Meridional Wind Components Research Articles

Comparison of high-latitude thermospheric meridionalwinds I: optical and radar experimental comparisons

Abstract. Thermospheric neutral winds at Kiruna, Sweden (67.4°N, 20.4°E) are compared using both direct optical Fabry-Perot Interferometer (FPI) measurements and those derived from European incoherent scatter radar (EISCAT) measurements. This combination of experimental data sets, both covering well over a solar cycle of data, allows for a unique comparison of the thermospheric meridional component of the neutral wind as observed by different experimental techniques. Uniquely in this study the EISCAT measurements are used to provide winds for comparison using two separate techniques: the most popular method based on the work of Salah and Holt (1974) and the Meridional Wind Model (MWM) (Miller et al., 1997) application of servo theory. The balance of forces at this location that produces the observed diurnal pattern are investigated using output from the Coupled Thermosphere and Ionosphere (CTIM) numerical model. Along with detailed comparisons from short periods the climatological behaviour of the winds have been investigated for seasonal and solar cycle dependence using the experimental techniques. While there are features which are consistent between the 3 techniques, such as the evidence of the equinoctial asymmetry, there are also significant differences between the techniques both in terms of trends and absolute values. It is clear from this and previous studies that the high-latitude representation of the thermospheric neutral winds from the empirical Horizontal Wind Model (HWM), though improved from earlier versions, lacks accuracy in many conditions. The relative merits of each technique are discussed and while none of the techniques provides the perfect data set to address model performance at high-latitude, one or more needs to be included in future HWM reformulations. Key words. Meteorology and atmospheric dynamics (thermospheric dynamics), Ionosphere (ionosphere-atmosphere interactions, auroral ionosphere)

The upwelling regime off the West Portuguese Coast, 1941–2000

AbstractThe evolution of the upwelling regime off west Portugal between 1941 and 2000 was investigated. Monthly averages of the longshore (meridional) wind component at four coastal stations of the Institute for Meteorology were computed and subject to linear regression analysis. Several comparisons were made among stations until a final regression model was reached. The resulting residuals were checked for the presence of red noise, and pairwise correlation coefficients were estimated for residuals of different stations. To complement this study, monthly sea‐surface temperature averages were computed for six regions off west Portugal and subject to a similar procedure. In both analyses, it was concluded that the Portuguese upwelling regime has weakened since the 1940s. The waning of the northerly, upwelling‐favourable winds was significant throughout the traditional upwelling season (April–September). Sea‐surface temperature showed a steady year‐round increase from 1941 onwards, in both offshore (+0.002°C/year) and coastal (+0.010°C/year) regions. Copyright © 2004 Royal Meteorological Society

African easterly waves and their association with precipitation

Summer tropical synoptic‐scale waves over West Africa are quantified by the 850 mb meridional wind component from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project. Their relationships with surface precipitation patterns are explored by applying the data from the Tropical Rainfall Measuring Mission satellite in combination with other satellite observations during 1998–2002. Evident wavelet spectral power peaks are seen within a period of 2.5–6 days in both meridional wind and precipitation. The most intense wave signals in meridional wind are concentrated along 15°N–25°N. Wave signals in precipitation and corresponding wavelet cross‐spectral signals between these two variables, however, are primarily located at 5°N–15°N, the latitudes of major summer rain events. Southerly wind perturbations tend to lag (lead) precipitation signals south (north) of 15°N. In some cases either an in‐phase or out‐of‐phase relationship can even be found, suggesting two distinct relationships between the waves and convection. Moreover, the lagging relationship (and/or the out‐of‐phase tendency) is only observed south of 15°N during July–September, indicating a strong seasonal preference. This phase relationship is generally consistent with the horizontal wave structures from a composite analysis.

Coupled modes of large-scale climatic variables and regional precipitation in the western Mediterranean in autumn

