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Cecilia G. Griffith and William Lee Woodley

Abstract

As part of a program to estimate rain from satellite observations, and to test the program's assumption that the highest clouds are the brightest, a correlation between cloud height and cloud brightness in the South Florida area was made 43 days during the summer of 1972. Brightness was determined from ATS-3 transparencies using a 32-step color densitometer. Echo heights within 100 mi of Miami were measured with the WSR-57, 10-cm radar of the National Hurricane Center in Miami. An unequivocal positive correlation is shown.

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Stephanie P. Browner, William L. Woodley, and Cecilia G. Griffith

Abstract

An analysis of 16 days from eight Atlantic storms, two in 1974 and six in 1975, objectively quantified a suspected diurnal oscillation of tropical storm cirrus cloud cover. The oscillation shows a maximum area at approximately 1700 local mean solar time and a minimum area at 0300 local mean solar time. The average ratio of the maximum area to the minimum area is 1.65.

SMS infrared imagery was analyzed with a scanning false-color densitometer to obtain area measurements of the cloudiness associated with the storms. These measurements were made approximately every 1½ h at three temperature thresholds: 253, 239 and 223 K.

Two tests were performed to rule out the possibility of the oscillation being due either to the satellite sensor or to image processing. Measurement of the ocean surface temperature, was made with SMS-I to determine whether the sensor detected a constant ocean temperature. The second test compared simultaneous area measurements obtained by SMS-I and SMS-II. The results of these tests support the storm oscillation detected.

Two other related phenomena were also observed: 1) the amplitude of the area oscillation is apparently inversely proportional to the intensity of the storm, and 2) a time-dependent, shorter period oscillation is superimposed on the daily oscillation. Inferences of causality are made.

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William L. Woodley, Cecilia G. Griffith, Joseph S. Griffin, and Scott C. Stromatt

Abstract

Quantitative precipitation estimates have been made for the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE) from geosynchronous, infrared satellite imagery and a computer-automated technique that is described in this paper. Volumetric rain estimates were made for the GATE A scale (1.43 × 107 km2) and for a 3° square (1.10 × 105 km2) that enclosed the B scale for time frames ranging from all of GATE (27 June—20 September 1974) down to 6 h segments. The estimates for the square are compared with independent rain measurements made by four C-band digital radars that were complemented by shipboard raingages. The A-scale estimates are compared to rainfall estimates generated by NASA using Nimbus 5 microwave imagery. Other analyses presented include: 1) comparisons of the satellite rain estimates over Africa with raingage measurements, 2) maps of satellite-inferred locations and frequencies of new cumulonimbus cloud formation, mergers and dissipations, 3) latitudinal precipitation cross sections along several longitudes and 4) diurnal rainfall patterns.

The satellite-generated B-scale rainfall patterning is similar to, and the rain volumes are within a factor of 1.10, of those provided by radar for phases 1 and 3. The isohyetal patterns are similar in phase 2, but the satellite estimates are low, relative to the radar, by a factor of 1.73. The B-scale disparity in phase 2 is probably due to the existence of rather shallow but rain-productive convective clouds in the B scale. This disparity apparently does not carry over to the A scale in phase 2. Comparison of NASA Electronically Scanning Microwave Radiometer (ESMR) rain estimates with ours for several areas within the A scale for all GATE suggests that the former is low relative to the latter by a factor of 1.50. The satellite estimates of rainfall in Africa are similar to measurements by raingages in all phases of GATE up to 11°N and progressively greater than the gage measurements north of this latitude toward the Sahara desert.

The diurnal rainfall studies suggest a midday (about 1200 GMT) maximum of rainfall over the water areas and a late evening maximum (about 0000 GMT) over Africa and the northern part of South America. The latitudinal cross sections along several longitudes of phase rainfall clearly show the west-southwest/east-northeast orientation of the Intertropical Convergence Zone (ITCZ), the diminution of the rainfall west-southwestward from Africa into the Atlantic, and the northward progression of the ITCZ from phase 1 into phases 2 and 3. The center of action for cloud formation, merger and dissipation, and the area of maximum rainfall (>1600 mm for all of GATE) occur along the southwest African coast near 11°N. This agrees with past climatologies for this region. Superposition of the satellite-generated rainfall maps and sea surface temperature maps by phase suggests a strong relationship between the two. Almost all of the rainfall occurs within 26°C sea surface temperature envelope. The mean daily coverage of rainfall and the mean rainfall in the raining areas for the A scale for all GATE are 20% and 14.1 mm day−1, respectively. These and other results are discussed.

