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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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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. 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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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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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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S. D. Schubert
,
Y. Chang
,
H. Wang
,
R. D. Koster
, and
A. M. Molod

Abstract

We outline a framework for identifying the geographical sources of biases in climate models. By forcing the model with time-averaged short-term analysis increments [tendency bias corrections (TBCs)] over well-defined regions, we can quantify how the associated reduced tendency errors in these regions manifest themselves both locally and remotely through large-scale teleconnections. Companion experiments in which the model is fully corrected [constrained to remain close to the analysis at each time step, termed replay (RPL)] in the various regions provide an upper bound to the local and remote TBC impacts. An example is given based on MERRA-2 and the NASA/GMAO GEOS AGCM used to generate MERRA-2. The results highlight the ability of the approach to isolate the geographical sources of some of the long-standing boreal summer biases of the GEOS model, including a stunted North Pacific summer jet, a dry bias in the U.S. Great Plains, and a warm bias over most of the Northern Hemisphere land. In particular, we show that the TBC over a region that encompasses Tibet has by far the largest impact (compared with all other regions) on the NH summer jets and related variables, leading to significant improvements in the simulation of North American temperature and, to a lesser degree, precipitation. It is further shown that the results of the regional TBC experiments are for the most part linear in the summer hemisphere, allowing a robust interpretation of the results.

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Y. Chang
,
S. D. Schubert
,
R. D. Koster
,
A. M. Molod
, and
H. Wang

Abstract

We revisit the bias correction problem in current climate models, taking advantage of state-of-the-art atmospheric reanalysis data and new data assimilation tools that simplify the estimation of short-term (6 hourly) atmospheric tendency errors. The focus is on the extent to which correcting biases in atmospheric tendencies improves the model’s climatology, variability, and ultimately forecast skill at subseasonal and seasonal time scales. Results are presented for the NASA GMAO GEOS model in both uncoupled (atmosphere only) and coupled (atmosphere–ocean) modes. For the uncoupled model, the focus is on correcting a stunted North Pacific jet and a dry bias over the central United States during boreal summer—long-standing errors that are indeed common to many current AGCMs. The results show that the tendency bias correction (TBC) eliminates the jet bias and substantially increases the precipitation over the Great Plains. These changes are accompanied by much improved (increased) storm-track activity throughout the northern midlatitudes. For the coupled model, the atmospheric TBCs produce substantial improvements in the simulated mean climate and its variability, including a much reduced SST warm bias, more realistic ENSO-related SST variability and teleconnections, and much improved subtropical jets and related submonthly transient wave activity. Despite these improvements, the improvement in subseasonal and seasonal forecast skill over North America is only modest at best. The reasons for this, which are presumably relevant to any forecast system, involve the competing influences of predictability loss with time and the time it takes for climate drift to first have a significant impact on forecast skill.

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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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Michael J. S. Belton
,
Gerald R. Smith
,
Gerald Schubert
, and
Anthony D. Del Genio

Abstract

We provide morphological and kinematic desc6ptions of the UV markings seen in the Mariner 10 imagery of Venus: the dark horizontal Y, bow-like waves, circumequatorial belts, subsolar disturbance, spiral streaks and bands, polar ring and polar region. The dark horizontal Y is interpreted as a westward-propagating planetary wave with zonal wavenumber 1 and period ∼4.2 days; it may he the superposition of a Rossby-Haurwitz wave dominant at mid-latitudes and a Kelvin wave dominant in equatorial regions. Bow-like waves may be true bow waves formed by the interaction of the rapid zonal flow with internal gravity waves of lower horizontal phase speeds generated by the subsolar disturbance. Circumequatorial belts are interpreted as internal gravity waves with horizontal wavelength ∼500 km and zonal extent ∼5000 km. They are essentially parallel to latitude circles and propagate southward at about 20 m s−1. Cellular features in the subsolar region undoubtedly imply convection there. The identificatiod of both bright- and dark-rimmed cells, with horizontal scales of about 200 and 500 km, respectively, implies a 15 km deep convective layer, based on an analogy with mesoscale convection in the terrestrial maritime atmosphere. The dark areas of the cells may be regions of downwelling. Variability in the location and intensity of the polar ring may be caused by a zonally propagating disturbance, perhaps related to the planetary wave producing the Y in lower latitudes. Circulation patterns and other atmospheric processes in the polar region may be rather different from elsewhere on the planet; only in the polar region are UV markings also visible in the orange.

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