Tropical Cyclone Rainfall As Measured by the Nimbus 5 Electrically

Home , Nimbus 5

Lewis J. Allison1 tropical cyclone rainfall as NASA/Goddard Space Flight Center Edward B. Rodgers2 measured by the Nimbus 5 Environmental Research and Technology, Inc. Thomas T. Wilheit1 electrically scanning NASA/Goddard Space Flight Center and Robert W. Fett3 microwave radiometer Environmental Prediction Research Facility

Abstract 2. The Nimbus 5 electrically scanning radiometer A selected group of 1973 North Pacific Ocean tropical (ESMR) cyclones was studied by using data from the Nimbus 5 A prototype of the Nimbus 5 ESMR was flight tested by Electrically Scanning Microwave Radiometer (ESMR), Catoe et al. (1967) and Nordberg et al. (1969 and 1971). the Temperature-Humidity Infrared Radiometer (THIR), Additional flights by Wilheit et al. (1972), Gloersen et al. NOAA-2 and USAF DMSP imageries. From the unique (1972), and Schmugge et al. (1972) recorded and mapped combination of infrared, visible, and microwave data, passive microwave radiation data (in brightness tem- it was possible during various stages of storm develop- perature TB) to delineate rain areas, to demarcate areas ment to differentiate between dense cirrus outflow and of sea ice and open sea in the polar regions regardless rain areas, to identify centers of circulation and areas of of cloud cover, and to test the feasibility of inferring low-level moisture, and by the use of a theoretical model soil moisture providing no heavy clouds exist over the to estimate semi-quantitatively areas of light, moderate, and area of interest. The description of the ESMR and the heavy rainfall rates. fundamental physical relationships are discussed by Wilheit (1972). The Nimbus 5 ESMR (Fig. 1) measures 1. Introduction the earth and atmospheric radiation in the 19.35 GHz Tropical cyclone rainfall measurements and estimations (1.55 cm) region from a polar orbit of approximately have been made by conventional meteorological net- 1100 km. It scans the earth every 4 sec from 50° to the works using ground, ship, and aircraft observations; rain- left to 50° to the right through nadir in 78 steps with gages; and weather radars. With the advent of the mete- some overlap. The scanning process is controlled by an orological satellite, some new techniques have been de- onboard computer which permits the recording of bright- veloped to estimate average hourly, daily, and monthly ness temperature within a NEAT of approximately 2K rainfall from satellite cloud photography, using cloud from a 25 X 25 km scan spot at nadir to a 45 X 160 km categories and brightness, cloud top temperatures, and scan spot at 50° nadir angle. monthly cloud nephanalyses (Follansbee, 1973; Gruber, The brightness temperatures (TB) measured by the 1973; Martin and Scherer, 1973; Sikdar, 1972; Woodley horizontally polarized ESMR is dependent upon the et al, 1972; Griffith and Woodley, 1974; Scherer and radiation emerging from the earth's surface and the in- Hudlow, 1971; Barrett, 1970). tervening atmosphere between the satellite and the The Electrically Scanning Microwave Radiometer earth. The microwave energy emitted from the earth's (ESMR), which was carried aloft aboard Nimbus 5 on surface at 19.35 GHz is dependent upon the earth's 11 December 1972, recorded microwave radiometric mea- thermodynamic temperature and its emissivity. The surements through clouds. This instrument now makes it emissivity may vary due to the change in dielectric con- possible to measure directly new meteorological and stant of the emitting surface and the radiometer viewing hydrological parameters that were not detectable by angle. For example, non-vegetating land areas that con- previous satellite-borne visible and infrared radiation tain soils with a low dielectric constant have a high sensors. (Allison et al., 1974; Schmugge et al., 1974; Skid- emissivity of ca. 0.95 but water surfaces with a high di- more and Purdom, 1973.) electric constant will have a low emissivity of ca. 0.40. The purpose of this paper is to describe briefly the This results in low brightness temperatures (120 to 170K) ESMR experiment, to evaluate the theoretical deriva- being recorded over water and high brightness tempera- tion of rainfall rate from the ESMR data, and to examine tures (215 to 300K) over land. Brightness temperature rainfall and cloud patterns over a select group of 1973 contrast between land and ocean can be greater than North Pacific tropical cyclones using Nimbus 5 ESMR 100K under clear sky conditions. Over ocean areas TB at and supporting imagery. this frequency does not vary due to salinity changes, since the emissivity does not change. Sea surface temperature 1 Greenbelt, Maryland 20771. 2 Lexington, Massachusetts 02173. changes, i.e., 9 to 28C, through the Gulf Stream were not 3 Naval Postgraduate School, Monterey, Calif. 93940. detected by ESMR since emissivity varies inversely with

