Zonal Direction Research Articles

The behaviors of optimal precursors during wintertime Eurasian blocking onset

In this paper the optimal precursors for wintertime Eurasian blocking onset are acquired by solving a nonlinear optimization problem whose objective function is constructed based on a blocking index with a triangular T21, three-level, quasi-geostrophic global spectral model. The winter climatological state is chosen as the reference basic state. Numerical results show that the optimal precursors are characterized by a baroclinic pattern with a westward tilt with height, which are mainly located upstream of the blocking region. For an optimization time of 5 days, these perturbations are mainly localized over the Northeast Atlantic Ocean and continental Europe. With the extension of the optimization time to 8 days, these perturbations are distributed more upstream and extensively in the zonal direction. Wave spectrum analysis reveals that the optimal precursors are composed of not only synoptic-scale (wave numbers 5–18) waves, but planetary-scale (wave numbers 0–4) waves as well. The synoptic-scale optimal precursors are mainly located in the mid-latitude area, while the planetary-scale optimal precursors focus primarily on the high-latitude region. The formation of a strong planetary-scale positive blocking anomaly is accompanied by the reinforcement of synoptic-scale perturbations and further fragmentation into two branches, in which the northern branch is generally stronger than the southern one. The eddy forcing arising from the self-interaction of synoptic-scale disturbances is shown to be crucial in triggering the dipole blocking anomaly, and the planetary-scale optimal precursor provides the initial favorable background conditions for blocking onset.

Equatorial ionospheric electrodynamics associated with high‐speed solar wind streams during January–April 2007

High‐speed solar wind streams cause recurrent geomagnetic activity and ionospheric disturbances. In this study, we analyze the equatorial ionospheric ion drift measured by the Defense Meteorological Satellite Program (DMSP) satellites near dusk when high‐speed solar wind streams with a period of 13.5 days occurred during January–April 2007. A well‐defined quantitative correlation between the solar wind velocity and the equatorial ionospheric ion drift is identified for the first time. The plasma drift in the dusk equatorial ionosphere induced by high‐speed solar wind streams is eastward in the zonal direction and downward in the vertical direction at the altitude of DMSP orbit (∼840 km) during this low solar activity period (January–April 2007). The zonal component of the equatorial ionospheric ion drift is inversely correlated with the vertical component. The ionospheric ion zonal drift varies, on average, from −40 to 40 m s−1 when the solar wind velocity varies from 300 to 700 km s−1 over a 13.5 day period, and the ion vertical drift varies from 10 to −10 m s−1. The quantitative correlations between the solar wind velocity and ionospheric ion drift and between the vertical and zonal components of the ion drift velocity are important for understanding the equatorial ionospheric electrodynamics associated with high‐speed solar wind streams and for space weather prediction.

Hemispheric transport and influence of meteorology on global aerosol climatology

Abstract. Based on a 10-yr simulation with the global air quality modeling system GEM-AQ/EC, the northern hemispheric aerosol transport with the inter-annual and seasonal variability as well as the mean climate was investigated. The intercontinental aerosol transport is predominant in the zonal direction from west to east with the ranges of inter-annual variability between 14% and 63%, and is 0.5–2 orders of magnitude weaker in the meridional direction but with larger inter-annual variability. The aerosol transport is found to fluctuate seasonally with a factor of 5–8 between the maximum in late winter and spring and the minimum in late summer and fall. Three meteorological factors controlling the intercontinental aerosol transport and its inter-annual variations are identified from the modeling results: (1) Anomalies in the mid-latitude westerlies in the troposphere. (2) Variations of precipitation over the intercontinental transport pathways and (3) Changes of meteorological conditions within the boundary layer. Changed only by the meteorology, the aerosol column loadings in the free troposphere over the source regions of Europe, North America, South and East Asia vary inter-annually with the highest magnitudes of 30–37% in January and December and the lowest magnitudes of 16–20% in August and September, and the inter-annual aerosol variability within the boundary layer influencing the surface concentrations with the magnitudes from 6% to 20% is more region-dependent. As the strongest climatic signal, the El Niño-Southern Oscillation (ENSO) can lead the anomalies in the intercontinental aerosols in El Niño- and La Niña-years respectively with the strong and weak transport of the mid-latitude westerlies and the low latitude easterlies in the Northern Hemisphere (NH).

