pro...