This study uses four-year radar-based precipitation organization and reanalysis datasets to study the mechanisms that lead to the abrupt springtime onset of precipitation associated with isolated storms in the Southeast United States (SE US). Although the SE US receives relatively constant precipitation year-round, previous work demonstrated a “hidden” summertime maximum in isolated precipitation features (IPF) whose annual cycle resembles that of monsoon climates in the subtropics. In the SE US, IPF rain abruptly ramps up in May and lasts until sometime between late August and early October. This study suggests that the onset of the IPF season in the SE US is brought about by a combination of slow thermodynamic processes and fast dynamic triggers, as follows. First, in the weeks prior to IPF onset, a gradual seasonal build-up of convective available potential energy (CAPE) occurs in the Gulf of Mexico. Then, in one-to-two pentads prior to onset, the upper-tropospheric jet stream shifts northward, favoring the presence of slow-moving frontal systems in the SE US. This poleward shift in the jet stream location in turn allows the establishment of the North Atlantic subtropical high western ridge over the SE US which, with associated poleward transport of high CAPE air frommore »
Springtime Onset of Isolated Convection Precipitation across the Southeastern United States: Framework and Regional Evolution
This study examines the geographic and temporal characteristics of the springtime transition to the summer precipitation regime of isolated convection in the southeastern (SE) United States during 2009–12, using a high-resolution surface radar-based precipitation dataset. Isolated convection refers herein to isolated elements or small clusters of precipitation in radar imagery less than 100 km in horizontal dimension. Though the SE United States does not have a monsoon climate, it is useful to apply the established framework of monsoon onset to study the timing and regional variation of the onset of the summer isolated convection regime. Overall, isolated convection rain onset in the SE U.S. domain occurs in late May. Onset begins in south Florida in mid-April, continuing nearly simultaneously across the southeastern coastal plain in early to mid-May. In the northern domain, from Virginia to the Ohio Valley, onset generally occurs much later (mid-June to early July) with more variable onset timing. The sharpness of onset timing is most evident in the coastal plain and Florida. Results suggest the hypothesis, to be examined in a forthcoming study, that the timing of isolated convection onset in the spring may be triggered by specific synoptic-scale events within gradual seasonal changes in atmospheric more »
- Award ID(s):
- 1660049
- Publication Date:
- NSF-PAR ID:
- 10133720
- Journal Name:
- Monthly Weather Review
- Volume:
- 148
- Issue:
- 3
- Page Range or eLocation-ID:
- p. 891-906
- ISSN:
- 0027-0644
- Publisher:
- American Meteorological Society
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract This study analyzes the effect of the location of the North Atlantic Subtropical High (NASH) western ridge on the daily variability of precipitation organization in the southeastern United States (SE US). The western side of the NASH, also known as the NASH western ridge, plays an important role in the variability of summertime precipitation in this region. In this study, the mean summertime position of the NASH western ridge was determined and used to classify each summer day during 2009–2012 into one of four quadrants. Composites of synoptic‐scale circulation and precipitation from mesoscale and isolated precipitation features (MPF and IPF) were calculated for each NASH western ridge quadrant. MPF contributed most (about 65%) of the total summertime precipitation and accounted for most of the differences between the four NASH quadrants. Domain‐averaged precipitation was highest (lowest) during NASH‐SW (NASH‐NW) when IPF (MPF) precipitation was strongest (weakest). The regionality of MPF precipitation maxima was generally associated with the location of low‐level jets and upper‐level troughs. For instance, positive MPF anomalies occurred across the SE US during NASH‐SW when the Great Plains low‐level jet turned eastward bringing moisture to fuel convection in the SE US. In contrast, IPF rain was distributed moremore »
-
