Geostationary Satellite Imagery Research Articles

The North Pacific Experiment (NORPEX-98): Targeted Observations for Improved North American Weather Forecasts

Abstract The objectives and preliminary results of an interagency field program, the North Pacific Experiment (NORPEX), which took place between 14 January and 27 February 1998, are described. NORPEX represents an effort to directly address the issue of observational sparsity over the North Pacific basin, which is a major contributing factor in short-range (less than 4 days) forecast failures for land-falling Pacific winter-season storms that affect the United States, Canada, and Mexico. The special observations collected in NORPEX include approximately 700 targeted tropospheric soundings of temperature, wind, and moisture from Global Positioning System (GPS) dropsondes obtained in 38 storm reconnaissance missions using aircraft based primarily in Hawaii and Alaska. In addition, wind data were provided every 6 h over the entire North Pacific during NORPEX, using advanced and experimental techniques to extract information from multispectral geostationary satellite imagery. Preliminary results of NORPEX dat...

Land-surface–atmosphere interactions associated with deep convection in Illinois

Geostationary operational environmental satellite (GOES East) imagery are used to investigate a possible spatial clustering of free convective cloud masses along land-cover-type boundaries in the state of Illinois on days with weak synoptic-scale flow. Soil moisture data and a surface cover-type classification system are employed to identify likely cloud formation zones (as a function of non-classical mesoscale circulations (NCMCs)) forced by heterogeneous surface fluxes of sensible and latent heat. The emphasis of this research is on documenting the existence of convective cloud clusters and determining locations where they would be expected to occur if land-surface–atmosphere forcing were involved. This research shows that a statistically significant spatial clustering of free convective cloud masses does exist, and that these clusters occur along land-cover-type and soil-order boundaries. The three most conspicuous locations for both the initiation and vertical enhancement of convective clouds are along: (i) the agricultural and mixed forest–agricultural boundary that extends west–east from near St. Louis, Missouri, to Terre Haute, Indiana; (ii) along the urban/suburban Chicago metropolitan area and agricultural boundary in northeast Illinois; and (iii) along the agricultural and agricultural–mixed forest boundary along the Illinois River in east-central Illinois. It is also apparent from composite rawinsonde soundings that a majority of free convective cloud masses develop along land-cover-type boundaries on weak synoptic-scale flow days when planetary boundary layer moisture levels are large. This provides further support for the proposal that non-classical mesoscale circulations (NCMCs) provide the mass convergence necessary to force moist air parcels to the level of free convection along land-cover-type boundaries. These results also suggest that human-modification of the Earth'rsquo;s surface may play a significant role in local-scale climate change. © 1998 Royal Meteorological Society

PICTURE OF THE QUARTER: Nighttime Detection of Low-Level Thunderstorm Outflow Using a GOES Multispectral Image Product

Low-level thunderstorm outflow (hereafter referred to as LTO) boundaries can interact with the environment or with other storms in a variety of ways. Studies have shown that convection can be initiated, intensified, or weakened through interaction with an LTO boundary depending on local environmental conditions (Purdom 1976; Weaver 1979; Maddox et al. 1980; Wilson and Wilk 1981; Weaver and Nelson 1982; Wilson and Schreiber 1986; Weaver and Purdom 1995). The interface separating the environmental and LTO air masses can often be detected by Doppler radar (Wilson and Wilk, 1981). Although radar’s ability to detect low-level boundaries is range limited, when boundaries are detectable by Weather Surveillance Radar-1988 Doppler (WSR-88Ds) they can be observed every 5–6 min. Cloud lines generated along LTO boundaries can also be detected using satellite imagery. Those cloud lines, sometimes called arc clouds, frequently develop as a curved line of clouds along the boundary interface (Purdom 1973). In satellite imagery, stratiform low-level billow clouds (sometimes referred to as stable wave clouds) have also been observed to form at the top of the cold-air dome associated with the LTO. Geostationary satellite imagery can be used to observe the development and evolution of low-level cloud boundaries across a broad range of scales, including the mesoscale.