Solar migrating atmospheric tides in the winds of the polar region of Venus

We study the effects of migrating solar tides on the winds at the cloud tops of the polar region of Venus. The winds were measured using cloud tracking on images obtained at wavelengths of 3.9 and 5.0μm by the instrument VIRTIS-M onboard Venus Express. These wavelengths probe about the same altitude close to the cloud tops, allowing for the first time to retrieve winds simultaneously in the day and nightside of the planet. We use a dataset with observations from 16 orbits, covering a time span of 289days and a latitude range between 70°S and 85°S, the region where the so called cold collar resides. Diurnal and quarter-diurnal tides (wavenumbers 1 and 4) were detected in the wind field, with a decoupled influence on the zonal and meridional directions. The diurnal tide is the dominant harmonic with amplitudes of about 4.7m/s exclusively affecting the meridional component of the wind and forcing a solar-to-antisolar circulation at the polar region. The quarter-diurnal mode is only apparent in the zonal wind in a restricted latitude range with amplitudes ∼2.2m/s. The spatial structure of the diurnal tide has also been investigated, obtaining a vertical wavelength of about 8km, in accordance with predictions by models. Finally, a theoretical relation between the amplitudes of tidal temperature and tidal wind has been derived and its validity tested with models and results from previous missions.

On the influence of a β‐effect on Lagrangian diffusion

Geophysical turbulence is strongly influenced by the variation of the Coriolis force with latitude (β‐effect): when this effect is significant an anisotropic inverse cascade is developed since energy is preferentially transferred towards zonal modes. The consequent emergence of flow structures along the zonal direction can strongly impact turbulent transport and modify meridional scalar diffusion. In this letter we investigate zonal and meridional diffusion in laboratory experiments of turbulence affected by aβ‐effect. The degree of anisotropy and the flow characteristic scales have been quantified according to a Lagrangian approach based on the reconstruction of tracer trajectories, i.e., in analogy with oceanic drifters data, and on the Finite‐Scale Lyapunov Exponent (FSLE) analysis. An experimental confirmation of a recently introduced scaling law for the meridional diffusion in the marginally zonostrophic regime is also presented.

Comparison of a simple 2‐D Pluto general circulation model with stellar occultation light curves and implications for atmospheric circulation

We use a simple Pluto general circulation model (sPGCM) to predict for the first time the wind on Pluto and its global, large‐scale structure, as well as the temperature and surface pressure. Wind is a fundamental atmospheric variable that has previously been neither measured nor explicitly modeled on Pluto. We ran the sPGCM in 2‐D mode (latitude, height, and time varying) using the Massachusetts Institute of Technology general circulation model dynamical core, a simple radiative‐convective scheme, and no frost cycle. We found that Pluto's atmosphere is dynamically active in the zonal direction with high‐speed, high‐latitude jets that encircle the poles in gradient wind balance and prograde with Pluto's rotation. The meridional and vertical winds do not show evidence for a Hadley cell (or other large‐scale structure) due to the low‐altitude temperature inversion. The horizontal variation in surface pressure is a small fraction of the previously derived interannual variation in surface pressure. The simple general circulation model output was validated with stellar occultation light curve data from the years 1988, 2002, 2006, and 2007. For 2006 and 2007, the best fit global mean surface pressure was 24 microbar, in 2002 it was 22 microbar, and in 1988 it was 12 microbar (1 microbar error bars). For all years the methane mixing ratio was 1% (0.2% error bars). This work is a first step for future Pluto, Triton, and Kuiper Belt object atmosphere general circulation models that will also include longitudinal variations and a volatile cycle.

