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Qi Hu

Abstract

Recent studies have identified two sources alternating their dominant roles in the interannual summer rainfall variations in the central United States. One is the ENSO cycle in the tropical Pacific, and the other is the interannual variability in the intensity of the southerly flow from the Gulf of Mexico. The ENSO cycle affected the rainfall variation through an atmospheric teleconnection, which was particularly strong in 1871–1916 and 1948–78. When the teleconnection weakened in 1917–47 and 1979–2002, the southerly flow from the Gulf of Mexico invigorated its effect on the interannual rainfall variation. Because the effect of the two sources could result in different rainfall processes in the central United States, their alternation should have built similar variation into the region’s summer season diurnal rainfall pattern.

An hourly rainfall dataset was used to examine this hypothesis. Results showed a multidecadal variation in the diurnal rainfall pattern. In the decades when the southerly flow dominated the rainfall variation, the diurnal pattern had large rainfall in late night/morning hours with a sharp rainfall peak in the midnight hour. In the decades when the southerly flow effect weakened, a different diurnal pattern emerged, with small late night/morning hour rainfall and a broad plateau of rainfall in the late night/early morning hours. This diurnal pattern change was happening simultaneously with variations of the southerly moisture flux and moisture convergence in the central United States. These coherent variations show that the summer season diurnal rainfall pattern varied at a multidecadal scale consistent with the alternation frequency of the two sources. In addition to showing the variation in the diurnal rainfall pattern, results of this study provide the knowledge for understanding the climate of extreme events, particularly heavy rainfall and floods, in the central United States.

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Qi Hu

Abstract

The author presents a cumulus parameterization that uses a cloud model that describes atmospheric convection as consisting of a sequence of intermittently rising thermals. The total mass of thermals in a convection event is determined by the amount of convective available potential energy in local soundings. In the cloud model, it is assumed that a thermal entrains environmental air only at a thin layer around the top frontier of its rising body. The entrained air mass mixes with the thermal’s air and produces “mixtures” that then detach themselves from the thermal. This limited mixing prevents deep erosion to the thermal’s buoyancy by entrainment and mixing processes. The remainder of the thermal continues rising to higher levels and forming more mixtures on its way to its own level of neutral buoyancy. The mixtures also rise or sink from the levels where they form to their level of neutral buoyancy.

Evaluation of this scheme using Global Atmospheric Research Program Atlantic Tropical Experiment data shows that the parameterized convective heating and drying rates are consistent with observations. The calculated convective precipitation also shows a distribution similar to the observed total precipitation, except at the trough of the easterly waves where calculated precipitation is smaller than observed. The capability of this scheme in describing cumulus convection is further tested in a fully prognostic one-dimensional climate model. Results from this evaluation show reasonable climatological temperature and relative humidity profiles in the troposphere.

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Qi Hu

Abstract

The author revisits the singular value decomposition (SVD) method and shows that the nonuniqueness of the left and right singular vectors related to SVD posts limitations on applications of the method. Caution should be observed when the heterogeneous and homogeneous correlation maps are applied to interpret the relationship between two meteorological data series.

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Qi Hu and Song Feng

Abstract

Interannual and multidecadal time-scale anomalies in sea surface temperatures (SST) of the North Atlantic and North Pacific Oceans could result in persistent atmospheric circulation and regional precipitation anomalies for years to decades. Understanding the processes that connect such SST forcings with circulation and precipitation anomalies is thus important for understanding climate variations and for improving predictions at interannual–decadal time scales. This study focuses on the interrelationship between the Atlantic multidecadal oscillation (AMO) and El Niño–Southern Oscillation (ENSO) and their resulting interannual to multidecadal time-scale variations in summertime precipitation in North America. Major results show that the ENSO forcing can strongly modify the atmospheric circulation variations driven by the AMO. Moreover, these modifications differ considerably between the subtropics and the mid- and high-latitude regions. In the subtropics, ENSO-driven variations in precipitation are fairly uniform across longitudes so ENSO effects only add interannual variations to the amplitude of the precipitation anomaly pattern driven by the AMO. In the mid- and high latitudes, ENSO-forced waves in the atmosphere strongly modify the circulation anomalies driven by the AMO, resulting in distinctive interannual variations following the ENSO cycle. The role of the AMO is shown by an asymmetry in precipitation during ENSO between the warm and cold phases of the AMO. These results extend the outcomes of the studies of the recent Climate Variability and Predictability (CLIVAR) Drought Working Group from the AMO and ENSO effects on droughts to understanding of the mechanisms and causal processes connecting the individual and combined SST forcing of the AMO and ENSO with the interannual and multidecadal variations in summertime precipitation and droughts in North America.

