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Andrew Crook and John D. Tuttle

Abstract

Numerical simulations of three gust-front cases that occurred in northeastern Colorado during the summers of 1991 and 1992 am presented. The simulations are initialized with radar-derived winds and, for the two cases in 1992, measurements from a surface mesonet. Thermodynamic retrieval is used to calculate the buoyancy in the boundary layer. The sensitivity of the retrieved buoyancy to the various constraints of real data was examined in Part I of this study.

In the first case, a large-scale gust front moved southward over the Denver region at a speed of 8–9 m s−1. The retrieved buoyancy field for this case exhibits a broad baroclinic zone, with a width of approximately 20 km centered about the radar-detected fine line. This baroclinic zone collapses to a width of about 5 km as the numerical model is integrated forward. The simulated gust front propagates at 7 m s−1, which is slightly less than the observed speed.

For the second and third cases, data from a 50-station surface mesonet were also available. In the second case, two gust fronts converged in the region of Mile High Radar but failed to generate significant convection. In the third case, three gust fronts converged and generated strong convection. Numerical simulations for both of these cases using surface and radar-derived winds are presented.

A verification analysis is performed on the forecasts of the two cases from 1992. Both surface observations and analyzed convergence fields are used to verify the forecasts. For the two cases, the numerical forecast of surface winds at 60 min improved over persistence by an average of 30%. The forecast surface convergence and temperature fields improved over persistence by an average of 25% and 28%, respectively.

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John D. Tuttle and Chris A. Davis

Abstract

Traveling deep tropospheric disturbances of wavelengths ~1500 km (short waves) have long been known to play an important role in the initiation and maintenance of warm-season convection. To date, relatively few studies have formally documented the climatology of short waves and their relationship to the diurnal heating cycle, the topography, and the diurnal cycle of precipitation. Those that did had to rely on low-resolution global analyses and often could not track short waves across mountain barriers. In this study, 10 yr of the (32 km) NCEP North American Regional Reanalysis (NARR) are used to objectively identify and track short waves in the North American domain. Statistics of short-wave span, duration, phase speed, latitudinal extent, and locations of preferred intensification/decay are presented. Some of the key findings from the climatology include that the lee (windward) of mountain barriers are preferred regions of intensification (decay) and short waves show little evidence of a diurnal cycle and can pass a given point at any time of the day. The second part of the study focuses on the role that short waves play in modulating the diurnal cycle of propagating convection east of the Rocky Mountains. Depending on the timing of short-wave passage, short waves may either significantly enhance the precipitation above the mean or completely disrupt the normal diurnal cycle, causing precipitation to develop at times and locations where it normally does not. While short waves play an important role in modulating the mean precipitation patterns their role is considered to be secondary in nature. The diurnal precipitation signature is prominent even when short waves are not present.

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John D. Tuttle and Chris A. Davis

Abstract

During the warm season in the central United States there often exists a corridor of precipitation where a succession of mesoscale convective systems (MCSs) follow similar paths lasting several days. The total cumulative rainfall within a corridor can be substantial while precipitation at nearby regions may be below normal. Understanding the nature of the corridors and the environmental factors important for their formation thus has important implications for quantitative precipitation forecasting and hydrological studies. In this study a U.S. national composite radar dataset and model-analyzed fields are used for the 1998–2002 warm seasons (July–August) to understand the properties of corridors and what environmental factors are important for determining when and where they develop. The analysis is restricted to a relatively narrow longitudinal band in the central United States (95°–100°W), a region where convection often intensifies and becomes highly organized. It is found that ∼68% of MCSs were members of a series and that corridors typically persist for 2–7 days with an extreme case lasting 13 days. Cumulative radar-derived maximum rainfall ranges from 8 to 50 cm, underscoring the fact that corridors can experience excessive rainfall. Combining radar with Rapid Update Cycle model kinematic and thermodynamic fields, 5-yr composites are presented and stratified according to the environmental conditions. While the corridors show the expected association with areas of enhanced CAPE and relatively strong northwesterly/westerly shear, the strongest association is with the northern terminus region of the nocturnal low-level jet (LLJ). Furthermore, the relative intensity of the rainfall is positively correlated with the strength of the LLJ. The LLJ is thought to play a role through enhanced convergence and lifting, moisture transport, and frontogenesis. In the five years analyzed, the large-scale environment varied considerably, but the role of the LLJ in the formation of corridors remained persistent.

