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Arthur Y. Hou

Abstract

The hypothesis is advanced that a latitudinal shift in the tropical convective heating pattern can significantly alter temperatures in the extratropics. Results of a simplified GCM show that the shift of a prescribed tropical heating toward the summer pole, on time scales longer than a few weeks, leads to a more intense cross-equatorial “winter” Hadley circulation, enhanced upper-level tropical easterlies, and a slightly stronger subtropical winter jet, accompanied by warming at the winter middle and high latitudes as a result of increased dynamical heating. The indications are that there is a robust connection between the net dynamic heating in the extratropics and the implied changes in the subtropical wind shear resulting from adjustments in the Hadley circulation associated with convective heating displacements in the tropics. The implications are that (i) the low-frequency temporal variability in the Hadley circulation may play an important role in modulating wave transport in the winter extratropics, (ii) the global climate may be sensitive to those processes that control deep cumulus convection in the tropics, and (iii) systematic temperature biases in GCMs may be reduced by improving the tropical rainfall simulation.

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Arthur Y. Hou

Abstract

The parametric behavior of an axially symmetric circulation induced by heat and momentum sources is analyzed in the context of a simple Boussinesq model. Implications for the Venus atmosphere are examined in the light of recent data.

For nearly inviscid flows in a stably stratified atmosphere, this work extends the analysis of Held and Hou to large thermal Rossby numbers (slowly rotating atmospheres). For parametric values appropriate to the Venus atmosphere, heat flux by the Hadley circulation leads to a temperature distribution nearly uniform with latitude, close to the asymptotic limit for a nonrotating atmosphere, while the zonal wind shows a strong polar jet but does not superrotate at the equator, contrary to what is observed on Venus.

For the Venus atmosphere to superrotate above the cloud base, where most of the solar radiation is deposited, momentum sources and sinks must be provided by asymmetric motions to offset transports by the mean meridional circulation. Diagnostically, the eddy momentum source/sink pattern required by a given meridional cell depends upon both the sign of the vertical zonal wind shear and the direction of the meridional flow. For a realistic vertical zonal wind profile, the implication is that the observed superrotation on Venus can be supported by alternating layers of sources and sinks.

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Arthur Y. Hou

Abstract

Studies based on GCM ensemble forecasts have shown that an intensification of the cross-equatorial Hadley circulation associated with a latitudinal displacement of the zonally averaged convective heating in the Tropics can lead to remote warming in the winter high latitudes. This work further investigates this tropical–extratropical connection in a perpetual winter experiment using an idealized GCM without orography to focus on the role of transient eddies, and tests against observations using a multiyear reanalysis produced by the Goddard Earth Observing System-Version 1 (GEOS-1) Data Assimilation System.

The GCM results show that the intensification and poleward expansion of the cross-equatorial Hadley cell induced by a tropical heating shift can lead to westerly acceleration in the winter subtropics and enhanced vertical shear of the zonal wind in the subtropics and midlatitudes. The increased baroclinicity outside the Tropics is accompanied by reduced meridional temperature and potential vorticity (PV) gradients, consistent with enhanced PV mixing and increased poleward heat transport by baroclinic eddies. But if the changes in the Hadley cell are such that they produce a deceleration of the zonal wind in the winter subtropics, stronger temperature and PV gradients result in the winter extratropics. The midlatitude response to Hadley acceleration of the subtropical jet is dominated by enhanced power in low-frequency planetary-scale waves that peaks at zonal wavenumber 2 with a period of 40 days.

