Midlatitude Atmosphere Research Articles

Mechanisms Determining the Atmospheric Response to Midlatitude SST Anomalies

Previous GCM experiments demonstrated that a model atmosphere produces two different responses to a midlatitude warm SST anomaly over the Pacific under perpetual January and February conditions. To elucidate the mechanisms responsible for the different GCM responses and their dependence on the background flow, experiments with two idealized models are conducted. Experiments with a linear baroclinic model reveal that the GCM responses at equilibrium are primarily maintained by the anomalous eddy forcing. The anomalous flow induced directly by an idealized initial heat source exhibits little sensitivity to the background flow. Eddy feedbacks on the heating-induced anomalous flow are examined using a linear storm track model. The anomalous eddy forcing produced by the storm track model is sensitive to the basic state. The eddy forcing in January acts to shift the heating-induced upper-level ridge toward the northeast of the Gulf of Alaska, while in February it acts to reinforce the ridge. This suggests that the differences in the GCM responses are primarily associated with differences in the response of synoptic eddies to the presence of an anomalous ridge at the end of the Pacific storm track. The idealized model experiments are also performed with the observed winter mean flow. The eddy feedbacks depend on the position of the heating relative to the storm track. With the heating centered over the western Pacific the eddy-driven anomalous flow reinforces the ridge over the Pacific, similar to that in GCM February, but much stronger. No such reinforcement by the transients is found with the heating shifted over the eastern Pacific. These results suggest that SST anomalies over the western Pacific perhaps play a more active role in midlatitude atmosphere–ocean interactions.

Spectral characteristics of solar near‐infrared absorption in cloudy atmospheres

Theoretical and experimentally derived estimates of the atmospheric absorption of solar energy in the presence of clouds have been reported to be at variance for quite a long time. A detailed set of near‐monochromatic computations of the reflectance, transmittance, and absorptance of a standard midlatitude atmosphere with embedded water clouds is used to identify spectral features in the solar near‐infrared that can be utilized to study this discrepancy. The results are framed in terms of the cloud radiative forcing both at the surface and at the top of the atmosphere, and it is shown that water vapor windows are the most sensitive to variations in cloud optical properties and cloud placement in the vertical. The ratio of the cloud radiative forcing at the surface to that at the top of the atmosphere, R, varies from near zero in the band centers at small wavenumbers for high clouds to ∼1 in the band centers at larger wavenumbers for low clouds and to values in excess of 2 in the water vapor windows at small wavenumbers. The possibility of using measurements from space with the future Moderate Resolution Imaging Spectroradiometer (MODIS) and simultaneous surface measurements is discussed. It is also shown that horizontal inhomogeneities in the cloud layers do not alter appreciably the estimates of the R factor, but areal mean cloud absorption is lower for an inhomogeneous cloud having the same mean liquid water as the corresponding homogeneous cloud.

Atmosphere–Ocean Interaction in the North Atlantic: Near-Surface Climate Variability

The impact of an interactive ocean on the midlatitude atmosphere is examined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 208 and 608N) coupled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle of sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months. Coupling does not change the mean wintertime model climatology (December‐February); however, it does have a significant impact on model variance. Air temperature and mixing ratio variance increase while total surface heat flux variance decreases. In addition, it is found that air‐sea interaction has a greater impact on seasonally averaged variance than monthly variance. There is an enhancement in the persistence of air temperature anomalies on interannual timescales as a result of coupling. In the North Atlantic sector, surface air and ocean temperature anomalies during late winter are uncorrelated with the following summer but are significantly correlated (0.4‐0.6) with anomalies during the following winter. These autocorrelations are consistent with the ‘‘re-emergence’’ mechanism, where late winter ocean temperature anomalies are sequestered beneath the shallow summer mixed layer and are reincorporated into the deepening fall mixed layer. The elimination of temperature anomalies from below the mixed layer in a series of uncoupled sensitivity experiments notably reduces the persistence of year-to-year anomalies. The persistence of air temperature anomalies on monthly timescales also increases with coupling and is likely associated with ‘‘decreased thermal damping.’’ When coupled to the atmosphere, the ocean is able to adjust to the overlying atmosphere so that the negative feedback associated with anomalous heat fluxes decreases, and air temperature anomalies decay more slowly.