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Ronald E. Rinehart and John D. Tuttle

Abstract

The detection of hail with a dual-wavelength radar system can succeed only when the two essentially independent radars used are correctly calibrated, when attenuation is correctly handled, and when the radars sample the same volume in space. The primary point of this paper is to examine the effects of mismatched antenna beam patterns on dual-wavelength processing. We examine and develop techniques to handle calibration problems, range differences, scan-rate-dependent pointing errors, and attenuation. We also develop a technique (using a ground target) to determine the antenna beam patterns of the two antennas and use these to simulate numerically the effects of mismatched antenna beam patterns on dual-wavelength hail signals. With the National Center for Atmospheric Research CP-2 (10.7 cm wavelength) and M33 (3.2 cm wavelength) dual-wavelength radar system, mismatched beam patterns produce numerous erroneous hail signals of large amplitude. Beam patterns must be well matched to avoid producing erroneous hail signals. Mismatched beam patterns may have contributed to erroneous interpretations in several studies using dual-wavelength data.

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John D. Tuttle and G. Brant Foote

Abstract

In the warm season the optically clear boundary layer often contains scatterers that can be detected by sensitive radars to distances of 50–100 km. Inhomogeneities in the field of reflectivity lead to echo patterns that have some persistence over periods as long as 5–10 minutes. These echo patterns translate with the local wind and can, for example, be easily followed by eye on PPI playback loops reviewing 15–20 minutes of data. In this paper a technique is discussed that determines the wind field by objectively identifying and tracking local echo. patterns. The technique, called TREC (Tracking Radar Echoes by Correlation), involves the cross-correlation of the echo features measured at two times a few minutes apart. The translation of a local feature during the measurement interval then determines the local wind. Use of this technique in the clear boundary layer means that problems associated with noisy data (reflectivities just above the minimum detectable signal) and ground clutter will be common. A number of methods for dealing, with these matters are presented. The use of TREC in clear air rather than in storms carries the advantage that misleading results associated with the sedimentation of hydrometeors in a sheared flow are avoided.

In this study TREC is applied to data collected during the Convective Initiation and Downburst Experiment (CINDE) in northeastern Colorado. The method is shown to provide horizontal winds in the boundary layer (the region of significant clear-air echo) over areas 100 to 150 km on a side with a resolution of about 10 km. TREC can be used with either conventional or Doppler radars sensitive enough to detect clear-air echo, though there are some advantages in using single Doppler measurements to improve the reliability of the technique. Applications of the present method are anticipated in a number of research and forecast areas.

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Ronald E. Rinehart and John D. Tuttle

Abstract

No abstract available.

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John D. Tuttle and Richard E. Carbone

Abstract

In a recent radar-based climatological study of warm-season precipitation over the continental United States, Carbone et al. found a high frequency of long-lived coherent rainfall episodes. Many of the events were of longer duration than normally associated with mesoscale convective complexes and exhibited phase speeds ∼10 m s−1 in excess of the phase speed associated with synoptic systems. The observations led to the speculation that cold pool dynamics and wavelike propagation mechanisms were responsible for the longevity of the systems. One of the long-lived episodes included in the statistics of the Carbone et al. study is described here. Occurring on 14–15 July 1998, the system lasted ∼50 h and traveled over 2800 km. At its peak intensity the system was a bow echo producing damaging wind, large hail, and local flash flooding. An interesting aspect of the event was an abrupt 90° turn in the storm's orientation and propagation vector midway through its life. The environmental factors that led to the observed behavior are investigated.

The episode consisted of two mesoscale convective systems (MCSs) and owed its longevity to a coherent regeneration process, with favorable cold pool–wind shear interactions playing a dominant role. Synoptic-scale forcing indirectly played a role by moistening the environment and creating a favorable wind shear region. An important observation is an E–W spatial displacement (∼200 km) between the N–S corridor of maximum low-level moisture/CAPE and the maximum low-level wind shear/system relative inflow, and the fact that the storm followed the most favorable wind shear corridor. High moisture/instability alone were not enough to ensure the longevity of this system.