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Qi Hu and Song Feng

Abstract

Although affected by atmospheric circulations, variations in soil temperature result primarily from the radiation and sensible and latent heat exchanges at the surface and heat transfer in the soils of different thermal properties. Thus, soil temperature and its variation at various depths are unique parameters that are useful in understanding both the surface energy processes and regional environmental and climatic conditions. Yet, despite the importance, long-term quality data of soil temperatures are not available for the United States. The goal of this study is to fill this data gap and to develop a soil temperature dataset from the historical data of U.S. cooperative stations. Cooperative station soil temperatures at various depths from 1967 to 2002 are collected and examined by a set of quality checks, and erroneous data of extended periods are estimated using methods constructed in this study. After the quality control, the data are used to describe the climatic soil temperature as well as soil temperature change in the contiguous United States. The 35-yr climatological dataset shows that the annual soil temperature at 10-cm depth, at which most stations have soil temperature measurements, decreases gradually from 297 K in the coastal areas along the Gulf of Mexico to below 281 K on the United States–Canada border. In seasonal variation, the largest change occurs from spring to summer, during which time soil temperatures are adjusted from the cold season to the warm season, particularly in snow-cover regions. Mild changes are observed from autumn to winter, during which time the soil heat storage still dominates the soil temperature variations. An analysis of the soil temperature variation reveals a warming trend of soil temperatures in most of the stations in the northern and northwestern United States and a large cooling trend in some stations in the southeastern United States. Significant warming is found in the winter and spring season. Potential effects of these trends on regional agriculture are discussed.

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Qi Hu and Gregory Buyanovsky

Abstract

Understanding climate effects on crop yield has been a continuous endeavor aiming at improving farming technology and management strategy, minimizing negative climate effects, and maximizing positive climate effects on yield. Many studies have examined climate effects on corn yield in different regions of the United States. However, most of those studies used yield and climate records that were shorter than 10 years and were for different years and localities. Although results of those studies showed various influences of climate on corn yield, they could be time specific and have been difficult to use for deriving a comprehensive understanding of climate effects on corn yield. In this study, climate effects on corn yield in central Missouri are examined using unique long-term (1895–1998) datasets of both corn yield and climate. Major results show that the climate effects on corn yield can only be explained by within-season variations in rainfall and temperature and cannot be distinguished by average growing-season conditions. Moreover, the growing-season distributions of rainfall and temperature for high-yield years are characterized by less rainfall and warmer temperature in the planting period, a rapid increase in rainfall, and more rainfall and warmer temperatures during germination and emergence. More rainfall and cooler-than-average temperatures are key features in the anthesis and kernel-filling periods from June through August, followed by less rainfall and warmer temperatures during the September and early October ripening time. Opposite variations in rainfall and temperature in the growing season correspond to low yield. Potential applications of these results in understanding how climate change may affect corn yield in the region also are discussed.

