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Robert F. Adler

Abstract

Atmospheric structure derived from satellite, multi-channel radiance data is used to calculate zonally- averaged vertical motions in the wintertime stratosphere of both hemispheres using a heat budget approach. The Northern Hemisphere calculations based on the satellite data are shown to compare favorably with a computation carried out with conventional data, and with results of previous studies. The mean Southern Hemisphere pattern for the month of July 1969 indicates a high-latitude cell with the axis of sinking motion at approximately 50°S, while the rising motion is centered at 70°S. Thus the antarctic stratosphere jet stream is associated with an indirect cell.

Two individual 10-day periods from July 1969 are examined to compare the mean meridional circulation and eddy heat flux patterns in the Southern Hemisphere during a minor midwinter warming and during a quiet period. Large eddy fluxes at 60°S and a strong indirect cell in the meridional circulation are associated with the minor warming. During the quiet period eddy fluxes at 60°S are relatively small and the mean meridional circulation appears to develop an additional cell in very high latitudes with sinking motion over the South Pole.

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Scott Curtis
and
Robert Adler

Abstract

In this study, gridded observed precipitation datasets are used to construct rainfall-based ENSO indices. The monthly El Niño and La Niña indices (EI and LI) measure the steepest zonal gradient of precipitation anomalies between the equatorial Pacific and the Maritime Continent. This is accomplished by spatially averaging precipitation anomalies using a spatial boxcar filter, finding the maximum and minimum averages within a Pacific and Maritime Continent domain for each month, and taking differences. The EI and LI can be examined separately or combined to produce one El Niño–Southern Oscillation (ENSO) precipitation index (ESPI). ESPI is well correlated with traditional sea surface temperature (e.g., Niño-3.4) and pressure indices [e.g., Southern Oscillation index (SOI)], leading Niño-3.4 by a month. ESPI has a tendency to produce stronger La Niñas than does Niño-3.4 and SOI. One advantage satellite-derived precipitation indices have over more conventional indices is describing the strength and position of the Walker circulation. Examples are given of tracking the impact of recent ENSO events on the tropical precipitation fields. The 1982/83 and 1997/98 events were unique in that, during the transition from the warm to the cold phase, precipitation patterns associated with El Niño and La Niña were simultaneously strong. According to EI and ESPI, the 1997/98 El Niño was the strongest event over the past 20 years.

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Robert F. Adler

Abstract

Certain aspects of the general circulations of the Northern and Southern Hemispheres are compared using atmospheric structure obtained from Nimbus 3 Satellite Infrared Spectrometer (SIRS) data. Comparisons between the hemispheres of zonal and eddy available potential energy (AZ and AE) and zonal and eddy kinetic energy (KZ and KE) indicate that the ratios of AZ to AE and KZ to KE are larger in the Southern Hemisphere.

The relative importance of standing and transient eddies in both hemispheres is investigated. The results show that standing eddies in the Southern Hemisphere contribute less to eddy available potential energy (AE) and eddy kinetic energy (KE) than in the Northern Hemisphere. The same type of inter-hemispheric distinction is true for the mid-latitude eddy heat flux. The distribution with latitude of the relative importance of standing and transient eddies is also studied.

Horizontal eddy heat fluxes in the upper troposphere of both hemispheres are examined and in mid-latitudes found to be approximately equal in magnitude when averaged over the summer and winter month. The Southern Hemisphere mid-latitude eddy heat flux is also shown to have significant longitudinal variations, apparently associated with the location of the southern continents.

