Search Results

You are looking at 1 - 9 of 9 items for :

  • Author or Editor: Sumant Nigam x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: All Content x
Clear All Modify Search
Sumant Nigam

Abstract

The minimum meridional resolution needed for an adequate numerical simulation of the linear and “quasi-linear” baroscopic vorticity dynamics in the vicinity of a critical latitude is determined by using a semi-spectral nondivergent barotropic model on a sphere. The high resolution barotropic calculations of Nigam and Held in which the stationary waves are forced by the earth's orography are repeated with several lower meridional resolutions. Comparison of the lower resolution simulations with the higher resolution ones (the “true solutions”) shows the quality of both the linear and the quasi-linear simulations to deteriorate with decreasing meridional resolution.

An unresolved critical latitude results in spurious sensitivity of the steady linear response to the tropical zonal wind structure, whereas a critical latitude resolved using a strong damping coefficient rather than a fine latitudinal grid may result in the attenuation of any genuinely reflected wave at the critical latitude. For a Rayleigh damping coefficient of (13.5 days)−1, a latitudinal resolution of Δθ <3° is found to be sufficient for an adequate simulation of planetary waves in the quasi-linear model; the linear model, for a commensurate quality of simulation, needs a Δθ< 2°. While this choice of the damping coefficient is arbitrary to some extent, the obtained solutions do have structure similar to that seen in the observed wintertime stationary planetary waves.

Full access
Sumant Nigam

Abstract

The latitude-height structure of variability of the monthly-mean zonally-averaged zonal wind (Ū) is objectively documented for the 9-year period (1980–88) during which both ECMWF and NMC global analyses are available. Modes, resulting from a rotated principal component analysis of the wintertime variability in each dataset, are compared not only with each other but also with those present in a longer dataset (1963–77) of NMC's geostrophically analyzed extratropical winds.

In the northern extratropics, there is considerable agreement between the two modern datasets on the structure of wintertime variability: the first two modes, which together account for over 58% of the integrated variance, have largest amplitudes (∼3 m s−1) at the tropopause level and little, if any, phase variation with height. The first mode, which explains over 40% of the variance (in the ECMWF, and over 32% in the NMC data), has meridionally a dipole structure centered approximately at the latitude of the subtropical jet—suggestive of small latitudinal shifts of the jet core. The dominant mode of fluctuation in the 14-year NMC's geostrophic wind record, however, has a node at ∼40°N, which is suggestive more of “in place” fluctuations in the jet speed rather than in the “jet-location.”

In the tropics and subtropics, the variability in both 9-year datasets is dominated by a mode that represents fluctuations in the intensity of tropical convection. The time series associated with this mode is rather intriguing.

An examination of variability in the winter troposphere/stratosphere in an 8-year (1978/79–1985/86) record of zonal-mean zonal winds, derived from “NMC/CAC-analyzed” geopotential heights, reveals interesting baroclinic-type modes of variability.

Full access
Eric DeWeaver
and
Sumant Nigam

Abstract

Seasonally averaged 200-mb circulations for recent winters (1987/88 and 1988/89) that represent opposite phases of El Niño and a zonal-mean zonal flow index cycle are diagnosed using data assimilated by the Goddard Earth Observing System (GEOS) and operational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). The comparison is undertaken to determine whether there are significant differences in the 200-mb vorticity dynamics implied by the mean meridional circulations in the two datasets and whether these differences can be related to the Incremental Analysis Update (IAU) method used in the GEOS assimilation.

The two datasets show a high degree of similarity in their depictions of the large-scale rotational flow, but there are substantial differences in the associated divergent circulations. For the zonal-mean flow, the zonal winds are substantially the same, but the meridional wind in the Tropics and subtropics is considerably weaker in the GEOS assimilation than its counterparts in both the ECMWF data and the GEOS analyses used to produce the assimilation.