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Qi Hu and Song Feng

Abstract

This study continues the investigation of causes of the interannual variations in summer rainfall in the central United States. A previous study by the authors showed that the ENSO teleconnection significantly affected the interannual variations in summer rainfall in the central United States in two epochs, 1871–1916 and 1949–78. The teleconnection effect weakened in the epochs 1917–48 and 1979–97. The current study partially answers the question: What affected the interannual summer rainfall variations in the two epochs when the ENSO teleconnection weakened? Its results showed that the low-level southerly flow from the Gulf of Mexico was another source of interannual summer rainfall variations. The southerly flow possessed significant interannual variations independent of the ENSO variation. In the epochs when the ENSO teleconnection broke down, the variations of the southerly flow amplified. In the meantime, the circulation anomalies in the lower troposphere in the central United States favored a convergence and an unstable thermal profile. They helped to engage the variations in the southerly flow and the summer rainfall variation in the central United States and to maintain the interannual summer rainfall variation. A coherent variation of this source and ENSO teleconnection in different epochs sustained the observed interannual variations of the summer rainfall in the central United States. The coherent variation of the role of these two different sources was in accord with the multidecadal variation in SST in the mid- and high-latitude North Pacific Ocean, supporting the notion that the multidecadal variation in the SST may have facilitated the coherent variation.

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Qi Hu and Song Feng

Abstract

Several studies of the established warm season climate records for eastern China (1470–1997) showed alternating dry and wet periods at centennial scales. The spatial patterns show that when a dry condition or drought was observed in southern China, a wet or flood situation was found in the northern part of eastern China and vice versa. These patterns suggest a meridional variation of the centennial-scale wet/dry anomalies.

This study analyzed the same data and showed that the dry and wet anomalies initially appeared in the northern part of eastern China and then migrated southward to affect the low latitudes. An extension of this analysis to the United States revealed a similar southward migration of dry/wet anomalies that first developed in the high latitudes in the western part of the country. The average speed of the migrations in both areas is about 3.0° of latitude per 10 years.

The results suggest that mechanisms in mid- and high latitudes may play critical roles in the development of drought in high- as well as subtropical-latitude regions. The findings also indicate key areas to monitor for prediction of extended periods of frequent droughts or floods in “downstream” regions in the migration of the centennial-scale anomalies.

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Qi Hu and Song Feng

Abstract

The “land memory” refers to an interseasonal predictability of the summer monsoon rainfall in the southwestern United States, describing a relationship of the summer monsoon rainfall anomaly with anomalies in the antecedent winter season snow and land surface conditions in the western United States. This relationship has varied, however, showing a peculiar on-and-off feature in the last century. It is important to understand this variation so that the relationship can be used to assist making predictions of the monsoon rainfall for that region. This note offers the evidence and shows that the change of the land memory may have been a reflection of an irregular variation in the persistence of the sea surface temperature anomaly (SSTA) in the North Pacific Ocean; in epochs when the SSTA persisted from winter through summer, the SSTA and related anomalies in atmospheric circulation could have dominated the summer monsoon variation, whereas in epochs when the persistence collapsed the SSTA effect weakened and the effect of the land processes on the summer monsoon rainfall became prominent.

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Song Feng and Qi Hu

Abstract

Observational studies have created a dilemma on how El Niño–Southern Oscillation (ENSO) may have affected interannual variations of summer rainfall in northern China; some suggested a consistent effect while others showed a complete lack of effect. This dilemma is resolved in this study, which shows that ENSO has affected the summer rainfall in northern China and the effect has varied at multidecadal scales. The question of how the ENSO teleconnection with northern China rainfall variation was established is addressed, and an answer pointing to the Indian summer monsoon as a “facilitator” connecting ENSO and northern China rainfall variation is examined. The Indian monsoon circulation interacted with the regional circulations in northern China in some epochs and such interaction was interrupted in other epochs. When the interaction was active, the Indian monsoon variations originating from ENSO, during El Niño or La Niña, was extended to affect the rainfall variation in northern China, creating a teleconnection of ENSO with northern China rainfall. When the interaction weakened or was inactive, the ENSO effect languished. Additional analyses were done to address the related question of why the interactions have alternated. The alternation was suggested to result from variations of the large-scale circulation in the Eurasian continent. The circulation anomalies showed lowering (rising) 500-hPa geopotential height centered at Mongolia and western China in some epochs, enhancing cyclonic (anticyclonic) rotation in mid- and low-level winds and creating (disrupting) a moisture convoy from the Indian monsoon region to northern China and synergetic convergence/divergence anomalies in the monsoon region and in northern China. Results of this study contribute to the understanding of interannual and multidecadal variations of the summer rainfall in the semiarid region of northern China.

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