The variability of autumn precipitation in the western Mediterranean and its relationship to the large-scale atmospheric variability during the period 1948 to 1989 is assessed. A singular value decomposition analysis is used to establish modes of coupled variability between regional precipitation and geopotential height (Z300), zonal (U-wind) and meridional (V-wind) wind components at the 300 hPa level. The Z300/precipitation coupling, which accounts for 52% of the total squared covariance, is strong during the autumn. The first Z300 coupled mode, in its positive phase, is characterised by a dipole structure with negative anomalies over Scandinavia and positive anomalies over the Iberian Peninsula in the Z300 and negative precipitation anomalies in the western Mediterranean. In its negative phase, a coupled pattern is found showing a high-over-low block and positive precipitation anomalies over the Mediterranean area. The coupling depicted by the second mode is weaker than that found in the first mode. The second coupled mode (21% of the total squared covariance) is characterised by negative anomalies in the eastern North Atlantic and positive ones over North Africa and the central Mediterranean in the Z300 and negative anomalies in the regional precipitation. Consistent with the results of the two first modes mentioned, the coupled patterns of either U-wind/precipitation or V-wind/precipitation are found to be coherent with those for Z300/precipitation. Composite maps were obtained to give a representation of the average circulation associated with coherent precipitation variability in the western Mediterranean. The regional impacts of both modes are investigated and the large-scale dynamic patterns presented are important modes of variability. Taking into account data for the whole of the twentieth century, results show that the first singular mode is responsible for the decadal trends and long term changes in precipitation. The late 1970s and 1980s is shown as the drier period and the late 1950s and early 1960s as the wettest years of the century.

Variational data assimilation in the tropics: The impact of a background‐error constraint

AbstractGeostrophic adjustment theory predicts that wind information is a primary information source in the tropics. We ask whether this is true when variational data assimilation including a background‐error constraint is applied. The question is investigated by carrying out three‐ and four‐dimensional variational (4D‐Var) data assimilation experiments with nonlinear shallow‐water equations and idealized waves. The contribution to the analysis from mass and wind field observations is contrasted to that of spectral characteristics and multivariate relationships of equatorial waves built into a background‐error covariance matrix. A background‐error term for the analysis is built by using the tropical eigenmodes and the observed variability is used to determine the relative weights for different modes.Single observation sensitivity experiments illustrate an important impact of Kelvin and mixed Rossby–gravity waves in reducing the coupling between the mass and wind fields, found when only equatorial Rossby waves are utilized in the background‐error constraint.The assimilation experiments demonstrate two main features. First, the wind field information is of greater value than the mass field information, in spite of the known error statistics. A second intriguing feature is that the assimilation of wind observations better resolves smaller‐scale features of the height field than the assimilation of height data. When height data are used, a somewhat different analysis response is obtained for the zonal and meridional wind components, due to the strong projection onto the Kelvin wave structure for height observations. Finally, a comparison of different time windows for 4D‐Var illuminates the sensitivity of height field assimilation to the length of the assimilation window. Copyright © 2004 Royal Meteorological Society

The influence of wind and topography on precipitation distribution in Sweden: statistical analysis and modelling

AbstractTo estimate daily catchment precipitation from point observations there is a need to understand the spatial pattern, particularly in mountainous regions. One of the most important processes occurring there is orographic enhancement, which is affected by, among other things, wind speed and wind direction. The objective of this paper was to investigate whether the relationship between precipitation, airflow and topography could be described by statistical relationships using data easily available in an operational environment. The purpose was to establish a statistical model to describe basic patterns of precipitation distribution. This model, if successful, can be used to account for the topographical influence in precipitation interpolation schemes. A statistical analysis was carried out to define the most relevant variables, and, based on that analysis, a regression model was established through stepwise regression. Some 15 years of precipitation data from 370 stations in Sweden were used for the analysis. The geostrophic wind, computed from pressure observations, was assumed to represent the airflow at the relevant altitude. Precipitation data for each station were divided into 48 classes representing different wind directions and wind speeds. Among the variables selected, the single most important one was found to be the location of a station with respect to a mountain range. On the upwind side, precipitation increased with increasing wind speed. On the leeward side there was less variation in precipitation, and wind speed did not affect the precipitation amounts to the same degree. For ascending air, slope multiplied by wind speed was another important factor. The effect of slope was enhanced close to the coast, and reduced for mountain valleys with upwind barriers. The stepwise procedure led to a regression model that also included the meridional and zonal wind components. Their inclusion might indicate the importance of air mass characteristics not explicitly accounted for. Copyright © 2003 Royal Meteorological Society

The 6–9 day wave and rainfall modulation in northern Africa during summer 1981

Zonal and meridional wind components and geopotential height from European Centre for Medium‐Range Weather Forecasts model analyses and daily rainfall data from the Institut Français de Recherche Scientifique pour le Développement en Coopération (ORSTOM) are used to study westward propagating 6–9 day waves and rainfall modulation in northern Africa during summer 1981. The 6–9 day wave structure is determined using a composite method. In this structure, there are two vortices of opposite circulation on either side of the latitude 12.5°N. The rainfall maxima are associated with cyclonic vortices and the rainfall minima with anticyclonic vortices, coinciding with the minima and the maxima of geopotential height anomalies, respectively. The composite variability shows that the 6–9 day wave is associated with positive rainfall anomalies in West Africa in the band of latitude 7.5°–17.5°N, in the western part of the area around Senegal and Guinea and in the center toward Lake Chad. The rainfall anomalies are linked to the zonal wind anomalies, and the increase in rainfall is associated with large modulation of the African Easterly Jet zonal wind component, mainly in the cyclonic circulation. The main zones of decreasing rainfall appear north of 17.5°–20°N, toward Sudan, and south of 8°N, near Ivory Coast.