The authors examine the assimilation of the Hadley circulation using a zonally symmetric f-plane model. For this model, the IAU method easily assimilates the rotational flow but fails to assimilate the divergent circulation. This deficiency of the IAU method may explain the weakness of the Hadley cell in the GEOS assimilation, at least in relation to the GEOS analysis.

For this simple model, an alternative assimilation method, based on constraints imposed by the analyzed potential vorticity and mean meridional circulation fields, is proposed that simultaneously assimilates both rotational and divergent flow components.

Barotropic modeling suggests that an accurate representation of mean meridional flow anomalies can be important for the diagnosis of both zonal-mean and eddy rotational flow perturbations, particularly during extreme phases of the zonal-mean zonal flow fluctuation.

Full access
Richard S. Lindzen
and
Sumant Nigam

Abstract

We examine the importance of pressure gradients due to surface temperature gradients to low-level (p ≥ 700 mb) flow and convergence in the tropics over time scales ≳ 1 month. The latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall.

Our approach is to consider a simple one-layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically—consistent with ECMWF analyzed data. The top of the layer is taken at 700 mb. The influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer. Horizontal convergence within the layer is taken up by cumulonimbus mass flux. However, the development of the cumulonimbus mass flux is associated with a short relaxation time [O(½ hr)] (roughly the development time for such convection). During this short time, horizontal convergence acts to redistribute mass so as to reduce horizontal pressure gradients. This effect proves important in the immediate neighborhood of the equator.

Our results show that flows forced directly by surface temperature are often comparable to observed low-level flows in both magnitude and distribution.

Full access
Sumant Nigam
and
Richard S. Lindzen

Abstract

A linear, primitive equation stationary wave model having high vertical and meridional resolution is used to examine the sensitivity of orographically forced (primarily by Himalayas) stationary waves at middle and high latitudes to variations in the basic state zonal wind distribution. We find relatively little sensitivity to the winds in high latitude but remarkable sensitivity to small variations in the subtropical jet. Fluctuations well within the range of observed variability in the jet can lead to large variations in the stationary waves of the high latitude stratosphere, and to large changes even in tropospheric stationery waves. Implications for both sudden warmings and large-scale weather are discussed.

Full access
Sumant Nigam
and
Isaac M. Held

Abstract

A nondivergent barotropic model on a sphere is used to study the effects of a critical latitude on stationary atmospheric waves forced by topography. Linear and “quasi-linear” calculations are performed with an idealized wavenumber 3 mountain and with realistic topography. Quasi-linear dynamics, where mean flow changes are due to momentum flux convergence, “form drag” and relation to a prescribed climatological mean flow, produces an S-shaped kink in the zonal mean absolute vorticity gradient near the critical latitude, resulting in enhanced reflection. The component of the quasi-linear solution resulting from enhanced reflection at the critical latitude is computed by taking the difference between the linear and the quasi-linear solutions. In a calculation with realistic topography and zonal flow, this reflected component is found to be dominated by a wave train emanating from the western tropical Pacific and propagating northward and then eastward across the Pacific 0cean and the North American continent. This wave train results from the reflection of the Himalayan wave train at the zero-wind latitude in the tropical winter troposphere.

The vorticity gradients in the monthly mean statistics of Oort (1983) show structure near the critical latitude similar to that produced in our quasi-linear model, suggesting that some reflection of incident Rossby waves is likely in the atmosphere, at least in the western Pacific, and that the wind structure responsible for this reflection may be created in part by the stationary Rossby waves themselves.

Full access
Sumant Nigam
,
Isaac M. Held
, and
Steven W. Lyons

Abstract

The validity of linear stationary wave theory in accounting for the zonal asymmetries of the winter-averaged tropospheric circulation obtained in a general circulation model (GCM) is ascertained. The steady linear primitive equation model used towards this end has the same vertical and zonal resolution as the spectral GCM, but is finite-differenced in the meridional direction. It is linearized about a zonally symmetric basic state and forced by topography and 3-dimensional diabatic heating and transient flux convergence fields, all of which are taken from the GCM. As in Part I, (in which we studied a GCM with a flat lower boundary) we obtained the best correspondence, between the GCM and the linear solutions when strong Rayleigh friction is included in the linear model not only near the surface, but in the interior of the tropical troposphere as well.