The extent to which this tropical–extratropical connection may be present in nature is tested using the GEOS-1 reanalysis for five austral winters from 1985 to 1989. Results show that the year-to-year variation in the zonally averaged extratropical temperature gradient in austral winters is correlated with the variation in the acceleration of the subtropical zonal wind by the winter Hadley cell. The anomaly correlation coefficients range from 0.80 to 0.92, depending on the statistical test. The positive Hadley acceleration anomaly in the subtropics during the 1988 austral winter is accompanied by stronger than normal zonal wind shears in the subtropics and midlatitudes, a colder troposphere in the midlatitudes, and a warmer pole. The extratropical temperature anomalies are associated with a reduced PV gradient, and the midlatitude geopotential height anomaly shows a spectral peak at wavenumbers 2–3 with periods between 40 and 60 days, similar to the idealized GCM results. The implication of this study is that the Hadley circulation may play a role in modulating the temperature difference between middle and high latitudes by modifying the zonal wind shear in the subtropics and midlatitudes.

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Arthur Y. Hou and Andrea Molod

Abstract

The hypothesis that the cross-equatorial Hadley circulation can modulate the poleward heat transport in the winter extratropics is investigated using the Goddard Earth Observing System (GEOS-1) GCM for 10 northern winter initial conditions. Three-month forecasts are compared with parallel runs from the same initial conditions but with a slightly perturbed radiative forcing in the Tropics. Analysis of the zonal-mean climates shows that, on timescales longer than a month, a persistent intensification in the winter Hadley circulation is positively correlated with increased dynamic cooling in the winter midlatitudes and warming at the high latitudes through-out the troposphere, signaling an increased beat transport toward the winter pole.

While the heating anomaly can undergo significant transient fluctuations in the winter extratropics, the variance of the time-averaged dynamic heating anomaly is dominated by contributions from low zonal wavenumbers (particularly wavenumbers 1 and 2), with minor contributions from wavenumbers 4 and higher, suggesting that the low-frequency planetary-scale waves are the primary vehicle for the increased poleward heat transport, with the synoptic-scale waves assuming a secondary role along the storm tracks.

These results support the earlier idealized GCM study by Hou showing that a stronger winter Hadley circulation induced by a latitudinal shift in tropical convection can lead to enhanced upper-level tropical easterlies and a slightly stronger subtropical winter jet attended by increased poleward heat transport in the winter extratropics. Specific examples were also found in which the zonally averaged response is dominated by regional changes (notably over the North Pacific), indicating these relations may hold locally, as suggested by Bjerknes.

The implication of this work is that the low-frequency variability in the Hadley circulation associated with persistent tropical rainfall anomalies may play an important role in global climate by modulating the subtropical wind shear and the energy available for baroclinic wave growth outside the Tropics.

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Timothy DelSole and Arthur Y. Hou

Abstract

The possibility of empirically correcting a nonlinear dynamical model is examined. The empirical correction is constructed by fitting a first-order Markov model to the forecast errors using initial conditions as predictors. The dynamical operator of the Markov model can then be subtracted from the original forecast model to correct the forecast. The procedure is based on an earlier work by Leith, suitably modified for practical applications. The effects of analysis errors and finite difference approximations are analyzed. It is shown that uncorrelated analysis errors produce spurious terms in the empirical correction operator that act to damp eddy variance at a rate inversely proportional to the lead time used to estimate the Markov model. The finite difference approximation is appropriate as long as the forecast errors grow linearly with lead time. These restrictions can be interpreted as setting lower and upper limits on the lead time, respectively, and can be checked without knowing the true state exactly. The procedure is sensitive to sampling errors, but this problem was not explored.

These formal conclusions are tested on a nonlinear quasigeostrophic model in which model error is created by changing the model parameters. A “systematic error” correction, which does not depend on state and is held constant throughout the integration, does not improve the forecast skill of the model because the contrived error is primarily state dependent. However, an empirical correction, which depends on instantaneous state, improves the forecast skill of the model by 10 days, and further iterations extend the skill to the limit imposed by observation error (about 20 days in this example).

