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R. W. Higgins and S. D. Schubert

Abstract

This study examines the role of synoptic-scale eddies during the development of persistent anticyclonic height anomalies over the central North Pacific in a general circulation model under perpetual January conditions. The GCM replicates the basic characteristics of the evolution of the anomaly patterns found in observations. The life cycle is characterized by the rapid establishment of the major anomaly center and considerably longer maintenance and decay phases, which include the development of downstream anomaly centers. The simulation also shows a realistic evolution of synoptic-scale activity beginning with enhanced activity off the east coast of Asia prior to onset, followed by a northward shift of the Pacific storm track, which lasts throughout the maintenance phase. The initial enhancement of synoptic-scale eddy activity is associated with a large-scale cyclonic anomaly that develops over Siberia several days prior to the onset of the main anticyclonic anomaly over the central North Pacific. The observations, however, show considerable interdecadel variability in the details of the composite onset behavior; it is unclear whether this variability is real or whether it reflects differences in the data assimilation systems.

The role of the time mean flow and synoptic-scale eddies in the development of the persistent Pacific anomalies is studied within the context of a kinetic energy budget in which the flow is decomposed into the time-mean, low-frequency (timescales longer than 10 days), and synoptic (timescales less than 6 days) components. The budget, which is carried out for the simulation at 500 mb, shows that the initial growth of the persistent anticyclonic anomalies is associated with barotropic conversions of energy, with approximately equal contributions coming from the mean flow and the synoptic-scale eddies. After onset the barotropic conversion from the mean flow dominates, whereas the decay phase is associated with baroclinic processes within the low-frequency flow.

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Joseph S. Wakefield and Wayne H. Schubert

Abstract

The cloud-topped mixed-layer theory developed by Lilly (1968) is employed in a slightly generalized form to simulate stratocumulus in the eastern North Pacific. The radiation parameterization involves cloud-top longwave cooling and mixed-layer shortwave heating. Long-wave emissivity and shortwave absorption are functions of cloud thickness. The model is integrated along climatological trajectories which traverse the area bounded by 145°W, 115°W, 20°N and 40°N. The results of the integration include fields of cloud-top and cloud-base height, mixed-layer thermodynamic structure, and convective and radiative fluxes. Where it is possible to compare model-generated fields with the observations of Neiburger et al. (1961), general agreement is found. The model-predicted location of the southern limit (due to cloud-top instability) of stratocumulus clouds is in agreement with observations.

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R. W. Higgins and S. D. Schubert

Abstract

The nature of low-frequency variations in synoptic-eddy activity over the North Pacific is examined in a general circulation model (GCM). A comparison with observations reveals that the GCM produces realistic time mean and low-frequency synoptic-eddy forcing of the 200-mb zonal wind. In the time mean, this forcing, which is computed as the divergence of the extended Eliassen–Palm (E–P) flux, shows an east-west dipole structure that tends to reduce the zonal wind over the western North Pacific and tends to enhance it to the east. This structure is consistent with the general picture of the life cycle of baroclinic waves, which show strong upward and eastward propagation in the western and central Pacific and meridional propagation to the east. The western and central Pacific synoptic-eddy forcing is dominated by the convergence of the baroclinic component of the E–P flux divergence, while over the eastern Pacific the divergence of the barotropic component is important. The dominant component of the low-frequency “envelope” (periods > 10 days) of synoptic-eddy forcing, computed as the first empirical orthogonal function (EOF) modulates the time mean synoptic-eddy forcing. This modulation is associated with low-frequency changes in the intensity of the synoptic-eddy activity and is only weakly tied to fluctuations in the low-frequency flow.

Composites of the hemispheric distribution of synoptic-eddy forcing in the GCM, based on the extremes of the dominant Pacific EOF, show a seesaw behavior with enhanced eddy forcing in the North Pacific basin associated with suppressed forcing in the North Atlantic basin, and vice versa. The link between the Pacific and the Atlantic basins appears to be due to the presence of eastward-traveling baroclinic wave packets that travel around the globe with a period of about 10 days. Some evidence is found for a similar behavior in the observations.

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R. W. Higgins and S. D. Schubert

Abstract

Evidence is presented from a composite analysis of a 14-year general circulation model simulation, that persistent North Pacific (PNP) circulation anomalies during boreal winter are part of a larger-scale meridional development extending into the Tropics and the Southern Hemisphere. Lagged composites suggest that the development is initiated over the tropical Pacific by anomalous convection (characterized by an east-west dipole structure centered at the date line) one to two weeks prior to the extratropical onset time. Relatively weak wave trains. extending from the region of anomalous convection into the extratropics, appear to set the stage for the subsequent rapid development of the PNP anomalies. After onset, the PNP anomalies extend into the Tropics and enhance moisture transports that tend to supply moisture to, and thus reinforce, the associated tropical precipitation anomalies. The mature stage is characterized by a strong coupling between hemispheres, including twin low-level cyclonic (anticyclonic) circulations straddling the equator with westerly (easterly) wind “bursts” on their equatorward flanks. The tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations reminiscent of the Madden–Julian oscillation.