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Robert F. Adler
and
Robert A. Mack

Abstract

Observational studies of thunderstorm cloud height-rainfall rate and cloud height-volume rainfall rate relations are reviewed with significant variations being noted among climatological regimes. Analysis of the Florida (summer) and Oklahoma (spring) relations are made using a one-dimensional cloud model to ascertain the important factors in determining the individual cloud-rain relations and the differences between the two regimes. In general, the observed relations are well simulated by the model-based calculations. The generally lower predicted rain rates in Oklahoma (as compared to Florida) result from lower precipitation efficiencies which are due to a combination of larger entrainment (related to larger vertical wind shear) and drier environment. The generally steeper slope of the Oklahoma rain rate height curves is shown to be due to a stronger variation in maximum vertical velocity with cloud top height, which, in turn, is related to the greater static stability in the range of cloud tops. The impact of the regime-to-regime variations on empirical rain estimation schemes based on satellite-observed cloud height or cloud temperature information is discussed and a rain estimation approach based on model-generated cloud-rain relations is outlined.

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Robert F. Adler
and
Robert A. Mack

Abstract

A Lagrangian model applicable to the overshooting region of thunderstorm tops is used to describe the temperature-height path taken by updraft core parcels as they penetrate above the tropopause, reach their maximum height and descend in the periphery of the convective tower. The model is run under a variety of ambient and in-cloud conditions in order to simulate certain temperature-height relationships observed in satellite observations.

Observations indicate that in the majority of observed storm tops, the satellite-observed cold point in the IR brightness temperature (TB ) field is collocated with the highest point in the convective overshooting region and the TB -height relations are near adiabatic. The parcel model quantitatively reproduces this type of relationship for model runs where the mixing parameter is relatively small.

Another type of storm has a close-in, cold-warm TB couplet with a dimension of approximately 20–40 km and a V-shaped cold TB pattern. In some cases of these V-shaped storms, the cold point is clearly located upwind of the high point. Model runs have been made to reproduce a number of these salient features for these types of storms. With larger values of the mixing parameters (presumably related to larger shear), the model produces temperature-height relationships that are, of course, much closer to ambient than to adiabatic, as is observed in these cases. With the larger mixing parameter, the cold-high offset is also produced, for model runs having a relatively large initial vertical velocity and under conditions of a strong inversion. The amount of the cold-high offset is shown to be a direct function of the strength of the inversion.

The cause of the close-in warm point is also explored with the simple model. As has been shown in three-dimensional cloud model results, the warm point in the cold-warm couplet can be related to internal cloud subsidence on the downwind side in association with mixing with the environment. This effect is also reproduced in the parcel model with the occurrence of a warm point being related to conditions of an intense updraft and strong mixing. The model also points to parcels subsiding from their maximum height and crossing the ambient lapse rate from negative to positive buoyancy on the downwind side and then coming into equilibrium at a relatively high level above the tropopause on the downwind side. This effect may be related to the top of the downwind anvil cloud being elevated significantly above the equilibrium point or tropopause. Another interpretation of this model result may be related to the above-anvil cirrus noted by a few investigators.

The temperature-height distributions produced by the model in a Lagrangian framework are converted to the spatial domain by the assumption of steady state conditions and are compared to temperature-height cross sections determined from GOES IR and stereoscopic height fields. The locations of cold points, high points, warm points, and the magnitude of cold-high offsets compare favorably between the model and the satellite observations.

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Andrew J. Negri
and
Robert F. Adler

Abstract

The relationships between satellite-viewed cloudy (or partly cloudy) grid cells and the variability of the precipitation contained therein are explored. Using a 32 km grid and 30 min interval visible, infrared and radar data, 5 days of the Florida Area Cumulus Experiment are examined. Cloud is delineated from no-cloud by an infrared threshold of 253 K.

While high rainrates are always associated with low temperatures, the reverse is not true: low temperatures do not always imply high rainrates. For partly cloudy cells, the percent-explained variance of rainrate by infrared parameters is low, with none of the parameters explaining more than 14% of the variance. The mean visible count explains slightly more variance, but it is not apparent that higher visible values are indicative of higher rainrates, because the higher resolution of those data introduces ground pixels into the average. When only completely cloudy cells are considered, the infrared parameters still explained about 14% of the variance, but with larger day-to-day variability. For those cells, the mean visible count explains less than 10% variance on 4 of the 5 days, due to its inability to discern rainrates in widespread cirrus anvils. The mean visible structure by itself explains 10%–26% of the rainrate variance for completely cloudy grid cells. Modest (4%–14%) increases in explained variance are shown when this quantity is then added as a second regression parameter.

