1. Introduction

We have noted with interest the series of scatterometer data studies carried out by Chelton, Freilich, and colleagues [see, e.g., Chelton et al. (2004), Freilich and Dunbar (1999), and in particular, Chelton and Freilich (2005, hereafter CF05)]. In CF05, the authors addressed at least three pressing needs. They provided a much-needed quantification of the gross, or bulk, error statistics of scatterometer observations. Equally as significant, they also showed that the use of scatterometer observations improved the accuracy of 10-m wind analyses and predictions in two of the major global numerical weather prediction (NWP) systems, namely those of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP). Finally, CF05 presented a lucid and in-depth description of scatterometer instrument technology, which we suspect will make CF05 required reading for users of scatterometer observations. Before CF05, most of the literature on these three topics was confined to conference proceedings and various technical and other reports.

Here, we present work that is complementary to that of CF05. We describe our experience with scatterometer data in both operations and research over a specific part of the globe. Our operational and research goals are very different from the topics covered in CF05. Unlike CF05, we do not focus on the gross statistics of the scatterometer observations, but instead on the capacity of scatterometer data to detect, monitor, and provide more accurate analyses and predictions of individual severe and extreme weather events on a routine basis. We do so largely by describing two case studies, chosen from many that we have investigated, which demonstrate clearly the value of scatterometer observations for weather systems in data-sparse oceanic and coastal areas. We welcome comments from the authors of CF05 on the results we present below.

Our domain of interest is not global, but it is still a substantial region, covering the East Indian Ocean, the Southern Ocean, and the southwest Pacific Ocean, including the Tasman Sea. The region also spans a large range of latitudes, extending from the high latitudes, through the subtropics to the Tropics, and has a longitudinal extent from about 65° to 170°E. This region is very active meteorologically; however, it has been relatively neglected until recent years. It contains a number of countries that are routinely and adversely affected by severe weather, and there has long been exceptionally poor oceanic data coverage. The consequences of the data paucity include inaccurate initial analyses and subsequent operational forecast failures of NWP predictions on short-, medium-, and extended-range time scales. These failures usually occur when success is most needed. There have also been almost insurmountable problems in developing reliable climatologies, both atmospheric and oceanographic.

In this correspondence, we concentrate only on the intense cyclonic systems that regularly generate severe and extreme weather, as these have the most impact and usually present the greatest analysis and forecast difficulties. The cyclonic systems affecting the region include the midlatitude systems that cross the southern Indian Ocean between southern Africa and Australia, similar systems that traverse the Southern Ocean and southwest Pacific Ocean poleward of Australia and New Zealand, tropical cyclones in the east Indian Ocean, and subtropical and tropical cyclones in the Coral and Tasman Seas between Australia and New Zealand. Such a wide range of severe storms is a major analysis and prediction challenge, with the systems typically being either operationally underpredicted in peak intensity or even missed completely when they are of small size or are severe only for a relatively short part of their life cycle.

As CF05 indicate, prior to the availability of the scatterometer instruments numerical analysis, prediction, and verification over the oceans was severely limited. The lack of reliable, timely, and accurate observations of near-surface winds was an insurmountable problem because of the problems of identifying the position, track, size, intensity, and other characteristics of oceanic and coastal cyclonic weather systems. Moreover, the information from sequential passes of the scatterometer instruments now allows the observation of changes in these characteristics. This is highly valuable information that previously was virtually impossible to detect and especially to quantify. We therefore have a greatly increased capacity to follow, and eventually to understand, the evolution of this wide range of severe cyclonic storms.

