In an effort to explain observed blocking phenomena, the work of Charney and DeVore (1979) and Hart (1979) has been extended to incorporate observed zonal topography in a barotropic nonlinear channel model. Multiple stationary equilibria are obtained, one of which, for an appropriate forcing, corresponds exactly to the “normal” winter flow predicted by Charney and Eliassen (1949) from the linearized version of the model. When this forcing is applied in the nonlinear model, other equilibria, related to resonances with the wevenumber 2 and 3 Fourier components of the zonal topography, occur. Wavenumber 1 and 4 resonances could also have occurred with slight modifications of the model. For comparison with observation, semi-objective criteria are adopted for identifying blocking events from daily 500 mb observations of 15 consecutive winter seasons. Following Dole (1979), we demand that there exist sufficiently large geopotential height anomalies for a sufficient length of time. Numerical values of the anomaly and duration criteria are determined from physical characteristics of observed blocks. Altogether, 34 blocking events were found by this process, and the hemispheric patterns associated with 19 of these appear to be explainable qualitatively as one or another of the calculated equilibria. Five of the remaining blocking events might also have been explained if the forcing and geometry were some-what altered. What is not explained is the localized character of the blocking ridge (or trough) and the mechanism of transition to and from a blocking configuration. The failure to explain the localized properties is attributed in part to the exclusion of longitudinal variations of forcing and dissipation and in part to limitations on north-south structure in the model. It is suggested that the generation and decay of blocks may occur by changes of external factors driving the flow closer to or farther from topographic resonance, or by strong, large-scale cyclonic development. Another possibility is that when the flow is driven into a superresonant configuration, form-drag instability may transform it either into a subresonant blocking configuration or a nonblocking configuration.

Gridded datasets produced for May-July 1979 from the FGGE data by the Goddard Laboratory for Atmospheres were used to study the characteristics of a well-documented seasonal transition, as revealed by weekly averages, from spring to summer in the Northern Hemisphere and from fall to winter in the Southern Hemisphere. Analysis of the weekly departures from the seasonal averages (i.e., anomalies) reveals an eastward-propagating divergence disturbance with wave-number one. The passage of this pulse over Central America appears to trigger the displacement of negative outgoing long-wave radiation (OLR) anomalies northward from the South American winter location. The center of maximum divergence is found over Asia during the onset of the summer monsoon and is also found in conjunction with strengthening of the negative OLR anomalies over this region.

Relationships between the interannual variability of the U.S. summer precipitation regime and the intensification, weakening, or changes in position of the climatological-mean circulation features that organize this regime are examined. The focus is on the atmospheric conditions over the conterminous United States relative to wet and dry monsoons over the southwestern United States. The onset of the monsoon in this region, which typically begins in early July, is determined using an index based on daily observed precipitation for a 32-yr (1963-94) period. Composites of observed precipitation and various fields from the National Centers for Environmental Prediction-National Center for Atmospheric Research Reanalysis for wet and dry monsoons are used to show that the interannual variability of the summer precipitation regime closely mimics the seasonal changes associated with the development of the North American monsoon system.The warm season precipitation regime is characterized by a continental-scale precipitation pattern consisting of an out-of-phase relationship between the Southwest and the Great plains/Northern Tier and an in-phase relationship between the Southwest and the East Coast. This pattern is preserved for both wet and dry monsoons, but the Southwest is relatively wetter and the Great Plains are relatively drier during wet monsoons. Wet (dry) monsoons are also associated with a stronger (weaker) upper-tropospheric monsoon anticyclone over the western United States, consistent with changes in the upper-tropospheric divergence, midtropospheric Vertical motion, and precipitation patterns. The intensity of the monsoon anticyclone over the western United States appears to be one of the most fundamental controls on summertime precipitation downstream over the Great Plains.Evidence is presented that the interannual variability of the U.S. warm season precipitation regime is linked to the season-to-reason "memory" of the coupled atmosphere-ocean system over the eastern tropical Pacific. In particular it is shown that SST anomalies in the eastern Pacific cold tongue and precipitation anomalies in the intertropical convergence zone, present during the winter and spring preceding the monsoon, are linked via an anomalous local Hadley circulation to the warm season precipitation regime over the United States and Mexico. Wet (dry) summer monsoons tend to follow winters characterized by dry (wet) conditions in the Southwest and wet (dry) conditions in the Pacific Northwest. This association is attributed, in part, to the memory imparted to the atmosphere by the accompanying Pacific SST anomalies.