Role of the Indian Ocean in the ENSO–Indian Summer Monsoon Teleconnection in the NCEP Climate Forecast System

Abstract The observed negative correlation between El Niño–Southern Oscillation (ENSO) and the Indian summer monsoon is not simulated by the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) coupled model. The correlation is partially restored in the simulations where the Indian Ocean (IO) sea surface temperature (SST) is prescribed with the daily mean or climatology. Comparison among the simulations suggests that ENSO-induced SST anomalies form a strong dipole pattern oriented along the zonal direction in the IO in the coupled model, preventing the ENSO signals from reaching the Indian monsoon region. In the model, the dipole develops early in the monsoon season and extends to the central equatorial IO while it is formed at the end of the season in observations. The dipole modifies low-level winds and surface pressure, and grows in a positive feedback loop involving winds, surface pressure, and SST. Examination of the mean state in the model reveals that the thermocline is relatively shallow in the eastern IO. This preconditions the ocean such that the atmospheric fluxes can easily impart fluctuations in the subsurface temperature and thereby in the SST. These results suggest that biases in the IO can adversely affect the ENSO–monsoon teleconnection in a coupled model.

Forecast of development of quasi-biennial oscillation in the equatorial stratospheric wind until April 2014

As is well known, the quasi-biennial oscillation (QBO) of wind in the equatorial stratosphere has a significant influence on the processes not only at tropical but also at middle and high latitudes in both stratosphere and troposphere. Consequently, a forecasting the dates of QBO-phase (W or E) transitions is required for the solution of many problems (for instance, for investigation of possible climate changes, for studying of solar activity effects in atmospheric phenomena, for predicting the trends of various atmospheric parameters, etc.). The type of QBO-phase is commonly determined by the zonal wind direction at fixed altitude. The quasi-biennial alternations of wind direction are related with a successive descent of easterly and westerly wind regimes. The paper describes the peculiarities of the wind evolution during QBO-cycle and reveals that the easterly regime propagates downward always with stop at some altitude in the range 20–40hPa. This time period (no less than three months) of the easterly descent interruption is termed as the stagnation stage. Seasonal regularities of stagnation stage cause the discreteness of QBO-cycle period. If the interval between beginnings of successive stagnation stages is identified as a QBO-cycle length, it appears that no QBO-period other than 24, 30, or 36 months can be realized. The QBO-cycle period is unambiguously related with the stagnation duration that allows making an early forecast of the QBO-phase transition dates. The paper discusses the stagnation stage features occurring during QBO-cycles of different period, and describes the technique of forecasting the QBO-cycle evolution by analyzing the vertical profiles of wind speed. The study is based on experimental observation of zonal wind in the equatorial stratosphere during 1953–2011. It is shown that the previous quasi-biennial cycle has begun in January 2009, lasted for 30 months and completed in June 2011. The current cycle has begun in July 2011, will last for 30 months and will end in December 2013. Subsequent cycle will begin in January 2014. The QBO-phase transition dates for different height levels are determined for all months up to April 2014.

Connecting Subseasonal Movements of the Winter Mean Ridge in Western North America to Inversion Climatology in Cache Valley, Utah

AbstractA 10-yr record of PM2.5 (particulate matter of aerodynamic diameter ≤ 2.5 μm), collected in Cache Valley near downtown Logan, Utah, reveals a strong peak in the PM2.5 concentration climatology that is tightly localized in mid-January. The cause of this subseasonal variation in the PM2.5 climatology is investigated through dynamical downscaling and large-scale diagnostics. Climatological analysis of the U.S. winter mean ridge reveals a mid-January subseasonal shift in the zonal direction, likely in response to variations in the Rossby wave source over the central North Pacific Ocean. This displacement of the winter mean ridge, in turn, has an impact on regional-scale atmospheric conditions—specifically, subsidence with local leeside enhancements and midlevel warming over Cache Valley. The analyses of this study indicate that the subseasonal peak of long-term mean PM2.5 concentrations in Cache Valley is linked to the large-scale circulations’ subseasonal evolution, which involves remote forcing in the circumpolar circulations as well as possible tropical–midlatitude interactions. This subseasonal evolution of the winter mean circulation also affects precipitation along the West Coast.

