Average Zonal Wind Research Articles

Intercomparison and analyses of the climatology of the West African Monsoon in the West African Monsoon Modeling and Evaluation project (WAMME) first model intercomparison experiment

This paper briefly presents the West African Monsoon (WAM) Modeling and Evaluation Project (WAMME) and evaluates WAMME general circulation models’ (GCM) performances in simulating variability of WAM precipitation, surface temperature, and major circulation features at seasonal and intraseasonal scales in the first WAMME experiment. The analyses indicate that models with specified sea surface temperature generally have reasonable simulations of the pattern of spatial distribution of WAM seasonal mean precipitation and surface temperature as well as the averaged zonal wind in latitude-height cross-section and low level circulation. But there are large differences among models in simulating spatial correlation, intensity, and variance of precipitation compared with observations. Furthermore, the majority of models fail to produce proper intensities of the African Easterly Jet (AEJ) and the tropical easterly jet. AMMA Land Surface Model Intercomparison Project (ALMIP) data are used to analyze the association between simulated surface processes and the WAM and to investigate the WAM mechanism. It has been identified that the spatial distributions of surface sensible heat flux, surface temperature, and moisture convergence are closely associated with the simulated spatial distribution of precipitation; while surface latent heat flux is closely associated with the AEJ and contributes to divergence in AEJ simulation. Common empirical orthogonal functions (CEOF) analysis is applied to characterize the WAM precipitation evolution and has identified a major WAM precipitation mode and two temperature modes (Sahara mode and Sahel mode). Results indicate that the WAMME models produce reasonable temporal evolutions of major CEOF modes but have deficiencies/uncertainties in producing variances explained by major modes. Furthermore, the CEOF analysis shows that WAM precipitation evolution is closely related to the enhanced Sahara mode and the weakened Sahel mode, supporting the evidence revealed in the analysis using ALMIP data. An analysis of variability of CEOF modes suggests that the Sahara mode leads the WAM evolution, and divergence in simulating this mode contributes to discrepancies in the precipitation simulation.

<b>PENGEMBANGAN MODEL PREDIKSI MADDEN JULIAN OSCILLATION (MJO) BERBASIS PADA HASIL ANALISIS DATA REAL TIME MULTIVARIATE MJO (RMM1 DAN RMM2)</b><br> (PREDICTION MODEL DEVELOPMENT MADDEN JULIAN OSCILLATION (MJO) BASED ON THE RESULTS OF DATA ANALYSIS ...

Background of this research is the importance of study on the Madden Julian Oscillation, the dominant oscillation in the equator area. MJO cycle showed by cloud cluster growing in the Indian Ocean then moved to the east and form a cycle with a range of 40-50 days and the coverage area from 10N-10S. Method that used to predict RMM is Box-Jenkins based on ARIMA (Autoregressive Integrated Moving Average) statistical analysis. The data used RMM daily data period 1 Maret 1979–1 Maret 2009 (30 years). RMM1 and RMM2 is an index for monitoring MJO. This is based on two empirical orthogonal functions (EOFs) from the combined average zonal 850hPa wind, 200hPa zonal wind, and satellite-observed Outgoing Longwave Radiation (OLR) data. The results in form of the Power Spectral Density (PSD) graph Real Time Multivariate MJO (RMM) and long wave radiation (OLR = Outgoing Longwave Radiation) at the position 100° BT, 120° BT, and 140°BT that show the wave pattern (spectrum pattern) and clearly shows the oscillation periods. There is a close relation between RMM1 with OLR at the position 100oBT that characterized the PSD value about 45 day. Through Box-Jenkins method, the prediction model that close to time series data of RMM1 and RMM2 is ARIMA (2,1,2), that mean the forecasts of RMM data for the future depending on one time previously and the error one time before. Prediction model for Zt = Zt = 1,681 Zt-1 – 0,722 Zt-2 - 0,02 at-1 - 0,05 at-2.. Prediction model for RMM2 is Zt = 1,714 Zt-1 – 0,764 Zt-2 - 0,109 at-1 - 0,05 at-2.. The flood case in Jakarta January-February 1996 and 2002 are one of real evidence that made the MJO prediction important. MJO with active phase dominant cover almost the entire Indonesia west area at that moment.

