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The Southern Hemisphere subtropical convergence zones are important regions of rainfall in the subtropics. The south Indian Ocean convergence zone (SICZ) has the strongest seasonality and exhibits substantial interannual variability in strength and position during austral summer. On synoptic time scales, the SICZ is a preferred region for the formation of tropical–extratropical (TE) cloud bands with local maxima over the southern African mainland and Madagascar. This study investigates how the seasonality in satellite-observed cloud band frequency emerges from the interplay between the asynchronous seasonal cycles in convective instability and upper-level flow, as represented by reanalysis data. These atmospheric mean states are diagnosed with a gross convective instability metric and a method to distinguish between subtropical and eddy-driven jet axes. Month-by-month analysis of these diagnostics elucidates how mean-state perturbations during ENSO events modify cloud band likelihood. Typically, 150%–200% more cloud bands develop during La Niña seasons supported by 5°–10° latitudinal separation between the local subtropical and eddy-driven jets and higher values of convective instability, especially in semiarid parts of mainland southern Africa. During El Niño events, fewer cloud bands develop over southern Africa in a more convectively stable environment without a distinct subtropical jet. However, east of Madagascar cloud bands are 150% more likely. Plausible teleconnection pathways based on these ENSO-related perturbations are discussed. The paper concludes with a conceptual framing of the seasonal cycle in the mean-state pertinent to TE cloud band likelihood. [ABSTRACT FROM AUTHOR]

Part I of this two-part paper treats Tropical Rainfall Measuring Mission (TRMM) radar, passive microwave, and lightning observations in hurricanes individually. This paper (Part II) examines relationships between these parameters (and implications of the relationships). Quantitative relationships between lightning occurrence and 85-GHz brightness temperature, 37-GHz brightness temperature, and radar reflectivity in the mixed phase region are established separately for hurricane eyewall regions, inner rainband regions, and outer rainband regions; other tropical oceanic regions; and tropical continental regions. When any of the brightness temperature or radar parameters are held constant as controls, lightning is more frequent in hurricane outer rainbands than elsewhere over tropical oceans, and more frequent over continents than even in the outer rainbands. Reflectivity profiles associated with specific brightness temperatures are presented, demonstrating a link between high-altitude ice phase precipitation and 85-GHz scattering and a link between lower-altitude precipitation and 37-GHz scattering. Based on the combination of radar, passive microwave, and lightning observations, it is proposed that supercooled cloud water occurs preferentially in outer rainbands compared to other tropical oceanic precipitation. The suspected microphysical differences produce only subtle differences in the remote sensing parameters other than lightning. [ABSTRACT FROM AUTHOR]

This paper aims at separating the respective influences of tropical and midlatitude variability on the development and life cycle of tropical temperate troughs (TTTs) over southern Africa in austral summer (November-February). Cluster analysis is applied to 1971-2000 40-yr ECMWF Re-Analysis (ERA-40) daily outgoing longwave radiation (OLR) anomalies to identify TTTs and monitor tropical convection. The same analysis applied to the zonal wind stretching deformation at 200 hPa (ZDEF) characterizes midlatitude transient perturbations. Results based on the comparison between these two classifications first confirm that midlatitude baroclinic waves are a necessary condition for TTT development, but they are not sufficient. Roughly 40% of those occurring in austral summer are associated with a TTT. They tend to be stronger than the baroclinic waves not associated with TTT development. In the tropics, additional conditions needed to form a TTT consist of an excess of latent energy over the Mozambique Channel, mostly because of moisture advections and convergence from the Atlantic and Indian Oceans. Taken together, these conditions are highly favorable for deep atmospheric convection over and near southern Africa and seem to explain a large fraction of TTT variability. [ABSTRACT FROM AUTHOR]

In a recent article, Li et al. examined changes in the summer-season location of the western ridge of the North Atlantic subtropical high from 1948 to 2007 because there has been an increase in interannual summer rainfall variability in the southeastern United States. The following major conclusions by Li et al. are incorrect: the western ridge has undergone a significant westward trend since the late 1970s; the western ridge had increased meridional movement during 1978-2007 compared to 1948-1977; and global warming appears to be contributing to the westward expansion of the western ridge. Results presented in this paper reveal that the western ridge has been moving eastward over the past three decades, there was no change in latitudinal variance, and a westward movement of the western ridge should not be linked to global warming. [ABSTRACT FROM AUTHOR]