Heat center of the western Pacific warm pool

A heat center (HC) of the western Pacific warm pool (WPWP) is defined, its variability is examined, and a possible mechanism is discussed. Analysis and calculation of a temperature dataset from 1945–2006 show that the mean position of the HC during this period was near 0.4°S/169.0°E, at 38.0 m depth. From a time series of the HC, remarkable seasonal variability was found, mainly in the meridional and vertical directions. Interannual variabilities were dominant in the zonal and vertical directions. In addition, semiannual variation in the HC depth was discovered. The longitude of the HC varies with ENSO events, and its latitude is weakly related to ENSO on time scales shorter than a decade. The variation of the HC longitude leads the Nino-3 index by about 3–4 months, and its depth lags the index for approximately 3 months. It is concluded that the HC depth results from a combination of its longitudinal and latitudinal variations. Low-pass-filtered time series reveal that the HC has moved eastward since the mid 1980s.

Gravity wave characteristics from OH airglow imager over Maui

[1] The gravity wave (GW) characteristics over Maui from 2002 to 2007 are deduced from OH airglow imager. The dominant horizontal wavelength falls between 15 and 30 km. Intrinsic phase speeds are mainly in the range of 50–100 m s−1. Observed phase speeds are mainly in the range of 20–70 m s−1. Intrinsic wave periods are mostly below 30 min with the majority clusters around 5–15 min. For intrinsic wave period larger than 5 min, as wave period increases, the number of wave events decreases. Observed wave periods fall in the range of 8–20 min. The vertical wavelength distribution peaks between 20 and 30 km and falls rapidly as wavelength decreases. GWs propagating against (along) background wind are Doppler shifted to a higher (lower) frequency, larger (smaller) vertical wavelength, and decreased (enhanced) chance of observation. During all seasons Doppler shifting is part of the explanation as to why the majority of observed GWs propagate against the background wind. The preferentially poleward or equatorward propagation of GWs during summer or winter observed by airglow imagers at Maui can be explained by the effects of critical layer filtering, ducting, and Doppler shifting. In March, when critical layer filtering effects are not important, Doppler shifting and ducting play dominant roles in determining preferred zonal propagation direction. In December, ducting and wave source location predominantly determine wave propagation direction. Momentum flux deduced from OH imager is also highly anticorrelated with background wind on both seasonal and diurnal time frames. Momentum flux is highly modulated by tidal wind. Waves with shorter vertical wavelengths have much smaller power in GW spectra. OH imager may capture the majority of momentum flux of high-frequency GW spectra through the airglow layer.

On the scaling laws derived from ice beacon trajectories in the southern Beaufort Sea during the International Polar Year - Circumpolar Flaw Lead study, 2007–2008

