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Abstract
Precipitating cumulus clouds occur regularly in the afternoon over the south Florida peninsula during summer months. A day without significant rainfall or radar echoes is rare. This paper discusses one such day, 23 July 1987, during which a dry, stable airmass covered the Florida peninsula. Nonprecipitating shallow cumulus formed in a few areas, but there were not any deep, precipitating cumulus clouds over land. The thermodynamic characteristics of the airmass are described and the synoptic-scale patterns that produced the airmass are presented.
Abstract
Precipitating cumulus clouds occur regularly in the afternoon over the south Florida peninsula during summer months. A day without significant rainfall or radar echoes is rare. This paper discusses one such day, 23 July 1987, during which a dry, stable airmass covered the Florida peninsula. Nonprecipitating shallow cumulus formed in a few areas, but there were not any deep, precipitating cumulus clouds over land. The thermodynamic characteristics of the airmass are described and the synoptic-scale patterns that produced the airmass are presented.
Abstract
Computations of peninsula-scale convergence in southern Florida reveal that daily-averaged surface convergence on sea-breeze days with relatively little rainfall is larger than on days with widespread rain. This negative correlation between surface convergence and area-averaged rainfall occurs as a result of significantly less surface convergence in the late afternoon and early evening on those days with considerable rainfall. The decrease in sea-breeze convergence during the late afternoon of the days with extensive rainfall is apparently a consequence of the downdrafts and thunderstorm-generated circus cloud cover produced by the deep convection that forms in the sea-breeze convergence zones. Before the typical midafternoon maximum of deep convection on sea-breeze days, there is no significant difference between the surface convergence averaged for days with widespread rain and for days with little rain. Important differences are observed, however, in the middle troposphere, where the sea-breeze days with widespread rain are more moist and have cooler temperatures than the days with little or no rain. The observations suggest that both the magnitude and timing of the convective response to the sea-breeze forcing during the afternoon are very sensitive to the moisture amount and some-what less sensitive to the thermal stability in the midtroposphere.
Abstract
Computations of peninsula-scale convergence in southern Florida reveal that daily-averaged surface convergence on sea-breeze days with relatively little rainfall is larger than on days with widespread rain. This negative correlation between surface convergence and area-averaged rainfall occurs as a result of significantly less surface convergence in the late afternoon and early evening on those days with considerable rainfall. The decrease in sea-breeze convergence during the late afternoon of the days with extensive rainfall is apparently a consequence of the downdrafts and thunderstorm-generated circus cloud cover produced by the deep convection that forms in the sea-breeze convergence zones. Before the typical midafternoon maximum of deep convection on sea-breeze days, there is no significant difference between the surface convergence averaged for days with widespread rain and for days with little rain. Important differences are observed, however, in the middle troposphere, where the sea-breeze days with widespread rain are more moist and have cooler temperatures than the days with little or no rain. The observations suggest that both the magnitude and timing of the convective response to the sea-breeze forcing during the afternoon are very sensitive to the moisture amount and some-what less sensitive to the thermal stability in the midtroposphere.
Abstract
A compositing technique has been used to analyze the wind field of easterly waves in the GATE region of western Africa and the eastern Atlantic. The compositing analysis is similar to the method of Reed and Recker (1971), but this study includes variations of the waves as a function of latitude in addition to variations as a function of east–west wavelength and height. A wave composited for the summer of 1974 has been isolated from the mean flow and analyzed on constant pressure maps. From these analyses, kinematic vertical motion has been computed in the lower and middle troposphere. The results show that the pattern of vertical motion calculated from the compositing agrees quite well with satellite cloud patterns.
Abstract
A compositing technique has been used to analyze the wind field of easterly waves in the GATE region of western Africa and the eastern Atlantic. The compositing analysis is similar to the method of Reed and Recker (1971), but this study includes variations of the waves as a function of latitude in addition to variations as a function of east–west wavelength and height. A wave composited for the summer of 1974 has been isolated from the mean flow and analyzed on constant pressure maps. From these analyses, kinematic vertical motion has been computed in the lower and middle troposphere. The results show that the pattern of vertical motion calculated from the compositing agrees quite well with satellite cloud patterns.
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Sanders designed a barotropic tropical cyclone (TC) track prediction model for the North Atlantic TC basin that became known as the Sanders barotropic (SANBAR) model. It predicted the streamfunction of the deeplayer mean winds (tropical circulation vertically averaged from 1000 to 100 hPa) that represents the vertically averaged tropical circulations. Originally, the wind input for the operational objective analysis (OA) consisted of winds measured by radiosondes and 44 bogus winds provided by analysis at the National Hurricane Center (NHC), which corresponded to the vertically averaged flow over sparsely observed tropical, subtropical, and midlatitude oceanic regions. The model covered a fixed regional area and had a grid size of ~ 154 km. It estimated the initial storm motion solely on the basis of the large-scale flow from the OA, not taking into account the observed storm motion.
During 1970, the SANBAR model became the first dynamical TC track model to be run operationally at NHC. Track forecasts of SANBAR were verified from the 1971 TC season when track model verifications began at NHC until its retirement after the 1989 Atlantic TC season. The average annual SANBAR forecast track errors were verified relative to Climatology and Persistence (CLIPER), the standard no-skill track forecast. Comparison with CLIPER determines the skill of track forecast methods. Verifications are presented for two different versions of the SANBAR model system used operationally during 1973–84 and 1985–89. In homogeneous comparisons (i.e., includes only forecasts for the same initial times) for the former period, SANBAR's track forecasts were slightly better than CLIPER at 24–48-h forecast intervals; however, from 1985 to 1989 the average SANBAR track forecast errors from 24–72 h were ~10% more skillful than homogeneous CLIPER track forecasts.