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John D. Tuttle and Ronald E. Rinehart

Abstract

In using a dual-wavelength radar system to detect hail, erroneous positive hail signals can result because of the stronger attenuation of the shorter wavelength radar beam. We present a simple technique to correct for attenuation in dual-wavelength analyses. The technique makes use of an attenuation-reflectivity relationship of the form, A = CZxp, where Zx is the S-band reflectivity, C is a coefficient which is determined on a ray-by-ray basis, and p is the exponent, which is assumed to be a constant. In situations where rays of radar data contain a mixture of rain and hail, the attenuation-correction scheme can erroneously apportion more of the attenuation to hail regions rather than to rain regions. The scheme is modified to account for such situations.

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John D. Tuttle, Richard E. Carbone, and J. J. Wang

Abstract

This note documents the effects of a radar calibration error that affected 5% of the data used in an earlier study by Carbone et al. While the original intent of this correction was merely to set the record straight, a significant finding emerged from the revised time series.

The main impact is to reveal a more rapid spatial decorrelation between island Froude number (Fr) and oceanic rainfall, a measure of the dynamical blocking effects upstream of Hawaii. There no longer exists a residual correlation between Fr and cumulative rainfall over the remote ocean, defined in this instance as distances >50 km from shore. This result both simplifies and strengthens the conclusions of the original work, which was based on a very short time series. The strong dependence of rainfall on Fr over the coastal lowlands and near shore remains unchanged from the results presented by Carbone et al. To the extent that remote oceanic rainfall is expected to be uncorrelated with Fr, there is increased confidence with respect to the causes and effects of island blocking on rainfall production over and near the island.

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Kevin R. Knupp, Bart Geerts, and John D. Tuttle

Abstract

The evolution of the mesoscale flow and precipitation distribution are investigated for a small mesoscale convective system (MCS) that evolved in a nearly barotropic environment exhibiting moderate instability and weak wind shear. Observations primarily from a single Doppler radar detail the growth of the MCS from the merger of several clusters and lines of vigorous convective cells into a mature state consisting of a weaker convective line trailed by an expanding stratiform precipitation region. Analysis of radar reflectivity reveals that this stratiform region formed in situ in the presence of weak mesoscale updraft as decaying convective cores coalesced, rather than through rearward advection of ice particles directly from the convective region. In the absence of sufficient low-level shear, the MCS collapsed rapidly as it assumed the structure of the archetypal convective line and trailing stratiform precipitation region.

Velocity–azimuth displays reveal mesoscale updrafts of about 70 cm s−1 during the active convective stage. In the mature stratiform region, the lower-tropospheric mesoscale downdraft (∼40 cm s−1) exceeded the mesoscale updraft (∼10 cm s−1) above it, and the level separating the two was relatively high at 6.5 km, about 2 km above the 0°C level. As the MCS cloud-top anvil area colder than −52°C peaked near 60000 km2, the cloud top descended at rates of 20–40 cm s−1 despite weak but sustained mesoscale updraft within the upper part of the cloud.

A rear inflow jet was observed before convective activity peaked, remained strong while the deep convection diminished, and became the main flow feature as the MCS decayed. This jet subsided from approximately 7 km at the rear end to near the surface at the leading edge of the convection. A weaker ascending front-to-rear current was found above this rear inflow jet.

No midlevel mesoscale cyclonic vortex was apparent in the echo structure of the maturing MCS. Indirect estimates of mesoscale vorticity, based on Lagrangian conservation of radar reflectivity, indicate that cyclonic rotation was present in the mesoscale downdraft region, and anticyclonic rotation occurred aloft. The magnitude of this vorticity is about half the Coriolis parameter. A positive potential vorticity anomaly is found at midlevels within the MCS, and this anomaly intensifies in depth and in strength as the system matures. This growth is consistent with the diabatic heating profile estimated from a 1D cloud model.

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