Abstract The influence of the Great Plains low-level jet (LLJ) on summertime precipitation and moisture transport over the central United States is examined in observations and in assimilated datasets recently produced by the NCEP/NCAR and the NASA/DAO. Intercomparisons between the assimilated datasets and comparisons with station observations of precipitation, winds, and specific humidity are used to evaluate the limitations of the assimilated products for studying the diurnal cycle of rainfall and the Great Plains LLJ. The winds from the reanalyses are used to diagnose the impact of the LLJ on observed nocturnal precipitation and moisture transport over a multisummer (JJA 1985–89) period. The impact of the LLJ on the overall moisture budget of the central United States is also examined. An inspection of the diurnal cycle of precipitation in gridded hourly station observations for 1963–93 reveals a well-defined nocturnal maximum over the Great Plains region during the spring and summer months consistent ...

A composite analysis of multiyear (1985-93) global reanalyses produced by the NCEP/NCAR and the NASA/DAO is used to show that the development of persistent North Pacific (PNP) circulation anomalies during NH winter is linked to tropical intraseasonal oscillations. The development is initiated over the tropical west Pacific by anomalous convection (characterized by an east-west dipole structure) one to two weeks prior to the extratropical onset time in both reanalyses. As tropical heating moves eastward toward the central Pacific, anomalous divergent outflow associated with the local Hadley circulation generates an anomalous Rossby wave sink (source) in the subtropics, consistent with the retraction (extension) of the Pacific jet. Prior to onset the signature of the forced anomalies is a pair of cyclonic (anticyclonic) circulation anomalies centered near the node of the tropical heating dipole. Wave trains extending from the region of anomalous convection into the extratropics set the stage for the subsequent rapid development of the PNP anomalies. After onset, the mature PNP anomalies extend equatorward to feed back (through modifications to the moisture transport) on the tropical precipitation anomalies. Throughout the evolution, the tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations in both reanalyses.

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.

In preparation for the execution of the National Meteorological Center and National Center for Atmospheric Research (NMC/NCAR) Reanalysis Project, which will cover the period 1958-93, the impact of satellite data on both analyses and forecasts has been assessed. This was done by diagnosing two sets of analyses and forecasts made with and without the use of satellite data (SAT and NOSAT) within the data assimilation. The analyses and forecasts were performed using a state-of-the-art global data assimilation system and were evaluated for August 1985.The impact of satellite data is smaller than that obtained in previous impact studies during the First GARP (Global Atmospheric Research Program) Global Experiment (FGGE) that took place in 1979, reflecting the effect of improvements that have been implemented in the global analysis scheme and the model. In the Northern Hemisphere (NH), there are no significant differences between SAT and NOSAT analyses for both primary variables and eddy transports. The satellite impact on the forecasts in the NH is positive but very small, reaching about 1% in the 5-day forecasts, as measured by the average rms errors and anomaly correlations. In the Southern Hemisphere (SH), the difference between the SAT and NOSAT analyses is estimated to be equivalent to the difference between 1.5-day SAT forecasts and the verifying analyses. After 5 days, the SAT forecasts are shown to be superior to the NOSAT forecasts by about 1 day, an advantage apparent whether they are verified against SAT or NOSAT analyses. A comparison of SAT and NOSAT analyses suggests that the NOSAT captures well over 90% of the variance of monthly mean stationary waves of the SAT analyses in most of the Tropics and Southern Hemisphere from 20 degrees to 60 degrees S. The daily variability is captured at 70%-90% in the Tropics and Southern Hemisphere, except above 200 hPa and south of 60 degrees S.In several earlier satellite data impact studies performed using FGGE (1979) data, it was observed that satellite data, which cannot resolve smaller-scale features, have a damping effect on the apparent atmospheric circulation. With the improvements in data assimilation methods, it is seen that the smoothing effect is much less apparent. A comparison of the SAT and NOSAT monthly tropical precipitation derived from the 0-6-h forecast cycle shows a general agreement with the rain estimates from satellite data.Overall, these results are very encouraging, indicating that a reanalysis spanning the years before and after satellite data was available should be useful. In the NH, the analyses are basically unaffected by the satellite data. Even in the SH a large component of both the monthly and the daily anomalies can be captured in the absence of the satellite data, except in the stratosphere and Antarctic region.