There is sufficient quantitative correspondence between the GCM and the linear solution to justify decomposing the linear simulation into parts forced by different processes, although in some regions, such as over North America, the simulation is unsatisfactory. Different fields give different impressions as to the relative importance of orography, heating, and transients. The eddy zonal velocity field in the upper troposphere shows the orographic and thermal plus transient contributions to he nearly equal in amplitude, whereas the eddy meridional velocity field, dominated by shorter zonal scales, shows the orographic contribution to be decisively dominant. Although there is no systematic phase relationship between these two contributions, they are roughly in phase over the cast Asian coast, where each of them is largest. They also contribute roughly equal amounts to the low level Siberian high.

Other findings are that (i) the 300 mb extratropical response to tropical forcing reaches 50 gpm over Alaska (given our frictional parameterization), which is smaller than the response to local thermal forcing, (ii) the responses to sensible heating and lower tropospheric thermal transients are strongly anticorrelated, and (iii) the circulation in the vicinity of the Andes in the GCM is not attributable to direct mechanical forcing by the mountains.

Full access
Sumant Nigam
,
Isaac M. Held
, and
Steven W. Lyons

Abstract

The quantitative validity of linear stationary wave theory is examined by comparing the results from a linear primitive equation model on the sphere with the stationary eddies produced by a general circulation model (GCM). The GCM simulated has a flat lower boundary, so that the stationary eddies can be thought of as forced by heating (sensible. latent and radiative) and time-averaged transient eddy flux convergences. Orographic forcing is examined in the second part of this study. The distribution of the diabatic heating and transient eddy flux convergences and the zonally symmetric basic state are taken directly from the GCM's climatology for Northern winter (DJF). Strong Rayleigh friction is included in the linear model wherever the zonal mean wind is small, as well as near the surface.

The linear model is found to simulate the stationary eddy pattern of the GCM with considerable skin in both midlatitudes and the tropics. Some deficiencies include the inaccurate simulation of the upper tropospheric geopotential over North America and distortion of the wind field near the low-level zero-wind line in the subtropics. Decomposition of the linear solution shows that 1) the extratropical upper tropospheric eddy pattern generated by tropical forcing is significant but smaller than due to extratropical forcing, 2) the upper-level extratropical pattern deteriorates somewhat when forcing by transients is removed, while the low-level pattern deteriorates dramatically and 3) there is considerable compensation between the effects of low-level thermal transients and extratropical sensible heating, to the point that we argue that this decomposition is not physically meaningful. The sensitivity of the results to the Rayleigh friction.formulation is discussed, as is the effect of replacing the transients with thermal damping.

Full access
Isaac M. Held
,
Steven W. Lyons
, and
Sumant Nigam

Abstract

A baroclinic stationary wave model linearized about a zonally symmetric flow is used to interpret the extra-tropical atmospheric response to El Niño produced by a general circulation model. When forced by the anomalous diabatic beating and tendency due to transients, the linear model provides a useful simulation of this response. The direct response to anomalous diabatic heating is found to be small in the extratropics; the dominant term is the response to the anomalous transients, particularly the anomalous upper tropospheric transients in the vorticity equation. These results are complementary to those obtained with a nonlinear barotropic model by Held and Kang, and indicate that the anomalous subtropical convergence which plays a key role in that study is itself primarily forced by the anomalous transients. One can distinguish between two distinct parts of the response of the transients to the tropical heating: the movement of the Pacific storm track associated with the anomalous extratropical wave train, and changes in the penetration of Rossby waves into the tropics resulting from the modified tropical winds.

Full access