Parametric smoothness and self-scaling of the statistical properties of a minimal climate model: What beyond the mean field theories?

A quasi-geostrophic intermediate complexity model of the mid-latitude atmospheric circulation is considered, featuring simplified baroclinic conversion and barotropic convergence processes. The model undergoes baroclinic forcing towards a given latitudinal temperature profile controlled by the forced equator-to-pole temperature difference T E . As T E increases, a transition takes place from a stationary regime–Hadley equilibrium–to a periodic regime, and eventually to a chaotic regime where evolution takes place on a strange attractor. The attractor dimension, metric entropy, and bounding box volume in phase space have a smooth dependence on T E , which results in power-law scaling properties. Power-law scalings with respect to T E are detected also for the statistical properties of global physical observables — the total energy of the system and the averaged zonal wind. The scaling laws, which constitute the main novel result of the present work, can be thought to result from the presence of a statistical process of baroclinic adjustment, which tends to decrease the equator-to-pole temperature difference and determines the properties of the attractor of the system. The self-similarity could be of great help in setting up a theory for the overall statistical properties of the general circulation of the atmosphere and in guiding–on a heuristic basis–both data analysis and realistic simulations, going beyond the unsatisfactory mean field theories and brute force approaches. A leading example for this would be the possibility of estimating the sensitivity of the output of the system with respect to changes in the parameters.

Seasonal behavior of meteor radar winds over Wuhan

Abstract A newly installed meteor radar has been installed to measure winds in the mesosphere and lower thermosphere (MLT) over Wuhan (114.4°E, 30.6°N). In the present study, a database of the first 25 months (February 2002–February 2004) of observations has been analyzed to investigate the climatology of mean winds and tides. The daily average zonal wind is charactered by a strong shear in solstices and an intense eastward flow in summer. The daily average meridional wind is northward in winter and southward in other seasons. There are some discrepancies between the radar mean winds and the HWM93 model winds. The summer zonal winds and meridional winds from the model are obviously weaker than our observations. The analysis on tides indicates that the diurnal tide is dominant at Wuhan. The seasonal variability is observed in both the diurnal and semidiurnal tidal amplitudes with the maximum values occurring usually near the equinoxes. Compared with the Global Scale Wave Model (GSWM00), the observed results generally show a smaller diurnal tidal amplitude and a larger semidiurnal tidal amplitude.

The South America Monsoon Circulation and Its Relationship to Rainfall over West-Central Brazil

The mean atmospheric circulation for the rainy season over west-central Brazil (20°–10°S, 60°–50°W) is investigated for a 21-yr period (July 1979–June 2000). The NCEP–NCAR reanalysis, pentad outgoing longwave radiation (OLR), and daily gridded precipitation analyses from NCEP are used in this study. The analysis presented here provides an improved description of the mean annual cycle based on daily and pentad (5-day) rainfall and atmospheric circulation data. The pentad precipitation averaged for the study region shows a rapid increase during the spring months (September–November) and a rapid decrease in April. The rainiest period (more than 8 mm day−1) is from December to February. There are distinct wet (7 months) and dry (5 months) seasons, with approximately 90% of the total annual rainfall occurring during October–April. The rainy season begins following the period when temperature is at its seasonal maximum. The pre-rainy-season warmth acts to destabilize the atmosphere and to create a reversal in the low-latitude temperature gradient. This results in a reversal of the vertical shear of the zonal wind from increasing (decreasing) westerly (easterly) winds with height to decreasing (increasing) westerly (easterly) winds with height. The examination of the vertical structure of the average zonal wind over the area reveals that prior to the onset of the rainy season easterlies prevail in the lower troposphere (up to 600 hPa) and westerly winds prevail at upper levels with maximum speed in the layer 200–150 hPa. During the rainy season the vertical structure of zonal wind is opposite to that of winter (June–August). Within each rainy season, west-central Brazil experiences anomalously wet and dry periods. Wet periods are associated with low-level northwesterly and westerly flow east of the Andes that converges on west-central Brazil. Dry periods are associated with enhanced northwesterly flow over Paraguay and northern Argentina, and easterly anomalies over west-central Brazil.