Classification of the mean rainrate into six groups and the subsequent computation of a mean infrared parameter for each class shows statistically significant differences in the mean infrared parameters among classes. Assigning independent observations to classes becomes unsatisfactory given the distribution of the rain classes themselves. Variability (between days) in the mean temperature of each rainrate class is often as great as the variability (of the mean temperature) among rain classes on any given day. Relationships are clearly dependent on where in the convective cycle they occur, and this cycle is itself variable from day to day. Extensive cold anvils often produce widespread stratiform rain late in the day, while earlier these same temperatures produced intense convective rain. On the scales examined here, the results indicate that useful, accurate rainfall estimates beyond rain/no-rain discrimination are unlikely.

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Andrew J. Negri
and
Robert F. Adler

Abstract

This study examines the relationships between satellite infrared clouds and rainfall, and infrared-threshold visible clouds and rainfall. Clouds are defined by the outline of the 253 K isotherm. Cloud infrared area was highly correlated with rain area (ρ = 0.85) and with volume rainrate (ρ = 0.81). It was poorly correlated with mean cloud rainrate (ρ = −0.28). One-parameter models were as effective in explaining the variance of cloud volume rainrate as multiparameter methods, due to the high correlations between visible brightness, mean cloud temperature and cloud area. An exception was found for clouds >10 000 km2, where area and temperature were uncorrelated, and mean temperature was more effective in discriminating among classes of volume rain than was cloud area. Statistical separation of five- of six-volume rain classes was achieved with mean temperature; however, the probability of occurrence of the classes effectively reduced this to a four-class problem.

Due to the high correlation between visible brightness and infrared temperature, visible data provided largely redundant information. Using a mean cloud brightness threshold of 148 counts, rain/no-rain separation was effected with a POD, FAR, and CSI of 0.98, 0.13, and 0.86, respectively. An infrared threshold (mean temperature of 241 K) produced statistics of 0.88, 0.07 and 0.83, respectively for the POD, FAR and CSI. The standard deviation of visible counts (used as a measure of cloud structure) was poor in explaining the variance of rainrate, yielding no better than rain/no-rain separation.

Time series of the cloud evolution showed that rain volume fluctuations were better “mirrored” by cloud temperature fluctuations than by cloud area. Contrary examples could be found and inconsistency between days was noted. The apportionment of rain volume (assigning rainrates to areas) remained a difficult problem, with significant variability, both within clouds of the same size and between clouds of different size. The coldest 10% cloud area was found to contain 11%–23% of the total rain volume while the coldest 50% area contained 60%–70–. This is in contrast to the rain apportionment used in the Griffith-Woodley Technique.

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Robert F. Adler
and
Andrew J. Negri

Abstract

This paper describes a new method of estimating both tropical convective precipitation and stratiform precipitation (produced under the anvils of mature and decaying convective systems) from satellite infrared data. The method, denoted CST (Convective-stratiform Technique) locates, in an array of infrared data, all local minima in the brightness temperature field (T min. After an empirical screening to eliminate cirrus, these points are assumed to be convective centers. Rainrate and rain area are assigned to each minimum point as a function of its T min, based on one-dimensional cloud model results. A stratiform rain algorithm, using a brightness temperature threshold based on the mode temperature of thunderstorm anvils, completes the convective/stratiform rain estimation.

Individual CST rain fields wore spatially most similar to the radar for young, isolated storms, and most dissimilar in capturing linear features such as squall lines. Some convective features were missed, while others (generally cirrus debris) were sometimes misrepresented as active convection. Stratiform estimates generally corresponded to the radar-derived 1 mm h−1 contour.

