scholarly journals The Convective Instability Pathway to Warm Season Drought in Texas. Part I: The Role of Convective Inhibition and Its Modulation by Soil Moisture

2010 ◽  
Vol 23 (17) ◽  
pp. 4461-4473 ◽  
Author(s):  
Boksoon Myoung ◽  
John W. Nielsen-Gammon
Keyword(s):  
Soil Moisture ◽  
Convective Instability ◽  
Linear Regression Analysis ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Precipitable Water ◽  
Back Trajectory ◽  
Atmospheric Processes ◽  
Dewpoint Temperature ◽  
Convective Inhibition

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.

The Convective Instability Pathway to Warm Season Drought in Texas. Part II: Free-Tropospheric Modulation of Convective Inhibition

2010 ◽  
Vol 23 (17) ◽  
pp. 4474-4488 ◽  
Author(s):  
Boksoon Myoung ◽  
John W. Nielsen-Gammon
Keyword(s):  
United States ◽  
Convective Instability ◽  
Large Scale ◽  
Potential Temperature ◽  
Warm Season ◽  
Air Transport ◽  
Southern United States ◽  
Back Trajectory ◽  
Convective Inhibition ◽  
Upper Level

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 Part I 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.

scholarly journals Sensitivity of Monthly Convective Precipitation to Environmental Conditions

2010 ◽  
Vol 23 (1) ◽  
pp. 166-188 ◽  
Author(s):  
Boksoon Myoung ◽  
John W. Nielsen-Gammon
Keyword(s):  
Convective Instability ◽  
Large Scale ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Precipitable Water ◽  
Convective Precipitation ◽  
Tropospheric Temperature ◽  
Monthly Basis ◽  
Convective Parameter ◽  
Tightly Coupled

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.

NAM Model Forecasts of Warm-Season Quasi-Stationary Frontal Environments in the Central United States

2010 ◽  
Vol 25 (4) ◽  
pp. 1281-1292 ◽  
Author(s):  
Shih-Yu Wang ◽  
Adam J. Clark
Keyword(s):  
United States ◽  
Mesoscale Model ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Correlation Coefficients ◽  
Precipitable Water ◽  
Wind Fields ◽  
Vapor Flux ◽  
Convective Systems ◽  
Central United States

Abstract Using a composite procedure, North American Mesoscale Model (NAM) forecast and observed environments associated with zonally oriented, quasi-stationary surface fronts for 64 cases during July–August 2006–08 were examined for a large region encompassing the central United States. NAM adequately simulated the general synoptic features associated with the frontal environments (e.g., patterns in the low-level wind fields) as well as the positions of the fronts. However, kinematic fields important to frontogenesis such as horizontal deformation and convergence were overpredicted. Surface-based convective available potential energy (CAPE) and precipitable water were also overpredicted, which was likely related to the overprediction of the kinematic fields through convergence of water vapor flux. In addition, a spurious coherence between forecast deformation and precipitation was found using spatial correlation coefficients. Composite precipitation forecasts featured a broad area of rainfall stretched parallel to the composite front, whereas the composite observed precipitation covered a smaller area and had a WNW–ESE orientation relative to the front, consistent with mesoscale convective systems (MCSs) propagating at a slight right angle relative to the thermal gradient. Thus, deficiencies in the NAM precipitation forecasts may at least partially result from the inability to depict MCSs properly. It was observed that errors in the precipitation forecasts appeared to lag those of the kinematic fields, and so it seems likely that deficiencies in the precipitation forecasts are related to the overprediction of the kinematic fields such as deformation. However, no attempts were made to establish whether the overpredicted kinematic fields actually contributed to the errors in the precipitation forecasts or whether the overpredicted kinematic fields were simply an artifact of the precipitation errors. Regardless of the relationship between such errors, recognition of typical warm-season environments associated with these errors should be useful to operational forecasters.

scholarly journals Diurnal Cycle of Convective Instability around the Central Mountains in Japan during the Warm Season

