Richard E. Carbone

Inferences of Predictability Associated with Warm Season Precipitation Episodes

Herein preliminary findings are reported from a radar-based climatology of warm season precipitation ‘‘episodes.’’ Episodes are defined as time‐space clusters of heavy precipitation that often result from sequences of organized convection such as squall lines, mesoscale convective systems, and mesoscale convective complexes. Episodes exhibit coherent rainfall patterns, characteristic of propagating events, under a broad range of atmospheric conditions. Such rainfall patterns are most frequent under ‘‘weakly forced’’ conditions in midsummer. The longevity of episodes, up to 60 h, suggests an intrinsic predictability of warm season rainfall that significantly exceeds the lifetime of individual convective systems. Episodes are initiated primarily in response to diurnal and semidiurnal forcings. Diurnal forcing is dominant near the Rocky and Appalachian Mountains, whereas semidiurnal forcing is dominant between these cordilleras. A most common longitude of origin is at or near the east slope of the Continental Divide (1058W). These observations are consistent with a condition of continual thermal forcing, widespread hydrodynamic instability, and the existence of other processes that routinely excite, maintain, and regenerate organized convection. The propagation speed of major episodes is often in excess of rates that are easily attributable either to the phase speeds of large-scale forcing or to advection from low- to midlevel ‘‘steering’’ winds. It is speculated that wavelike mechanisms, in the free troposphere and/or the planetary boundary layer, may contribute to the rates of motion observed. Once understood, the representation of such mechanisms in forecast models offers the opportunity for improved predictions of warm season rainfall.

Improving quantitative precipitation forecasts in the warm season: A USWRP research and development strategy

Warm-season quantitative precipitation forecasts (QPFs) are the poorest performance area of forecast systems worldwide. They stubbornly fall further behind while other aspects of weather prediction...

A Severe Frontal Rainband. Part I. Stormwide Hydrodynamic Structure

Abstract A narrow cold frontal band of intense precipitation is examined by means of triple Doppler radar and supporting observations. As the band passed through the Central Valley of California, it was accompanied by strong gusty winds, electrical activity, tornadoes and pressure jumps. Part I delineates the stormwide kinematic and thermodynamic structure. A highly two-dimensional pre-frontal updraft of 15–20 m s−1 results primarily from intense planetary boundary layer forcing of a low-level jet by the gravity-current propagation mechanism. Maximum updraft speed occurs at 2.1 km and the maximum radar echo depth is 6.6 km. Diabatic cooling, due to melting hydrometers, is proposed as a likely mechanism for control of gravity current depth and maintenance of density contrast together with synoptic-scale cold air advection. Available convective potential energy is shown to be small and kinetic energy of the environmental vertical wind shear is proposed as a likely source of energy on the updraft scale. Torn...

Characteristics through the Melting Layer of Stratiform Clouds

Abstract Thermodynamic and hydrometeor measurements from an aircraft flown through the melting layer of stratiform clouds over the California Valley are discussed and are compared with radar observations. An isothermal layer ∼200 m thick existed at 0°C, and radar bright bands up to 36 dB(Ze) were measured. The largest concentrations of ice particles occurred near −5°C and snowflakes melted by ∼2°C. Aggregation, and possibly ice multiplication, contributed to the characteristics of the radar bright band.

Rainfall Occurrence in the U.S. Warm Season: The Diurnal Cycle*

The diurnal occurrence of warm-season rainfall over the U.S. mainland is examined, particularly in light of forcings at multiple scales. The analysis is based on a radar dataset of 12-seasons duration covering the U.S. mainland from the Continental Divide eastward. The dataset resolves 2-km features at 15-min intervals, thus providing a detailed view of both large- and regional-scale diurnal patterns, as well as the statistics of events underlying these patterns. The results confirm recent findings with respect to the role of propagating rainfall systems and the high frequency at which these are excited by sensible heating over elevated terrain. Between the Rockies and the Appalachians, 60% of midsummer rainfall occurs in this manner. Most rainfall in the central United States is nocturnal and may be attributed to the following three main forcings: 1) the passage of eastward-propagating rainfall systems with origins near the Continental Divide at 105°W; 2) a nocturnal reversal of the mountain–plains solenoid, which is associated with widespread ascent over the plains; and 3) the transport of energetic air and moisture convergence by the Great Plains low-level jet. Other features of interest include effects of the Appalachians, semidiurnal signals of regional significance, and the impact of breezes along the Gulf of Mexico. A modest effort was put forth to discern signals associated with El Nino and the Southern Oscillation. While tendencies in precipitation patterns are observed, the record is too short to draw conclusions of general significance.