Mesoanalysis of Summertime Convergence Zones in Central and Eastern North Carolina

Abstract It is demonstrated that it is possible to perform informative mesoanalysis of summertime convergence boundaries in the southeastern United States by combining capabilities of the new WSR-88D Doppler radar with Geostationary Operational Environmental Satellite imagery and conventional surface data. Observed phenomena are identified as thunderstorm outflow boundaries, sea-breeze fronts, horizontal convective rolls, deep synoptic-scale fronts, prefrontal troughs, shallow fronts (airmass boundaries lacking upper-level support), stationary and propagating boundaries of unknown origin, and the “Piedmont trough,” which is apparently a new feature discovered in the course of this research. The transition zone between the Piedmont and the Coastal Plain was found to be a preferred location for convergence boundaries. An unexpectedly far inland advance of the sea breeze to central North Carolina occurred in some instances. The very sensitive “clear air mode” of the WSR-88D radar, when used in combination wi...

The Generation and Assimilation of Cloud-Drift Winds in Numerical Weather Prediction (gtSpecial IssueltData Assimilation in Meteology and Oceanography: Theory and Practice)

This paper describes the methods used to produce cloud-drift winds (CDWs), concentrating, in particular, on their generation from sequential Geostationary Meteorological Satellite (GMS) imagery. It discusses the estimation of these motion vectors from both infrared (IR) and visible imagery at high spatial and temporal resolution and also records their accuracy and utility. The paper then discusses the assimilation of CDW data for numerical weather prediction (NWP). It does this by looking at studies, both in the Australian Region and over a larger domain, showing the impact of CDWs on operational NWP using current conventional data assimilation techniques. Subsequent to this, the use of CDWs is examined in the context of tropical cyclone motion prediction, where intermittent assimilation, nudging and the use of a full variational technique are contrasted, using examples from the Tropical Cyclone Motion-90 (TCM-90) experiment in the tropical North-West Pacific and by examining the impact of hourly CDWs in the Australian region. It was found that the high spatial and temporal resolution winds clearly have the potential to improve the accuracy of NWP, however, full exploitation of their information content appears to require appropriate assimilation techniques such as the variational method employed here.

Links between Tropical Convection and Variations of the Extratropical Circulation during TOGA COARE

Data from the Intensive Observation Period of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (November 1992-February 1993) have been used to investigate the links between intraseasonal variations in tropical convection and those in forcing of upper-tropospheric Rossby waves in the extratropics. The primary databases are Geostationary Meteorological Satellite imagery and tropical wind analyses from the Bureau of Meteorology, Australia. A number of 5-day periods showing convection in different locations were chosen. For each period, mean fields of divergence, cloud-top temperature, and upper-tropospheric Rossby wave source are presented. Vorticity budgets are used to demonstrate the processes responsible for the Rossby wave source patterns. The approach follows earlier studies of links between interannual variations of tropical convection associated with the Southern Oscillation and variations of the extratropical circulation. It is shown that the regions of tropical convection correspond to longitudinally localized Hadley cells. In the subtropics, at the higher-latitude end of each cell, there is a Rossby wave source dipole with anticyclonic and cyclonic forcing. The anticyclonic forcing of Rossby waves is associated with advection of vorticity by the divergent outflow, while the cyclonic forcing is due to the region of convergence immediately above the downward branch of the local Hadley cell. Hence, the authors provide a dynamical basis for tropical-midlatitude interactions associated with intraseasonal variations of tropical convection.

A Three-Dimensional Analysis of the Outflow Layer of Supertyphoon Flo (1990)

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 consi...