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Xin Lin and Arthur Y. Hou

Abstract

This study compares instantaneous rainfall estimates provided by the current generation of retrieval algorithms for passive microwave sensors using retrievals from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and merged surface radar and gauge measurements over the continental United States as references. The goal is to quantitatively assess surface rain retrievals from cross-track scanning microwave humidity sounders relative to those from conically scanning microwave imagers. The passive microwave sensors included in the study are three operational sounders—the Advanced Microwave Sounding Unit-B (AMSU-B) instruments on the NOAA-15, -16, and -17 satellites—and five imagers: the TRMM Microwave Imager (TMI), the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) instrument on the Aqua satellite, and the Special Sensor Microwave Imager (SSM/I) instruments on the Defense Meteorological Satellite Program (DMSP) F-13, -14, and -15 satellites. The comparisons with PR data are based on “coincident” observations, defined as instantaneous retrievals (spatially averaged to 0.25° latitude and 0.25° longitude) within a 10-min interval collected over a 20-month period from January 2005 to August 2006. Statistics of departures of these coincident retrievals from reference measurements as given by the TRMM PR or ground radar and gauges are computed as a function of rain intensity over land and oceans. Results show that over land AMSU-B sounder rain retrievals are comparable in quality to those from conically scanning radiometers for instantaneous rain rates between 1.0 and 10.0 mm h−1. This result holds true for comparisons using either TRMM PR estimates over tropical land areas or merged ground radar/gauge measurements over the continental United States as the reference. Over tropical oceans, the standard deviation errors are comparable between imager and sounder retrievals for rain intensities above 5 mm h−1, below which the imagers are noticeably better than the sounders; systematic biases are small for both imagers and sounders. The results of this study suggest that in planning future satellite missions for global precipitation measurement, cross-track scanning microwave humidity sounders on operational satellites may be used to augment conically scanning microwave radiometers to provide improved temporal sampling over land without degradation in the quality of precipitation estimates.

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Timothy DelSole and Arthur Y. Hou

Abstract

Two Markov models with different dynamics and forcing are used to model the transient eddy statistics of an idealized general circulation model (GCM). The first Markov model employs a physically based dynamical operator composed of the linearized primitive equations plus spatially uniform damping. This model, when driven by spatially uncorrelated forcing, failed to produce reasonable fluxes and variances, despite tuning of the damping and forcing coefficients. This result contrasts with previous studies that have used the linearized quasigeostrophic equations to model extratropical eddy statistics.

The second Markov model is constructed empirically from the time-lagged covariances of the GCM time series. This empirical Markov model could reproduce the dominant covariances over a range of time lags, provided it contained a sufficiently large number of degrees of freedom. It could not, however, reproduce the time-lag evolution of the trailing EOFs contained in the model. The errors in the trailing EOFs displayed a systematic behavior that could be explained by assuming that the “effective noise”—the noise required to reproduce the full covariances—is correlated over timescales comparable to the smallest e-folding time of the eigenmodes. Under this assumption, the effective noise is not white, but, for sufficiently large model dimension, the dominant disturbances still can be modeled appropriately by a Markov model because their associated decorrelation rates are small compared to the decorrelation rate of noise. This explanation is illustrated using a three-variable Markov model. These results suggest the following criteria for Markov model estimation: the lag and number of EOFs should be chosen such that the least damped modes show little or no dependence on lag and that none of the imaginary eigenvalues are aliased (in a sense defined in the paper). The resulting Markov model for the dominant disturbances is not sensitive to EOF truncation or choice of time lag, except for the structure of the singular vectors and adjoints, and the ordering of the eigenmodes. The sensitivity in singular vectors and adjoints is a plausible consequence of nonnormality, as nonnormality of the underlying physical system leads to singular vectors differing considerably from the normal modes and EOFs.

