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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

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 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

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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Michael Veres and Qi Hu

Abstract

Idealized model experiments using the NCAR CESM1.0.5 under equinox conditions are designed and performed to address two fundamental questions about the effects of the sea surface temperature (SST) variation associated with the Atlantic multidecadal oscillation (AMO) on circulation and precipitation in North America and Europe: 1) Is the observed relationship between the AMO SST and the warm-season precipitation in North America a statistical coincidence? and 2) Why is the response of negative precipitation anomaly to warm SST in the AMO fairly uniform across most of North America, whereas the positive precipitation anomaly in the cold SST rather spotty? Model experiments are done with either a warm or cold SST anomaly in an aquaplanet, a planet with idealized continents, and a planet with both idealized continents and orography. Major results show that the atmospheric response to warm SST anomaly in the North Atlantic is fairly similar among the three sets of experiments. In the lower troposphere, the response has a significant negative geopotential anomaly from the SST anomaly center to the east and a positive geopotential anomaly in upstream North America. However, the response to the cold SST anomaly changes considerably among these experiments, particularly in North America. These results provide a foundation to answer the abovementioned two questions. First, they show that there is physical connection of the AMO SST and atmospheric circulation anomalies in North America. Moreover, the rather stable atmospheric response to the warm SST may explain the observed largely consistent response to the warm SST anomaly. The varying responses of the atmosphere to the cold SST indicate a strong sensitivity of the atmosphere to other forcings during the cold SST anomaly in the North Atlantic. This sensitivity could explain the varying and less stable response of the atmosphere to the cold SST during the AMO.

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Song Feng and Qi Hu

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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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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

The following questions are addressed in this study using an array of data and statistical methods: 1) does the North American monsoon region have a single dominant monsoon system; 2) if it has more than one, what are they; and 3) what are major causes of interannual monsoon rainfall variations in these systems? Results showed two dominant summer monsoon systems in the region: one in south-central Mexico, south of the 26°N, and the other in the southwestern United States and northwestern Mexico. Monsoon rainfall variations in these regions are usually opposite to each other and have different causes. The interannual variations in monsoon rainfall in south-central Mexico were highly affected by interannual variations in the intertropical convergence zone (ITCZ) in the eastern tropical Pacific. A northern (southern) position of the ITCZ, often related to cooler (warmer) than normal sea surface temperatures in the eastern tropical Pacific Ocean, corresponded to strong (weak) monsoon.

The “land memory effect” was evident in interannual variations of monsoon rainfall in the southwestern United States, shown by strong correlations of the summer rainfall variation versus antecedent winter precipitation anomalies in the western United States. However, the effect was not robust but varied fairly regularly. It was strong from approximately 1920 to 1930 and disappeared from 1931 to 1960. It regained its strength from 1961 to 1990 but has weakened again since 1990. The forcing of this variation was identified as a multidecadal variation in atmosphere circulations in the North Pacific–North American sector and the land memory effect was part of this variation. This multidecadal variation has to be included in prediction methods in order for them to correctly describe seasonal and interannual variations in summer rainfall in the North American monsoon region.

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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

Summer rainfall in the central United States has singular interannual variations of a 3–6-yr period. Identifying the causes of these variations assures improvement in predictions of summer rainfall in the region.

A review of previous studies revealed a puzzling situation: the outstanding interannual variations of the summer rainfall in the central United States showed no persistent correlations with known influential interannual variations in the Northern Hemisphere and the El Niño–Southern Oscillation (ENSO). This study was undertaken to identify the cause of this situation and ultimately explain the causes of the observed interannual summer rainfall variations. Its results showed a teleconnection of the ENSO with the summer rainfall in the central United States. The intensity of which has varied over the last 125 years. The teleconnection was active in two epochs, 1871–1916 and 1948–78, and absent in the two epochs 1917–47 and 1979–present. This variation was associated with a multidecadal variation in both sea surface temperature and sea level pressure in the mid- and high-latitude North Pacific. In the epochs of active teleconnection, the circulation in the warm phase of ENSO favored a deformation field in the lower troposphere in the central United States causing wet summers and a reversed circulation in cold phase of ENSO yielding dry summers, a process that partially explains the interannual summer rainfall variations.

The result also showed that the variations of the teleconnection were “in phase” with the variation in the average surface temperature of the Northern Hemisphere. When the “abrupt warming” of the surface temperature developed in 1917–47 and the most recent two decades, the teleconnection broke down. Because of the limitation in data record length, this observed relationship and the persistence of the variation in the teleconnection need further investigations when additional data are available.

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