Midwinter Suppression of Northern Hemisphere Storm Track Activity in the Real Atmosphere and in GCM Experiments

Abstract The seasonal cycle of the North Pacific and the North Atlantic storm track activity is investigated on the basis of daily National Meteorological Center (now known as NCEP) upper-air analyses (1946–89) and of data from the ECHAM3 T42 atmospheric general circulation model. Emphasis is put on the midwinter suppression of the Pacific storm track. This feature of seasonal variability is not sensitive to a particular definition of midlatitude synoptic wave activity, as is shown by applying a common definition of area mean storm track intensity. The suppression is reproduced by the atmospheric model with very similar characteristics. It is attributed to a negative correlation between the storm track intensity and the speed of the subtropical jet at 250 hPa for average zonal winds exceeding the threshold of approximately 45 m s−1, contrasting with a positive correlation below this value. The lack of an analogous behavior over the Atlantic may be assigned to the lower wind speeds there. In a 3·CO2 time-s...

Hubble space telescope observations of the 1990 equatorial disturbance on Saturn: Zonal winds and central Meridian albedos

Two sets of Hubble Space Telescope (HST) data, obtained in August 1990 and November 1990, are compared with Voyager 1 and 2 data acquired in 1980 and 1981. A rare disturbance developed in the equatorial region of Saturn in late September 1990. Due to the large equatorial wind shear, this disturbance generated inhomogeneous bright cloud structures in the equatorial region that extended to 25° north latitude in November 1990. Meridional albedo scans and wind measurements from the Voyager northern spring equinox and the HST northern summer observations are compared. Differences in average zonal winds in the images obtained with the green F547M and the methane F889N filters are interpreted as evidence of vertical shear in the zonal winds. Longitudinal variations in the equatorial zonal winds are interpreted as evidence of interaction between the storm nucleus, generated during the disturbance, and the local wind field.

The behavior of the high-latitude F-region neutral thermosphere in relation to IMF parameters

Ground based incoherent scatter radar (ISR) and Fabry Perot interferometer (FPI) studies in the northern high latitudes during the period 1983 to 1989 have shown that the F-region neutral wind field pattern depends upon the sign of the IMF parameters. For example, the cell structure of the northern hemisphere high latitude neutral wind field during periods of low geomagnetic activity depends to a large degree upon the sign of the IMF B y parameter. Long term monitoring of the F-region thermosphere by FPI in Thule, Greenland, and by both FPI and ISR in Søndre Strømfjord, Greenland, have made it possible to produce maps of average meridional and zonal wind fields for various IMF configurations for northern high latitudes. Comparison of observations with theoretical wind field modelling, such as the Vector Spherical Harmonic model, indicates that most observed features are consistent with the models.