The Basic Effects of Atmosphere–Ocean Thermal Coupling on Midlatitude Variability*

Starting from the assumption that the atmosphere is the primary source of variability internal to the midlatitude atmosphere–ocean system on intraseasonal to interannual timescales, the authors construct a simple stochastically forced, one-dimensional, linear, coupled energy balance model. The coupled system is then dissected into partially coupled and uncoupled systems in order to quantify the effects of coupling. The simplicity of the model allows for analytic evaluation of many quantities of interest, including power spectra, total variance, lag covariance between atmosphere and ocean, and surface flux spectra. The model predicts that coupling between the atmosphere and ocean in the midlatitudes will enhance the variance in both media and will decrease the energy flux between the atmosphere and the ocean. The model also demonstrates that specification of historical midlatitude sea surface temperature anomalies as a boundary condition for an atmospheric model will not generally lead to a correct simulation of low-frequency atmospheric thermal variance. This model provides a simple conceptual framework for understanding the basic aspects of midlatitude coupled variability. Given the simplicity of the model, it agrees well with numerical simulations using a two-level atmospheric general circulation model coupled to a slab mixed layer ocean. The simple model results are also qualitatively consistent with the results obtained in several other studies in which investigators coupled realistic atmospheric general circulation models to ocean models of varying complexity. This suggests that the experimental design of an atmospheric model coupled to a mixed layer ocean model would provide a reasonable null hypothesis against which to test for the presence of distinctive decadal variability.

ベンガル湾及び南シナ海上の夏季モンスーンの早期開始におけるチベット高原の温度上昇の役割

In this study we examine the mechanisms of the onset of the Southeast Asian monsoon (SEAM) over the Bay of Bengal and the South China Sea in terms of thermal contrast between the Tibetan Plateau and surrounding ocean based upon 5-day mean ECMWF circulation field data (1980-89) and 5-day mean GMS equivalent black body temperature (T BB ) data. The early onset of the SEAM is recognizable at Pentad 28 (May 16-20) with accelerated low-level monsoon westerlies followed by second enhancement of the monsoon activities in early June. The warming over the Tibetan Plateau from spring to summer is found in the 200 500 hPa thickness data oil about 15-day intervals. Of importance is the observational evidence that the warming phase over tle Tibetan Plateau around mid-May is concurrent with the early onset of the SEAM. Thus, the thermal contrast between the Tibetan Plateau and the adjacent ocean is likely to induce the acceleration and eastward extension of the low-level monsoon flow, causing the abrupt commencement of the SEAM including onset, of the South China Sea monsoon (SCSM). This relationship between low-level wind over the key region (10°-20°N, 80°-120°E) and 200-500 hPa thickness over the Tibetan Plateau is also confirmed based on the correlation analysis in the interannual variabilities. An influence for the mid-latitude atmosphere, stationary Rossby waves are generated over the South China Sea and propagate in a northeastward direction toward Japan because of the cyclonic vorticity and the tropical heat source associated with the onset of the SCSM. As a result of this wave propagation, a high pressure anomaly appears over Japan, which is consistent with a singularity of clear skies around Japan in mid-May (Kawamura and Tian, 1992).

Transitions between blocked and zonal flows in a rotating annulus with topography

The mid-latitude atmosphere is dominated by westerly, nearly zonal flow. Occasionally, this flow is deflected poleward by blocking anticyclones that persist for 10 days or longer. Experiments in a rotating annulus used radial pumping to generate a zonal jet under the action of the Coriolis force. In the presence of two symmetric ridges at the bottom of the annulus, the resulting flows were nearly zonal at high forcing or blocked at low forcing. Intermittent switching between blocked and zonal patterns occurs because of the jet's interaction with the topography. These results shed further light on previous atmospheric observations and numerical simulations.