[1] Sea ice motion is an important element in mass balance calculations, ice thermodynamic modeling, ice management plans for industry, and ecosystems studies. In the historical literature, sea ice motion in the Beaufort Sea was characterized by a predominantly anticyclonic motion during winter months, with episodic reversals to cyclonic activity during summer. However, recent studies have shown an increase in cyclonic activity throughout the annual cycle. In this paper we examine circulation in the Beaufort Sea based on the trajectories of 22 ice beacons launched in the Franklin Bay area during the International Polar Year - Circumpolar Flaw Lead (IPY-CFL) study during an over-wintering experiment in 2007–2008. Dispersion characteristics of ice motion show that absolute zonal dispersion follows a t2 scaling law characteristic of advection associated with Beaufort Gyre circulation, whereas absolute meridional dispersion follows a scaling law of t5/4 characteristic of floaters and dispersion in 2-D turbulence. Temporal autocorrelations of ice velocity fluctuations highlight definitive timescales with values of 1.2 (0.7) days in the zonal (meridional) direction. Near-Gaussian behavior is reflected in higher-order moments for ice velocity fluctuation probability density functions (pdfs). Non-Gaussian behavior for absolute displacement pdfs indicates spatial heterogeneity in the ice motion fields. Atmospheric forcing of sea ice is explored through analysis of daily North American Regional Reanalysis and in situ wind data, where it is shown that ice in the CFL study region travels with an average speed of approximately 0.2% and an average angle of 51.5° to the right of the surface winds during the 2007–2008 winter. The results from this analysis further demonstrate seasonality in ice drift to wind ratios and angles that corresponds to stress buoy data indicative of increases in internal ice stress and connectivity due to consolidation of the seasonal ice zone to the coast and perennial ice pack during winter in the Beaufort Sea region.

Superrotation of the ionosphere and quiet time zonal ion drifts at low and middle latitudes observed by Republic of China Satellite-1 (ROCSAT-1)

[1] Local-time variations of zonal drifts at different latitudes, longitudes, and seasons are investigated using ion drift measurements from the Republic of China Satellite-1 (ROCSAT-1). We select a quiet time period defined when Dst > −100 nT and Kp ≤ 3. The ion drifts in the magnetic zonal direction are latitude (apex height) dependent and differ for different longitude sectors and seasons. Superrotation of the ionosphere is observed in the zonal drift measurements, particularly at the lower latitudes. The ionospheric superrotation generally decreases with increasing latitude and is attributed mainly to the F region dynamo where the features of the neutral wind and conductance affect the characteristics of the superrotation at different latitudes, longitudes, and seasons. We suggest that the latitudinal variation of the superrotation is mainly produced by the decrease in latitude of the neutral winds in the F region. A decrease of conductance with increasing latitude is also present, but evidence shows that neutral winds decrease faster with latitude than the conductance for high solar flux conditions. The longitudinal and seasonal variations of the superrotation are more influenced by changes in the F region conductance that is modified by vertical E × B ion drift motions and neutral wind-induced ion drift motions along magnetic field lines.

The fine-scale structure of the global tropopause derived from COSMIC GPS radio occultation measurements

[1] The spatiotemporal structure of the lapse‐rate tropopause is examined by using state‐of‐the‐art Global Positioning System radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Formosa Satellite Mission 3 mission. The high temporal and spatial resolutions of the data reveal the detailed structure of tropopause properties such as pressure (pt), temperature (Tt), and sharpness (Nt ) and their relationships to upper tropospheric and lower stratospheric processes. The overall results are generally in good agreement with previous studies. The climatology of all three tropopause properties shows largely homogeneous structure in the zonal direction: noticeable asymmetries are found only in the tropics and the Northern Hemisphere extratropics during boreal winter owing to localized tropospheric processes. This contrasts with the seasonal cycles of tropopause properties which are significantly influenced by stratospheric processes such as the Brewer‐Dobson circulation, the polar vortex, and the radiative processes near the tropopause. On intraseasonal time scales, pt and Tt exhibit significant variability over the Asian summer monsoon and the subtropics where double tropopauses frequently occur. In contrast, Nt shows maximum variability in the tropics where pt and Tt have minimum variability, possibly a consequence of vertically propagating waves. The tropopause properties derived from COSMIC observations are further applied to evaluate tropopause data directly available from the NCEP‐NCAR Reanalysis (NNR). Although the NNR tropopause data have been widely usedinclimatestudies,theyarefoundtohavesignificantandsystematicbiases,especiallyin the subtropics. This suggests that the NNR tropopause data should be treated with great caution in any quantitative studies.