The impact of the sea surface temperature (SST) anomalies on predictions in the extratropics has been studied by comparing circulation changes in general circulation model experiments generated with observed and climatological sea surface temperatures for warm and cold Southern Oscillation events. The small samples may be insufficient for drawing firm conclusions, but results suggest that the linkage between tropical and extratropical circulations in the model resembles observed relationships.As the atmosphere responds to the warm (cold) tropical SSTs, the convection in the Pacific intensifies (diminishes). The enhanced (suppressed) convection in the tropics enhances (suppresses) the local Hadley circulation and changes the position and strength of the divergent outflow. This in turn changes the position, shape, and strength of the upper-level subtropical jet streams. After the jets move to their new positions, synoptic eddies organize themselves at the exit regions of the jets.The response time for the upper-level streamfunction in the tropics is about 10 days, but the changes in the position of the subtropical jets occur after 15-20 days. The largest impact on predictions is located in the tropics and downstream in the Pacific-North America and the Pacific-South America regions.

This paper describes the horizontal structure of ozone and 50 mb height waves for the 1986-1989 springs of the Southern Hemisphere using global NMC height analyses and Nimbus 7 TOMS Grid-T Version 6 data. Empirical Orthogonal Functions (EOFs) are obtained for the ozone and height data independently to identify the meridional structure of observed long waves as well as their propagation characteristics. It is found that the leading EOFs for both the ozone and 50 mb height waves exhibit wavenumber 1 and 2 patterns. The wave 1 pattern is usually centered at 60-65-degrees-S and it either propagates eastward with a period of 30 days or longer (1986, 1987, 1989) or it is quasi-stationary (1988). This wave is weak during 1987, a year with low ozone values which persisted into summer. The wave 2 pattern travels eastward with a period of 10-12 days. It is best defined while the polar vortex is strong and its amplitude in the ozone field is observed to decay sharply after onset of the summer regime. The correlation coefficients between ozone and 50 mb height principal components are high. These indicate high/low ozone values associated with high/low values of 50 mb heights. This distribution is consistent with the advection of ozone by planetary waves south of the ozone maximum, as shown by a simple linear wave model. This model consists of an equivalent barotropic wave between 5 and 150 mb. The observed total ozone amplitudes for waves 1 and 2 for most years are consistent with the total ozone predictions by the wave model. In contrast, during 1989, a year with a well defined vertically propagating wave 1, the linear model overpredicts the observed ozone wave amplitude by a factor of 2.

This paper describes the horizontal structure of ozone and 50 mb height waves for the 1986-1989 springs of the Southern Hemisphere using global NMC height analyses and Nimbus 7 TOMS Grid-T Version 6 data. Empirical Orthogonal Functions (EOFs) are obtained for the ozone and height data independently to identify the meridional structure of observed long waves as well as their propagation characteristics. It is found that the leading EOFs for both the ozone and 50 mb height waves exhibit wavenumber 1 and 2 patterns. The wave 1 pattern is usually centred at 60-65°S and it either propagates eastward with a period of 30 days or longer (1986, 1987, 1989) or it is quasi-stationary (1988). This wave is weak during 1987, a year with low ozone values which persisted into summer. The wave 2 pattern travels eastward with a period of 10-12 days. It is best defined while the polar vortex is strong and its amplitude in the ozone field is observed to decay sharply after onset of the summer regime. The correlation coefficients between ozone and 50 mb height principal components are high. These indicate high/low ozone values associated with high/low values of 50 mb heights. This distribution is consistent with the advection of ozone by planetary waves south of the ozone maximum, as shown by a simple linear wave model. This model consists of an equivalent barotropic wave between 5 and 150 mb. The observed total ozone amplitudes for waves 1 and 2 for most years are consistent with the total ozone predictions by the wave model. In contrast, during 1989, a year with a well defined vertically propagating wave 1, the linear model overpredicts the observed ozone wave amplitude by a factor of 2.

Eighteen 30-day integrations with the NMC global atmospheric model (T40 resolution) were performed in order to test the impact of sea surface temperature anomalies (SSTAs) on 30-day forecasts for the Northern Hemisphere early summer. The years considered-1987, 1988, and 1989-correspond to a warm El Nino-Southern Oscillation (ENSO) event, a cold ENSO event, and a normal (non-ENSO year), respectively. For each year, 30-day forecasts were started on three successive days around 22 May, using climatological SSTs, and repeated using SSTAs fixed at their initial values.The results indicate that SSTAs have a clear positive impact on the tropical forecasts and surface fluxes. The impacts on the extratropical forecasts, on the other hand, tend to be positive but small. Larger positive impacts in midlatitudes are obtained only in a case in which the atmospheric anomalous circulation is apparently driven by the ocean anomalies. A simple rule of thumb to distinguish whether quasi-stationary atmospheric anomalies are the cause or the result of SSTAs is discussed. It is also found that ensemble averaging results in a modest improvement in forecast skill. Moreover, in areas where the ensemble forecast anomalies are found to be significantly different from zero in a statistical sense, the anomalies tend to verify well, suggesting a method to estimate a priori regional skill. Overall, the Southern Hemisphere forecasts are more skillful than those in the Northern Hemisphere, perhaps because of a seasonal effect.