Cloud-Tracked Winds fromPioneer VenusOCPP Images

Analysis of ultraviolet image sequences, obtained from the Pioneer Venus Orbiter Cloud Photopolarimeter and covering five 80-day periods from 1979–1985, provides the first climatological description of the cloud top circulation on Venus. The average zonal winds can be characterized as a 5-day retrograde rotation of the whole cloud-level atmosphere with weak “jets” at middle to high latitudes. Both the midlatitude and equatorial zonal winds vary by about 5–8 m s−1 over time spans of 1–6 years. The average meridional circulation is poleward in both hemispheres up to at least 60° latitude, consistent with the presence of a thermally direct Hadley circulation associated with the clouds. The strength of the Hadley circulation also varies with time. Four wave modes are clearly identified: a diurnal solar tide, a semi-diurnal solar tide, a “4-day equatorial” wave, and a “5-day midlatitude” wave. The semidiurnal tide appears to have an amplitude of about 5 m s−1 and to be approximately constant with time; the diurnal tide varies in amplitude from about 10 m s−1 to less than 5 m s−1. Both tides have phases such that maximum zonal windspeeds occur near the evening terminator. The “4-day” wave is wavenumber 1 and has an amplitude of about 5 m s−1 that peaks at the equator and varies with time; in 1982 no wave with this period was apparent in the data. This wave mode is identified as a Kelvin mode by Del Genio and Rossow. The “5-day” wave is wavenumber 1 and has an amplitude of about 5 m s−1 that peaks at midlatitudes and varies in time: in 1982 no wave with this period was apparent. This wave mode is identified as an internal Rossby-Haurwitz mode.

Seasonal variations in the equatorial thermospheric wind measured at Arequipa, Peru

Studies have been carried out at Arequipa, Peru, of the seasonal variations in the thermospheric winds at moderate solar flux levels and low geomagnetic activity. Fabry‐Perot interferometer measurements of the Doppler shifts in the 630.0 nm nightglow emission line from March to August 1983 and from April to October 1988 have yielded monthly‐average meridional winds that are nearly zero (<50 m/s) and possibly fluctuating in direction through much of the night but develop a southward flow at 50–100 m/s in the early and the late night from May onward. The average zonal winds are eastward throughout the night, reaching peak velocities before local midnight and then decreasing. The peak velocities increase to a maximum around the June solstice. The winds are generally stronger in 1988 than in 1983, even though the solar EUV fluxes are comparable for both years. Comparison of the present results with earlier satellite measurements, as embodied in the Horizontal Wind Model of Hedin et al., reveals generally satisfactory agreement at the equinox and June solstice, except for the June 1988 period. Killeen et al.'s vector spherical harmonic form of the NCAR Thermospheric General Circulation Model, calculated for similar solar flux levels, yields meridional and zonal wind variations which exhibit the same temporal behaviors but generally smaller values than the present measurements. The present wind patterns are also compared with those measured during the same years at Arecibo, Puerto Rico; one finds oppositely directed meridional flows and similar, eastward zonal flows at the two locations, as expected from a consideration of solar EUV heating as the principal driving force.

Finite-amplitude baroclinic instability with periodic forcing

A low-order model is used to study the behavior of large-scale planetary waves arising from the baroclinic instability of an east-west zonal wind that includes a weak modulation in time. The finite-amplitude equilibration model is equivalent to the modulated Lorenz equations, containing an externally forced time-periodic Rayleigh parameter. Without modulation the baroclinic instabilities equilibrate to steady waves for all values of the external parameters reasonably near the linear neutral curve. However, a small modulation with a period of the same order as the internal dynamical timescale can completely destabilize the non-linear system, leading to chaotic solutions even for small supercriticality. Low-frequency seasonal modulations give rise to tori in the solution phase space. As the amplitude of the periodic forcing increases to about 10% of the average zonal wind shear, the tori break up and chaotic orbits are again found. This latter behavior is interpreted by studying the effect of seasonal forcing on the adiabatic invariants of the system.

