Steven E. Koch

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...

The Synoptic Setting and Possible Energy Sources for Mesoscale Wave Disturbances

Abstract Thirteen case studies of mesoscale wave disturbances (characterized by either a singular wave of depression or wave packets with periods of 1–4 h, horizontal wavelengths of 50–500 km, and surface pressure perturbation amplitudes of 0.2–7.0 mb) are reviewed to isolate common synoptic features for these cases and to shed light on possible energy sources for the waves. A strong thermal inversion in the lower troposphere (north of a frontal boundary) and a jet streak propagating toward a ridge axis in the upper troposphere are commonly observed in all the cases. In general, the area of wave activity is bounded by the jet axis to the west or northwest, a surface front to the southeast, an inflection axis (between the trough and ridge axes) to the southwest and the ridge axis to the northeast. The conditions specified by Lindzen and Tung as being necessary to form a wave duct, which include the existence of the lower-tropospheric inversion, seem to be met in many of these cases. This suggests that a du...

The Impact of Different WRF Model Physical Parameterizations and Their Interactions on Warm Season MCS Rainfall

In recent years, a mixed-physics ensemble approach has been investigated as a method to better predict mesoscale convective system (MCS) rainfall. For both mixed-physics ensemble design and interpretation, knowledge of the general impact of various physical schemes and their interactions on warm season MCS rainfall forecasts would be useful. Adopting the newly emerging Weather Research and Forecasting (WRF) model for this purpose would further emphasize such benefits. To pursue this goal, a matrix of 18 WRF model configurations, created using different physical scheme combinations, was run with 12-km grid spacing for eight International H2O Project (IHOP) MCS cases. For each case, three different treatments of convection, three different microphysical schemes, and two different planetary boundary layer schemes were used. Sensitivity to physics changes was determined using the correspondence ratio and the squared correlation coefficient. The factor separation method was also used to quantify in detail the impacts of the variation of two different physical schemes and their interaction on the simulated rainfall. Skill score measures averaged over all eight cases for all 18 configurations indicated that no one configuration was obviously best at all times and thresholds. The greatest variability in forecasts was found to come from changes in the choice of convective scheme, although notable impacts also occurred from changes in the microphysics and planetary boundary layer (PBL) schemes. Specifically, changes in convective treatment notably impacted the forecast of system average rain rate, while forecasts of total domain rain volume were influenced by choices of microphysics and convective treatment. The impact of interactions (synergy) of different physical schemes, although occasionally of comparable magnitude to the impacts from changing one scheme alone (compared to a control run), varied greatly among cases and over time, and was typically not statistically significant.

Operational Forecasting and Detection of Mesoscale Gravity Waves

Abstract Mesoscale gravity waves display periods of 1–4 h, have wavelengths of 50–500 km, and can have important effects upon the sensible weather. Real-time prediction, detection, and nowcasting of these mesoscale phenomena is shown to be feasible, due to recent major advances in operational observing and modeling systems. The ability to predict the likelihood of a gravity wave event rests upon recognizing the synoptic flow pattern in which such waves are consistently found to occur. The delineation of the most likely region for wave activity can be further refined by computing simple indicators of unbalanced flow and conducting a cursory search for a suitable wave “duct” with meso-Eta Model data. Particular emphasis should be placed on propagating unbalanced fields. Whenever and wherever a suitable gravity wave environment is found, the Automated Surface Observing System pressure data should be carefully monitored for evidence of gravity wave activity. An automated gravity wave detection system is devel...

A mesoscale gravity wave event observed during CCOPE. III - Wave environment and probable source mechanisms

Abstract Synoptic and special mesoscale observations taken during the Cooperative Convective Precipitation Experiment (CCOPE) are used to describe the multiscale environment of a gravity wave event, understand the wave-environment interactions that led to the development of severe thunderstorms, and asses possible wave-generation mechanisms. The storms formed sequentially as a packet of gravity waves propagated across a stationary thunderstorm outflow boundary. Convection developed most rapidly in that part of the mesonetwork in which existed the combination of relatively high parcel buoyant energy, weak restraining inversion, strong storm downdraft potential, and substantial vertical wind shear (associated with a mesoscale jet streak). Synoptic-scale analysis reveals that the waves were excited north of a stationary front and within the right exit region of the jet streak as it approached a stationary ridge in the 300 mb height field. Strong indications of unbalanced flow were diagnosed within the gravit...