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FIG. 1. The Nimbus 5 spacecraft with associated experiments. surface temperature changes (Wilheit, 1972; Allison et al Wilheit, 1972; Aerojet Electrosystems Co., 1973; Paris, 1974). However, the TB does increase with ocean sur- 1969). Microwave radiation can penetrate non-raining face roughness, wind-driven foam, streaks, and white clouds, although there is some minor attenuation (West- caps. Nordberg et al. (1971) showed from NASA Con- water, 1972). Ice crystal clouds (ambient air tempera- vair 990 flight measurements that there was a linear ture, < —20C) are relatively transparent to microwave relationship between wind speed (>7 m sec-1) and en- energy and are not detected in the tropical analyses de- hanced TB. For example, with sea surface winds of 20 1 scribed in a later section of this paper. Areas of precipi- m sec' , an increase of 15K for the sea surface TB values was noted for nadir viewing at 19.35 GHz. tation (high TB values) can be detected by the ESMR Within the intervening atmosphere, the three major over an ocean background, which has a uniform low atmospheric constituents that contribute to the upward emissivity (low TB values) (Theon, 1973; Wilheit et al., flux of energy in the 19.35 GHz region are molecular 1973). Over land, where the emissivity is high and vari- oxygen, water vapor, and liquid water. Of the three able within the rainfall TB range, the detection of rain constituents, liquid water, particularly large droplets may be more difficult (Sabatini and Merritt, 1973). and rain with radii greater than 1 mm, emit more energy In this study an evaluation was made of rainfall rates due to the enhanced absorption effect. This effect is by the use of theoretical calculations derived from a caused by the effective size dimension being comparable numerical model (Gaut and Reifenstein, 19734), with to wavelength divided by the large index of refraction of water at microwave frequencies (Gunn and East, 1954; 4 Private correspondence.

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Unauthenticated | Downloaded 10/09/21 08:10 AM UTC Vol. 55, No. 9, September 1974 rainfall contribution in the model later revised (Isaacs, 1974 4). The numerical model solves the equation of radia- tive transfer at points along the line of sight from a satellite looking toward the earth in the 19.35 GHz region, through a tropical maritime standard atmosphere con- taining precipitation at various rainfall rates. The model contains 30 layers, 1 km thick from the surface to 30 km, in which the temperature, pressure, density, water vapor, and molecular oxygen for each layer is deter- mined from the mean of the parameter in the layer. Saturation is assumed and liquid water is considered constant for a given rainfall rate up to the freezing level (approx. 4 km). The rain model contains one layer in which a uniform Marshall-Palmer droplet size and distribution is assumed for a given rainfall rate from the surface to the freezing level. The temperature of the liquid droplets assumes the ambient temperature of the layer. A "calm" ocean surface at 300K is used in the model. This assumes a smooth surface for which re- flectivity can be calculated using the Fresnel equations for either horizontally or vertically polarized radiation. There is no scattering source function and no order of multiple scattering is assumed for the larger raindrops FIG. 3. Nimbus 5 ESMR (19.35 GHz) weighting function -1 1 (>1 mm in radius). For the larger raindrops, scattering curves with rainfall rates from 1 mm hr to 100 mm hr- , (Isaacs Model, 1974). extinction is computed as an equivalent absorption (Isaacs, 19744). The results of the theoretical data are weighting function at 19.35 GHz which is defined as the shown in Fig. 2. Limited calculations have been per- source region for the contribution to the ESMR bright- formed including all orders of scattering which agree well ness temperatures (Isaacs, 19744). The weighting func- -1 with this model for rain rates up to 10 mm hr (Chang tion at this frequency is primarily a function of atmo- 4 and Curran, 1974 ). The main characteristics of the curve, spheric water vapor and liquid water. As the atmosphere which relates ESMR TB values at 19.35 GHz and rain- changes from clear to cloudy conditions with given rain- 1 fall rates (mm hr" ), are that the TB increases rapidly fall rates, the contribution from the surface decreases for low rainfall rates and becomes asymptotic to 280K at considerably. This is particularly true for rain rates >10 higher rates. Thus unfortunately, only lower rainfall rates mm hr-1. In tropical cyclones where surface winds are -1 (<10 mm hr ) can be distinguished by the ESMR TB much above 7 m sec-1, sea roughness and foam may en- values. For the higher rain rates, the extinction within hance the TB considerably (Porter and Wentz, 1972). the precipitating cloud is so large that only the freezing However, as seen from Fig. 3, for moderate (2 to 7 mm level at the top of the rain column is observed. To illus- hr-1) to heavy rainfall rates (>7 mm hr"1) which occur trate this point further, Fig. 3 shows the atmospheric within 100-200 km of the storm center, the sea roughness emission is greatly attenuated or eliminated. In view of these uncertainties in the model the rainfall rate will only be semi-quantitatively characterized.

