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.

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.

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.

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.

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.

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.

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.

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.

Abstract

Recent studies have identified a connection between the summer monsoon rainfall in the southwest United States and anomalies of the antecedent winter precipitation and snowpack in the northwest United States. This connection shows a seasonal-scale predictability of the precipitation and indicates a seasonal predictability of the land–atmosphere system (the “land memory”) in the western United States. Although some efforts have been devoted to understanding this predictability, the physical processes constituting it remain unexplained. In this empirical study, a potential source, the soil enthalpy, and its role in land memory are examined for the recent epoch of a strong land memory (1961–90). The rationale is that the soil enthalpy variation has magnitudes comparable to the atmospheric enthalpy changes at various time scales, and the soil enthalpy anomaly in the top 20–50-cm soil column can persist for 2–3 months. As shown by the major results of this study, a persistent negative anomaly of the soil enthalpy in the northwest United States is related to negative anomalies of the surface and the lower-troposphere temperature in that region. Subsequently, the lower-troposphere temperature and related higher-atmospheric pressure anomalies in the northwest United States during late spring and the early summer months encourage a northward position of the lower-troposphere monsoon ridge in the western United States and, therefore, create a circulation that favors an above-average monsoon rainfall in the southwest United States. A weaker summer monsoon occurs when a sequence of opposite anomalies develops after a warm and dry winter in the northwest United States. In this regard, the soil enthalpy variations may serve to “record” the winter precipitation and temperature anomalies and “release” their effects on summer monsoon rainfall through interactions of soil enthalpy with the surface and lower-troposphere temperatures.

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.