The “Striated Delta” Cloud System–A Satellite Imagery Precursor to Major Cyclogenesis in the Eastern Australian-Western Tasman Sea Region

A distinctive upper-tropospheric cloud pattern, frequently a precursor to major extratropical cyclogenesis within the eastern Australian–western Tasman Sea region, has been identified from an examination of Geostationary Meteorological Satellite imagery. This cloud feature is usually triangular- (or delta-) shaped, approximately symmetrical, composed of cirrus level cloud tops, and associated with transverse banding. Thirty-seven of these cloud systems, referred to as “striated deltas” (SDs), were detected within the study region between 1980 and 1991. Approximately 75% of these were associated with the development of storms of at least gale force intensity within 24 h of formation of the SD. Most of these intense developments were characterized by a 12-h period of explosive deepening. Three types of SD development are observed. The cloud system usually forms as a discrete feature; this process can be either rapid or gradual. The SD can also form as a subcomponent of a major meridional cloud band. Striated deltas associated with intense cyclogenesis are normally generated east of a high-amplitude 1000-500-hPa thickness trough and within diffluent north to northwesterly flow in the exit region of a strong thickness jet streak. The relationship between the SD and the surface pattern differs substantially between oceanic and continental event. Case studies in which major surface low deepening occurred are presented to demonstrate these variations. Events in which intense cyclogenesis does not occur are also examined. The synoptic and geographic environment in which such cloud systems form are found to vary from that observed in intense developments. A qualitative investigation into the source mechanism of the striations suggests that their most probable cause is gravity wave activity due to the geostrophic adjustment process.

The Preconvective Environment of Summer Thunderstorms over the Florida Panhandle

The preconvective environment of summer thunderstorms over the Florida Panhandle is investigated. Geostationary satellite imagery as well as surface and radiosonde data were examined during the summers of 1990 and 1991. Days were classified either as synoptically disturbed or undisturbed based on the imagery. The undisturbed days then were subjectively subdivided into those having strong, weak, or no convection. Composite sounding profiles of various meteorological parameters were constructed for each category of the undisturbed days. Composites of various stability indexes also were calculated. Midtropospheric moisture (particularly from 700 to 500 mb) and low-level instability were the best thermodynamic parameters for forecasting convection over the Florida Panhandle. The surface-based lifted index was the most useful stability index for predicting convective development. Wind direction also was related to the degree of convective activity in the Florida Panhandle. The strong convection days tended to have low-level winds from the south or southwest. Low-level winds on the driest days generally had northerly and easterly components.

Classification-Based Rainfall Estimation Using Satellite Data and Numerical Forecast Model Fields

Using Global Precipitation Climatology Project data gathered during June, July, and August 1989 over Japan, rainfall estimates are examined from both geostationary satellite imagery using a multifeature classification approach, and from short-term weather prediction model fields. Additionally, the utility of combining model forecast information within such a classifier to improve the final estimate is investigated. During both months satellite estimates are superior to model forecasts in detecting heavy rain events associated with extremely cold cloud tops, and in identifying cloud-free regions. Model estimates are superior to satellite retrievals in terms of dynamic range and regional bias. Addition of visible data to an infrared-only scheme improved monthly rainfall estimates during June, and hourly estimates during both months. In June it is shown that a combined satellite-model method clearly yields improved retrievals of rainfall relative to those obtained by using either satellite data or model forecasts alone at both monthly and hourly time scales. However, in July and August, satellite retrievals largely underestimated monthly rainfall, and the model produced poor hourly forecasts. Generally, a good model forecast of rainfall can enhance the satellite estimate, while a poor forecast will degrade it. If results obtained during June are found to be valid for other regions of the globe, such a method could be used to develop rainfall climatologies. It could also be used in a real-time operational numerical weather prediction environment since it is computationally rapid, with only geostationary satellite observations and model-predicted fields needed to derive the estimates.

The Mexican Monsoon

Abstract The pronounced maximum in rainfall during the warm season over southwestern North America has been noted by various investigators. In the United States this is most pronounced over New Mexico and southern Arizona; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the “Mexican monsoon,” is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data. In particular, the authors note the geographical extent and magnitude of the summer rains, the rapidity of their onset, and the timing of the month of maximum rainfall. Finally, the difficulty in explaining the observed precipitation distribution and its timing from monthly mean upper-air wind and moisture patterns is discussed.