The Influence of Midlatitude Ocean–Atmosphere Coupling on the Low-Frequency Variability of a GCM. Part I: No Tropical SST Forcing*

Abstract This study examines the extent to which the thermodynamic interactions between the midlatitude atmosphere and the underlying oceanic mixed layer contribute to the low-frequency atmospheric variability. A general circulation model, run under perpetual northern winter conditions, is coupled to a motionless constant-depth mixed layer in midlatitudes, while elsewhere the sea surface temperature (SST) is kept fixed; interannual tropical SST forcing is not included. It is found that coupling does not modify the spatial organization of the variability. The influence of coupling is manifested as a slight reddening of the spectrum of 500-mb geopotential height and a significant enhancement of the lower-tropospheric thermal variance over the oceans at very low frequencies by virtue of the mixed-layer adjustment to surface air temperature variations that occurs on those timescales. This adjustment effectively reduces the thermal damping of the atmosphere associated with surface heat fluxes (or negative ocea...

Land surface emissivity and temperature determination in the whole HAPEX-Sahel area from AVHRR data

Most of the developed methods for land surface temperature retrieval have been derived for mid-latitude atmospheres and their application in tropical areas is very limited. The aim of this paper is to discuss the applicability of a global split-window method, developed by the authors, in a tropical environment as well as to analyse the accuracy in determining LST in tropical areas. The accuracy of this split-window method is about 2degK, which is a reasonable value taking into account the large atmospheric attenuation in tropical environments. We have performed this study in the framework of the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel), showing the results for the whole HAPEX-Sahel area (13 -14 N; 2-3 E). To complete the study, an intercomparison of the existing split-window methods has also been carried out in this area.

Interactions between Global SST Anomalies and the Midlatitude Atmospheric Circulation

Abstract A review is given of the processes contributing to variability of the atmosphere–ocean system on interannual timescales. Particular emphasis is placed on the relationships between midlatitude atmospheric fluctuations and sea surface temperature (SST) anomalies in various geographical sites. Various hypotheses are tested using output from a coordinated set of general circulation model experiments, which are subjected to time-varying SST forcing observed during 1946–88 in different parts of the world's oceans. It is demonstrated that tropical Pacific SST fluctuations associated with El Nino–Southern Oscillation (ENSO) episodes produce a strong extratropical response in the model atmosphere, whereas the atmospheric signal associated with midlatitude SST anomalies is less robust. Analysis of a 100-yr control experiment, which is conducted in the absence of any interannual SST forcing, indicates that a substantial fraction of the simulated atmospheric variability may be attributed to internal dynamica...

Line‐by‐line calculation of atmospheric fluxes and cooling rates: 2. Application to carbon dioxide, ozone, methane, nitrous oxide and the halocarbons

A line‐by‐line model (LBLRTM) has been applied to the calculation of clear‐sky longwave fluxes and cooling rates for atmospheres including CO2, O3, CH4, N2O, CCl4, CFC‐11, CFC‐12, and CFC‐22 in addition to water vapor. The present paper continues the approach developed in an earlier article in which the radiative properties of atmospheres with water vapor alone were reported. Tropospheric water vapor continues to be of principal importance for the longwave region due to the spectral extent of its absorbing properties, while the absorption bands of other trace species have influence over limited spectral domains. The principal effects of adding carbon dioxide are to reduce the role of the water vapor in the lower troposphere and to provide 72% of the 13.0 K d−1 cooling rate at the stratopause. In general, the introduction of uniformly mixed trace species into atmospheres with significant amounts of water vapor has the effect of reducing the cooling associated with water vapor, providing an apparent net atmospheric heating. The radiative consequences of doubling carbon dioxide from the present level are consistent with these results. For the midlatitude summer atmosphere the heating associated with ozone that occurs from 500 to 20 mbar reaches a maximum of 0.25 K d−1 at 50 mbar and partially offsets the cooling of 1.0 K d−1 contributed by H2O and CO2 at this level. In the stratosphere the 704 cm−1 band of ozone, not included in many radiation models, contributes 25% of the ozone cooling rate. Radiative effects associated with anticipated 10‐year constituent profile changes, 1990–2000, are presented from both a spectral and spectrally integrated perspective. The effect of the trace gases has been studied for three atmospheres: tropical, midlatitude summer, and midlatitude winter. Using these results and making a reasonable approximation for the polar regions, we obtain a value for the longwave flux at the top of the atmosphere of 265.5 W m−2, in close agreement with the clear‐sky Earth Radiation Budget Experiment (ERBE) observations. This agreement provides strong support for the present approach as a reference method for the study of radiative effects resulting from changes in the distributions of trace species on global radiative forcing. Many of the results from the spectral calculations reported here are archived at the Carbon Dioxide Information and Analysis Center for use by the community.

