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Christopher S. Velden

Abstract

The evolution of upper-tropospheric thermal patterns associated with extratropical cyclone events is often not well represented by the conventional observational network, especially in marine situations. In this paper, a potential tool for qualitatively analyzing tropopause-level thermal structure and variations based on remotely sensed passive microwave data from satellites is examined. Specifically, warm anomalies associated with tropopause undulations in upper-tropospheric waves are captured in imagery from the 54.96-GHz channel of the Microwave Sounding Unit (MSU) onboard the current series of NOAA polar-orbiting satellites. Examples of this imagery during selected western North Atlantic cyclone events are presented, and the potential usefulness of these observations in analysis and forecasting is discussed.

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Jeffrey Hawkins and Christopher Velden

Atmospheric and oceanographic field experiments are an important part of research programs aimed at enhancing observational analyses of meteorological and oceanic phenomena, validating new datasets, and/or supporting hypotheses. The Bulletin of the American Meteorological Society (BAMS) has chronicled many field programs, with a primary focus on the enhanced observational assets that were assembled to enable the projects' investigations. However, these field program summaries often overlook the multiple roles that satellite digital data, multispectral imagery, and derived products can play in premission planning, real-time forecasting and mission guidance, and extensive post–field phase analysis. In turn, these intensive observing periods often serve as crucial validation datasets for remotely sensed products and derived fields.

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Christopher S. Velden

Abstract

Passive microwave observations from the current NOAA series of polar-orbiting satellites of a large sample of North Atlantic tropical cyclones are qualitatively and quantitatively analyzed. Microwave observations can penetrate the cloud cover associated with tropical cyclones and capture the upper-level warm temperature anomaly, which is characteristic of these storms. The data are used to develop a statistical algorithm for estimating surface intensity. Based upon hydrostatic assumptions, linear regression relationships are developed between the satellite-depicted horizontal temperature gradient of the upper-level warm core (ΔT 250), and the surface intensity (ΔP SFC) as measured by reconnaissance reports. A good correlation is found to exist. Results indicate that standard errors of estimate of 8 mb and 13 kts are found for surface pressure and maximum winds, respectively. These errors are reduced when the effects of storm latitude, eye size, and surface-pressure tendency on the relationship are included. Knowledge gained in examining the accuracies and limitations of the current microwave sounders in tropical cyclone applications will be helpful in setting quantitative observational guidelines for future instruments.

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John Sears and Christopher S. Velden

Abstract

Fields of atmospheric motion vectors (AMVs) are routinely derived by tracking features in sequential geostationary satellite infrared, water vapor, and visible-channel imagery. While AMVs produced operationally by global data centers are routinely evaluated against rawinsondes, there is a relative dearth of validation opportunities over the tropical oceans—in particular, in the vicinity of tropical disturbances when anomalous flow fields and strongly sheared environments commonly exist. A field experiment in 2010 called Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) was conducted in the tropical west Atlantic Ocean and provides an opportunity to evaluate the quality of tropical AMVs and analyses derived from them. The importance of such a verification is threefold: 1) AMVs often provide the only input data for numerical weather prediction (NWP) over cloudy areas of the tropical oceans, 2) NWP data assimilation methods are increasingly reliant on accurate flow-dependent observation-error characteristics, and 3) global tropical analysis and forecast centers often rely on analyses and diagnostic products derived from the AMV fields. In this paper, the authors utilize dropsonde information from high-flying PREDICT aircraft to identify AMV characteristics and to better understand their errors in tropical-disturbance situations. It is found that, in general, the AMV observation errors are close to those identified in global validation studies. However, some distinct characteristics are uncovered in certain regimes associated with tropical disturbances. High-resolution analyses derived from the AMV fields are also examined and are found to be more reflective of anomalous flow fields than the respective Global Forecast System global model analyses.

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Christopher S. Velden and John Sears

Abstract

Vertical wind shear is well known in the tropical cyclone (TC) forecasting community as an important environmental influence on storm structure and intensity change. The traditional way to define deep-tropospheric vertical wind shear in most prior research studies, and in operational forecast applications, is to simply use the vector difference of the 200- and 850-hPa wind fields based on global model analyses. However, is this rather basic approach to approximate vertical wind shear adequate for most TC applications? In this study, the traditional approach is compared to a different methodology for generating fields of vertical wind shear as produced by the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (CIMSS). The CIMSS fields are derived with heavy analysis weight given to available high-density satellite-derived winds. The resultant isobaric analyses are then used to create two mass-weighted layer-mean wind fields, one upper and one lower tropospheric, which are then differenced to produce the deep-tropospheric vertical wind shear field. The principal novelty of this approach is that it does not rely simply on the analyzed winds at two discrete levels, but instead attempts to account for some of the variable vertical wind structure in the calculation. It will be shown how the resultant vertical wind shear fields derived by the two approaches can diverge significantly in certain situations; the results also suggest that in many cases it is superior in depicting the wind structure's impact on TCs than the simple two-level differential that serves as the common contemporary vertical wind shear approximation.

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Christopher S. Velden and Graham A. Mills

Abstract

On 1 December 1987, an unusual midlatitude cyclone affected much of southeastern Australia. The storm was characterized by unforced rapid deepening to a near record low (locally) mean sea-level pressure, high winds, anomalously cold surface temperatures, and near-record rainfall in some areas. The storm resulted in extensive damage, including a massive livestock kill. Comparison with storm tracks over southern Australia from the past 20 years shows that the path of this storm was quite unusual for this time of year.

