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Boksoon Myoung
and
John W. Nielsen-Gammon

Abstract

Identifying dynamical and physical mechanisms controlling variability of convective precipitation is critical for predicting intraseasonal and longer-term changes in warm-season precipitation and convectively driven large-scale circulations. On a monthly basis, the relationship of convective instability with precipitation is examined to investigate the modulation of convective instability on precipitation using the Global Historical Climatology Network (GHCN) and NCEP–NCAR reanalysis for 1948–2003. Three convective parameters—convective inhibition (CIN), precipitable water (PW), and convective available potential energy (CAPE)—are examined. A lifted index and a difference between low-tropospheric temperature and surface dewpoint are used as proxies of CAPE and CIN, respectively.

A simple correlation analysis between the convective parameters and the reanalysis precipitation revealed that the most significant convective parameter in the variability of monthly mean precipitation varies by regions and seasons. With respect to region, CIN is tightly coupled with precipitation over summer continents in the Northern Hemisphere and Australia, while PW or CAPE is tightly coupled with precipitation over tropical oceans. With respect to seasons, the identity of the most significant convective parameter tends to be consistent across seasons over the oceans, while it varies by season in Africa and South America. Results from GHCN precipitation data are broadly consistent with reanalysis data where GHCN data exist, except in some tropical areas where correlations are much stronger (and sometimes signed differently) with reanalysis precipitation than with GHCN precipitation.

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Boksoon Myoung
and
John W. Nielsen-Gammon

Abstract

This research is designed to investigate how convective instability influences monthly mean precipitation in Texas in the summertime and to examine the modulation of convective instability and precipitation by local and regional forcings. Since drought results from the accumulated effects of deficient precipitation over time, this study is expected to shed light on the physical and dynamical mechanisms of the initiation and maintenance of serious droughts as well. The focus in Part I of this two-part study is on identification of the controlling convective parameters and, in turn, the surface-based processes that cause variations in these parameters. NCEP–NCAR reanalysis data and observed precipitation data, correlation analysis, multiple linear regression analysis, and back-trajectory analysis are used to reveal the underlying dynamics of their linkage and causality.

Monthly mean precipitation is modified mainly by convective inhibition (CIN) rather than by convective available potential energy (CAPE) or by precipitable water. Excessive CIN is caused by surface dryness and warming at 700 hPa, leading to precipitation deficits on a monthly time scale. While the dewpoint temperature and thermodynamics at the surface are greatly affected by the soil moisture, the temperature at 700 hPa was found to be statistically independent of the surface dewpoint temperature since the 700-hPa temperature represents free-atmospheric processes. (These free-atmospheric processes are the focus of the companion paper.) Finally, the strong correlations among precipitation, soil moisture, and CIN, as well as their underlying physical processes, suggest that the tight linkage between precipitation and soil moisture is not only due to the impacts of precipitation on soil moisture but also to the feedbacks of soil moisture on precipitation by controlling CIN.

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Boksoon Myoung
and
John W. Nielsen-Gammon

Abstract

This study is concerned with the modulation by convective instability of summertime precipitation in Texas as a mechanism for maintaining or enhancing drought. The important role of convective inhibition (CIN), its dependence on the temperature at 700 hPa and the surface dewpoint, and the mechanism by which soil moisture modulates precipitation through CIN were described in of this two-part series study. This study, Part II, examines the dynamic and physical processes controlling the temperature at 700 hPa and elucidates the large-scale influences on convective instability and precipitation integrating the principal processes found in both Parts I and II.

Back-trajectory analysis indicates that a significant contributor to warming at 700 hPa is the inversion caused by warm air transport from the Rocky Mountains and the Mexican Plateau where the surface potential temperature is greater than 307.5 K, rather than by subsidence. It was found that downward motion and warm air transport are enhanced in Texas when an upper-level anticyclonic circulation develops in the southern United States.

Upper-level anticyclonic circulations in the southern United States, one of the distinctive features of central U.S. droughts, strongly affect Texas summertime precipitation by modulating the thermodynamic structure of the atmosphere and thus convective instability. Stationary anticyclonic anomalies increase CIN not only by enhancing warm air transport from the high terrain but also by suppressing the occurrence of traveling disturbances. The resulting reduced precipitation and dry soil significantly modulate surface conditions, which elevates CIN and decreases precipitation. The aforementioned chain reaction of upper-level anticyclone influences that is expected to play an important role in initiating and maintaining Texas summer droughts can be understood within the context of CIN.

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Martin Hoerling
,
Arun Kumar
,
Randall Dole
,
John W. Nielsen-Gammon
,
Jon Eischeid
,
Judith Perlwitz
,
Xiao-Wei Quan
,
Tao Zhang
,
Philip Pegion
, and
Mingyue Chen

Abstract

The record-setting 2011 Texas drought/heat wave is examined to identify physical processes, underlying causes, and predictability. October 2010–September 2011 was Texas’s driest 12-month period on record. While the summer 2011 heat wave magnitude (2.9°C above the 1981–2010 mean) was larger than the previous record, events of similar or larger magnitude appear in preindustrial control runs of climate models. The principal factor contributing to the heat wave magnitude was a severe rainfall deficit during antecedent and concurrent seasons related to anomalous sea surface temperatures (SSTs) that included a La Niña event. Virtually all the precipitation deficits appear to be due to natural variability. About 0.6°C warming relative to the 1981–2010 mean is estimated to be attributable to human-induced climate change, with warming observed mainly in the past decade. Quantitative attribution of the overall human-induced contribution since preindustrial times is complicated by the lack of a detected century-scale temperature trend over Texas. Multiple factors altered the probability of climate extremes over Texas in 2011. Observed SST conditions increased the frequency of severe rainfall deficit events from 9% to 34% relative to 1981–2010, while anthropogenic forcing did not appreciably alter their frequency. Human-induced climate change increased the probability of a new temperature record from 3% during the 1981–2010 reference period to 6% in 2011, while the 2011 SSTs increased the probability from 4% to 23%. Forecasts initialized in May 2011 demonstrate predictive skill in anticipating much of the SST-enhanced risk for an extreme summer drought/heat wave over Texas.

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