3. Tropical cyclone rainfall characteristics Previous studies of hurricane rainfall by Schoner and Molansky (1956), Ackerman (1963), Gentry (1964), Fujita et al. (1967), Goodyear (1968), Cry (1967), have indicated that very heavy rainfall (>100 to 150 mm/24 hr) from the eye wall clouds and feeder band convection was concentrated within 100-150 km of the storm track (Gray, 1973). Energy budget studies by Hawkins and Rubsam (1968), Riehl and Malkus (1961), and Miller (1958) estimated 6 to 12 inches (150 to 300 mm) of rainfall within the inner 60 to 100 n mi of the storm center during the storm's passage. Aircraft radar observations by Fujita and Black (1970) and Sheets (1968, 1974) indicate from FIG. 2. Nimbus 5 ESMR brightness temperature (K) versus the spatial distribution of the radar echoes around rainfall rate in mm hr-1 (Gaut and Reifenstein Model, 1973). Hurricane Debbie, 1969, and Dora, 1964, that the maxi-

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Data Catalog (1973) were omitted to save space. The THIR 11 fim data essentially provide night and day cloud top and surface temperature (uncorrected for water vapor attenuation) with a ground resolution of 5 n mi (ERTS/Nimbus Project, 1972). This imagery effectively

exhibits 16 shades of gray for a TB range from 200 to 300K, of which 6 to 8 are detectable by eye. A hand analysis of ESMR data on 7 June 1973 over Hurricane Ava, drawn from a computer-produced grid

print map of TB values is shown in the lower half of Fig. 5. The TB values were converted to rainfall rates using the Gaut-Reiferi stein model and curve shown in Fig. 2. Three rainfall rates are depicted by gray shades in this type of analysis. The rain rates will be termed light, moderate, and heavy for the <2 mm hr"1, 2-7 mm hr~\ and >7 mm hr-1, respectively, with the understanding that they were underestimated due to the large ESMR ground resolution mentioned earlier. FIG. 4. Composite aircraft radar echoes over Hurricane The infrared and visible images (Figs. 6, 7) over Debbie, 1969, 1616-1712 GMT, 20 August 1969, (Fujita and Ava show a circular uniformly-textured dense overcast Black, 1970). with a spectacular view of the eye (Fig. 7), as it was recorded by the high resolution (1/3 n mi) visual chan- nel of the U.S. Air Force DMSP (Meyer, 1973). It is ap- mum width of the rain bands that may contain the parent from an analysis of the cirrus striations appearing comparable rain rates mentioned earlier, were approxi- in these data that Ava was capped by a strong upper- mately 6 to 22 km. Figure 4 shows a representative level anticyclone. However, the extent of this outflow scale of the rain bands as observed from aircraft radar region cannot be appreciated until one examines the (from Fujita and Black, 1970). Ground radar rain band ESMR imagery in the 138 to 210K range (Fig. 5a). Note pictures of Hurricane Betsy, 1965, over Florida indicate the shade of grey which begins south of the Baja Penin- similar features (Sugg, 1966). sula and encircles the storm hundreds of miles out in the Since the ESMR field of view is approximately 25 km form of an elliptical arc. The ESMR is insensitive to the 2 X 25 km (ca. 500 km ) at nadir and greater at higher high level cirrus but can detect the lower level water nadir angles, the recorded brightness temperatures will vapor which is trapped in the subsiding environment of tend to be an average of the heavy, moderate, and light the storm's outer periphery. This subsidence has also rainfall from all cumulonimbus "towers" and stratified- tended to dissipate the stratocumulus cloudiness in the layer clouds between feeder rain bands. northern quadrants, the edge of which tends to parallel In the following section ESMR data over selected the curvature of the northward extent of water vapor, as tropical cyclones in the North Pacific Ocean during the shown in the ESMR imagery (Fig. 5a). The ESMR rain- 1973 season will be evaluated especially with respect to fall chart and images (Fig. 5) shows the heaviest rainfall their asymmetrical rain patterns and variable rainfall to be concentrated in rainbands circling the eye. Moder- intensities. ate rain is found in the single band extending to the southwest joining with the Intertropical Zone of Con- vergence at 5 to 7N. Note that the NOAA-2 infrared 4. Tropical cyclone cases images (Fig. 6, center) appears light grey over this band a. Hurricane Ava (lower cloud tops) as compared with the whiter (higher The Nimbus 5 ESMR and Temperature-Humidity In- cloud tops) over the storm center, yet moderate raii: frared Radiometer (THIR, 11 /mi) images shown in exists in both areas. This confirms the fact that in the this paper were obtained from "quick look" photo- tropics moderate rainfall can occur from clouds with facsimile processes. Three dynamic ranges of the same lesser vertical development. ESMR data, 138 to 210K, 194 to 266K, 254 to 290K are The cyclone had reached superhurricane status (137 shown in the facsimile pictures of Hurricane Ava on 7 kt winds) on 6 June (Fig. 7). Its peak winds dropped to June 1973 (Fig. 5a, b, c). Range (a) shows the lowest 120 kt on 7 June, the date of this analysis, after which brightness temperature which indicates changes in atmo- it weakened to 40 kt by 10 June 1973 (Mariners Weather spheric moisture content and light rainfall areas over the Log, 1973). NOAA scientists using a passive microwave ocean. Range (b) indicates regions of moderate to heavy radiometer (13 GHz) aboard a C-130 aircraft also made rainfall and range (c) indicates regions of heavy rainfall. coincident measurements with the SKYLAB of Hurri- The latitude-longitude grids that are normally shown cane Ava's winds (130 kt) and wave height (40 ft) next to these three facsimile images in the Nimbus 5 on 6 June (Ross, 1974; Cardone et al, 1973).