The Impact of Assimilating Satellite-Derived Precipitation Rates on Numerical Simulations of the ERICA IOP 4 Cyclone

Abstract The present study uses a regional-scale numerical model to test the impact of dynamically assimilating, satellite-derived precipitation rates on the numerical simulations of one of the deepest extratropical cyclones to develop south of 40°N in this century. This cyclone event occurred during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) intensive observing period 4 and has been selected because of the strength of the cyclone and the availability of the special ERICA data in addition to the Special Sensor Microwave/Imager (SSM/I) and Geostationary Operational Environmental Satellite (GOES) infrared (IR) satellite data. The unique methodology developed herein to synthesize the SSM/I and GOES IR satellite data produces precipitation estimates that have realistic spatial and temporal structure. The assimilation of satellite-derived precipitation is accomplished by scaling the internally generated model profiles of total latent heating. At points where the model is not prod...

Structure of an Internal Bore and Dissipating Gravity Current as Revealed by Raman Lidar

Abstract Detailed moisture observations from a ground-based Raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented. A synthesis of the lidar data with conventional meteorological data, in conjunction with theoretical calculations and comparison to laboratory studies, leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore. The disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site. Bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current. Analysis suggests that the bore was generated by the collapse of the gravity current into a stable, nocturnal inversion layer, and subsequently propagated along this wave guide at nearly twice the speed of the gravity current. The Raman lidar provided detailed measurements of the vertical structure o...

Turbulence and Gravity Waves within an Upper-Level Front

Abstract High-resolution dropwindsonde and in-flight measurements collected by a research aircraft during the Severe Clear-Air Turbulence Colliding with Aircraft Traffic (SCATCAT) experiment and simulations from numerical models are analyzed for a clear-air turbulence event associated with an intense upper-level jet/frontal system. Spectral, wavelet, and structure function analyses performed with the 25-Hz in situ data are used to investigate the relationship between gravity waves and turbulence. Mesoscale dynamics are analyzed with the 20-km hydrostatic Rapid Update Cycle (RUC) model and a nested 1-km simulation with the nonhydrostatic Clark–Hall (CH) cloud-scale model. Turbulence occurred in association with a wide spectrum of upward propagating gravity waves above the jet core. Inertia–gravity waves were generated within a region of unbalanced frontogenesis in the vicinity of a complex tropopause fold. Turbulent kinetic energy fields forecast by the RUC and CH models displayed a strongly banded appeara...

The Influence of Mesoscale Humidity and Evapotranspiration Fields on a Model Forecast of a Cold-Frontal Squall Line

Abstract Satellite imagery and rain gauge data are combined to create mesoscale detail in the initial states of relative humidity (RH) and surface moisture availability (M) for a mesoscale model simulation. The most profound impact of inserting the mesoscale initial fields was the development of a strong vertical circulation transverse to an intensifying cold front that triggered an intense frontal rainband similar to a severe squall line that was observed to develop explosively. This paper explores the causative factors leading to the formation of this intense circulation and the sensitivity of the model to the mesoscale initial fields. A substantial gradient in the initialized RH and M fields occurred across the cold front in the region where the observed frontal squall line formed. In contrast to the control run, the model simulations that incorporated the mesoscale initial analysis displayed considerable daytime warming just ahead of the front. This warming was due principally to a reduction in the RH...

A Nonclassical Cold Front Observed during COPS-91: Frontal Structure and the Process of Severe Storm Initiation

Abstract This case study addresses the issue of gravity current and bore development at surface cold fronts, and the role of these phenomena in the generation of severe frontal convection. The event investigated occurred on 27 April 1991 during the Cooperative Oklahoma Profiler Studies 1991 field project. The development of a bore from a gravity current–like structure along a cold front, the subsequent propagation of the bore ahead of the front on a low-level inversion, and the process of severe thunderstorm development along the front are revealed by a dense network of remote sensing and other special observations. Evidence for the gravity current and bore is strengthened by comparisons made between the synthesized observations and theory. The bore developed after a nocturnal inversion, which acted as a waveguide, had become established. The bore and gravity current were both evident as “fine lines” in the radar reflectivity displays. A microscale envelope of enhanced water vapor with an embedded roll cl...