Abstract

We have calculated the seasonal variation of frequency of blocking and its geographical location by examining the grid point values of daily 500 mb geopotential height over the Northern Hemisphere for 15 consecutive years (1963–77). Blocking events are objectively identified by requiring that a large positive anomaly of a specified magnitude persist for seven days or more. The magnitude of the threshold anomaly is assumed to be 200 gpm for winter, 100 gpm for summer, and 150 gpm for fall and spring. It is found that the geographical locations of the maximum frequency, characterized by three distinctly different maxima, remain nearly the same in all the four seasons. These maxima which occur in the Pacific to the west of the Rockies, in the Atlantic to the west of the Alps, and over land to the west of the Ural mountains, coincide with the maxima of the low frequency and total variance. If the persistence criterion is changed to 1–3 days, the geographical distribution of frequency for winter is very similar to the 2–6 day band-pass variance, showing maximum values in the areas of storm tracks. Large persistent negative anomalies during the winter season are found to be mostly associated with local high index flow, and in a few instances with a neighboring blocking ridge.

We have also examined the seasonal variability of persistent characteristics of wave numbers 1, 2, 3, 4 for 500 mb geopotential height between 50–70°N. It is found that the large scale planetary waves have preferred phase locations for persistence beyond seven days. It is also found, from the grid point analysis, as well as from the wave number analysis, that seasonal mean anomalies account for let than 25% of blocking events.

Abstract

Local autocorrelation statistics for the Northern Hemisphere geopotential height field are presented for different seasons and tropospheric pressure levels. Values are generally much higher than those calculated from time series of outgoing infrared radiation or eddy heat fluxes. The largest one-day lag autocorrelation values are observed over the polar regions and eastern oceans. Well-defined minima exist off the cast coasts of Asia and North America. The minima are most distinct in winter, and are shifted northward and weaker during summer. Otherwise, the pattern does not change drastically with season. These geographical patterns do not change much with height, but the minima are less sharply defined at upper tropospheric levels. At longer lags, the extratropical maxima remain distinct, but the minima shift westward over the continents and cover a broader area, as the autocorrelation approaches zero with increasing lag. Values remain high over the tropics, where height anomalies tend to persist for many days. A comparison of observed autocorrelations with red noise decay suggests that geographical variations in decay rates are associated with the relative importance of baroclinic and barotropic components to the total height variability and with advective effects on the local statistics.

Abstract

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.

Abstract

The focus of this Paper is on the frequency and spatial distributions of blocking and persistent anomalies of geopotential height over the Southern Hemisphere. The analysis is based upon daily height fields at 1000 and 500 mb for both summer and winter. Histogram frequency distributions of height anomalies and maps of the skewness and kurtosis have been computed. Blocking events are objectively defined by requiring a large positive anomaly to exist for 5 days or more. Composite flow and anomaly fields for several cases are presented and examined in detail. The geographical distribution of the frequency of lame amplitude ⩾150 gpm anomalies at 500 mb that persist for only 1–3 days is very similar to that of the high-frequency band (2–8 day period) variances that identity the storm tracks in the Southern Hemisphere.

The primary location for blocking in the Southern Hemisphere is in the New Zealand sector and blocking occurs through a local enhancement of the climatological split in the mean westerlies on a spatial scale of 60° longitude. Other maxima occur southeast of South America and over the southern Indian Ocean. Sporadically, multiple blocking events occur at more than one location and one triple blocking event is examined in detail. Zonal wave 3 is strongly evident in such cases; it also plays a dominant role in the majority of blocking events. However, on most occasions, blocking occurs in isolation as a local phenomenon, and it appears that the local wave 3 may be but part of a wave train with a great circle rather than zonal orientation. Transient eddies appear to play an important role in sustaining a blocking event by either continually reinforcing the anticyclone on its western flank or by quickly reestablishing a new anticyclone as the old one breaks down or moves away.

Abstract

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 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.

Abstract

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.

Abstract

Drought indices derived from the Climate Forecast System Reanalysis (CFSR) are compared with indices derived from the ensemble North American Land Data Assimilation System (NLDAS) and the North American Regional Reanalysis (NARR) over the United States. Uncertainties in soil moisture, runoff, and evapotranspiration (E) from three systems are assessed by comparing them with limited observations, including E from the AmeriFlux data, soil moisture from the Oklahoma Mesonet and the Illinois State Water Survey, and streamflow data from the U.S. Geological Survey (USGS). The CFSR has positive precipitation (P) biases over the western mountains, the Pacific Northwest, and the Ohio River valley in winter and spring. In summer, it has positive biases over the Southeast and large negative biases over the Great Plains. These errors limit the ability to use the standardized precipitation indices (SPIs) derived from the CFSR to measure the severity of meteorological droughts. To compare with the P analyses, the Heidke score for the 6-month SPI derived from the CFSR is on average about 0.5 for the three-category classification of drought, floods, and neutral months. The CFSR has positive E biases in spring because of positive biases in downward solar radiation and high potential evaporation. The negative E biases over the Great Plains in summer are due to less P and soil moisture in the root zone. The correlations of soil moisture percentile between the CFSR and the ensemble NLDAS are regionally dependent. The correlations are higher over the area east of 100°W and the West Coast. There is less agreement between them over the western interior region.

Abstract

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 valiance of monthly mean stationary waves of the SAT analyses in most of the Tropics and Southern Hemisphere from 20° to 60°S. The daily variability is captured at 70%–90% in the Tropics and Southern Hemisphere, except above 200 hPa and south of 60°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 am 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.

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 with earlier observational studies. During summer in excess of 25% more precipitation falls during the nighttime hours than during the daytime hours over a large portion of the Great Plains, with a commensurate decrease in the percentage amount of nocturnal precipitation along the Gulf Coast. Inspection of the nighttime precipitation by month shows that the maximum in precipitation along the Gulf Coast slowly shifts northward from the lower Mississippi Valley to the upper Midwest during the late spring and summer months and then back again during the fall.

Both reanalyses produce a Great Plains LLJ with a structure, diurnal cycle, and frequency of occurrence that compares favorably to hourly wind profiler data. Composites of observed nighttime rainfall during LLJ events show a fundamentally different pattern in the distribution of precipitation compared to nonjet events. Overall, LLJ events are associated with enhanced precipitation over the north central United States and Great Plains and decreased precipitation along the Gulf Coast and East Coast; nonjet events are associated with much weaker anomalies that are generally in the opposite sense. Inspection of the LLJ composites for each month shows a gradual shift of the region of enhanced precipitation from the northern tier of states toward the south and east in a manner consistent with the anomalous moisture transport. LLJ-related precipitation is found to be associated most closely with the strongest, least frequent LLJ events.

The moisture transport in the reanalyses compares favorably to radiosonde data, although significant regional differences exist, particularly along the Gulf Coast during summer. The diurnal cycle of the low-level moisture transport is well resolved in the reanalyses with the largest and most extensive anomalies being those associated with the nocturnal inland flow of the Great Plains LLJ. Examination of the impact of the LLJ on the nighttime moisture transport shows a coherent evolution from May to August with a gradual increase in the anomalous westerly transport over the southeastern United States, consistent with the evolution of the precipitation patterns. The impact of the LLJ on the overall moisture budget during summer is considerable with low-level inflow from the Gulf of Mexico increasing by more than 45%, on average, over nocturnal mean values.