Zonal wavenumber determination of the semi‐diurnal oscillation in the midlatitude upper atmosphere from single‐station optical measurements

Single station optical observations of the motions of the midlatitude upper atmosphere at Fritz Peak (39.88°N, 105.5°W), using the [OI] 17924K (557.7 nm) emission line as a tracer, are employed to experimentally determine the zonal wavenumber character and direction of travel of the observed oscillations of 12‐hr periodicity. The results show that this 12‐hr oscillation is westward traveling and of zonal wavenumber character two, and they describe a solar migrating semidiurnal tide.

Convectively induced mesoscale weather systems in the tropical and warm‐season midlatitude atmosphere

As anticipated by Nelson [1991] in the last U.S. National Report, mesoscale meteorology has continued to be an area of vigorous research activity. Progress is evinced by a growing number of process‐oriented studies capitalizing on expanded observational capabilities, as well as more theoretical treatments employing numerical simulations of increasing sophistication. While the majority of papers within the scope of this review fall into the category of basic research, the field's maturation is evident in the emergence of a growing number of applications to operational weather forecasting.Even as our ability to anticipate shifts in synoptic scale upper‐air patterns and associated baroclinic developments has steadily improved, lagging skill with regard to quantitative forecasts of precipitation—particularly in situations where deep moist convection is prevalent—has sustained research in warm‐season mesoscale meteorology. Each spring and summer midlatitude populations are exposed to life‐threatening natural weather phenomena in the form of lightning, tornadoes, straight‐line winds, hail, and flash floods. This point was driven home during the summer of 1993, when an extraordinarily persistent series of mesoscale convective systems (MCSs) led to unusually severe and widespread flooding throughout the Mississippi and Missouri river basins. In addition to this obvious impact on regional climate, the 1990's have brought an increased appreciation for the less direct yet potentially significant role that tropical convection may play in shaping global climate through phenomena such as the El Niño‐Southern Oscillation (ENSO).

Climatic trends of the mid-latitude upper atmosphere and ionosphere

Long-term variations of electron concentration in the mid-latitude ionosphere, independent of heliogeophysical conditions and the meteorological characteristics of the atmosphere, have been determined. It is suggested that a long-term decrease of atomic and molecular oxygen concentrations in the mid-latitude thermosphere is the most possible reason for the trends found in ionospheric parameters.

Microweve Radiative Transfer through Clouds Composed of Realistically Shaped Ice Crystals. Part II. Remote Sensing of Ice Clouds

Abstract This paper presents the results of polarized microwave radiative transfer modeling of cirrus clouds containing five different particle shoes and 18 Gamma size distributions. Upwelling brightness temperatures for tropical and midlatitude winter atmospheres are simulated at 85.5, 157, 220, and 340 GHz using scattering properties computed with the discrete dipole approximation (described in Part I). The key parameter for the results is the sensitivity (ΔTb/IWP), which relates the modeled brightness temperature depression to the ice water path. It is shown that for the higher frequencies or distributions of larger particles (i.e., in the scattering regime) the sensitivity is nearly independent of cloud temperature and details of the underlying atmosphere. As expected from the single-scattering results, the characteristic particle size has a large effect on the sensitivity, while the distribution width has only a minor effect. The range in sensitivity over the five particle shapes is typically a facto...