The stable eigenmodes resemble the leading EOFs used to construct the empirical model, but differ considerably from the unstable eigenmodes of the linearized GCM. This difference is attributed to nonlinear processes implicitly represented in the Markov model. The dissipation and stochastic forcing were concentrated in the Tropics and subtropics, in contrast to the eddy variance and fluxes, which were concentrated in subtropics and midlatitudes. The associated singular vectors also were localized initially in the Tropics and subtropics, but eventually develop into robust extratropical disturbances. Interestingly, the dominant singular vectors undergo a growth and decay life cycle characteristic of the classic nonlinear life cycle, with time-averaged fluxes in close agreement with those diagnosed from the GCM climatology. The fact that the forcing, dissipation, and initial singular vectors are concentrated in the same vicinity suggests a dynamical feedback between Tropics and subtropics.

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Xin Lin and Arthur Y. Hou

Abstract

A high-resolution surface rainfall product is used to estimate rain characteristics over the continental United States as a function of rain intensity. By defining data at 4-km horizontal resolutions and 1-h temporal resolutions as an individual precipitating or nonprecipitating sample, statistics of rain occurrence and rain volume including their geographical and seasonal variations are documented. Quantitative estimations are also conducted to evaluate the impact of missing light rain events due to satellite sensors’ detection capabilities.

It is found that statistics of rain characteristics have large seasonal and geographical variations across the continental United States. Although heavy rain events (>10 mm h−1) only occupy 2.6% of total rain occurrence, they may contribute to 27% of total rain volume. Light rain events (<1.0 mm h−1), occurring much more frequently (65%) than heavy rain events, can also make important contributions (15%) to the total rain volume.

For minimum detectable rain rates setting at 0.5 and 0.2 mm h−1, which are close to sensitivities of the current and future spaceborne precipitation radars, there are about 43% and 11% of total rain occurrence below these thresholds, and they respectively represent 7% and 0.8% of total rain volume. For passive microwave sensors with their rain pixel sizes ranging from 14 to 16 km and the minimum detectable rain rates around 1 mm h−1, the missed light rain events may account for 70% of rain occurrence and 16% of rain volume.

Statistics of rain characteristics are also examined on domains with different temporal and spatial resolutions. Current issues in estimates of rain characteristics from satellite measurements and model outputs are discussed.

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Arthur Y. Hou and Richard M. Goody

Abstract

We have extended our previous calculations of zonally averaged temperature, circulation, and eddy source requirements for the Venus atmosphere to include the region from the surface to 95 km, using a Curtis matrix method for the radiation calculation. We conclude: (i) The cloud top circulation is not significantly changed from our previous calculations based on a radiative-relaxation method, but large differences occur in the lower atmosphere. (ii) The physical and dynamical states of the atmospheric regions above and below the cloud base are effectively independent and are equally important for accounting for the 4-day circulation at the cloud tops. (iii) Above the cloud base, the circulation is effectively inviscid and the eddy source requirements at the low latitudes are consistent with the mean-flow forcing by the semidiurnal tide. (iv) The circulation below the clouds is important compared to viscous dissipation above the lowest scale height. We discuss possible mechanisms for satisfying the tropical eddy source requirements in the lower atmosphere.

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Arthur Y. Hou and Richard M. Goody

Abstract

For the solar heating and zonal wind profiles observed in the Venus atmosphere (below 80 km) we have calculated the eddy source pattern required to maintain the zonally averaged circulation.

In the cloud-top region (45-75 km) the calculated residual meridional circulation corresponds to multiple direct and indirect cells in the vertical, whose depths are controlled by the scales of solar heating and eddy sources. For the amount of small-scale diffusion suggested by in situ measurements, the circulation is close to the nearly inviscid limit, and advections by the mean circulation must be balanced by eddy sources. In the presence of mean meridional transports, the observed zonal superrotation can be supported by alternating layers of eddy sources and sinks (i.e., Eliassen-Palm flux divergences or convergences), which may possibly be caused by thermal tides.

Below the cloud decks, the effect of meridional motions is small, and eddy sources are required to balance diffusion (if diffusion is as large as measurements indicate). Near the lower model boundary, differential solar heating forces a shallow Hadley cell which controls surface winds and ensures that the net surface torque vanishes in a steady state.

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