2005 ◽  
Vol 62 (5) ◽  
pp. 1626-1636 ◽  
Author(s):  
Tomonori Sato ◽  
Fujio Kimura
Keyword(s):  
Water Vapor ◽  
Diurnal Cycle ◽  
Convective Instability ◽  
Potential Temperature ◽  
Warm Season ◽  
Precipitable Water ◽  
Specific Humidity ◽  
High Temporal Resolution ◽  
Mountainous Areas ◽  
Diurnal Cycles

Abstract Convective rainfall often shows a clear diurnal cycle. The nighttime peak of convective activity prevails in various regions near the world's mountains. The influence of the water vapor and convective instability upon nocturnal precipitation is investigated using a numerical model and observed data. Recent developments in GPS meteorology allow the estimation of precipitable water vapor (PWV) with a high temporal resolution. A dense network has been established in Japan. The GPS analysis in August 2000 provides the following results: In the early evening, a high-GPS-PWV region forms over mountainous areas because of the convergence of low-level moisture, which gradually propagates toward the adjacent plain before midnight. A region of convection propagates simultaneously eastward into the plain. The precipitating frequency correlates fairly well with the GPS-PWV and attains a maximum value at night over the plain. The model also provides similar characteristics in the diurnal cycles of rainfall and high PWV. Abundant moisture accumulates over the mountainous areas in the afternoon and then advects continuously toward the plain by the ambient wind. The specific humidity greatly increases at about the 800-hPa level over the plain at night, and the PWV reaches its nocturnal maximum. The increase in the specific humidity causes an increase of equivalent potential temperature at about the 800-hPa level; as a result, the convective instability index becomes more unstable over the plain at night. These findings are consistent with the diurnal cycle of the observed precipitating frequency.

scholarly journals Does Afternoon Precipitation Occur Preferentially over Dry or Wet Soils in Oklahoma?

2015 ◽  
Vol 16 (2) ◽  
pp. 874-888 ◽  
Author(s):  
Trent W. Ford ◽  
Anita D. Rapp ◽  
Steven M. Quiring
Keyword(s):  
Soil Moisture ◽  
Great Plains ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Causal Link ◽  
Atmospheric Conditions ◽  
Low Level ◽  
Low Level Jet ◽  
The Impact ◽  
Soil Moisture Feedback

Abstract Soil moisture is an integral part of the climate system and can drive land–atmosphere interactions through the partitioning of latent and sensible heat. Soil moisture feedback to precipitation has been documented in several regions of the world, most notably in the southern Great Plains. However, the impact of soil moisture on precipitation, particularly at short (subdaily) time scales, has not been resolved. Here, in situ soil moisture observations and satellite-based precipitation estimates are used to examine if afternoon precipitation falls preferentially over wet or dry soils in Oklahoma. Afternoon precipitation events during the warm season (May–September) in Oklahoma from 2003 and 2012 are categorized by how favorable atmospheric conditions are for convection, as well as the presence or absence of the Great Plains low-level jet. The results show afternoon precipitation falls preferentially over wet soils when the Great Plains low-level jet is absent. In contrast, precipitation falls preferentially over dry soils when the low-level jet is present. Humidity (temperature) is increased (decreased) as soil moisture increases for all conditions, and convective available potential energy prior to convection is strongest when atmospheric humidity is above normal. The results do not demonstrate a causal link between soil moisture and precipitation, but they do suggest that soil moisture feedback to precipitation could potentially manifest itself over wetter- and drier-than-normal soils, depending on the overall synoptic and dynamic conditions.