Longwave radiative cooling rates in axially symmetric clouds

The effects of cloud sides and of cloud top perturbations on radiative cooling rates are examined using an approximate model of longwave radiative transfer that ignores the effect of scattering on net flux divergence. Tuned monochromatic calculations are used for isothermal clouds, and spectral integration from 8 to 13.4 μm for nonisothermal clouds. Water vapor and cloud droplet effects within the cloud are included. Atmospheric conditions outside the cloud are fixed appropriate to a midlatitude summer atmosphere. The distribution of cooling rates in isolated cylindrical clouds shows local maxima at the cloud top and sides as well as modest warming of the base. These rates also depend on cloud and surface temperature. For a 286 K cloud above a 302 K ground, the local cooling rates were typically −34 K h−1 at the cloud top and −14 K h−1 at the cloud sides, with a typical local heating rate of 8 K h−1 at the cloud base. The gradient in cooling rate near the cloud boundaries is very sharp and high rates occur mainly within ≈20 m of the boundary. The possibility of sustained cloud side cooling leading to low‐level convergence and enhanced cloud development is noted. Changes in cloud top cooling rates due to positive (lifting) or negative (sinking), axially symmetric perturbations to the top of horizontally extensive stratiform clouds were also considered. Significant effects were found that do not cancel when averaged over both positive and negative perturbations. A lifting of the cloud top does not increase the cooling rate, whereas the peak cooling rate decreases rapidly in depressions. For small perturbations, ≈10 m, this may tend to inhibit the growth of negative perturbations. For larger perturbations, ≈100 m, changes to the radiative cooling rates within the positive perturbations and next to the negative perturbations may act to sustain the perturbation and promote its growth.

Winds and vertical wavelengths deduced from the ground‐based measurement of the Doppler shifts of the O2(b ¹∑g+ ‐ X ³∑g−), OI(¹D2 ‐ ¹S0), and the X ²Π OH (6–2) Band P1(2)c,d line emissions in the midlatitude upper middle atmosphere

Simultaneous ground‐based measurements of the upper middle atmosphere motions, using the OI(¹D2 ‐¹S0) emission and the O2 (b¹∑g+ ‐ X³∑g−) atmospheric band emission as tracers, indicate that the height separation between the emission layers is small, in comparison with the vertical wavelength oscillations present in that atmospheric region, and in agreement with earlier results obtained with rocket measurements. The finding of a long vertical wavelength is corroborated by simultaneous wind motion measurements obtained with the emissions of O[I] and OH (6–2) as tracers, where a vertical wavelength of about 90 km has been determined. This new optical, ground‐based, capability to determine motions and vertical wavelengths in the upper middle atmosphere from simultaneous measurements of multiple emission layers provides a new optical tool to investigate the oscillations present in this region of the upper atmosphere.

Vertical structure of the midlatitude temperature from stratosphere to mesopause (30–105 km)

Data sets from Rayleigh lidar (1979–93) at the Observatory of Haute‐Provence (44°N, 6°E) and at Biscarrosse (44°N, 1°W) of southern France, and from narrowband Na lidar (1991–4) at Fort Collins, Colorado (41°N, 105°W) are used to deduce the vertical temperature structure of the midlatitude middle atmosphere from 30–105 km. Nightly averaged temperatures measured across the Atlantic at comparable latitudes between 81 and 90 km showed seasonal variations tracking one another. Harmonic analyses of the measured profiles at these sites have been compared to the reference atmosphere, CIRA 1986. Considerable differences are noted in the annual mean temperatures in the mesopause region. Two temperature minima at 86 and 99 km are observed, suggesting heating and cooling mechanisms not previously incorporated into the reference atmosphere. Using the new 3D TIME‐GCM of Roble and Ridley which includes both dynamical and chemical processes, we account for this anomalous behavior by model simulation for the first time. Except the existence of strong semi‐annual oscillations, the observed annual and semi‐annual temperature variations are compatible with CIRA 1986. Observed annual temperature and solar flux forcing are out‐of‐phase between 65 and 99 km, reflecting a dynamics dominated atmosphere at these altitudes.

