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Siegfried D. Schubert and Man Li Wu

Abstract

The predictability of the 1997 and 1998 south Asian summer monsoon winds is examined from an ensemble of 10 atmospheric general circulation model simulations with prescribed sea surface temperatures (SSTs) and soil moisture. The simulations have no memory of atmospheric initial conditions for the periods of interest.

The model simulations show that the 1998 monsoon is considerably more predictable than the 1997 monsoon. During May and June of 1998 the predictability of the low-level wind anomalies is largely associated with a local response to anomalously warm Indian Ocean SSTs. Predictability increases late in the season (July and August) as a result of the strengthening of the anomalous Walker circulation and the associated development of easterly low-level wind anomalies that extend westward across India and the Arabian Sea. During these months the model is also the most skillful, with the analyses showing a similar late-season westward extension of the easterly wind anomalies.

The model shows little predictability or skill in the monthly mean low-level winds over Southeast Asia during 1997. Predictable wind anomalies do occur over the western Indian Ocean and Indonesia; however, over the Indian Ocean the predictability is artificial, because the model is responding to SST anomalies that were wind driven. The reduced predictability in the low-level winds during 1997 appears to be the result of a weaker (as compared with 1998) simulated anomalous Walker circulation, and the reduced skill is associated with pronounced intraseasonal activity that is not captured well by the model. It is remarkable that the model does produce an ensemble mean Madden–Julian oscillation (MJO) response, though it is approximately in quadrature with, and much weaker than, the observed MJO anomalies during 1997.

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Man-Li C. Wu, Oreste Reale, and Siegfried D. Schubert

Abstract

This study shows that the African easterly wave (AEW) activity over the African monsoon region and the northern tropical Atlantic can be divided in two distinct temporal bands with time scales of 2.5–6 and 6–9 days. The results are based on a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The novel result of this investigation is that the 6–9-day waves appear to be located predominantly to the north of the African easterly jet (AEJ), originate at the jet level, and are different in scale and structure from the well-known low-level 2.5–6-day waves that develop baroclinically on the poleward flank of the AEJ. Moreover, they appear to interact with midlatitude eastward-propagating disturbances, with the strongest interaction taking place at the latitudes where the core of the Atlantic high pressure system is located. Composite analyses applied to the mode decomposition indicate that the interaction of the 6–9-day waves with midlatitude systems is characterized by enhanced southerly (northerly) flow from (toward) the tropics. This finding agrees with independent studies focused on European floods, which have noted enhanced moist transport from the ITCZ toward the Mediterranean region on time scales of about a week as important precursors of extreme precipitation.

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C-H. Sui, X. Li, K-M. Lau, and D. Adamec

Abstract

Two distinct intraseasonal oscillations (ISO) are found in the tropical ocean atmosphere in the western Pacific region during Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The ISO is characterized by cycles of dry–wet phases in the atmosphere due to the passage of Madden–Julian oscillations, and corresponding warming/shoaling–cooling/deepening cycles in the ocean mixed layer (OML). During the wet phase, 2–3-day disturbances and diurnal variations in the atmosphere are pronounced. During the dry phase, diurnal cycles in sea surface temperature (SST) is much enhanced while the OML is shallow.

These multiscale coupled air–sea variations are further investigated with an ocean mixed-layer model forced by the observed surface heat, water, and momentum fluxes. The variations of ocean mixed layer are shown to be crucially dependent on the vertical distribution of solar radiation, that is, diurnal SST variability primarily determined by the absorbed solar radiation in the surface layer (∼1 m), and intraseasonal variations determined by penetrating solar radiation below the surface layer. Results further reveal that the accumulative effect of diurnal mixing cycles (solar heating/nocturnal deepening) is essential to maintain a stable temperature stratification and a realistic evolution of mixed-layer depth and temperature at the intraseasonal scale. The nonlinear response of the ocean mixed layer to the surface heat and momentum fluxes indicates the need to resolve the high-frequency response including diurnal atmospheric radiative–convective processes and ocean mixing processes in a coupled model to simulate the whole spectrum of multiscale variations within ISOs.