Effects of mean winds and dissipation on the diurnal propagating tide: An analytic approach

The effects of mean winds and dissipation on the vertical and horizontal structures of the diurnal propagating tide are investigated analytically using a set of “modified” (complex) Hough functions, velocity expansion functions, and equivalent depths. These are derived by introducing a doppler-shifted complex frequency σ ∗ into the “large ϵ” approximation of Longuet-Higgins (1968, Phil. Trans. R. Soc. (London) A262, 511) for his “Type 1” waves. An equivalent result can be obtained by utilizing an equatorial betaplane approximation. The imaginary part of σ ∗ represents effective Rayleigh friction and Newtonian cooling coefficients which parameterize diffusion of momentum (mechanical dissipation) and heat (thermal dissipation), respectively, and the real part of σ ∗ is doppler-shifted according to an average (latitude-dependent) zonal wind speed. Our results are utilized to interpret salient features of recent numerical simulations of the propagating diurnal tide, including formation of an amplitude peak near 90 km, coupling into evanescent modes above this altitude, amplitude and phase asymmetries about the equator, and amplitude broadening with latitude. We suggest that these functions can be utilized to interpret tidal observations in the 70–100 km region, and perhaps to serve as basis functions for fitting (decomposing) such data from a latitude chain of stations.

The Effect of Vertical Finite Difference Approximations on Simulations with the NCAR Community Climate Model

Two commonly used vertical finite difference approximations produce markedly different simulations when adapted to the nine-level Community Climate Model assembled at the National Center for Atmospheric Research. The differences are conveniently illustrated by considering the zonal average temperature and zonal wind, but these different zonal averaged are also associated with differences in the stationary and transient waves in the model. The hydrostatic equation and vertical temperature advection are the main contributors to the differences in the simulations. Other terms produce only minor differences. Except above the equatorial tropopause, the two schemes converge to the same solution with significantly higher vertical resolution. In many respects, this convergent simulation is closer to that produced by one of the approximations on the original nine levels than to that produced by the other. However, the resemblance is not adequate to justify use of that scheme on the coarse grid when other aspects of the simulation are also considered. Higher resolution should be used so that the simulation becomes insensitive to the vertical finite difference approximations.

Observations and Simulation of the Variability of the Stratosphere and Troposphere in January

Abstract A sufficiently long record of satellite data is now available for the upper stratosphere to make preliminary estimates of the interannual variability of the circulation possible. Global analyses from the lower troposphere to the stratopause for 7 Januaries (1979–85) are used to calculate the ensemble means of the geopotential height field and the zonally averaged temperature and zonal wind fields, together with their daily and interannual variability. These statistics are compared with corresponding statistics from a general circulation model experiment. The model generally simulates the ensemble mean fields reasonably well although several differences are found when the variability of the model is compared to the observations. In particular, the interannual variability of the model is considerably larger than observed in the high latitude winter stratosphere. This excessive variability may be related to the perpetual January methodology used in the model simulation. Statistical significance test...

High‐latitude mesopause neutral winds and geomagnetic activity: A cross‐correlation analysis

A cross‐correlation analysis of horizontal winds at 87 km obtained by the Poker Flat mesosphere‐stratosphere‐troposphere (MST) radar and College magnetometer horizontal component fluctuations has been performed on 3 years of summertime data. No consistent, significant peak in the average 4‐day estimate cross‐correlation results is apparent from year to year. Individual estimates similarly do not show evidence of a peak in cross correlation at a variable time lag during geomagnetically active intervals. In addition, the 1‐day 24‐ and 12‐hour wave amplitudes and phases and average zonal and meridional winds do not vary significantly and consistently with the daily averaged magnetometer horizontal component fluctuations or with the standard deviation thereof. The interval following the July 13, 1982, polar cap absorption event and geomagnetic storm, however, shows a different cross‐correlation estimate behavior and is characterized by significantly more westward zonal winds and enhanced 24‐hour zonal wind amplitude.

Comparison of horizontal winds from the LIMS satellite instrument with rocket measurements

Statistical results are given for a comparison between horizontal geostrophic winds computed from satellite height data and all available in situ rocket wind soundings during a 7‐month period. The satellite data are the daily mapped fields from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) instrument, which extend from 100 to 0.1 mbar. Results indicate that in both the tropics and the extratropical northern hemisphere, the average zonal and meridional wind speeds agree to within 2–4 m s−1 throughout the stratosphere. The rms differences are much larger, with values of 5–10 m s−1 in the lower stratosphere, increasing to 20–40 m s−1 in the lower mesosphere. Time series show that LIMS and rocketsonde zonal wind speeds show coherent variations with temporal periods of 1–2 weeks and more, and both exhibit irregular variations on time scales of less than 1 week.

