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Max Popp and Levi G. Silvers


A major bias in tropical precipitation over the Pacific in climate simulations stems from the models’ tendency to produce two strong distinct intertropical convergence zones (ITCZs) too often. Several mechanisms have been proposed that may contribute to the emergence of two ITCZs, but current theories cannot fully explain the bias. This problem is tackled by investigating how the interaction between atmospheric cloud-radiative effects (ACREs) and the large-scale circulation influences the ITCZ position in an atmospheric general circulation model. Simulations are performed in an idealized aquaplanet setup and the longwave and shortwave ACREs are turned off individually or jointly. The low-level moist static energy (MSE) is shown to be a good predictor of the ITCZ position. Therefore, a mechanism is proposed that explains the changes in MSE and thus ITCZ position due to ACREs consistently across simulations. The mechanism implies that the ITCZ moves equatorward if the Hadley circulation strengthens because of the increased upgradient advection of low-level MSE off the equator. The longwave ACRE increases the meridional heating gradient in the tropics and as a response the Hadley circulation strengthens and the ITCZ moves equatorward. The shortwave ACRE has the opposite effect. The total ACRE pulls the ITCZ equatorward. This mechanism is discussed in other frameworks involving convective available potential energy, gross moist stability, and the energy flux equator. It is thus shown that the response of the large-scale circulation to the shortwave and longwave ACREs is a fundamental driver of changes in the ITCZ position.

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Levi G. Silvers and Wayne H. Schubert


The subject of this study is topographically bound low-level jets, such as the South American summertime low-level jet on the eastern side of the Andes and its companion, the Chilean low-level jet on the western side of the Andes. These jets are interpreted as balanced flows that obey the potential vorticity invertibility principle. This invertibility principle is expressed in isentropic coordinates, and the mathematical issue of isentropes that intersect the topography is treated by the method of a massless layer. In this way, the low-level jets on the western and eastern sides of the Andes can both be attributed to the infinite potential vorticity that lies in the infinitesimally thin massless layer on the topographic feature. To obtain a cyclonic flow centered on the topographic feature, the mountain crest must have been heated enough to draw down the overlying isentropic surfaces; otherwise, isentropic surfaces bend upward at the mountain crest and an anticyclonic flow is produced. Both anticyclonic and cyclonic solutions are obtained here using analytical and numerical methods to solve the invertibility principle. The summertime topographically bound flows discussed here are quite distinct from the wintertime Rossby wave train patterns that occur when strong westerlies impinge on the topography.

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Gan Zhang, Levi G. Silvers, Ming Zhao, and Thomas R. Knutson


Earlier studies have proposed many semi-empirical relations between climate and tropical cyclone (TC) activity. To explore these relations, this study conducts idealized aqua-planet experiments using both symmetric and asymmetric sea surface temperature (SST) forcings. With zonally symmetric SST forcings that have a maximum at 10°N, reducing meridional SST gradients around an Earth-like reference state leads to a weakening and southward displacement of the intertropical convergence zone. With nearly flat meridional gradients, warm-hemisphere TC numbers increase by nearly 100 times due particularly to elevated high-latitude TC activity. Reduced meridional SST gradients contribute to an poleward expansion of the tropics, which is associated with a poleward migration of the latitudes where TCs form or reach their lifetime maximum intensity. However, these changes cannot be simply attributed to the poleward expansion of Hadley circulation. Introducing zonally asymmetric SST forcings tends to decrease the global TC number. Regional SST warming—prescribed with or without SST cooling at other longitudes—affects local TC activity but does not necessarily increase TC genesis. While regional warming generally suppresses TC activity in remote regions with relatively cold SSTs, one experiment shows a surprisingly large increase of TC genesis. This increase of TC genesis over relatively cold SSTs is related to local tropospheric cooling that reduces static stability near 15°N and vertical wind shear around 25°N. Modeling results are discussed with scaling analyses and have implications for the application of the “convective quasi-equilibrium and weak temperature gradient” framework.

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Songmiao Fan, Paul Ginoux, Charles J. Seman, Levi G. Silvers, and Ming Zhao


Mixed-phase clouds are frequently observed in the atmosphere. Here we present a parameterization for ice crystal concentration and ice nucleation rate based on parcel model simulations for mixed-phase stratocumulus clouds, as a complement to a previous parameterization for stratus clouds. The parcel model uses a singular (time independent) description for deposition nucleation and a time-dependent description for condensation nucleation and immersion freezing on mineral dust particles. The mineral dust and temperature-dependent parameterizations have been implemented in the Geophysical Fluid Dynamics Laboratory atmosphere model, version 4.0 (AM4.0) (new), while the standard AM4.0 (original) uses a temperature-dependent parameterization. Model simulations with the new and original AM4.0 show significant changes in cloud properties and radiative effects. In comparison to measurements, cloud-phase (i.e., liquid and ice partitioning) simulation appears to be improved in the new AM4.0. More supercooled liquid cloud is predicted in the new model, it is sustained even at temperatures lower than −25°C unlike in the original model. A more accurate accounting of ice nucleating particles and ice crystals is essential for improved cloud-phase simulation in the global atmosphere.

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