June 1988 has been classified as one of the hottest and driest months on record in the United States. This study used the NMC Medium-Range Forecast (MRF) T40 model to simulate circulation features of June 1988 and to investigate the relationship between sea surface temperature anomalies (SSTA) and circulation patterns in the Northern Hemisphere. Three control experiments have been performed using three different initial conditions, separated by one day (21, 22, and 23 May 1988) and using SSTA fixed at the starting date. The three forecasts, and their average, are remarkably skillful in the Northern Hemisphere. The observed anomaly of June 1988, a wave train with a persistent ridge in the north-central United States and a northward shifting of the jet stream in the Pacific-North America area, is very well simulated in each of the integrations. All three experiments were repeated using the same initial conditions, but with climatological SST. The wave train generated is similar to that in the control experiments, but it is not as robust. The simulated jet streams are also similar to those in the control experiments. Two experiments with the 1988 SSTA, but with initial conditions of 22 May 1987 and 22 May 1989 were also run. The circulation patterns generated by these runs are very different from those of 1988, indicating that the persistence of the anomalous ridge in the north-central United States after late May 1988 was not due to the SSTA of the May 1988 alone.A barotropic analysis was done to obtain the normal modes associated with the 300-mb streamfunction of the June climatology. The analysis indicates the existence of a slowly growing mode with structure similar to the anomalies of 1988. This result, as well as the numerical experiments, suggests that the persistence of the June 1988 wave train may be associated with initial conditions, which were in a rather stable regime. The SSTA may have helped to strengthen the pattern, but the wave train associated with the 1988 drought could not have been generated by SSTA alone.

The NMC Global Spectral Model was integrated for one year. The model used is the same as the 1989 operational medium range forecast model except that the horizontal resolution was reduced from T80 to T40. Overall, the model was very successful in reproducing most of the characteristics of the atmospheric circulation and its seasonal evolution.A comparison with the summer and winter integrations of Kinter et al., which were performed with the NMC model operational in 1985, shows that the changes made in the last few years in the NMC model have significantly improved its ability to reproduce the atmospheric circulation, particularly in the tropics and in the summer hemisphere. The simulation of precipitation is also much more realistic with the present model.We also performed a 150 day simulation with a lower resolution (R16) version of the model. The stationary and transient eddy simulations were similar to that of T40 model but the zonal circulation was much poorer in the R16 model, particularly in the Southern Hemisphere. This indicates that for a global simulation study a horizontal resolution of at least T40 is necessary.

We present a new empirical orthogonal function (EOF) analysis of winter 500 mb geopotential height anomalies in the Southern Hemisphere. An earlier EOF analysis by two of the present authors prefiltered the anomalies to exclude wavenumbers 5 and higher; we do not. The different preprocessing of data affects the results. All three distinct planetary flow regimes identified in the winter circulation of the Southern Hemisphere by a pattern correlation method are captured by the new set of EOFs; only two of those regimes were captured by the earlier set. The new results, therefore, lend further support to the idea that EOFs point to distinct planetary

Abstract Early results are presented of an experimental program in Dynamical Extended Range Forecasting at the National Meteorological Center. The primary objective of this program is to assess the feasibility of extending operational numerical weather prediction beyond the medium range to the monthly outlook problem. Additionally, the extended integrations provide greater insight into systematic errors and climate drift and thereby feedback to model development. In this paper the principal focus is upon assessment of a contiguous set of 108 thirty-day integrations generated with the then operational Medium Range Forecast model from initial conditions 24 hours apart between 14 December 1986 and 31 March 1987. Results indicate some serious model deficiencies such as the tendency for zonalization, i.e., systematically stronger midlatitude zonal flow than observed, and a stratospheric cold bias, which continues to grow through the 30--day integrations. In the 1–30 day mean Northern Hemisphere 500 mb height f...