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R. E. Carbone

Abstract

A method of radar scan is proposed to efficiently acquire scalar velocity components and generate a natural coordinate system which simplifies and minimizes spatial filtering requirements for certain applications. Tetrahedral scan is shown to permit direct computation of horizontal divergence from the sampled grid points and subsequently estimate vertical air motion. Existing radar systems can perform tetrahedral scan provided they have computerized scan control with suitable software.

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Yanping Li and R. E. Carbone

Abstract

The authors have examined 4 years of satellite-derived SST and rainfall data in anticipation of a relationship between SST structure and the excitation of convective rainfall. The results exhibit a strong excitation signal consistent with the presence of mesoscale SST gradients in about 75% of approximately 10 000 rainfall onset events. Rainfall onset events occur at locations with enhanced horizontal convergence, as inferred by the Laplacian of SST on scales of order 100 km. The daily SST field exhibits multiscale patchiness, spanning a 2+°C range. The signal is disproportionately large at SSTs that are 0.25°C above the mean, near 29.5°C; disproportionately weak for SST ≤ 28.8°C; and proportionately neutral for SST ≥ 30.3°C. The calculations suggest that a characteristic strength of this lower-boundary forcing (~3 × 10−5 s−1) is approximately one order of magnitude stronger than the mean regional background forcing (~3 × 10−6 s−1). The periphery of warm oceanic patches exhibits both convergent and divergent Laplacian values of similar frequency and magnitude; however, rainfall onset favors the locally convergent locations by a 3:1 ratio.

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R. E. Carbone and Yanping Li

Abstract

Based upon on the findings of Y. Li and R. E. Carbone, the association of tropical rainfall with SST structure is further explored, with emphasis on the MJO passband. Analyses include the tropical Indian Ocean, Maritime Continent, and tropical western Pacific regions. The authors examine the anomalies of and correlations between SST structure, the frequency of rainfall events, and rainfall amount. Based on detailed examination of a 49-month time series, all findings are statistical inferences and interpretations consistent with established theory.

The statistical inferences are broadly consistent with a pivotal role played by the convergent Laplacian of SST together with an expected, but somewhat indirect, role of SST itself. The main role of SST in the MJO passband appears limited to production of moist static energy, which is highly correlated with cumulative precipitation, yet bears a decidedly conditional relationship to the occurrence of rainfall. If rain occurs, then more rain is likely over warmer SST. The convergent Laplacian of SST is strongly associated with the onset of rainfall, apparently through its capacity to induce vertical air motion with sufficient kinetic energy to overcome convective inhibition in a conditionally unstable troposphere. The convergent Laplacian of SST is directly associated with the location and the variability of rainfall event frequency while having a less direct relationship to cumulative rainfall. These nuanced interpretations of rainfall forcing by the Laplacian of SST, and conditional modulation of cumulative rainfall by SST, may underlie systematic errors in highly parameterized models as a consequence of variable asymmetry in the field of Laplacian anomalies.

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Yanping Li and R. E. Carbone

Abstract

This work focuses on the seaward propagation of coastal precipitation with and without mountainous terrain nearby. Offshore of India, diurnal propagation of precipitation is observed over the Bay of Bengal. On the eastern side of the bay, a diurnal but nonpropagating signal is observed near the west coast of Burma. This asymmetry is consistent with the inertio-gravity wave mechanism. Perturbations generated by diurnal heating over the coastal mountains of India propagate offshore, amplify in the upwind direction, and dissipate in the downwind direction relative to the steering wind, owing to critical-level considerations. A linear model is applied to evaluate sensitivity to gravity waves, as these affect deep moist convection and propagation. Analyses are performed for various heating depths, mountain widths, stability, Coriolis effect, background mean wind, and friction. Calculations reveal how these factors affect the amplitude, dissipation, initiation phase, and propagation speed of the diurnal disturbance. The propagation of precipitation triggered by land–sea breezes is distinguishable from that triggered by a mountain–plains circulation. Convection resulting purely from mountain heating begins earlier, propagates slower, and damps faster than that of the land–sea breeze. For mountains near a coast, slower propagation and stronger earlier convection result from a resonance-like combination of two dynamical mechanisms. The propagation of precipitation is initially triggered by the mountain breeze near the coastal mountain. Over the open ocean, the dominant signal propagates as that of the land breeze but with stronger convection.

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R. E. Carbone and Alan R. Bohne

Abstract

Snow generating cells were observed by vertically pointing Doppler radar. Analysis was carried out in three distinct regions. The region exhibiting cellular structure was found to consist of convective elements with vertical velocities ±1.5 m s−1 in magnitude. A generation region was found to exist beneath the convective region with updrafts of 0.5 in s−1 sustained over a 7 km path length which originated at a generating level less than 75 m in depth associated with an inversion layer. The trail region below exhibited maximum downdrafts of 0.5 m s−1 or no net vertical motions.

Maximum ice content was estimated to be no larger than 0.24 g m−2 in the convective cells. Growth appeared to take place throughout the rise and fall of the snow crystals in the updraft regions.