Abstract Latewood width tree‐ring chronologies from arid‐site conifers in the southwestern United States are correlated with precipitation during portions of the summer monsoon season. The onset date and length of the monsoon season varies across the region, and these regional differences in summer rainfall climatology may impact the strength and timing of the warm season precipitation response of latewood chronologies. The optimal latewood response to summer precipitation is computed on a daily basis using 67 adjusted latewood chronologies (LWa) from the southwestern United States, adjusted to remove correlation with preceding earlywood growth. Most LWa chronologies are significantly correlated with precipitation summed over a period of approximately 4 weeks (29 days) in early summer. This early summer precipitation signal is present in most ponderosa pine chronologies across the study area. It is also evident in Douglas‐fir chronologies, but only from southern Arizona and New Mexico. The Julian date of summer precipitation onset increases from south to north in the instrumental precipitation data for the southwestern United States. The timing of the early summer season precipitation response in most LWa chronologies also tends to occur later in the summer from southeastern Arizona into northern New Mexico and eastern Colorado. Principal components analysis of themore »
-
The northeast monsoon (NEM) brings the bulk of annual rainfall to southeastern peninsular India, Sri Lanka, and the neighboring Southeast Asian countries. This October–December monsoon is referred to as the winter monsoon in this region. In contrast, the southwest summer monsoon brings bountiful rainfall to the Indo-Gangetic Plain. The winter monsoon region is objectively demarcated from analysis of the timing of peak monthly rainfall. Because of the region’s complex terrain, in situ precipitation datasets are assessed using high-spatiotemporal-resolution Tropical Rainfall Measuring Mission (TRMM) rainfall estimates, prior to their use in monsoon evolution, variability, and trend analyses. The Global Precipitation Climatology Center’s in situ analysis showed the least bias from TRMM.
El Niño–Southern Oscillation’s (ENSO) impact on NEM rainfall is shown to be significant, leading to stronger NEM rainfall over southeastern peninsular India and Sri Lanka but diminished rainfall over Thailand, Vietnam, and the Philippines. The impact varies subseasonally, being weak in October and strong in November. The positive anomalies over peninsular India are generated by anomalous anticyclonic flow centered over the Bay of Bengal, which is forced by an El Niño–related reduction in deep convection over the Maritime Continent.
The historical twentieth-century climate simulations informing the Intergovernmental Panel on Climatemore »
-
Abstract The Indian monsoon is of utmost concern to agriculture, the economy, and the livelihoods of billions in South Asia. However, little attention has been paid to the possibility of distinct subseasonal episodes phase-locked in the Indian monsoon annual cycle. This study addresses this gap by utilizing the self-organizing map (SOM) method to objectively classify six distinct subseasonal stages based on the 850-hPa wind fields. Each subseasonal stage ranges from 23 to 90 days. The Indian summer monsoon (ISM) consists of three substages, the ISM-onset, ISM-peak, and ISM-withdrawal, altogether contributing to 82% of the annual precipitation. The three substages signify the rapid northward advance, dominance, and gradual southward retreat of southwesterlies from mid-May to early October. The winter monsoon also comprises three substages (fall, winter, and spring), distinguishable by the latitude of the Arabian Sea high pressure ridge and hydrological conditions. This study proposes two compact indices based on zonal winds in the northern and southern Arabian Sea to measure the winter and summer monsoons, respectively. These indices capture the development and turnabouts of the six SOM-derived stages and can be used for subseasonal monsoon monitoring and forecasts. The spring and the ISM-onset episodes are highly susceptible to compound hazardsmore »
Significance Statement This research explores the existence of subseasonal features in the Indian monsoon annual cycle. Through the use of machine learning, we discover that the Indian summer monsoon and winter monsoon each consist of three substages. These substages’ evolution can be measured by two compact indices proposed herein, which can aid in subseasonal monsoon monitoring and forecasts in South Asia. Pertaining to hazard adaptations, this work pinpoints the subseasonal episodes most susceptible to droughts, heatwaves, floods, and tropical storms. High correlations are obtained when validating the substages’ yearly start and end dates against those documented in the existing literature, offering credibility to the subseasonal features of the Indian monsoon.