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Lei Zhang, Weiqing Han, Yuanlong Li, and Eric D. Maloney

Abstract

Air–sea coupling processes over the north Indian Ocean associated with the Indian summer monsoon intraseasonal oscillation (MISO) are investigated. Observations show that MISO convection anomalies affect underlying sea surface temperature (SST) through changes in surface shortwave radiation and surface latent heat flux. In turn, SST anomalies may also affect the MISO precipitation tendency (dP/dt). In particular, warm (cold) SST anomalies can contribute to increasing (decreasing) precipitation rate through enhanced (suppressed) surface convergence associated with boundary layer pressure gradients. These air–sea interaction processes are manifest in a quadrature relation between MISO precipitation and SST anomalies. A local air–sea coupling model (LACM) is formulated based on these observed physical processes. The period of the LACM is proportional to the square root of seasonal mixed layer depth H, assuming other physical parameters remain unchanged. Hence, LACM predicts a relatively short (long) MISO period over the north Indian Ocean during the May–June monsoon developing (July–August monsoon mature) phase when H is shallow (deep). This result is consistent with observed MISO characteristics. A 30-day-period oscillating external forcing is also added to the LACM, representing intraseasonal oscillations propagating from the equatorial Indian Ocean to the north Indian Ocean. It is found that resonance will occur when H is close to 25 m, which significantly enhances the MISO amplitude. This process may contribute to the higher MISO amplitude during the monsoon developing phase compared to the mature phase, which is associated with the seasonal cycle of H.

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J. Li, D. J. W. Geldart, and Petr Chýlek

Abstract

To investigate the photon transport in inhomogeneous clouds, a Monte Carlo cloud model with internal variation of optical properties is developed. The data for cloud vertical internal inhomogeneity are chosen from published observations. Parameterization of the solar radiative properties of clouds is used in the form of the liquid water content and the effective radius of cloud droplet. The Monte Carlo simulations show that for overcast stratocumulus clouds, the differences in reflectance between the vertical inhomogeneous clouds and their planeparallel counterpart are very small (only about 1%). These differences can be enhanced up to 10% for large solar zenith angles, when the overcast clouds are separated into broken cloud fields. If the cloud coverage is large, the vertical inhomogeneity of clouds can cause about 7% increase in cloud absorption, which may help to explain the cloud absorption anomaly. Also, the parameterization of effective cloud amount for cloud absorption is discussed.

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Yi Chao, Zhijin Li, John D. Farrara, and Peter Hung

Abstract

A two-dimensional variational data assimilation (2DVAR) method for blending sea surface temperature (SST) data from multiple observing platforms is presented. This method produces continuous fields and has the capability of blending multiple satellite and in situ observations. In addition, it allows specification of inhomogeneous and anisotropic background correlations, which are common features of coastal ocean flows. High-resolution (6 km in space and 6 h in time) blended SST fields for August 2003 are produced for a region off the California coast to demonstrate and evaluate the methodology. A comparison of these fields with independent observations showed root-mean-square errors of less than 1°C, comparable to the errors in conventional SST observations. The blended SST fields also clearly reveal the finescale spatial and temporal structures associated with coastal upwelling, demonstrating their utility in the analysis of finescale flows. With the high temporal resolution, the blended SST fields are also used to describe the diurnal cycle. Potential applications of this SST blending methodology in other coastal regions are discussed.

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J. Li, D. J. W. Geldart, and Petr Chýlek

Abstract

For a vertical homogeneous plane-parallel layer with horizontal cosinusoidal periodic variations of the extinction coefficient, k = k 0{1 + ε[cos(ax) + cos(by)]}, the first-order perturbation solution of the three-dimensional radiative transfer equation has been obtained. The first-order perturbation correction in cloud albedo cancels when a horizontal domain averaging is done. A correspondence exists between the distribution of the extinction coefficient and the distribution of the upwelling intensity. However, under certain conditions, the distribution of the upwelling intensity is opposite to the distribution of the extinction coefficient. If the solar zenith angle is large, shifts in the configurations of the distribution of the upwelling intensity may appear. The single scattering parameters can influence the distribution of the diffuse radiative intensity. The distribution of the heating rate inside the cloud and the distribution of the extinction coefficient are nearly coincident with each other.