Results from a similar composite analysis of a shorter (5 year) assimilated atmospheric dataset are generally consistent with the simulated results, despite the substantially smaller sample size. The assimilation, however, positions the tropical heating dipole farther west, in better agreement with previous observational studies of intraseasonal tropical extratropical teleconnections. As a consequence. the pre-onset extratropical “response” to the tropical anomalies in the simulation has significant phase errors. The remarkably similar evolution in the extratropics after onset suggests that the tropical forcing acts primarily as a catalyst for the development of the PNP anomalies and that the most useful predictors of PNP events may lie not in the extratropics but in the tropical western and central Pacific.

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R. W. Higgins, K. C. Mo, and S. D. Schubert

Abstract

The moisture budget of the central United States during May is examined using multiyear (1985–89) assimilated datasets recently produced by NASA/DAO and NCEP/NCAR. Intercomparisons and comparisons with station observations are used to evaluate the limitations of the assimilated products for studies of the atmospheric component of the U.S. hydrologic cycle. Attempts are made to reconcile differences in terms of disparities in the analysis systems.

Both reanalyses overestimate daily mean precipitation rates by a factor of almost 2 over the southeastern United States. This is associated with much larger than observed afternoon convective rain and a substantial overestimate of the number of days with precipitation. Both products capture the transition to the much drier conditions over the western United States, though the NCEP/NCAR product extends moderate rain rates too far to the northwest. Over the Great Plains, the reanalyses capture observed synoptic-scale precipitation events quite well, but the variability of the daily mean precipitation is underestimated; this is particularly true for the NASA/DAO analysis, which has difficulty capturing the extreme rain rates. The NCEP/NCAR product shows generally higher correlation's with the observed precipitation, though the fluctuations in the two assimilation products are more similar to each other than they are to the observations.

The moisture transport in the reanalyses compares favorably to gridded rawinsonde data though there are some significant regional differences particularly along the Gulf Coast. Examination of the overall moisture budget for the central United States shows that the observations act as a significant local source of moisture, reflecting model bias in the first-guess fields. In both products the analysis increments act to remove water over much of the northern and western part of the country, apparently counteracting excessive evaporation in those regions, especially in the NASA/DAO. Perhaps most disturbing are the substantial differences between the two reanalyses in the moisture divergence fields since these are the most strongly constrained by the observations.

Both reanalyses capture the basic temporal and structural characteristics of the Great Plains low-level jet (LLJ) documented in previous observational studies. Composites of the nocturnal fluxes of moisture during LLJ events reveal a horizontally confined region of strong southerly transport to the east of the Rocky Mountains that is sandwiched between well-defined synoptic-scale cyclonic (anticyclonic) circulations to the northwest (southeast). Low-level inflow from the Gulf of Mexico increases by more than 50% over nocturnal mean values in both reanalyses, though the excess inflow is more than 30% stronger in the NCEP/NCAR reanalysis. While both analyses underestimate the nocturnal maximum in precipitation over the Great Plains, the pattern of precipitation anomalies associated with LLJ events compares favorably to observations.

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R. D. Koster, S. D. Schubert, and M. J. Suarez

Abstract

The hydroclimatic conditions under which a seasonal meteorological drought (below-normal seasonal rainfall) can induce an increase in seasonal air temperature are investigated, first with an atmospheric general circulation model (AGCM) and then with observations. Geographical differences in the dryness–warmth connection abound in the AGCM; in the United States, for example, identified evaporative controls tend to tie meteorological droughts to warmer temperatures in the South but not in the Northeast. The strong agreement between AGCM and observations-based geographical patterns of drought-induced warming supports the idea that the same evaporative controls are also present in nature. A powerful side benefit of the analysis of drought-induced warming is a Northern Hemisphere map, derived solely from observations, showing where total boreal summer evaporation is controlled by soil moisture, energy availability, or both.

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Bruce A. Albrecht, Richard S. Penc, and Wayne H. Schubert

Abstract

The turbulence and mean structure of oceanic stratocumulus was studied using aircraft data collected during the summer of 1976 off the coast of California. Three cloud-topped mixed layers were studied in detail. They consisted of 1) a thin cloud capped by an inversion at a height of ∼1000 m, 2) a relatively thick but broken cloud layer capped by a weak inversion at ∼600 m and 3) a solid cloud capped by a strong inversion at ∼600 m. The mean temperature, moisture, liquid water and radiative characteristics obtained for these three cases were compared. Heat and moisture fluxes were also calculated and compared.