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FIG. 5. Nimbus 5 ESMR (19.35 GHz) photofacsimile pictures of Hurricane Ava, D/O 2936, 7 June 1973. Gray scale dynamic range of TB values (a) 138 to 210K, (b) 194 to 266K, (c) 254 to 290K. Lower half of figure: Hand drawn grid print map analysis, TB (K), 1:2.5 million Mercator, 1903 to 1913 GMT.

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FIG. 6. NOAA-2 VHRR visual and IR and Nimbus 5 THIR (11 finL) imagery of Hurricane Ava on 7 June 1973.

b. Typhoon Nora It is indeed fortunate that Nimbus 5 ESMR and THIR coverage was available from 29 September and 1-3 October 1973 to show the developing sequence of this cyclone from the formative stage to that of a tropical storm (Fig. 8). The rainfall rate maps of the storm, as extracted from the theoretical numerical model, are shown in Figs. 9 and 10. Nora formed as a weak surface low in the monsoon trough 120 mi south of Yap Island on 30 September 1973 (U.S. Fleet Weather Center/Joint Typhoon Warning Center, 1973). The first image and analysis of the formative stage of Nora were available on 29 September (Figs. 8 and 9, top). A large area of cloud clusters with little organization is shown in the THIR imagery. The ESMR analog image and rainfall rate map show that these cloud clusters contain light to moderate precipitation, the latter extending 1° to 3° in latitude and longitude. The images obtained two days later on 1 October, showed Nora as a tropical depression. A center of circulation would be difficult to deduce from the infrared picture of Nora alone (Fig. 8, bottom). However, the curved band appearing in the ESMR data (Fig. 8, top; Fig. 9, bottom) on the west side of the large N-S oriented rain area suggests a center of circulation at 11N, 136.5E. The accuracy of this positioning would have to be considered only fair due to the lack of definitive banding and the 25 FIG. 7. U.S. Air Force Data Acquisition and Processing Pro- km ground resolution at nadir of the ESMR data. Nora, gram (DAPP) (now Defense Meteorological Satellite Program, at this stage, conforms to Stage B or C, "Comma Con- DMSP), very high resolution, visual imagery, 1/3 n mi resolution of Hurricane Ava on 6 June 1973. Arrows indicate figuration" categories of tropical cyclone development anticyclone outflow aloft.

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FIG. 8. Nimbus 5 ESMR (19.35 GHz) photofacsimile imagery (194 to 266K gray scale) of the development of Nora, 29 September, 1-3 October 1973 (top); Nimbus 5 THIR (11 /im) photofacsimile imagery of the same dates, (bottom).

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FIG. 9. Nimbus 5 ESMR (10.35 GHz) hand-drawn grid print map analysis, in rainfall rates (mm hr-1), 1:2.5 million Mercator. Top: 29 September 1973, D/O 3923, 1405-1415 GMT, Formative Stage of Nora. Bottom: 1 October 1973, D/O 3950, 1417-1425 GMT, Tropical Depression Nora with U.S. Air Force (SWAN) weather reconnaissance winds (700 mb level) for 1 and 2 October 1973.

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FIG. 10. Nimbus 5 ESMR (19.35 GHz) hand-drawn grid print map analysis, in rainfall rates (mm hr_1), 1:2.5 million Mercator. Top: 2 October 1973, D/O 3957, 0200-0208 GMT, Tropical Depression Nora, with U.S. Air Force (SWAN) weather reconnaissance winds (700 mb level), for 1 and 2 October 1973. Bottom: 3 October 1973, D/O 3978, 1432-1439 GMT, Tropical Storm Nora.