Statistical reliability and the seasonal cycle: comments on “Bottom pressure measurements across the antarctic circumpolar current and their relation to the wind”

The influence of the seasonal cycle on the sample correlation between two time series is discussed. It is shown that any narrow band energetic signal common to both time series can dramatically reduce the statistical reliability of time-lagged sample cross correlation values by reducing the effective number of degrees of freedom in the records. The results are used to discuss the statistical analysis of wind-forced transport of the Antarctic Circumpolar Current through Drake Passage given in a recent paper by Wearn and Baker (Deep-Sea Research, 27, 875–888, 1980). It is suggested that their high correlation between Drake Passage transport and circumpolar averaged zonal wind stress may largely be due to the presence of a strong semi-annual signal in both time series. This reduces the statistical significance of their conclusion that this reflects a dynamical coupling between the atmosphere and the ocean. Their conclusion that the southern hemisphere analyzed with fields are of high quality (based on the high correlation with Drake Passage transport) is also questioned.

Nighttime thermospheric winds at high latitudes

Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high‐resolution Fabry‐Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 Å) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components at F‐region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three‐dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds at F‐region heights in the high‐latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical model and ion drag.

A climatological Study of the Atmospheric Circulation in the Southern Hemisphere during the IGY, Part I: 1 July 1957–31 March 1958

Time sections of monthly average zonal geostrophic wind at sea level and 500 mb in the Southern Hemisphere confirm that the strongest zonal westerlies usually are found in summer, but since the westerlies expand over a bigger area in the colder part of the year their average relative angular momentum is greater in winter than in summer. For the earth as a whole the westerly relative angular momentum at sea level and 500 mb is greater in January than in July. The highest frequency of surface fronts in the Southern Hemisphere forms a belt round the hemisphere in middle latitude in summer and coincides with the strongest zonal wind at sea level and 500 mb, and with the steepest temperature gradient in the lower half of the troposphere. In winter there are two separate zones of highest frequency of fronts. The frequencies of sea level highs, lows and cyclogenesis plotted as a function of latitude have their peaks in three different latitudes zones so arranged that the peak frequency of highs is farthest equatorward and that of the lows farthest poleward. The meridional geostrophic flow at 500 mb between the 30th and 60th parallels, weighted according to area, was used as a relative measure of the mass flux across the given latitude belt. It was found that the flux is considerably greater during the southern than during the northern summer, but little greater during the northern than during the southern winter. The shape of the meridional profiles of mass exchange is the same in the. northern and southern summers and in the northern and southern winters. An analysis of the 500-mb data on Hovmöller diagrams shows that horizontal dispersion of energy, seen as the downstream intensification of alternating troughs and ridges, took place on several occasions during the period investigated.

Mid-Season Average Zonal Winds at Sea Level and at 500 mb South of 25 Degrees South, and a Brief Comparison with the Northern Hemisphere

Abstract Meridional profiles of sea-level and 500-mb average zonal winds at mid-season in both hemispheres are presented and described. Over much of the southern hemisphere the mean zonal westerly flow changes little during the year, and in each season it is always stronger for the hemisphere as a whole in the southern than in the northern hemisphere. The southern hemisphere circulation and its seasonal changes are shown to be closely related to the surface temperature distribution. It is pointed out that despite the seemingly symmetrical distribution of heat and cold sources over the southern hemisphere, regional differences in the strength and distribution of the mean zonal wind do exist, particularly during the colder part of the year. Thus, although on the whole there is a strong control arising from the ocenic dominance of the hemisphere, there are, especially in the Australian sector, large disturbances connected with continental influences on heating and cooling. The strongest zonal westerly mean c...