Simulation study of mid-latitude ionosphere fluctuations observed at Millstone Hill

Quasi‐periodic variations in ionospheric F‐region parameters associated with interplanetary magnetic field fluctuations have been reported by Huang et al. [2002]. Two mechanisms that could generate these ionospheric fluctuations are a time‐varying electric field or neutral wind. We present simulation results of the response of the mid‐latitude F‐region ionosphere to these drivers using the NRL ionosphere model SAMI2. We obtain good agreement with Millstone Hill density and velocity observations for a zonal F‐region electric field E ≃ ±2.4 mV/m. We also obtain good agreement with density observations if the neutral wind fluctuates ∼±40 m/s along the geomagnetic field. However, this neutral wind variation corresponds to ∼±140 m/s in the meridional component of the horizontal neutral wind, and the fluctuating ion velocity is ∼30% larger than those observed. This suggests that a fluctuating penetration electric field is probably responsible for the Millstone Hill observations.

Simulation of marine boundary layer characteristics using a 1-D PBL model over the Bay of Bengal during BOBMEX-99

The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July–August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX.

Midwinter Suppression of the Pacific Storm Track Activity as Seen in Aircraft Observations

Abstract Unassimilated observational data, from the years 1979 to 1993, are analyzed and compared to the NCEP–NCAR reanalysis data to confirm the magnitude of the seasonal cycle in Pacific storm track activity. Such a comparison is necessary since recent publications have indicated that the reanalysis data may contain biases and spurious jumps that may affect climate signals, and eddy variance/covariance statistics are particularly sensitive to model biases, as well as changes in data coverage and quality. High-pass-filtered variance in the 250-hPa meridional wind component is taken to be the indicator of upper-tropospheric storm track intensity. Rawinsonde observations located mainly over the land masses, and aircraft observations over the oceans, both display seasonal cycles very similar to that seen in the reanalysis data. Both the midwinter suppression in Pacific storm track activity, as well as the more recent finding that the Pacific storm track is significantly stronger during the late 1980s and ea...

On large variations in atmospheric CO2 concentration observed over the central and western Pacific Ocean

In order to examine distribution of atmospheric CO2 concentration over the central and western Pacific Ocean and elucidate factors governing the distribution, shipboard measurements were made between 1992 and 1996. Large variations in the concentration were observed in the northernmost and the tropical regions during the cruises. Also, year‐to‐year differences in the latitudinal CO2 distribution were detected. To interpret these variations, analyses were performed in terms of atmospheric transport from various source regions using meteorological and climatological data. The following results were obtained: (1) Episodic large CO2 variations appeared north of 30°N over the central and western Pacific during the periods of ship cruises. These variations were related with exchanges of continental and marine air masses associated with movements of weather systems. (2) Discontinuous changes of CO2 concentration were observed in the equatorial region during spring cruises, reflecting suppression of interhemispheric air mixing. However, the magnitude, the sharpness, and the location of these discontinuous changes were associated with variations in the strength of air convergence and the movement of enhanced convective areas, and were quite variable. (3) Differences in the latitudinal CO2 gradient from northern low latitudes to southern low latitudes were found between each spring cruise. Comparison with climatological data suggests that the difference in the CO2 gradient is related to variation in the anomaly of meridional wind component distribution over the central and western Pacific at low latitudes.

Mesoscale Correlation Length Scales from NSCAT and Minimet Surface Wind Retrievals in the Labrador Sea