Use of Enhanced IR/Visible Satellite Imagery to Determine Heavy Snow Areas

Abstract Interpretation techniques are established which relate heavy snow (100 mm or more in a 12 h period) areas to real-time infrared and visible geostationary satellite imagery. An initial collection of cases totaled about 75 during the period from the fall of 1979 to the spring of 1984. The main area of concern extended from the Rockies to the Appalachians. By using conventional surface and upper air data and numerical model output from the LFM-II relationships were established between heavy snow occurrences and cloud patterns in the satellite imagery. The different cloud patterns and flow regimes suggest a classification of heavy snow events into four main types: High Plains cyclogenesis, northwest and southwest flow short-wave troughs, and orographic. Each type has preferred geographical locations, common snowfall durations, storm totals, and band widths and lengths. The snowfall maximum with each storm is influenced by the strength and movement of the system and the structure of the moist layer. S...

The Relationship between Large-Scale Convective Rainfall and Cold Cloud over the Western Hemisphere during 1982-84

Estimates of areal- and time-averaged convective precipitation derived from geostationary satellite imagery using a simple thresholding technique are presented. The estimates are based on measurements of the monthly mean fraction of 2.5° × 2.5° areas covered by clouds whose equivalent blackbody temperature in infrared imagery is below 235 K. The transformation between fractional coverage and rainfall amount is based upon comparisons of fractional coverages using a variety of temperature thresholds and spatial and temporal averaging scales with areal averaged rainfall from the GARP Atlantic Tropical Experiment. Three-year means of the estimated precipitation for the period December 1981-November 1984 are shown for each of the (3-month) calendar seasons and compared with published descriptions of the long-term seasonal mean rainfall fields. Over the tropical oceans agreement is quite good with no evidence of any systematic errors. Over the Americas, long-term means derived from station observations of rainfall show less extensive areas of heavy rainfall than those derived here, and a slight tendency for lower peak values during the rainy season. The interannual variability during the 3-yr period is described and compared with station observations of rainfall. The relationship between cloud cover and rainfall in the tropics (30d°N-30°S) is found to be similar to that found in previous studies, with a threshold of 235 K giving highest correlations, while observations between 30° and 50° were best correlated with a threshold of 220 K. The large changes in rainfall distribution over South America associated with the 1982-83 ENSO episode and the breaking of the drought in Northeast Brazil during 1984 are clear in the estimates presented here, but the amplitude of the changes is somewhat over-estimated. Warm season rainfall observed over the United States is less than the estimates, except near the Gulf of Mexico and southeast United States coast where the degree of overestimation increases away from the coast.

Grid History: A Geostationary Satellite Technique for Estimating Daily Rainfall in the Tropics

Abstract A new technique is described for estimating daily rainfall by means of visible and infrared geostationary satellite imagery. It is designed for the tropics and warm-season midlatitudes. Because it operates on a grid of points and measures time changes at these points, the technique has been named “grid history.” It is assumed that at any grid point in some image belonging to a sequence, by means of spectral, textural and evolutionary information, it is possible to classify instantaneous rain rate as nil, light, moderate or heavy. Then the total rainfall over a day is the sum over three classes of the product of frequency and class average rate. The class average rates have been determined by least-squares multivariate linear regression of frequencies on observed rainfalls. The areas treated are South China Sea, India, Arabian Sea, tropical North Atlantic Ocean and Amazonia. Inland India had the lowest (driest) class average rates (coefficients), coastal India the largest (wettest) coefficients. D...

The earth location of geostationary satellite imagery

Spin scan cloud pictures from NASA's Applications Technology Satellites are being computer processed as an operational experiment. Details of this near-real-time image manipulating activity are described. Processing involves the treatment of horizon and landmark features in support of the photogrammetric aspects and the subsequent transformation of images onto standard map projections.