We recently published a preliminary climatology, based heavily upon scatterometer and other satellite data, for south Indian Ocean and Southern Ocean midlatitude cyclones (Buckley and Leslie 2004). The QuikSCAT data have become sufficiently established to allow the development of this limited climatology, which is based on a careful interpretation of almost 5 yr of QuikSCAT scatterometer data. This climatology continues to be updated as the length of the record increases. We have almost completed a similar exercise for southwest Pacific Ocean cyclones. The south Indian and Southern Oceans have long been known to generate midlatitude marine cyclones of intensities seldom experienced in cyclonic systems that form away from the Tropics. Ship reports, buoy data, and the few available oil platform–, island-, and land-based station reports led to unsupported speculation amongst operational forecasters that these storms were far stronger than they had been defined in operational analyses, and in the few research studies available. The consequences of such an underestimation of the frequency and intensity of these cyclones have been profound for short- and medium-range NWP, and also for climate studies that employ these archived analyses. Chelton et al. (2004) clearly are aware of similar problems elsewhere. They listed a number of implications, including a lack of detail in the wind patterns both in the oceans and in coastal waters, leading to significant errors in computing the wind stresses that drive the ocean circulation and also feed back to the atmospheric models. It was not until the late 1990s, when scatterometer observations from the Seawinds scatterometer instrument aboard the National Aeronautics and Space Administration (NASA) QuikSCAT satellite became routinely available, that there was a sudden increase in regular and relatively reliable wind observations over these traditionally data-sparse oceanic areas. As a direct consequence, the above-mentioned suspicion that the cyclones were far stronger, and more frequent, than hitherto analyzed was confirmed. The scatterometer datasets now underpin the operational forecast and warning services of various national weather services that have high seas warning responsibilities in these waters.

Studies other than CF05 have also pointed out that accurate, near surface, and wind data are vital for the routine operational analysis and prediction of severe cyclonic systems, for case studies aimed at gaining increased knowledge of their life cycles, and for the compilation of climatological datasets. Chelton et al. (2004) have utilized the evolving high-resolution scatterometer dataset to identify persistent small-scale features in ocean surface winds, particularly those affected by proximity to the coast. Atlas (2004) has shown the importance of scatterometer data in numerical weather prediction, with the effects propagating well away from the swathe of the instrument within the first 6 to 18 h of the model prediction. Liu et al. (2004) have stressed the importance of high-resolution scatterometer wind vector observations in determining the ocean's response to high-resolution wind forcing.

To illustrate the improvement in analyzing and predicting severe cyclonic systems in our region of interest, we discuss briefly two case studies that illustrate the importance of the scatterometer data. This point is recognized only implicitly, rather than emphasized, in CF05. We present some ground truth observations from an island station and a gas production platform that support the growing confidence in the accuracy of the scatterometer data. A caveat is that the scatterometer-derived winds must be analyzed very carefully, with experience in the selection of the correct solution for the data and a detailed knowledge of the weather systems of interest both being important factors at this point in time.

2. Case studies

These cases of severe weather systems were detected and analyzed with a level of accuracy possible only since the advent of scatterometer data. Case 1 is an intense cyclone in the Southern Ocean, whereas case 2 is a severe tropical cyclone that traversed the near-coastal waters of the tropical east Indian Ocean over northwest Western Australia. Both cases were chosen because they provide rare examples of where high quality ground truth observations were available to verify the intensity and some features of these systems.

3. Conclusions

CF05 stressed the importance of scatterometer-derived data by quantifying the accuracy of the observations, by demonstrating the positive impact of these data on the skill of global weather analysis and prediction models, and by providing a detailed description the functioning and characteristics of the scatterometer instrument and the data it generates. Here, we describe a very different role of scatterometer data that we believe to be equally as important. Our aim is to demonstrate the large positive impact of scatterometer data in improving the routine analysis and prediction of the tracks, intensities, and life cycles of individual severe and extreme atmospheric and oceanic systems over some of the world's most data-sparse ocean and coastal regions. Specifically, our work concentrates on the intense midlatitude cyclones, subtropical cyclones, and tropical cyclones that form over and traverse the East Indian Ocean, Southern Ocean, and southwest Pacific Ocean. We have shown, by comparison with ground-based observations, that the scatterometer data is very accurate and that its inclusion in numerical analyses has an impact on all time scales, from hours and days, through to the development of climatologies. In particular, the most powerful of the midlatitude storms, which we refer to here as super cyclones, is a primary focus of our work, as they have only recently been accepted as reaching the intensities that have been suspected for some time. We have found support in the work of Chelton, Freilich, and colleagues that scatterometer data, despite several known remaining problems, have now made possible a large step toward providing the accurate and timely warnings necessary to minimize loss of life and damage to property that can be caused by such severe systems. We have provided support for this view in the form of two examples in which ground truth was available for validation of the scatterometer data. These warnings are a vital and immediate requirement owing to their immense destructive potential and the fact that they form, develop, and move over data-sparse oceans before they make landfall. The continued development of techniques for optimizing the application of scatterometer-derived data is central to continued success.