Equilibrium dynamics in a forced-dissipative f-plane shallow-water system

The equilibrium dynamics in a homogeneous forced-dissipative f-plane shallow-water system is investigated through numerical simulations. In addition to classical two-dimensional turbulence, inertio-gravity waves also exist in this system. The dynamics is examined by decomposing the full flow field into a dynamically balanced potential-vortical component and a residual ‘free’ component. Here the potential-vortical component is defined as part of the flow that satisfies the gradient-wind balance equation and that contains all the linear potential vorticity of the system. The residual component is found to behave very nearly as linear inertio-gravity waves. The forcing employed is a mass and momentum source balanced so that only the large-scale potential-vortical component modes are directly excited. The dissipation is provided by a linear relaxation applied to the large scales and by an eighth-order linear hyperdiffusion. The statistical properties of the potential-vortical component in the fully developed flow were found to be very similar to those of classical two-dimensional turbulence. In particular, the energy spectrum of the potential-vortical component at scales smaller than the forcing is close to the ∼ k−3 expected for a purely two-dimensional system. Detailed analysis shows that the downscale enstrophy cascade into any wavenumber is dominated by very elongated triads involving interactions with large scales. Although not directly forced, a substantial amount of energy is found in the inertio-gravity modes and interactions among inertio-gravity modes are principally responsible for transferring energy to the small scales. The contribution of the inertio-gravity modes to the flow leads to a shallow tail at the high-wavenumber end of the total energy spectrum. For parameters roughly appropriate for the midlatitude atmosphere (notably Rossby number ∼ 0.5), the break between the roughly ∼ k−3 regime and this shallower regime occurs at scales of a few hundred km. This is similar to the observed mesoscale regime in the atmosphere. The nonlinear interactions among the inertio-gravity modes are extremely broadband in spectral space. The implications of this result for the subgrid-scale closure in the shallow-water model are discussed.

Simulations of the Effects of Water Vapor, Cloud Liquid Water, and Ice on AMSU Moisture Channel Brightness Temperatures

Radiative transfer simulations are performed to determine how water vapor and nonprecipitating cloud liquid water and ice particles within typical midlatitude atmospheres affect brightness temperatures TB's of moisture sounding channels used in the Advanced Microwave Sounding Unit (AMSU) and AMSU-like instruments. The purpose is to promote a general understanding of passive top-of-atmosphere TB's for window frequencies at 23.8, 89.0, and 157.0 GHz, and water vapor frequencies at 176.31, 180.3 1, and 182.31 GHz by documenting specific examples. This is accomplished through detailed analyses of TB's for idealized atmospheres, mostly representing temperate conditions over land. Cloud effects are considered in terms of five basic properties: droplet size distribution, phase, liquid or ice water content, altitude, and thickness. Effects on TB of changing surface emissivity also are addressed. The brightness temperature contribution functions are presented as an aid to physically interpreting AMSU TB's. Both liquid and ice clouds impact the TB's in a variety of ways. The TB's at 23.8 and 89 GHZ are more strongly affected by altostratus liquid clouds than by cirrus clouds for equivalent water paths. In contrast, channels near 157 and 183 GHz are more strongly affected by ice clouds. Higher clouds have a water impact on 157- and 183-GHz TB's than do lower clouds. Clouds depress TB's of the higher-frequency channels by suppressing, but not necessarily obscuring, radiance contributions from below. Thus, TB's are less closely associated with cloud-top temperatures than are IR radiometric temperatures. Water vapor alone accounts for up to 89% of the total attenuation by a midtropospheric liquid cloud for channels near 183 GHz. The Rayleigh approximation is found to be adequate for typical droplet size distributions; however, Mie scattering effects from liquid droplets become important for droplet size distribution functions with modal radii greater than 20 µm near 157 and 183 GHz, and greater than 30–40 µm at 89 GHz. This is due mainly to the relatively small concentrations of droplets much larger than the mode radius. Orographic clouds and tropical cumuli have been observed to contain droplet size distributions with mode radii in the 30–40-µm range. Thus, as new instruments bridge the gap between microwave and infrared to frequencies even higher than 183 GHz, radiative transfer modelers are cautioned to explicitly address scattering characteristics of such clouds.

Tropopause Folds and Synoptic-Scale Baroclinic Wave Life Cycles

Abstract The linear stability of three midlatitude zonal mean states of varying baroclinicity and barotropy is examined using a primitive equation stability analysis under the anelastic approximation. Each of these basic states includes both tropospheric and stratospheric regions with realistic properties compared with those of the earth's atmosphere in midlatitudes. In all three cases, the fastest growing synoptic-scale modes delivered by the linear stability analysis have wavelengths of approximately 4000 km. The nonlinear evolution of these synoptic-scale modes is explored using a three-dimensional anelastic finite-difference model whose zonal scale is chosen to be equal to the wavelength of the fastest growing mode of linear theory. During the nonlinear evolution of the wave, deep tropopause folds are shown to form (generically) and the depth to which the fold penetrates the troposphere is seen to increase with the baroclinicity of the mean state. These folds, descending along the sloping frontal zone...