Statistical characteristics of gravity wave activities observed by an OH airglow imager at Xinglong, in northern China

Abstract. An all-sky airglow imager (ASAI) was installed at Xinglong, in northern China (40.2° N, 117.4° E) in November 2009 to study the morphology of atmospheric gravity waves (AGWs) in the mesosphere and lower thermosphere (MLT) region. Using one year of OH airglow imager data from December 2009 to November 2010, the characteristics of short-period AGWs are investigated and a yearlong AGW climatology in northern China is first ever reported. AGW occurrence frequency in summer and winter is higher than that in equinoctial months. Observed bands mainly have horizontal wavelengths from 10 to 35 km, observed periods from 4 to 14 min and observed horizontal phase speeds in the range of 30 to 60 m s−1. Most of the bands propagate in the meridional direction. The propagation directions of the bands show a strong southwestward preference in winter, while almost all bands propagate northeastward in summer. Although the wind filtering in the middle atmosphere may control AGW propagations in the zonal direction, the non-uniform distribution of wave sources in the lower atmosphere may contribute to the anisotropy in the meridional direction in different seasons. Additionally, as an indication of local instability, the characteristics of ripples are also analyzed. It also shows seasonal variations, occurring more often in summer and winter and mainly moving westward in summer and eastward in winter.

Nonlinearity and finite-time instability in a T21L3 quasigeostrophic model

In this paper, a nonlinear optimization method is used to explore the finite-time instability of the atmospheric circulation with a three-level quasigeostrophic model under the framework of the conditional nonlinear optimal perturbation (CNOP). As a natural generalization of linear singular vector (SV), CNOP is defined as an initial perturbation that makes the cost function the maximum at a prescribed forecast time under certain physical constraint conditions. Special attentions are paid to the different structures and energy evolutions of the optimal perturbations. The results show that the most instable region of the global atmospheric circulation lies in the midlatitude Eurasian continent. More specially, SV and CNOP in the total energy norm with an optimization time of 2 days both present localness: they are mainly located in the midlatitude Asian continent and its east coast. With extension of the optimization time, SVs are more upstream and less localized in the zonal direction, and CNOPs differ essentially from SVs with broader zonal and meridional coverages; as a result, CNOPs acquire larger kinetic and available potential energy amplifications than SVs in the nonlinear model at the corresponding optimization time. For the climatological wintertime flow, it is seen that the baroclinic terms remain small over the entire time evolution, and the energy production comes essentially from the eddy kinetic energy, which is induced by the horizontal shear of the basic flow. In addition, the effects of SVs and CNOPs on the Eurasian atmospheric circulation are explored. The results show that the weather systems over the Eurasian continent in the perturbed fields by CNOPs are stronger than those by SVs at the optimization time. This reveals that the CNOP method is better in evaluating the instability of the atmospheric circulation while the SV method underestimates the possibility of extreme weather events.

Stochastic Forcing of Potential Vorticity: Observations

Abstract The linear theory of point correlation maps for synoptic systems relies so far mainly on specifications of stochastic forcing due to nonlinear processes that are not based on observations. Forty-year ECMWF Re-Analysis (ERA-40) data are used to derive time series of the forcing terms in a potential vorticity equation for a correlation point in the North Atlantic storm-track region. It is found that the forcing correlations are restricted to distances less than 1500 km to the correlation point in zonal direction and just a few hundred kilometers in meridional direction. The forcing is not even approximately white in time. Covariances of forcing and potential vorticity are presented as well. An advection equation with simple damping and realistic stochastic forcing is solved to approximate the observed covariances of forcing and potential vorticity.