Utilizing a series of analyses prepared from an incremental limited area data assimilation system, it is shown that: 1) an amplifying upper-tropospheric wave influenced the initial development and path of the cyclone as it crossed the southern coast of Australia, 2) transverse circulations associated with two juxtaposed upper-level jet streaks embedded in the wave focussed upper-level divergence and midlevel ascent over the low during its rapid intensification phase, and 3) a distinct upper-tropospheric isentropic potential vorticity maximum was identified well upstream of the developing low, but with no evidence of an extrusion of this air penetrating and enhancing the low-level circulation as has been found in other cases of rapid cyclogenesis.

Given that inadequate operational numerical weather prediction (NWP) guidance was partially to blame for the underforecast of this event, the operational limited area NWP forecasts are presented and compared with forecasts based on the research analyses from the assimilation system. 11 is shown that improved forecasts of cyclone intensification and of precipitation result when the model is initialized with the assimilation analyses. Further improvements are obtained when the grid resolution of the forecast model is increased. With the operational implementation of the assimilation system into the Australian Bureau of Meteorology (BOM) in 1989, the improved guidance resulting from the assimilated analyses is currently available to forecasters in Australia.

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Christopher S. Velden and William L. Smith

Abstract

NOAA satellite microwave soundings, which penetrate high clouds, delineate the development and dissipation of the upper tropospheric warm core associated with a tropical cyclone. The storm's “core” may be detected from microwave imagery. Vertical cross sections reveal the intensification of the upper tropospheric warm core as the storm developes, and the downward propagation of the warm core as the storm dissipates. Excellent correlation is found between the horizontal Laplacian of an upper tropospheric temperature field and the intensity of the storm, as categorized by its surface central pressure and maximum sustained wind speed at the eye wall. The microwave monitoring of tropical cyclones is achieved in real time at the University of Wisconsin's Space Science and Engineering Center through high-speed teleconnections to direct readout receiving systems at Wallops Island, Virginia and Redwood City, California.

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Anthony J. Wimmers and Christopher S. Velden

Abstract

Conventional methods of viewing and combining retrieved geophysical fields from polar-orbiting satellites often complicate the work of end users because of the erratic time differences between overpasses, the significant time gaps between elements of a composite image, or simply the different requirements for interpretation between contributing instruments. However, it is possible to mitigate these issues for any number of retrieved quantities in which the tracer lifetime exceeds the sampling time. This paper presents a method that uses “advective blending” to create high-fidelity composites of data from polar-orbiting satellites at high temporal resolution, including a characterization of error as a function of time gap between satellite overpasses. The method is especially effective for tracers with lifetimes of longer than 7 h. Examples are presented using microwave-based retrievals of total precipitable water (TPW) over the ocean, from the Cooperative Institute for Meteorological Satellite Studies (CIMSS) Morphed Integrated Microwave Imagery at CIMSS TPW product (MIMIC-TPW). The mean average error of a global 0.25° × 0.25° product at 1-h resolution is 0.5–2 mm, which is very reasonable for most applications.

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Robert T. Merrill and Christopher S. Velden

Abstract

Isentropic coordinate analyses of rawinsondes and cloud motion wind vectors derived from geostationary satellite imagery are employed to describe the three-dimensional upper-tropospheric and lower-stratospheric circulation attending western North Pacific Supertyphoon Flo during September 1990. Outflow from the storm is concentrated in several evolving channels in the horizontal. In terms of vertical structure, net outflow evaluated at 6° latitude (666 km) radius is found to occur at higher levels and over an increasing range of potential temperature θ as the tropical cyclone intensifies. Outflow on the equatorward side of the tropical cyclone tends to occur at greater θ values (higher altitudes) than poleward outflow. Potential vorticity also decreases within the corresponding isentropic layers associated with the outflow. The implications of the vertical variability of outflow structure in terms of the interactions between storm and environment, and effects on storm structural changes, are considered briefly.

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Christopher S. Velden and John A. Young

Abstract

The 1992/93 Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (COARE) was specifically designed to monitor multiscale interactions between the atmosphere and ocean over the western Pacific warm pool. To help meet this objective, satellite observations were used to augment the enhanced COARE conventional data array in both space and time.

In this paper the authors present a descriptive overview of convective cloud variability and sea surface temperature during the four-month intensive observational period (IOP) as revealed by satellite. Time series of Geostationary Meteorological Satellite infrared brightness temperatures are evaluated at selected equatorial locations in the western Pacific and eastern Indian Oceans. Intraseasonal modes of transient convection/cloudiness are revealed, with two eastward-propagating Madden-Julian oscillations identified. Spectral analysis on the time series data indicates that higher-frequency variations in regional convective activity are also found to occur.

Several satellite cloud signatures and patterns were detected during a strong west wind burst event in late December (1992), and these are described in detail. Time-composited sea surface temperature (SST) fields derived from satellite radiances indicate that significant regional variations in SST occurred during the passage of the west wind event. The satellite-derived SST fields compiled during the IOP are validated against in situ observations in the COARE domain, with a 0.25°C warm bias noted in the composited satellite data.

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