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(Fett, 1966a). In this stage of development, the major cloudy or rain area is elongated N-S in the shape of a broad-tailed comma and the center of circulation is at the head of the comma, generally in the clear, unobscured by heavy, dense cloudiness. Aircraft reconnaissance re- ports of 1 and 2 October 1973 superimposed on the ESMR rainfall rate chart of 1 October 1973 (Fig. 9, bottom), define the basic 700 mb wind flow pattern in the lighter rain area, around this center of circulation. FIG. 11. Tracks and maximum winds of Typhoon Nora The infrared image of Nora on 2 October 1973 from 2 to 4 October 1973. (Fig. 8, bottom) shows increased evidence of banding and cirroform cloud production suggesting further in- mi in diameter, which was beyond the resolution capabil- tensification. The presence of an upper-level anticyclone ity of the ESMR. The rainfall rate map (Fig. 12, over the storm area may be deduced from the alignment bottom), however, does show two light precipitation areas of the cirrus striations. A center of circulation is sug- near the storm center, the eastern-most of which closely gested in the northwest quadrant. The position is more approximates the eye position. Note the east-west orienta- definitely indicated near 11.5N, 135E in the ESMR tion of the heavy rainfall areas north and south of the image (Fig. 8, top). Aircraft wind data for 1 and 2 storm center. Nora continued to move westward toward October 1973 superimposed on the 2 October 1973 the Philippines by 6 October 1973, when her maxi- ESMR rainfall rate chart (Fig. 10, top) show that the mum winds rose to 160 kt and central pressure dropped center of circulation still has not become embedded in to 877 mb, a near record low. the area of moderate to heavy precipitation but is adja- c. Typhoon Ruth cent to it. This configuration is similar to that of Hurri- cane Gladys, 1968, whose center was adjacent to a cloud Ruth formed in the monsoon trough over the western of great vertical development with a saucer-shaped top, Caroline Islands on 10 October 1973 and intensified the "Circular Exhaust Cloud," (Gentry et al., 1970). slowly to tropical storm strength until it passed over the This type of cloud appeared to be representative of Philippines where it weakened due to terrain effects. rapidly intensifying cyclones passing from weaker cate- On 16 October, on a west-northwesterly track, it en- gories to hurricane intensity. Reconnaissance data indi- tered the South China Sea where it reintensified and cated that Nora had maximum flight level winds of 40 kt reached typhoon strength (90 kt) by 17 October 1973. at the 700 mb level on 2 October which increased to 75 The THIR imagery (Fig. 13, top) shows banding fea- kt the next day, and thus support this contention. tures curving cyclonically inwards towards a large eye Figure 8 (bottom), the infrared picture of Nora on 3 estimated by aircraft reconnaissance to be 40 n mi in October, shows the obvious intensification of the storm diameter (U.S. Fleet Weather Center/Joint Typhoon from the preceding day. The band curvature is more Warning Center, 1973). The ESMR imagery (Fig. 13, pronounced and the center appears to be partially ob- top) and rainfall rate map (Fig. 13, bottom) emphasize scured by overcast clouds. The ESMR data (Fig. 8, top) banding feature asymmetries not apparent in the THIR confirm this pronounced rainband curvature, indicating data. For example, the outermost rainbands northeast a center of circulation near 12N, 132E. Note that most of of the center are concentrically oriented rather than the cloudiness in the northeastern quadrant is cirro- spiral in nature. Similar "outer convective bands" were form and transparent to the microwave sensor. The described by Fett (1964). The inner rainbands are also rainfall rate map of 3 October (Fig. 10, bottom) re- concentric, suggesting that the typhoon is essentially in veals the rainbands under the cirrus cloud shield. Maxi- gradient balance and not undergoing rapid changes in mum rainfall is displaced north of the apparent center, intensity. In fact Ruth did not begin to lose intensity with only moderate rainfall in the two feeder bands. (75 kt) until 12 hr later on 18 July 1973. The heaviest If one superimposed rainfall map of Nora on 2 October precipitation appears on the west side of the storm in a with that of 3 October, rotating them to achieve best fit, N-S orientation while the typhoon was on a westerly a cyclonic rotation of 23° would be noted. Recent re- track. Note that Supertyphoon Nora, on 5 October (Fig. search has indicated that such a rotation is often asso- 12, bottom) also was on a westerly track but in this case ciated with a similar change in storm direction of motion the heaviest rainfall is found to the north and south during the same period (Fett and Brand, 1974). Nora of the eye in an E-W orientation. turned strongly cyclonically and did, in fact, complete a loop during the 36-hr period from 3-4 October 1973 d. Tropical storm Ellen (Fig. 11). Ellen developed in the trailing convergence area, south- By 5 October 1973, Nora had reached supertyphoon east of Typhoon Billie and is shown in the ESMR and status with maximum sustained surface winds of over THIR imagery (Fig. 14, top) as it reached tropical 130 kt. The THIR imagery reveals a small eye deeply storm intensity on 17 July 1973. The ESMR rainfall embedded within an almost circular overcast (Fig. 12, rate map (Fig. 14, bottom) shows an area of moderate to top). Reconnaissance aircraft reported a circular eye 10 heavy precipitation that is possibly associated with a

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FIG. 12. Nimbus 5 ESMR, 19.35 GHz, (194 to 266K gray scale) and THIR (11 /mi) photofacsimile imagery of Super- typhoon Nora, 5 October 1973, D/O 4004 (top). Nimbus 5 ESMR (19.35 GHz), hand-drawn grid print map analysis of Supertyphoon Nora, in rainfall rates, (mm hr"1), 1:2.5 million Mercator, 5 October 1973, D/O 4004, 1525-1535 GMT (bottom).

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FIG. 13. Nimbus 5 ESMR, 19.35 GHz, (194 to 266K gray scale) and THIR (11 fim) photofacsimile imagery of Typhoon Ruth on 17 October 1973, D/O 4166 (top). Nimbus 5 ESMR (19.35 GHz), hand-drawn grid print map analysis of Typhoon Ruth, in rainfall rates (mm hr_1), 1:2.5 million Mercator, 17 October 1973, D/O 4166, 1625-1635 GMT (bottom).

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FIG. 14. Nimbus 5 ESMR, 19.35 GHz, (194 to 266K gray scale) and THIR (11 fim) photofacsimile imagery of Tropical Storm Ellen on 17 July 1973, D/O 2930 (top). Nimbus 5 ESMR (19.35 GHz), hand-drawn grid print map analysis of Tropical Storm Ellen, in rainfall rates (mm hr"1), 1:2.5 million Mercator, 17 July 1973, D/O 2930, 1430-1440 GMT (bottom).