Dynamics of a Thunderstorm Outflow

Abstract The kinematic and thermodynamic structures of a thunderstorm outflow are examined by means of dual Doppler radar analysis, mesonet, lower, and sounding data. The data were collected in the Denver, Colorado area during June 1984. The dual-Doppler analysis shows that the cold outflow is ducted beneath the PBL inversion. Along the gust front there is a narrow quasi-two-dimensional updraft. Kelvin-Helmholtz instability (KHI) developed along the top of the gust front head near the surface front, and propagated backwards, dissipating in the wake of the head region. An isothermal layer aloft appears to have limited billow amplification to the quasi-neutral layer below. The gust fronts leading edge had numerous inflections which are believed to result from barotrophic instabilities. Small vortices develop at some of the inflection points. Detailed analysis of one such circulation shows evidence of the formation of two enhanced updrafts separated by an occlusion downdraft. These observations are the firs...

A Climatology of Warm-Season Cloud Patterns over East Asia Based on GMS Infrared Brightness Temperature Observations

Abstract In the present study, hourly infrared (IR) brightness temperatures observed by the Geostationary Meteorological Satellite (GMS) over the region 20°–40°N, 95°–145°E in May–August 1998–2001 are used to compile a climatology of warm-season cloud/precipitation episodes over east Asia. With a goal to better understand the characteristics of warm-season convection and the behavior of these episodes, results are compared with those obtained in North America using radar-derived data. The convection in east Asia, similar to that in North America, is shown to also exhibit coherent patterns and characteristics of propagating events in the longitude–time (Hovmoller) space, with a preferred phase speed of ∼10–25 m s−1, considerably faster than warm-season synoptic-scale waves. Near the eastern edge of the Tibetan Plateau, convection was most active with a strong diurnal signal, peaking in late afternoon or early evening then propagating eastward. The zonal span and duration of episodes could reach 3000 km and...

Tropical Island Convection in the Absence of Significant Topography. Part I: Life Cycle of Diurnally Forced Convection

Abstract Diurnally forced convection was observed over the Tiwi Islands, north of the Australian continent, as part of the Maritime Continent Thunderstorm Experiment. Immature peninsula-scale (5–15 km) sea breezes were observed to initiate moist convection early each day, principally through convergence that results from the confluence or collision of peninsula breeze fronts. Convection initiated by peninsula-scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event. Mature island-scale (∼100 km) breezes develop by late morning and subsequently play a pivotal role in the forcing and evolution of organized convection. The initiation of mesoscale convective systems (MCSs) is observed to be a direct consequence of breeze front collisions for only ∼20% of the days on which organized convection develops. This is referred to as “type A” forcing and it occurs when normal convective development is delayed or otherwise suppressed. Type A forcing is nature’s backup mech...

The NAME 2004 Field Campaign and Modeling Strategy

The North American Monsoon Experiment (NAME) is an internationally coordinated process study aimed at determining the sources and limits of predictability of warm-season precipitation over North America. The scientific objectives of NAME are to promote a better understanding and more realistic simulation of warm-season convective processes in complex terrain, intraseasonal variability of the monsoon, and the response of the warm-season atmospheric circulation and precipitation patterns to slowly varying, potentially predictable surface boundary conditions. During the summer of 2004, the NAME community implemented an international (United States, Mexico, Central America), multiagency (NOAA, NASA, NSF, USDA) field experiment called NAME 2004. This article presents early results from the NAME 2004 campaign and describes how the NAME modeling community will leverage the NAME 2004 data to accelerate improvements in warm-season precipitation forecasts for North America.

The Maritime Continent Thunderstorm Experiment (MCTEX): Overview and some results

A description is given of the Maritime Continent Thunderstorm Experiment held over the Tiwi Islands (12°S, 130°E) during the period November–December 1995. The unique nature of regularly occurring storms over these islands enabled a study principally aimed at investigating the life cycle of island-initiated mesoscale convective systems within the Maritime Continent. The program objectives are first outlined and then selected results from various observationally based and modeling studies are summarized. These storms are shown to depend typically on island-scale forcing although external mesoscale disturbances can result in significant storm activity as they pass over the heated island. Particular emphasis is given to summarizing the environmental characteristics and the impact this has on the location of storm development and the associated rainfall distribution. The mean rainfall production from these storms is shown to be about 760 × 105 m3, with considerable variability. The mesoscale evolution is summ...