An Analysis of the Soil Moisture Feedback on Convective and Stratiform Precipitation

2008 ◽  
Vol 9 (2) ◽  
pp. 280-291 ◽  
Author(s):  
Lorenzo Alfieri ◽  
Pierluigi Claps ◽  
Paolo D’Odorico ◽  
Francesco Laio ◽  
Thomas M. Over
Keyword(s):  
Soil Moisture ◽  
Land Surface ◽  
Water Cycle ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Daily Rainfall ◽  
Convective Precipitation ◽  
Antecedent Soil Moisture ◽  
Rainfall Occurrence ◽  
Moisture Conditions

Abstract Land–atmosphere interactions in midlatitude continental regions are particularly active during the warm season. It is still unclear whether and under what circumstances these interactions may involve positive or negative feedbacks between soil moisture conditions and rainfall occurrence. Assessing such feedbacks is crucially important to a better understanding of the role of land surface conditions on the regional dynamics of the water cycle. This work investigates the relationship between soil moisture and subsequent precipitation at the daily time scale in a midlatitude continental region. Sounding data from 16 locations across the midwestern United States are used to calculate two indices of atmospheric instability—namely, the convective available potential energy (CAPE) and the convective inhibition (CIN). These indices are used to classify rainfall as convective or stratiform. Correlation analyses and uniformity tests are then carried out separately for these two rainfall categories, to assess the dependence of rainfall occurrence on antecedent soil moisture conditions, using simulated soil moisture values. The analysis suggests that most of the positive correlation observed between soil moisture and subsequent precipitation is due to the autocorrelation of long stratiform events. The authors found both areas with positive and areas with negative feedback on convective precipitation. This behavior is likely due to the contrasting effects of soil moisture conditions on convective phenomena through changes in surface temperature and the supply of water vapor to the overlying air column. No significant correlation is found between daily rainfall intensity and antecedent simulated soil moisture conditions either for convective or stratiform rainfall.

Convective Instability Associated with the Eastward-Propagating Rainfall Episodes over Eastern China during the Warm Season

2014 ◽  
Vol 27 (6) ◽  
pp. 2331-2339 ◽  
Author(s):  
Guixing Chen ◽  
Ryuhei Yoshida ◽  
Weiming Sha ◽  
Toshiki Iwasaki ◽  
Huiling Qin
Keyword(s):  
Convective Instability ◽  
Eastern China ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Leading Edge ◽  
Low Level ◽  
Climate Anomalies ◽  
Late Night ◽  
Forecast Models ◽  
Episode Duration

Abstract Analysis of the latest satellite rainfall and reanalysis datasets from 1998 to 2012 demonstrates that eastward-propagating rainfall episodes, which typically occur in late night and morning, are determinant factors for the rainfall diurnal cycle and climate anomalies over eastern China. The episode growth and propagation are facilitated by an elevated layer of conditionally unstable air in a mesoscale zone at their eastern leading edge. The convective available potential energy (CAPE), despite convection consumption and nocturnal cooling, decreases only from a high value to a moderate one during episode duration. An estimate of the CAPE generation budget suggests that low-level horizontal advection and vertical lifting of the warm moist air can produce sufficient CAPE to balance other stabilization effects, sustaining the mesoscale maximum of convective instability ahead of rainfall episodes. These instability geneses are pronounced at the convection growth stage and linked closely to a mesoscale nocturnal low-level jet. Thus, a proper representation of them in forecast models is essential for improved prediction of the warm-season rainfall.

scholarly journals Atmospheric Contributors to Heavy Rainfall Events in the Arkansas-Red River Basin

2016 ◽  
Vol 2016 ◽  
pp. 1-15 ◽  
Author(s):  
Taylor A. McCorkle ◽  
Skylar S. Williams ◽  
Timothy A. Pfeiffer ◽  
Jeffrey B. Basara
Keyword(s):  
River Basin ◽  
Heavy Rainfall ◽  
Convective Available Potential Energy ◽  
Warm Season ◽  
Precipitable Water ◽  
Red River ◽  
Rainfall Events ◽  
Red River Basin ◽  
Thermodynamic Variables ◽  
Heavy Rainfall Events