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FIG. 15. Nimbus 5 ESMR, 19.35 GHz, (138 to 210K gray scale) and THIR (11 /mi) photofacsimile imagery of Tropical Depression Dot on 18 July 1973, D/O 2944 (top). Nimbus 5 ESMR (19.35 GHz), hand-drawn grid print map analysis of Tropical Depression Dot, in rainfall rates (mm hr*1), 1:2.5 million Mercator, 18 July 1973, D/O 2944, 1535-1540 GMT (bottom).

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"Circular Exhaust Cloud" (CEC) located just northeast these areas are often not detectable with other satellite of the center of circulation (21N, 139.3E, based upon sensors, a new means of studying tropical cyclones is DMSP imagery). The storm was rapidly intensifying at now available. These new observations of convergence this time and reached the typhoon stage within 12 hr. A phenomena should be related to changes in storm in- comparison of the THIR image with the ESMR dra- tensity and direction of motion. More direct methods of matically emphasizes the ability of the microwave radia- rainfall measurements over the ocean by high resolution tion to penetrate nonprecipitating clouds. Joint Typhoon active radar satellite-borne systems are now under study Warning Center analysis (not shown) indicates that a for Shuttle Spacelab Applications (Durrani et al., 1974; surface asymptote of convergence is well aligned with the Eckerman, 1974 5). northwest-southeast oriented precipitating rainband connecting Ellen to Billie. In addition, a 700 mb trough References overlies this asymptote. An intense area of precipitation Ackerman, B., 1963: The distribution of liquid water in is also produced where this convergence area intersects hurricanes. National Hurricane Research Project Report the clouds at the southwestern end of Ellen's circulation. No. 62, U.S. Dept. of Commerce, Wash., D.C., 41 pp. Aerojet Electrosystems Co., 1973: Meteorological applications e. Tropical depression Dot of passive microwave radiometry. Final Report, SAMSO Dot became a typhoon on 16 July 1973 and diminished TR No. 73-206, Vol. Ill, Part I and II, 1750 FR-1, Azusa, Calif. in its intensity as it passed over Hong Kong on a north- Allison, L. J., E. B. Rodgers, T. T. Wilheit, and R. Wexler, eastly overland track. Figure 15 shows Dot in the weak- 1974: A multi-sensor analysis of Nimbus 5 data on 22 ening tropical depression stage over the East China Sea January 1973. NASA X-910-74-20, Goddard Space Flight with Tropical storm Billie farther to the north. The Center, 54 pp. center of circulation of Dot (30 kt winds) was located Barrett, E. C., 1970: The estimation of monthly rainfall from near 27.2N, 125E, which positions it on the western edge satellite data. Mon. Wea. Rev., 98, 322-327. of the major overcast cloudiness and heavier rainfall Cardone, V. J., W. J. Pierson, R. K. Moore, and J. D. region (Fig. 15, bottom) (U.S. Fleet Weather Center/ Young, 1973: Preliminary report on SKYLAB S-193 radscat Joint Typhoon Warning Center, 1973). This is in agree- measurements of hurricane Ava. CUNY UIO Report No. ment with the fact that in the weaker categories of storm 23, KU RSL Report 254-1, 16 pp. intensity the center of circulation is in an area of slight Catoe, C., W. Nordberg, P. Thaddeus, and G. Ling, 1967: winds with very light or no rainfall, and asymmetrically Preliminary results from aircraft flight tests of an electrically scanning microwave radiometer. NASA X-622-67-352, positioned with respect to the main overcast precipitat- Goddard Space Flight Center, Greenbelt, Md., 35 pp. ing body of the storm (Fett, 1966b). Cry, G. W., 1967: Effects of tropical cyclone rainfall on the One final point is of special interest. A line connecting distribution of precipitation over the eastern and southern Dot with Billie to the north (34N, 122.5E) should coin- United States. ESS A Professional Paper 1, U.S. Dept. of cide with the trough line between these two systems. Commerce, 67 pp. The ESMR image and rainfall rate map (Fig. 15) both Durrani, S., T. Golden, J. Morakis, R. Waetjen, and J. Des- indicate higher water vapor content east of the trough kevich, 1974: Potential shuttle spacelab applications. NASA line with dryer conditions to the west. This is suggestive X-950-74-70, Goddard Space Flight Center, 64 pp. of weakening storms reverting to asymmetrical wave-like Dvorak, V. F., 1973: A technique for the analysis and forecasting of tropical cyclone intensities from satellite pic- configurations with major areas of convergence and tures. NOAA Technical Memorandum NESS 45, Washing- moisture east of the trough axis and divergent—dryer ton, D.C. 19, pp. areas to the west (Riehl, 1954). ERTS/Nimbus Project, 1972: The Nimbus 5 users guide. NASA, Goddard Space Flight Center, 162 pp. 5. Conclusion Fett, R. W., 1964: Aspects of hurricane structure: New model It has been shown that Nimbus 5 ESMR data can be considerations suggested by TIROS and Project Mercury used to delineate rain areas and provide semi-quantitative observations. Mon. Wea. Rev., 92, 43-60. rainfall rates within oceanic tropical cyclones on a 12-hr , 1966a: Upper-level structure of the formative tropical basis, using a theoretical numerical model for calibra- cyclone. Mon. Wea. Rev., 94, 9-18. tion. The derivation of rainfall rates using a variety of , 1966b: Life cycle of Tropical Cyclone Judy as Re- vealed by ESSA 2 and Nimbus 2. Mon. Wea. Rev., 94, more realistic tropical models is now being evaluated 5 605-610. (Chang and Curran, 1974 ). Furthermore, it has been , and S. Brand, 1974: Tropical cyclone movement fore- demonstrated that the ESMR sensor is able to detect casts based on observations from satellite. ENVPREDRSH- areas of heavy concentrations of low level water vapor as FAC Tech. Paper 1-74, Naval Postgraduate School, Mon- demonstrated in Hurricane Ava and Tropical Depression terey, Calif., 70 pp. Dot. When ESMR data are combined with higher resolu- Follansbee, W. A., 1973: Estimation of average daily rainfall tion visual and infrared imagery, a better understanding from satellite cloud photographs. NOAA Technical Memo- of the dynamics of the storms can be obtained (Dvorak, randum NESS 44, 39 pp. 1973). The ESMR data uniquely define the precipitation areas where maximum convergence is occurring. Since s Private correspondence.