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R. E. Carbone and Loren D. Nelson

Abstract

Airborne optical spectrometer measurements of raindrop spectra were performed in Texas together with 10 and 3 cm radar observations. The observations reveal large drop spectra emanating from cloud base with low concentrations of small drops compared to the Marshall and Palmer (1948) distributions. Systematic patterns of exponential spectrum parameters (N 0 and λ) are revealed to constitute a “parametric cycle” in time and space. Drop spectrum form is attributed principally to updraft sorting of smaller drops. Subsequent to updraft sorting, the importance of the collision breakup process with respect to spontaneous breakup appears to be greatly diminished. Recent theoretical studies by Srivastava (1971, 1978) partially support conclusions drawn with respect to the relative importance of spontaneous versus collisional breakup.

A one-dimensional, time-dependent numerical model was employed for initial conditions simulating the observations. The resultant spectra emanating from the model were quantitatively similar to the measurements and exhibited temporal evolution consistent with the observed parametric cycle.

Implications for estimation of spectral moments such as liquid water content and radar reflectivity factor as well as rainfall rate are discussed briefly.

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WEATHER IMPACTS, FORECASTS, AND POLICY

An Integrated Perspective

Roger Pielke Jr. and R. E. Carbone

Society invests considerable resources into the science and technology of weather services. In order to effectively assess the market for weather services, and thus properly scale the level of resources that, for example, the U.S. Congress or a company ought to devote to serving this market, decision makers need information on the costs and benefits associated with alternative courses of action. To date such information has not been readily or systematically available, leaving unanswered questions about the effectiveness of investment in the science and technology of weather. We argue herein that the allocation of resources to weather in the public and private sectors is unlikely to become more effective or—of particular concern to the weather community—grow significantly unless the weather community takes an integrated perspective on weather impacts, forecasts, and policy that provides decision makers with reliable information on the costs and benefits of alternative courses of action. This paper suggests one such integrated perspective that might guide the provision of such information to decision makers. Two recommendations follow straightforwardly from the perspective offered herein:

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T. D. Keenan and R. E. Carbone

Abstract

Warm season cold cloud-top climatology in the Austral–Indonesian region is examined for evidence of propagating modes of precipitation that originate from elevated heat sources and the diurnal heating cycle. Using satellite-inferred cloudiness from the period 1996–2001 as a proxy for rainfall, this coherent regeneration process and subsequent event propagation is found to consistently occur from the midlatitudes (30°–40°S) to the tropics (10°–20°S) in the Austral region.

Given favorable environmental shear at midlatitudes, long-lived eastward-propagating events are observed to occur regularly with a span and duration typically larger than observed by Carbone et al. The genesis of these events, while intermittent, is directly related to elevated heat sources and the diurnal cycle, similar to the United States. However, given the relatively flat terrain of Australia, an elevated heat source is often insufficient, thus increasing the relative influence of transient synoptic forcing.

In the tropics, the thermal forcing associated with elevated terrain found over the islands of the Maritime Continent and the land–sea interface is increasingly dominant on daily basis. While eastward- and westward-propagating events are found in the more varied environment of the monsoon regime, evidence for meridionally propagating modes is also found. In this manner, complex interactions occur that modify the location and timing of clouds that develop over neighboring oceanic and continental locations. The impact of convection initially linked to the New Guinea highlands and subsequently impacting the Java Sea region is particularly evident affecting the observed diurnal cycle.

The subtropics show characteristics intermediate between the above extremes. With the seasonal cycle, the spring environment favors eastward-propagating events but in summer there is an increasing frequency of diurnally forced quasi-stationary development over elevated terrain enhanced by favorable synoptic conditions. Overall the subtropical summer events have a shorter duration and span than their spring counterparts. The increased environmental steering winds and shear in spring are thought to be the primary reason.

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R. E. Carbone and J. D. Tuttle

Abstract

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 Niño and the Southern Oscillation. While tendencies in precipitation patterns are observed, the record is too short to draw conclusions of general significance.

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J. Michael Fritsch and R. E. Carbone

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 steadily improve. Unless a major effort is mounted to overcome the impediments to improved prediction, it is certain to remain the Achilles' heel of weather prediction, at a progressively greater cost to society. For these reasons and others, the Office of the Lead Scientist, U.S. Weather Research Program (USWRP), commissioned a workshop to examine future courses of action to improve understanding and prediction of heavy warm-season rainfall and associated flood forecasts. The workshop was held in Boulder, Colorado, in March 2002. It was attended by 75 people and produced numerous “white papers” and panel reports, all of which are readily available to the reader.

Herein the major findings of the workshop are summarized, including an overarching strategy to achieve improved predictive skill and recommendations for future research and development. Improving warm-season QPFs requires a substantial and sustained commitment of resources focusing on a complex suite of issues. The basic strategy is to take those steps that will facilitate forecasting deep, moist convection in a fully probabilistic manner wherein the statistical properties of the forecast convection are similar to those observed in nature. A warm-season QPF program should be inclusive of a testbed framework, wherein development and testing of each and all components of the forecast system can be conducted; impediments to operations can be identified and corrected; and socioeconomic value, at the margin, can be researched and identified.

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