This paper describes an analysis of multiyear satellite datasets to characterize the modulations of convective versus stratiform rain, rain system size, and column water vapor by convectively coupled equatorial waves. Composites are built around space-time filtered equatorial-belt data from the Tropical Rainfall Measuring Mission (TRMM) 3B42 rainfall product, while TRMM Precipitation Radar (PR) and passive microwave data are the composited variables. The results are consistent with the more reanalysis-dependent findings in Part I, indicating that higher-frequency (or more divergent) waves, such as Kelvin and inertia-gravity families, modulate mesoscale convective systems and stratiform rain relatively more, whereas slower (or more rotational) types such as Rossby, mixed Rossby-gravity, and tropical depression (TD) or 'easterly' waves primarily modulate convective rain and smaller-sized precipitation systems. Column water vapor composites indicate that the more rotational wave types modulate the moisture field more pronouncedly than do the divergent waves, leading the authors to speculate that the slow/rotational versus fast/wavelike distinction in precipitation characteristics may correspond to the different balances of two main convective coupling mechanisms: moisture control of cumulus cells versus convective inhibition control (via low-level density waves) of mesoscale convective systems. The Madden-Julian oscillation (MJO) is unique in that it exhibits prominent modulation of both stratiform precipitation (like the fast divergent waves) and small-sized precipitation features, convective rainfall, and moisture (like the other low-frequency, rotational waves). A composite of other waves' amplitudes as a function of MJO amplitude and phase shows that divergent waves are more active in the developing phase and rotational waves are more active in the decaying rather than developing phase of the MJO. [ABSTRACT FROM AUTHOR]

This paper examines the sensitivity of short-term forecasts of the western North Pacific subtropical high (WNPSH) and rainfall to sea surface temperature (SST) uncertainty using the Coupled Ocean--Atmosphere Mesoscale Prediction System (COAMPS). A comparison of analyzed SSTs with satellite observations of SST indicates that SST analysis errors are particularly pronounced on horizontal scales from 100 to 200 km, similar to the mesoscale eddy scales in the Kuroshio region. Since significant oceanic variations occur on these scales, it is of interest to examine the effects of representing this small-scale uncertainty with random, scale-dependent perturbations. An SST ensemble perturbation generation technique is used here that enables temporal and spatial correlations to be controlled and produces initial SST fields comparable to satellite observations. The atmospheric model develops large uncertainty in the Korea and Japan area due to the fluctuation in the horizontal pressure gradient caused by the location of the WNPSH. This, in turn, increases the variance of the low-level jet (LLJ) over southeast China, resulting in large differences in the moist transport flux from the tropical ocean and subsequent rainfall. Validation using bin-mean statistics shows that the ensemble forecast with the perturbed SST better distinguishes large forecast error variance from small forecast error variance. The results suggest that using the SST perturbation as a proxy for the ocean ensemble in a coupled atmosphere and ocean ensemble system is feasible and computationally efficient. [ABSTRACT FROM AUTHOR]

This paper proposes a new method for investigating coupled land–atmosphere interactions. The method is to apply an empirical correction technique to distinct components of a model and then examine differences between forecasts of the empirically corrected models. The correction technique is based on adding a time-dependent term to the tendency equations that subtracts the estimated tendency error at every time step. This methodology can be interpreted more generally as a series of data assimilation experiments in which only certain components of a coupled model are assimilated at a time. The correction is applied to a state-of-the-art coupled land–atmosphere model in three different ways, namely, to the atmosphere only, to the land only, and to the land and atmosphere simultaneously. The land–atmosphere interactions are inferred from monthly-mean differences between experiments. The results suggest that the land–atmosphere coupling in midlatitudes can be understood from straightforward water balance considerations, whereas the coupling in the deep tropics involves a more complicated change in regional circulation. Specifically, in midlatitudes, moisture injected into the soil is transferred to the atmosphere directly above, which in turn advects downstream and subsequently moistens the atmosphere in the downwind regions to produce positive precipitation anomalies. In the deep tropics, the regional circulation, including precipitation, is sensitive to perturbations and has no obvious relation to corrections in the atmosphere or land. The similarity of biases among different models suggests that the conclusions and methodology may be relevant to other models. [ABSTRACT FROM AUTHOR]

Radar meteorologists have estimated, over a number of years, rainfall rate with ground-based radars. Since the 1970s, use of weather radar as an active technique to measure rainfall from space has been proposed, and a number of radar concepts, including modified radar altimeter design, were proposed. However, no attempt has yet been made to determine rain cell characteristics from available altimeter data. This paper presents a method to estimate rainfall rate and rain cell diameter from TOPEX/Poseidon dual frequency radar altimeter waveforms. The rain-affected waveforms are first selected using a criterion based on a departure from a normal Ku--C-band backscatter coefficient relationship and on a threshold of liquid water content estimated by the TOPEX microwave radiometer. The analytical representation of echo waveforms under rain conditions is used to model the effect of rain. A minimization procedure is then used to determine the model's best fit to the observed waveforms. Several case studies are presented to illustrate the method. These cases include squall lines, isolated light rain cells, and a tropical cyclone. The method, shown to give a high-resolution description of the rain distribution under various weather conditions, could be used to obtain global rain cell characteristics over the ocean. [ABSTRACT FROM AUTHOR]