John D. Tuttle

Corridors of warm season precipitation in the central United States

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 we...

A Single-Radar Technique for Estimating the Winds in Tropical Cyclones

A method for determining horizontal wind speeds in hurricanes using ground-based radars is presented and evaluated. The method makes use of the tracking reflectivity echos by correlation (TREC) method where individual features in radar reflectivity are tracked, from radar sweeps several minutes apart, by finding the maxima in the cross-correlation function between the two times. This method has been applied successfully in determining motions within the clear boundary layer where reflectors are insects and refractive index variations, but it generally has failed when applied to determining air motions by tracking precipitation elements in strong environmental shear. It appears to work in the lower few kilometers of the hurricane where the vertical wind shear is relatively weak. Examples are presented where the TREC algorithm is applied to three landfalling hurricanes: Hurricanes Hugo and Erin in the United States and Typhoon Herb in Taiwan. The results from Hugo, where the radar data were provided by a WS...

Long-Lived Mesoconvective Vortices and Their Environment. Part I: Observations from the Central United States during the 1998 Warm Season

Abstract Observations from the modernized United States National Weather Service (NWS) data network are used to assess the frequency and general characteristics of midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems (MCSs). Results from the 1998 convective season (15 May–15 September) over the central United States suggest that long-lived MCVs, which persist after the dissipation of the initiating MCS, are more common than previously documented. These MCVs occur in weaker ambient vertical shear (both in the lower troposphere and through a nominal vortex layer) than MCSs from which no detectable MCVs are spawned. An important aspect of MCVs is that they may focus subsequent convective development within long-lived discontinuous heavy precipitation episodes. Subsequent deep convection is observed in the vicinity of MCVs in slightly greater than 1/2 of the MCV cases. This subsequent convection occurs in thermodynamic environments of moderate-to-large convective available potent...

Small-Scale Spiral Bands Observed in Hurricanes Andrew, Hugo, and Erin

Abstract Analysis of radar data taken from the three intense hurricanes that passed close to WSR-57 and WSR-88D radar sites at their point of landfall illustrate small-scale spiral bands that are frequently observed within 100 km or so of the hurricane center. Faintly visible in the radar reflectivity images, these bands have scales of 10 km across the band and can extend for 100 km as they spiral outward in a clockwise fashion. They appear to move around the hurricane with speeds close to the tangential wind at the level of the bands and are characterized by enhanced updrafts with higher equivalent potential temperature in the regions of elevated reflectivity. They induce wind speed variations of at least 8 m s−1 across the bands. The authors suggest that these small-scale hurricane spiral bands are similar to boundary layer rolls although they extend through depths of 5–6 km, which is more than would be expected for rolls in the boundary layer near the sea. The data presented here are not sufficient to ...

Experiments of Hurricane Initialization with Airborne Doppler Radar Data for the Advanced Research Hurricane WRF (AHW) Model

Abstract Initialization of the hurricane vortex in weather prediction models is vital to intensity forecasts out to at least 48 h. Airborne Doppler radar (ADR) data have sufficiently high horizontal and vertical resolution to resolve the hurricane vortex and its imbedded structures but have not been extensively used in hurricane initialization. Using the Weather Research and Forecasting (WRF) three-dimensional variational data assimilation (3DVAR) system, the ADR data are assimilated to recover the hurricane vortex dynamic and thermodynamic structures at the WRF model initial time. The impact of the ADR data on three hurricanes, Jeanne (2004), Katrina (2005) and Rita (2005), are examined during their rapid intensification and subsequent weakening periods before landfall. With the ADR wind data assimilated, the three-dimensional winds in the hurricane vortex become stronger and the maximum 10-m winds agree better with independent estimates from best-track data than without ADR data assimilation. Through th...

Comparison of ground-based radar and geosynchronous satellite climatologies of warm-season precipitation over the United States

Abstract Studies in the past several years have documented the climatology of warm-season precipitation-episode statistics (propagation speed, span, and duration) over the United States using a national composited radar dataset. These climatological studies have recently been extended to other continents, including Asia, Africa, and Australia. However, continental regions outside the United States have insufficient radar coverage, and the newer studies have had to rely on geostationary satellite data at infrared (IR) frequencies as a proxy for rainfall. It is well known that the use of IR brightness temperatures to infer rainfall is subject to large errors. In this study, the statistics of warm-season precipitation episodes derived from radar and satellite IR measurements over the United States are compared and biases introduced by the satellite data are evaluated. It is found that the satellite span and duration statistics are highly dependent upon the brightness temperature threshold used but with the a...

Numerical Simulations Initialized with Radar-Derived Winds. Pail II: Forecasts of Three Gust-Front Cases

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 av...

Coherent regeneration and the role of water vapor and shear in a long-lived convective episode

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 storms orientation and propagation vector midway through its life. The environmental factors that led t...

Attenuation correction in dual-wavelength analyses

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.

The “Owl Horn” Radar Signature in Developing Southern Plains Supercells

Abstract During spring 2001 in the Southern Plains, a recurring, hitherto undocumented reflectivity signature that the authors have called the “Owl Horn” signature (because the radar reflectivity pattern resembles the profile of the Great Horned Owl) was observed on a mobile, X-band radar display. The reflectivity signature was always located at the rear side of a developing supercell, spanned the entire rear side of the storm, and was always seen on low-level plan position indicator (PPI) scans. It lasted on the order of only 5–10 min and was not an artifact of the radar. A study of the Owl Horn signature was undertaken using the Tracking Radar Echoes by Correlation technique (TREC) to estimate the wind field. TREC has previously been applied to clear-air and hurricane environments, and to the internal motions of severe storms, but not to their evolution. The characteristics of the signature are presented, and then, through the application of TREC to the radar reflectivity data (Doppler wind data were no...