The response of auroral absorption to substorm onset: Superposed epoch and propagation analyses

[1] The response of high-energy particle precipitation to substorm onset is investigated using observations of cosmic noise absorption by a 7 × 7 beam imaging riometer in Kilpisjarvi, Finland, and substorm onset information obtained from the IMAGE satellite. A new method is developed for automatic detection of absorption responses to substorm onsets. Superposed epoch analysis shows that absorption exhibits some fading prior to and a clear response preceding substorm onset. These features are interpreted as being due to the stretching of magnetic field lines and a rapid dipolarization during the late growth phase of a substorm. A distinct dependence on substorm onset location is discovered with response delay increasing with distance from substorm onset and all responses to closest substorms (within 250 km) preceding substorm onset by 1–5 min. The absorption propagation is further examined using the average velocity from the onset to the riometer location as well as instantaneous velocities for a short period of time when substorm-enhanced absorption reaches the field of view. Comparison of the two velocity estimates shows that the absorption propagation is slower and oriented in more poleward directions away from substorm onset, with some evidence of near instantaneous expansion in zonal directions over large distances of ∼2000 km from onset followed by a slower expansion primarily in poleward and westward directions. The observations suggest that during substorms precipitating electrons in the high-energy part of the spectrum behave differently from their low-energy counterparts and that they exhibit some evidence of predictable behavior.

Titan’s new pole: Implications for the Huygens entry and descent trajectory and landing coordinates

The European Space Agency’s Huygens probe separated from the NASA Cassini spacecraft on 25 December 2004, after having been attached for a 7-year interplanetary journey and three orbits around Saturn. The probe reached the predefined NASA/ESA interface point on 14 January 2005 at 09:05:52.523 (UTC). It performed a successful entry and descent sequence and softly landed on Titan’s surface on the same day at 11:38:10.77 (UTC) with a speed of about 4.54m/s. Since the publication of the official project entry and descent trajectory reconstruction effort by the Descent Trajectory Working Group in 2007 (referred to as DTWG#4) various other efforts have been performed and published. This paper presents an overview of the most relevant reconstructions and compares their methodologies and results. Furthermore, the results of a new reconstruction effort (DTWG#5) are presented, which is based on the same methodology as DTWG#4 but takes into account new estimates of Titan’s pole coordinates which were derived from radar images of different Cassini Titan flybys. It can be shown that the primary effect can be observed in the meridional direction which is represented by a stark southward shift of the trajectory by about 0.3deg. A much smaller effect is seen in the zonal direction (i.e., less than 0.01deg in the west to east direction). The revised probe landing coordinates are 192.335degW and 10.573degS. A comparison of these coordinates with results of recent landing site investigations using visual and radar images of the Cassini VIMS instrument shows excellent agreement of the two independently derived landing coordinates, i.e., longitude and latitude residuals of respectively 0.035deg and 0.007deg.

Spatial scales of climate response to inhomogeneous radiative forcing

The distances over which localized radiative forcing influences surface temperature have not been well characterized. We present a general methodology to analyze the spatial scales of the forcing/response relationship and apply it to simulations of historical aerosol forcing and response in four climate models. We find that the surface temperature response is not strongly sensitive to the longitude of forcing but is fairly sensitive to latitude. Surface temperature responses in the Arctic and the Southern Hemisphere extratropics, where forcing was small, show little relationship to local forcing. Restricting the analysis to 30°S–60°N, where nearly all the forcing was applied, shows that forcing strongly influences response out to ∼4500 km away examining all directions. The meridional length of influence is somewhat shorter (∼3500 km or 30°), while it extends out to at least 12,000 km in the zonal direction. Substantial divergences between the models are seen over the oceans, whose physical representations differ greatly among the models. Length scales are quite consistent over 30°S–60°N land areas, however, despite differences in both the forcing applied and the physics of the models themselves. The results suggest that better understanding of regionally inhomogeneous radiative forcing would lead to improved projections of regional climate change over land areas. They also provide quantitative estimates of the spatial extent of the climate impacts of pollutants, which can extend thousands of kilometers beyond polluted areas, especially in the zonal direction.