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Song Yang, X. Ding, D. Zheng, and Q. Li

Abstract

Several advanced analysis tools are applied to depict the time–frequency characteristics of the variations of Great Plains (GP) precipitation and its relationship with tropical central-eastern Pacific Ocean sea surface temperature (SST). These tools are advantageous because they reveal the detailed features of the dominant time scales of precipitation variations, the combined effects of multiscale oscillating signals on the intensity of precipitation, and the variations of SST–precipitation relationships in time and frequency domains. The variability of GP precipitation is characterized by strong annual and semiannual signals, which have the most stable oscillating frequencies and the largest amplitudes. However, nonseasonal signals, which are less oscillatory and have smaller amplitudes and more variable frequencies with time, also contribute significantly to precipitation variability and may modify the seasonal cycle of GP precipitation. The phase of these nonseasonal signals is in phase (out of phase) with that of seasonal signals during the periods of heavy (deficient) precipitation. Significant correlations exist between GP precipitation and Niño-3.4 SST, and the strongest relationship appears when the SST leads the precipitation by 1 month. The GP precipitation increases (decreases) during El Niño (La Niña) episodes. Significant relationships appear on semiannual and annual time scales in the 1950s and on interannual time scales in the 1910s, 1940s, and 1980s. A particularly significant relationship appears on biennial time scales in the 1980s. The revealed SST–precipitation relationship is strongly seasonally dependent, with the greatest significance in summer. Warming of tropical central-eastern Pacific SST weakens the overlying easterly trade winds and strengthens the northward moisture supply from Central America through the Gulf of Mexico to the Great Plains. This dominant SST influence prevails in all seasons. However, the moisture transport from the southwest coast and the Gulf of California also contributes to the variability of GP precipitation in September–November, December–February, and March–May. In June–August, the increase in GP precipitation is caused by convergence between anomalous northerly flow over the northern plains, associated with the warming in the northeastern Pacific, and southerly flow over the southern plains, associated with the warming in the tropical central-eastern Pacific.

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Zhanqing Li, H. O. Leighton, and Robert D. Cess

Abstract

A parameterization that relates the reflected solar flux at the top of the atmosphere to the net solar flux at the surface in terms of only the column water vapor amount and the solar zenith angle was tested against surface observations. Net surface fluxes deduced from coincidental collocated satellite-measured radiances and from measurements from towers in Boulder during summer and near Saskatoon in winter have mean differences of about 2 W m−2, regardless of whether the sky is clear or cloudy. Furthermore, comparisons between the net

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Yajie Li, Amanda Lee Hughes, and Peter D. Howe

Abstract

Message diffusion and message persuasion are two important aspects of success for official risk messages about hazards. Message diffusion enables more people to receive lifesaving messages, and message persuasion motivates them to take protective actions. This study helps to identify win–win message strategies by investigating how an underexamined factor, message content that is theoretically important to message persuasion, influences message diffusion for official risk messages about heat hazards on Twitter. Using multilevel negative binomial regression models, the respective and cumulative effects of four persuasive message factors—hazard intensity, health risk susceptibility, health impact, and response instruction—on retweet counts were analyzed using a dataset of heat-related tweets issued by U.S. National Weather Service accounts. Two subsets of heat-related tweets were also analyzed: 1) heat warning tweets about current or anticipated extreme heat events and 2) tweets about nonextreme heat events. This study found that heat-related tweets that mentioned more types of persuasive message factors were retweeted more frequently, and so were two subtypes of heat-related tweets. Mentions of hazard intensity also consistently predicted increased retweet counts. Mentions of health impacts positively influenced message diffusion for heat-related tweets and tweets about nonextreme heat events. Mentions of health risk susceptibility and response instructions positively predicted retweet counts for tweets about nonextreme heat events and tweets about official extreme heat warnings, respectively. In the context of natural hazards, this research informs practitioners with evidence-based message strategies to increase message diffusion on social media. Such strategies also have the potential to improve message persuasion.

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