Observations of the surface wind speed and direction in the Labrador Sea for the period October 1996‐May 1997 were obtained by the NASA scatterometer (NSCAT), and by 21 newly developed Minimet drifting buoys. Minimet wind speeds are inferred, hourly, from observations of acoustic pressure in the Wind-Speed Observation Through Ambient Noise (WOTAN) technology. Wind directions are inferred from a direction histogram, also accumulated hourly, as determined by the orientation of a wind vane attached to the surface floatation. Effective temporal averaging of acoustic pressure (20 min), and the interval over which the direction histogram is accumulated (160 s), are shown to be consistent with low-pass filtering to preserve mesoscale time- and spacescale signals in the surface wind. Minimet wind speed and direction retrievals in the Labrador Sea were calibrated with collocated NSCAT data. The NSCAT calibrations extend over the full field lifetimes of each Minimet (90 days on average). Wind speed variabilities of O(5 m s21) and wind direction variabilities of O(408) are evident on timescales of one to several hours in Minimet time series. Wind speed and direction rms differences versus spatial separation comparisons (from 0 to 400 km) for the NSCAT and Minimet records demonstrate similar rms differences in wind speed as a function of spatial separation, but O(208) larger rms differences in Minimet direction. These differences are consistent with spatial smoothing effects in the median filter step for wind direction retrievals within the NSCAT swath. Zonal and meridional surface wind components are constructed from the calibrated Minimet wind speed and direction dataset. Rms differences versus spatial separation for these components are used to estimate mesoscale spatial correlation length scales of 250 and 290 km in the zonal and meridional directions, respectively.

Rapid, large‐scale temperature changes in the polar mesosphere and their relationship to meridional flows

Mesospheric temperatures derived from spectroscopic measurements of the hydroxyl (OH) nightglow have been observed from Rothera (67.6°S, 68.1°W) and Halley (75.6°S, 26.6°W) stations in Antarctica during the 2002 austral winter season. In addition to the normal seasonal changes, the temperatures at both sites, separated by 1658 km, showed several simultaneous shifts in temperature of between 10 and 20 K. These changes abruptly occurred in the space of 2–3 days and lasted for several days. These rapid variations in temperature were associated with large swings observed in the meridional component of the mesospheric wind measured by the Rothera MF radar. As there appeared to be no phase shift between the temperature variations at the two longitudinally separated stations indicating that planetary‐waves caused the changes, these large‐scale changes have been interpreted as variations in the inter‐hemispheric meridional jet, and a corresponding modulation of the mesospheric descent and adiabatic heating rates over the polar region.

Intradiurnal wind variations in the midlatitude and high‐latitude mesosphere and lower thermosphere

MF radar observations of neutral zonal and meridional wind components in the mesosphere and lower thermosphere region (78–98 km) made at Adelaide (35°S, 138°E) and Davis (68°S, 78°E) from January 1994 to December 1999 are analyzed with an emphasis on intradiurnal wind variations common to both stations. Periodograms were used to investigate the spectral content of the wave motions from 6 hours to 20 days and reveal the presence of a rich spectrum of waves. Motions due to tides and intradiurnal oscillations (6–15 hour periods) are particularly evident. Cross‐spectral analyses were used to assess the global coherence of the intradiurnal oscillations. They are found to be short‐lived (∼2–4 days) and have wave amplitudes in the range of 10–20 ms−1. The cross‐spectral analyses revealed discrete intervals during which intradiurnal oscillations showed a high degree of coherence between the two stations. The cross‐spectral phase differences suggest that oscillations with periods between 8 and 14 hours are westward propagating with zonal wave number 1. These oscillations are identified as the atmospheric manifestation of gravitational normal modes or Lamb waves.

Meridional thermospheric winds over the Antarctic Peninsula longitude sector

Diurnal variations of the magnetic meridional component of the thermospheric neutral wind have been derived for Port Stanley (51.7°S;57.8°W), King George Island (62.2°S,58.8°W) and Argentine Islands (65.3°S;64.3°W). A servo-model based algorithm and two semiempirical procedures are used with ionosonde data input. Derived winds correspond to conditions of low geomagnetic activity and both low and high solar activity for months representative of autumn, winter, spring and summer. The shapes of the diurnal variations determined using the three different methods for a given location and month are fairly similar for all three locations and most months. Wind velocities at a given hour, however, may generally differ by 100 m/ s and by up to a few hundred m/s in a few cases. The amplitude of the diurnal variations is larger in winter than summer for all latitudes and at both low and high solar activity, and generally larger at low solar activity. Daily mean winds are poleward in winter and equatorward in summer at low solar activity for all latitudes but are poleward all the year round during high solar activity, and stronger during winter. There is almost no latitudinal dependence of the daily mean winds in summer and autumn, with velocities being less than about 25 m/ s . By contrast, winter daily mean winds change with latitude, particularly during low solar activity. Moreover, there seems to be a latitude at which the mean wind velocity is minimal both at low and high solar activity. A Fourier decomposition of the diurnal variations for all cases shows that all variations can be reproduced using at the most the first three Fourier components. Results are discussed with reference to other published wind determinations for the Antarctic Peninsula area and for the New Zealand longitude sector, and with reference to winds derived using the well-known HWM93 empirical model.