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Unauthenticated | Downloaded 10/09/21 08:10 AM UTC Bulletin Arnerican Meteorological Society

Fujita, T. T., and P. G. Black, 1970: In and outflow field nique. Final Report, Vol. 1, 2, Radiometric Technology of Hurricane Debbie as revealed by echo and cloud veloci- Inc., Contract 2-35309, for USDOC, NOAA, NESS. ties from airborne radar and ATS 3 pictures. SMRP Re- Riehl, H., 1954: Tropical meteorology. New York, McGraw- search Paper No. 93, University of Chicago, Chicago, Hill Book Co., Inc., 392 pp. (see p. 215). Illinois, 6 pp. , and J. Malkus, 1961: Some aspects of Hurricane Daisy, , T. Izawa, K. Watanabe, and I. Imai, 1967: A model 1958. Tellus, 13, 181-213. of typhoons accompanied by inner and outer rainbands. Ross, D., 1974: A remote sensing study of Pacific Hurricane /. Appl. Meteor., 6, 3-19. Ava. Proc. 9th International Symposium of the Remote Gentry, R. C., 1964: A study of hurricane rainbands. Na- Sensing of the Environment, Willow Run Laboratories, Ann tional Hurricane Research Project Report No. 69, U.S. Arbor, Michigan, 15-20 April, 1974. Dept. of Commerce, 85 pp. Sabatini, R. R., and E. S. Merritt, 1973: The Nimbus 5 ESMR , T. T. Fujita, and R. C. Sheets, 1970: Aircraft, space- and its application to storm detection. Final Report, Con- craft, satellite and radar observations of Hurricane Gladys, tract N62306-72-C-0153, EPRF, U.S. Navy, Monterey, Cali- 1968. J. Appl. Meteor, 9, 837-850. fornia, Earth Satellite Corp., Washington, D.C., 45 pp. Gloersen, P., W. Nordberg, T. J. Schmugge, T. T. Wilheit, Scherer, W. D., and M. D. Hudlow, 1971: A technique for and W. J. Campbell, 1972: Microwave signatures of first- assessing probable distributions of tropical precipitation year and multi-year sea ice. NASA X-652-73-341, Goddard echo lengths for X-band radar from Nimbus 3 HRIR Space Flight Center, 10 pp. Data. BOMEX Bulletin No. 10, NOAA, 63-68. Goodyear, H. V., 1968: Frequency and areal distribution of Schmugge, T., P. Gloersen, and T. T. Wilheit, 1972: Remote tropical rainfall in the U.S. coastal region on the Gulf of sensing of soil moisture with microwave radiometers. NASA Mexico. ESSA Technical Report WB-7, U.S. Dept. of X-652-72-305, Goddard Space Flight Center, 32 pp. Commerce, 33 pp. , A. Rango, L. J. Allison, and T. T. Wilheit, 1974: Hydro- Gray, W. M., 1973: Feasibility of beneficial hurricane modi- logic applications of Nimbus 5 ESMR data. NASA X-910- fication by carbon dust seeding. NOAA N 22-65-73 (G), 74-51, Goddard Space Flight Center, 21 pp. Dept. of Atmospheric Science, Colorado State University, Schoner, R. W., and S. Molansky, 1956: Rainfall associated Ft. Collins, Colorado, 130 pp. with hurricanes. National Hurricane Research Project Re- Griffith, C. G., and W. L. Woodley, 1974: Rainfall estimates port No. 3, Hydrologic Services Division, U.S. Weather from geosynchronous satellite imagery. Proc., 11th Space Bureau, 305 pp. Congress, Cocoa Beach, Florida. Sheets, R. C., 1968: The structure of Hurricane Dora, 1964. Gruber, A., 1973: Estimating rainfall in regions of active ESSA Technical Memo. ERL NHRL-83, 64 pp. convection. J. Appl. Meteor., 12, 110-118. , 1974: Unique data set obtained in Hurricane Ellen, Gunn, K. L. S., and T. U. R. East, 1954: The microwave 1973. Bull. Amer. Meteor. Soc., 55, 144-146. properties of precipitation particles. Quart. J. Roy. Sikdar, D. N., 1972: ATS-3 observed cloud brightness field Meteor. Soc., 80, 522-545. related to meso- to subsynoptic scale rainfall pattern. Hawkins, H. F., and D. Rubsam, 1968: Hurricane Hilda, Tellus, 24, 400-412. 1964. Part II, Mon. Wea. Rev., 96, 617-636. Skidmore, R. W., and J. F. W. Purdom, 1973: Application of Mariners Weather Log, 1973: Rough log, North Pacific Wea- meteorological satellite data in analysis and forecasting. ther, June-July 1973, 17, 333-334. Supplement 2 to ESSA Technical Report NESC 51, U.S. Dept. of Commerce, NOAA, NESS, 59 pp. Martin, D. W., and W. D. Scherer, 1973: Review of satellite Sugg, A. L., 1966: The hurricane season of 1965. Mon. Wea. rainfall estimation methods. Ball. Amer. Meteor. Soc., 54, Rev., 94, 183-191. 661-674. Theon, J., 1973: A multispectral view of the Gulf of Meyer, W. D., 1973: Data acquisition and processing pro- Mexico. Bull. Amer. Meteor. Soc., 54, 934-937. gram. Bull. Amer. Meteor. Soc., 54, 1251-1254. U.S. Fleet Weather Center/Joint Typhoon Warning Center, Miller, B. I., 1958: On the momentum and energy balance 1973: Annual typhoon report. Guam, Mariana Is., 99 pp. of Hurricane Helene, 1958. National Hurricane Research Westwater, E. R., 1972: Microwave emission from clouds. Project Report No. 53, U.S. Weather Bureau, 19 pp. NOAA Technical Report ERL 219-WPL 18. Nimbus 5 Data Catalog, 1973: Vols. 1 to 3, Goddard Space Wilheit, T. T., 1972: The electrically scanning microwave Flight Center. radiometer (ESMR) experiment. Nimbus 5 Users Guide, Nordberg, W., J. Conaway, and P. Thaddeus, 1969: Micro- NASA Goddard Space Flight Center, 55-105. wave observations of sea state from aircraft. Quart. J. Roy. , J. C. Blinn, W. Campbell, A. Edgerton, and W. Nord- Meteor. Soc., 95, 408 pp. berg, 1972: Aircraft measurements of microwave emis- , J. Conaway, D. B. Ross, and T. T. Wilheit, 1971: Mea- sion from Arctic Sea Ice. Remote Sensing of the Environ- surement of microwave emission from a foam-covered wind ment, 2, 129-139. driven sea. J. Atmos. Sci., 28, 429-435. , J. Theon, W. Shenk, and L. J. Allison, 1973: Mete- Paris, J. F., 1969: Microwave radiometry and its application orological interpretations of the images from Nimbus 5 to marine meteorology and oceanography. Ref. No. 69-IT, electrically scanned microwave radiometer. NASA X-651- Dept. of Oceanography, Texas A&M University, College 73-189, Goddard Space Flight Center, 21 pp. Station, Texas. Woodley, W. L., B. Sancho, and A. H. Miller, 1972: Rainfall Porter, R. A., and F. J. Wentz, III, 1972: Research to develop estimation from satellite cloud photographs. NOAA Tech- a microwave radiometric ocean temperature sensing technical Memo. ERL-OD-11, 43 pp.