The technique was tested for four south Florida cases during the second Florida Area Cumulus Experiment (FACE). Half-hourly estimates made in the FACE target area are verified against raingages and both unadjusted and gage-adjusted radar. When compared to three other infrared techniques applied to the same dataset, the CST had the lowest bias (−0.02 mm), lowest mean absolute difference (0.28 mm), lowest root mean square difference (0.39 mm), and lowest percent difference (41.2%) of any tested satellite technique.

The evolution of the precipitation averaged over the FACE target (104 km2), was well represented by the CST, particularly in capturing peak rainfall and the transition early and overestimate late, when compared to the gage-adjusted radar. Area-averaged estimates were in agreement with radar-based analyses, and comprised 10%30% of the total rainfall.

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Robert F. Adler
and
Douglas D. Fenn

Abstract

Digital infrared data from a geosynchronous satellite (SMS 2) on 6 May 1975 are used to study thunderstorm vertical growth rates and cloud top structure in relation to the occurrence of severe weather (tornadoes, hail and high wind) on the ground. All thunderstorms from South Dakota to Texas along a north–south oriented cold front are monitored for a 4 h period with 5 min interval data.

An examination of five cloud elements having eight tornadoes indicates that in seven of eight cases the first report of the tornado took place during, or just after, a period of cloud top ascent. This vertical velocity is applicable to an area of 15 km on a side.

Thunderstorm growth rate, as determined by the rate of blackbody temperature isotherm expansion and minimum cloud top temperature, are shown to be correlated with reports of severe weather on the ground. A time analysis indicates that the derived parameters reach critical values soon enough to provide a potential warning lead time of approximately 30 min.

Equations are derived relating the thunderstorm growth rate to vertical velocity and outflow layer divergence. Severe thunderstorm elements are shown to have mean vertical velocities approximately twice as large as the non-severe elements. The outflow layer divergence is calculated to be 1 × 10−3 s−1 for the severe thunderstorms.

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Andrew J. Negri
and
Robert F. Adler

Abstract

Quantitative observations of thunderstorms in the midwest United States made with short-interval (5 min) geosynchronous satellite data are examined in relation to concurrent digital radar observations for one case study over a limited area. Individual thunderstorms are defined in the satellite infrared (IR) data by the location of relative minima in the equivalent blackbody temperature (TBB ) field. In a large majority of cases, these satellite-defined thunderstorms coincide with individual radar echoes. This agreement allows comparison of digital satellite and radar data for individual thunderstorms.

The evolution of individual thunderstorms in terms of radar echo and satellite-observed cloud features is examined. An examination of a number of storms indicated that the first low-level radar echo (18 dBZ) appeared when the satellite observed cloud-top minimum TBB had a mean of 246 K (7.4 km). As the storms evolve, larger reflectivities appear as the cloud tops penetrate upward to colder temperatures. Larger reflectivity values (>50 dBZ) begin as the storms approach and penetrate the tropopause.

Maximum radar reflectivity is shown to be correlated with satellite-based estimates of thunderstorm intensity. Thunderstorm top ascent rates in the 235-240 K (∼8.8 km) region indicate the intensity of the initial storm updraft and are correlated with the maximum storm reflectivity with weak cells (-dTBB/dt of 1 K min−1) having maximum reflectivity of 30–40 dBZ and strong cells (3–4 K min−1) having echoes of ≥50 dBZ. The minimum TBB observed during the lifetime of the storm (Tmin ), indicative of maximum storm top height, is also correlated to maximum storm rainfall. Storms with tops colder (higher) than the tropopause (212 K) have the highest rainfall rates in the severe storm situation examined here. The parameter Tmin is also very well related to maximum volume rain rate as estimated from the radar data. Storms observed to reach temperatures lower than the tropopause temperature had volume rain rates of the order 103 m3 s−1, compared to 102 m3 s−1 for weaker storms.

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