This study analyzed the top 1% 24-hour rainfall events from 1994 to 2013 at eight climatological sites that represent the east to west precipitation gradient across the Arkansas-Red River Basin in North America. A total of 131 cases were identified and subsequently classified on the synoptic-scale, mesoscale, and local-scale to compile a climatological analysis of these extreme, heavy rainfall events based on atmospheric forcings. For each location, the prominent midtropospheric pattern, mesoscale feature, and predetermined thermodynamic variables were used to classify each 1% rainfall event. Individual events were then compared with other cases throughout the basin. The most profound results were that the magnitudes of the thermodynamic variables such as convective available potential energy and precipitable water values were poor predictors of the amount of rainfall produced in these extreme events. Further, the mesoscale forcings had more of an impact during the warm season and for the westernmost locations, whereas synoptic forcings were extremely prevalent during the cold season at the easternmost locations in the basin. The implications of this research are aimed at improving the forecasting of heavy precipitation at individual weather forecasts offices within the basin through the identified patterns at various scales.

scholarly journals Interactive Vegetation Phenology, Soil Moisture, and Monthly Temperature Forecasts

2015 ◽  
Vol 16 (4) ◽  
pp. 1456-1465 ◽  
Author(s):  
R. D. Koster ◽  
G. K. Walker
Keyword(s):  
Soil Moisture ◽  
Air Temperature ◽  
Warm Season ◽  
Small Extent ◽  
Vegetation Phenology ◽  
Seasonal Forecasts ◽  
Forecast System ◽  
Area Index ◽  
Atmospheric Processes ◽  
Phenology Model

Abstract The time scales that characterize the variations of vegetation phenology are generally much longer than those that characterize atmospheric processes. The explicit modeling of phenological processes in an atmospheric forecast system thus has the potential to provide skill to subseasonal or seasonal forecasts. We examine this possibility here using a forecast system fitted with a dynamic vegetation phenology model. We perform three experiments, each consisting of 128 independent warm-season monthly forecasts: 1) an experiment in which both soil moisture states and carbon states (e.g., those determining leaf area index) are initialized realistically, 2) an experiment in which the carbon states are prescribed to climatology throughout the forecasts, and 3) an experiment in which both the carbon and soil moisture states are prescribed to climatology throughout the forecasts. Evaluating the monthly forecasts of air temperature in each ensemble against observations—as well as quantifying the inherent predictability of temperature within each ensemble—shows that dynamic phenology can indeed contribute positively to subseasonal forecasts, though only to a small extent, with an impact dwarfed by that of soil moisture.

scholarly journals A 7-Year Climatology of Warm-Sector Heavy Rainfall over South China during the Pre-Summer Months

Atmosphere ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 914
Author(s):  
Tao Chen ◽  
Da-Lin Zhang
Keyword(s):  
South China ◽  
Heavy Rainfall ◽  
Large Scale ◽  
Convective Available Potential Energy ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Precipitable Water ◽  
Atmospheric Parameter ◽  
Low Level ◽  
Warm Sector

In view of the limited predictability of heavy rainfall (HR) events and the limited understanding of the physical mechanisms governing the initiation and organization of the associated mesoscale convective systems (MCSs), a composite analysis of 58 HR events over the warm sector (i.e., far ahead of the surface cold front), referred to as WSHR events, over South China during the months of April to June 2008~2014 is performed in terms of precipitation, large-scale circulations, pre-storm environmental conditions, and MCS types. Results show that the large-scale circulations of the WSHR events can be categorized into pre-frontal, southwesterly warm and moist ascending airflow, and low-level vortex types, with higher frequency occurrences of the former two types. Their pre-storm environments are characterized by a deep moist layer with >50 mm column-integrated precipitable water, high convective available potential energy with the equivalent potential temperature of ≥340 K at 850 hPa, weak vertical wind shear below 400 hPa, and a low-level jet near 925 hPa with weak warm advection, based on atmospheric parameter composite. Three classes of the corresponding MCSs, exhibiting peak convective activity in the afternoon and the early morning hours, can be identified as linear-shaped, a leading convective line adjoined with trailing stratiform rainfall, and comma-shaped, respectively. It is found that many linear-shaped MCSs in coastal regions are triggered by local topography, enhanced by sea breezes, whereas the latter two classes of MCSs experience isentropic lifting in the southwesterly warm and moist flows. They all develop in large-scale environments with favorable quasi-geostrophic forcing, albeit weak. Conceptual models are finally developed to facilitate our understanding and prediction of the WSHR events over South China.

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