Teleconnections between Indian monsoon and Sahel rainfall and the Mediterranean

AbstractThe teleconnections with Indian monsoon and Sahel rainfall indices are investigated here on an interannual time scale in terms of meteorological and marine dynamics over the Mediterranean area. Sea‐level pressure from gridded data sets and from individual stations, together with sea‐level data from stations all around the Mediterranean coastlines, are used.In summer (July–August–September, JAS) the sea‐level pressure field over the eastern Mediterranean anticorrelates with the Indian monsoon index (correlation coefficient C = −0.5 on average). A Mediterranean pressure index (MPI), defined as the standardized difference between sea‐level atmospheric pressure at Mersa Matruh (southeastern Mediterranean) and Marseille (northwestern Mediterranean) stations, anticorrelates with Indian monsoon index even more (C = −0.68). The MPI is proportional to the mean geostrophic surface flow field across an imaginary line joining the two stations and turns out to be significantly correlated with the meridional wind component over the eastern Mediterranean, known as the low‐level Etesian wind regime. This wind regime represents the inflow surface field into the African inter‐tropical convergence zone and, therefore, has an association with the Indian monsoon regime. The ocean response, evident by sea‐level anomalies at coastal stations, shows a maximum anticorrelation with Indian monsoon index in late summer and autumn (September–October–November, SON).The Sahel index anticorrelates with sea‐level pressure, with the maximum absolute value in June–July–August. This may be interpreted as a tendency of the Mediterranean sea‐level pressure anomalies to precede those of Sahel precipitation, which is characterized by maximum rainfall in July–September. The MPI anticorrelates with Sahel index during and before JAS, indicating that the Etesian wind regime intensity is connected to Sahel rainfall. The sea level again anticorrelates with the Sahel index, with the maximum absolute value in SON, as for the sea‐level–Indian monsoon correlation. Copyright © 2003 Royal Meteorological Society.

Momentum transport of wave zero during March: A possible predictor for the Indian summer monsoon

Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian Ocean during March.

Observations of Variability on Synoptic Timescales in the East Pacific ITCZ*

Abstract Data obtained in the eastern Pacific intertropical convergence zone (ITCZ) during the Tropical Eastern Pacific Process Study (TEPPS) show a 3–6-day variability. The NOAA ship Ronald H. Brown collected surface meteorological observations, C-band Doppler radar volumes, atmospheric soundings, and rainfall data while on station at 7.8°N, 125°W from 8–23 August 1997. The 3–6-day variability was a prominent timescale in the meridional wind and humidity data. The maxima of the surface-to-700-mb meridional wind anomalies were 5–12 m s–1. Maxima of the moisture anomalies at the same levels were ∼1–3 g kg–1, in phase with the low-level southerlies, while upper-level moisture lagged the southerlies by less than a day. The radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind, with the precipitation occurring when the surface southerlies were strongest. The meridional and zonal wind components had a maximum of 850–450-mb wind shear during the southerly e...

Relationship between easterly waves and mesoscale convective systems over the Sahel

Relationship between Sahelian mesoscale convective systems and easterly waves is studied at various time and space scales during 5 rainy seasons (1st July–15th September 1990–1994). This study focuses on a sub‐population of MCSs called organised convective systems (OCS) which account for most of the rainfall. Wave activity periods are detected by spectral analyses on the 700 hPa meridional wind component, information on the convective systems is derived from an automatic tracking algorithm and rainfall data of the EPSAT‐Niger experiment. We observe a modulation of OCS cloud cover at synoptic‐scale during easterly wave activity, with an increase of the cloud cover in and ahead of the trough but OCS rainfall efficiency is maximum behind of the wave trough. At seasonal scale, OCS number and cloud cover distributions are not significantly affected by easterly wave occurrences, which means that the latter are not directly associated to the interannual variability of rainfall.

Comparison of Na lidar and meteor radar wind measurements at Starfire Optical Range, NM, USA

Simultaneous wind measurements in the mesopause region at Starfire Optical Range near Albuquerque, NM with Na wind/temperature lidar and meteor radar have been performed and compared. 20 nights of hourly data recorded with these two instruments at two layers around 86 and 93 km altitude are compared for both zonal and meridional wind components. The mean values are found to have no statistically significant differences. The correlation coefficients vary between 0.63 and 0.70, indicating that the two sets of measurements are broadly consistent. When comparing the averaged variations over the night, the two measurements are highly correlated, with correlation coefficients varying from 0.84 to 0.95. It indicates that the strong tidal variation is well captured by both instruments. Differences are however significant at individual hours, which are believed to be mainly due to the fact that the meteor radar wind is an average over the entire sky while the lidar measures wind within a volume about 100 m in diameter.