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Unauthenticated | Downloaded 10/09/21 08:10 AM UTC —ACOUSTIC SOUNDER—

available from Science Assoc., Inc. • manufactured by AeroVironment Inc.

CATG. NO. 195 ACOUSTIC SOUNDER

When sound is propagated upward in the atmosphere, from a source that is co- located with a receiver, the intensity of the measured backscatter is related to small scale atmospheric temperature variations (similar to the manner in which a depth sounder identifies thermoclines in water). This is the principle of the Acoustic Sounder, some- times referred to as Acoustic Radar. It is a ground based remote sounding system for acquiring data that can be used—to identify inversion heights; to detect layers of turbu- lence when there is a discontinuity in temperature ; to outline the top of a layer of fog, or low clouds; to track wave motion; to measure convective plumes; and, to outline frontal surfaces. New applications are developing rapidly with experience, in techniques and equipment.

In the Model 300, which emits short bursts at 1,600 Hz., the sound source and the receiver are centered in a 4-ft. diameter parabolic reflector. The sound that is back- scattered from small scale temperature variations is recorded as a time-height plot of dis- continuities above the instrument, presently to heights of about 1,000 meters.

A complete system consists of a parabolic antenna and transducer; a control module and recorder, operating at 115 Y. 60 Hz. 50 W.; and an Acoustic Enclosure. System price, $8,400.00 (without Enclosure, $6,750.00). More details on request.

SCIENCE ASSOCIATES, INC., 230 NASSAU ST., PRINCETON, NJ. 08540 (609) 924-4470

1090 Vol. 55, No. 9, September 1974

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