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John A. Augustine, Cecilia G. Griffith, William L. Woodley, and JoséG. Meitín

Abstract

In the mean the Griffith/Woodley rain estimation technique underestimated the radar-measured rain of each of the three phases (a total of 56 days) of GATE, to varying degrees, and the satellite-derived isohyets were generally too extensive relative to radar-measured patterns. Three possible error sources are investigated in the present paper: 1) the method of apportionment of satellite-derived rain at the surface; 2) resolution degradation of the digital satellite imagery; and 3) anomalous behavior of convective clouds in the tropical Atlantic relative to those of the Florida derivation data set.

To correct the satellite-derived rain patterns, a new method of apportionment was tested by recomputing the GATE satellite rain estimates. Better volumetric comparisons between radar and satellite estimates were observed for 24 h and phase periods, and comparisons of isohyetal patterns improved on all time scales.

The relative error caused by resolution degradation was quantified by comparing rain estimates produced from full resolution imagery to estimates derived from degraded imagery for an 8° latitude by 12° longitude area in the eastern tropical Pacific ocean over a 54 h period. Results showed that the volumetric rainfall estimates made at 1/3° spatial and 1 h temporal resolution would be on the order of 10% lower than estimates made with the full resolution data (1/15° and 30 min).

The remaining differences between the GATE satellite and radar estimates are attributable to different conditions prevailing in Florida and in GATE. These include significant rain from clouds that do not grow above the −20°C level (“warm rain”) and very long-lived anvils.

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Cecilia Girz Griffith, William Lee Woodley, Pamela G. Grube, David W. Martin, John Stout, and Dhirendra N. Sikdar

Abstract

A diagnostic method to estimate rainfall over large space and time scales by the use of geosynchronous visible or infrared satellite imagery has been derived and tested. Based on the finding that arms of active convection and rainfall in the tropics are brighter or colder on the satellite visible or infrared photographs than inactive regions, ATS-3 and SMS/GOES images were calibrated with gage-adjusted 10 cm radar data over south Florida. The resulting empirical relationships require a time sequence of cloud area, measured from the satellite images at a specified threshold brightness or temperature to calculate rain volume over a given period.

Satellite rain estimates were made for two areas in south Florida that differ in size by an order of magnitude (1.3×104km2 vs 1.1×105km2) and verified by a combined system of gages and radar. Contrary to our expectations, the rain estimates for the smaller area agreed better with the raingage-radar groundtruth than the satellite rain estimates for the larger area. As expected, the accuracy of the rain estimates is a function of the period of rain estimation. The error and scatter of the hourly estimates are relatively large but both decrease as the estimates are accumulated with time. For periods of 6–9 h the mean absolute errors are factors of 1.50 and 1.90 using the visible and infrared imagery, respectively.

Additional tests for which groundtruth was available were also made to determine the applicability of this scheme to tropical areas other than the region of derivation. These include nine days over portions of Venezuela, five days over Honduras during the passage of Hurricane Fifi, and one day each of Hurricane Belle (1976) over the East Coast of the United States and Hurricane Agnes over Florida.

The potential of this method as a forecasting tool for hurricane-caused flooding is investigated in a study of selected Atlantic hurricanes between 1969 and 1977. Storms were ranked by satellite-calculated total daily rain volume and daily area-averaged rain depth. Relatively wet storms could be distinguished from the relatively dry storms. No correlation was found between volumetric storm rain output or area-averaged rainfall and storm intensity, which suggests that the location of latent heat releases and not its magnitude determines storm intensity.

Computer automation of the technique for real time and diagnostic estimates is discussed briefly. Satellite-inferred rains for the Big Thompson (Colorado) flood of 1976 and the Atlantic Ocean during GATE (summer 1974) are presented as examples of the real time and diagnostic computerized schemes, respectively.

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