Although there was considerable variation in the characteristics of the three cloud-topped mixed layers studied, all indicated the validity of the general approach used in simple mixed-layer models of stratocumulus. But for these models to be useful, they should be generalized to allow clouds other than solid clouds to be modeled. The measurements indicate that cloud microphysics may be important in regulating the structure of stratocumulus clouds and that the cloud structure is important in regulating the distribution of radiative cooling. The thin cloud case satisfied conditions for cloud-top entrainment instability, but no evidence of enhanced entrainment was observed. In the analysis it is shown that this cloud may have had insufficient liquid water available to drive the instability. Restrictions on cloud-top entrainment instability criteria are discussed, and it is shown that the instability may be neither a necessary nor a sufficient condition for the breakup of stratocumulus.

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A. S. Frisch, D. H. Lenschow, C. W. Fairall, W. H. Schubert, and J. S. Gibson

Abstract

A cloud-sensing Doppler radar is used with a vertically pointing antenna to measure the vertical air motion in clouds during the Atlantic Stratocumulus Transition Experiment. The droplet fall velocity contamination was made negligible by using only measurements during the time the reflectivity was below − 17 dBZ. During one day of measurements, the daytime character of the vertical velocity variance is different than that of the nighttime case. In the upper part of the cloud, the variance had a distinct maximum for both day and night; however, the nighttime maximum was about twice as large as the daytime case. Lower down in the cloud, there was a second maximum, with the daytime variance larger than the nighttime case. The skewness of the vertical velocity was negative near cloud top in both the day and night cases, changing to positive skewness in the lower part of the cloud. This behavior near cloud top indicates that the upper part of the cloud is behaving like an upside-down convective boundary layer, with the downdrafts smaller in area and more intense than the updrafts. In the lower part of the cloud, the behavior of the motion is more like a conventional convective boundary layer, with the updrafts smaller and more intense than the downdrafts. The upside-down convective forcing in the upper part of the cloud is due to radiative cooling, with the daytime forcing less because of shortwave warming.

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R. D. Koster, S. D. Schubert, H. Wang, S. P. Mahanama, and Anthony M. DeAngelis

Abstract

Flash droughts—uncharacteristically rapid dryings of the land system—are naturally associated with extreme precipitation deficits. Such precipitation deficits, however, do not tell the whole story, for land surface drying can be exacerbated by anomalously high evapotranspiration (ET) rates driven by anomalously high temperatures (e.g., during heat waves), anomalously high incoming radiation (e.g., from reduced cloudiness), and other meteorological anomalies. In this study, the relative contributions of precipitation and ET anomalies to flash drought generation in the Northern Hemisphere are quantified through the analysis of diagnostic fields contained within the MERRA-2 reanalysis product. Unique to the approach is the explicit treatment of soil moisture impacts on ET through relationships diagnosed from the reanalysis data; under this treatment, an ET anomaly that is negative relative to the local long-term climatological mean is still considered positive in terms of its contribution to a flash drought if it is high for the concurrent value of soil moisture. Maps produced in the analysis show the fraction of flash drought production stemming specifically from ET anomalies and illustrate how ET anomalies for some droughts are related to temperature and radiation anomalies. While ET is found to have an important impact on flash drought production in the central United States and in parts of Russia known from past studies to be prone to heat wave–related drought, and while this impact does appear stronger during the onset (first several days) of flash droughts, overall the contribution of ET to these droughts is small relative to the contribution of precipitation deficit.

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M. Hoerling, J. Eischeid, A. Kumar, R. Leung, A. Mariotti, K. Mo, S. Schubert, and R. Seager

Central Great Plains precipitation deficits during May–August 2012 were the most severe since at least 1895, eclipsing the Dust Bowl summers of 1934 and 1936. Drought developed suddenly in May, following near-normal precipitation during winter and early spring. Its proximate causes were a reduction in atmospheric moisture transport into the Great Plains from the Gulf of Mexico. Processes that generally provide air mass lift and condensation were mostly absent, including a lack of frontal cyclones in late spring followed by suppressed deep convection in the summer owing to large-scale subsidence and atmospheric stabilization.

Seasonal forecasts did not predict the summer 2012 central Great Plains drought development, which therefore arrived without early warning. Climate simulations and empirical analysis suggest that ocean surface temperatures together with changes in greenhouse gases did not induce a substantial reduction in sum mertime precipitation over the central Great Plains during 2012. Yet, diagnosis of the retrospective climate simulations also reveals a regime shift toward warmer and drier summertime Great Plains conditions during the recent decade, most probably due to natural decadal variability. As a consequence, the probability of the severe summer Great Plains drought occurring may have increased in the last decade compared to the 1980s and 1990s, and the so-called tail risk for severe drought may have been heightened in summer 2012. Such an extreme drought event was nonetheless still found to be a rare occurrence within the spread of 2012 climate model simulations. The implications of this study's findings for U.S. seasonal drought forecasting are discussed.

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