Abstract
Sanders designed a barotropic tropical cyclone (TC) track prediction model for the North Atlantic TC basin that became known as the Sanders barotropic (SANBAR) model. It predicted the streamfunction of the deeplayer mean winds (tropical circulation vertically averaged from 1000 to 100 hPa) that represents the vertically averaged tropical circulations. Originally, the wind input for the operational objective analysis (OA) consisted of winds measured by radiosondes and 44 bogus winds provided by analysis at the National Hurricane Center (NHC), which corresponded to the vertically averaged flow over sparsely observed tropical, subtropical, and midlatitude oceanic regions. The model covered a fixed regional area and had a grid size of ~ 154 km. It estimated the initial storm motion solely on the basis of the large-scale flow from the OA, not taking into account the observed storm motion.
During 1970, the SANBAR model became the first dynamical TC track model to be run operationally at NHC. Track forecasts of SANBAR were verified from the 1971 TC season when track model verifications began at NHC until its retirement after the 1989 Atlantic TC season. The average annual SANBAR forecast track errors were verified relative to Climatology and Persistence (CLIPER), the standard no-skill track forecast. Comparison with CLIPER determines the skill of track forecast methods. Verifications are presented for two different versions of the SANBAR model system used operationally during 1973–84 and 1985–89. In homogeneous comparisons (i.e., includes only forecasts for the same initial times) for the former period, SANBAR's track forecasts were slightly better than CLIPER at 24–48-h forecast intervals; however, from 1985 to 1989 the average SANBAR track forecast errors from 24–72 h were ~10% more skillful than homogeneous CLIPER track forecasts.
Abstract
Routine perusal of surface maps covering tropical and subtropical regions frequently reveals the presence of large-scale patterns in the 24 h pressure tendency. Previous analyses of surface pressure, 1000 mb heights and 500 mb heights have determined that these oscillations, have a preferred frequency and are caused by a global-scale wave with a wavelength equal to the circumference of the earth and a period of about 5 days. In order to investigate this wave motion in more detail, the troughs of 60 waves were followed from eastern Africa to the western Caribbean, and both surface and upper air data were composited relative to the troughs. The surface pressure analysis shows that the wave is nearly in phase from 30°N to 30°S and that the minimum amplitude occurs near the equator. Upper air observations in the region from 10–30°N indicate that the wave features do not tilt significantly in the vertical from the surface to 70 mb. Although the amplitude of the waves in the surface pressure field is less than 1 mb throughout the tropics; and the horizontal scale is zonal wavenumber 1 or 2, the data show that the wave affects the occurrence of both precipitation and thunder.
Abstract
Routine perusal of surface maps covering tropical and subtropical regions frequently reveals the presence of large-scale patterns in the 24 h pressure tendency. Previous analyses of surface pressure, 1000 mb heights and 500 mb heights have determined that these oscillations, have a preferred frequency and are caused by a global-scale wave with a wavelength equal to the circumference of the earth and a period of about 5 days. In order to investigate this wave motion in more detail, the troughs of 60 waves were followed from eastern Africa to the western Caribbean, and both surface and upper air data were composited relative to the troughs. The surface pressure analysis shows that the wave is nearly in phase from 30°N to 30°S and that the minimum amplitude occurs near the equator. Upper air observations in the region from 10–30°N indicate that the wave features do not tilt significantly in the vertical from the surface to 70 mb. Although the amplitude of the waves in the surface pressure field is less than 1 mb throughout the tropics; and the horizontal scale is zonal wavenumber 1 or 2, the data show that the wave affects the occurrence of both precipitation and thunder.
Abstract
This study uses both spectral and compositing analyses to determine the structure of North African easterly waves during the summers of 1968 and 1969. The average period and wavelength computed by the power-spectrum method are 4.5 days and 3800 km and for the compositing analysis 3.8 days and 3100 km. Although the period and wavelength of individual waves vary considerably, the spectral results are in qualitative agreement with the compositing results except in extreme northern Africa where eastward-moving disturbances of the same period affect the spectral computations. The maximum surface amplitudes of u, v, p and Td associated with the waves occur about 20N near the mean location of the east-west oriented region of minimum surface pressure. East of the Greenwich meridian the amplitude decreases and the wave motion is barely detectable with surface data near 25E. The compositing results show that the waves influence latitudes between 5 and 3ON and that wave features tilt from southwest to northeast. Both rainfall amount and thunderstorm activity are modulated by the passage of the waves with the rainfall maxima about 50% greater than the corresponding minima. Upper air calculations show that the vertical structure of the waves varies with the mean zonal wind in such a way that the meridional wind patterns tilt in the opposite direction to the mean zonal shear.
Abstract
This study uses both spectral and compositing analyses to determine the structure of North African easterly waves during the summers of 1968 and 1969. The average period and wavelength computed by the power-spectrum method are 4.5 days and 3800 km and for the compositing analysis 3.8 days and 3100 km. Although the period and wavelength of individual waves vary considerably, the spectral results are in qualitative agreement with the compositing results except in extreme northern Africa where eastward-moving disturbances of the same period affect the spectral computations. The maximum surface amplitudes of u, v, p and Td associated with the waves occur about 20N near the mean location of the east-west oriented region of minimum surface pressure. East of the Greenwich meridian the amplitude decreases and the wave motion is barely detectable with surface data near 25E. The compositing results show that the waves influence latitudes between 5 and 3ON and that wave features tilt from southwest to northeast. Both rainfall amount and thunderstorm activity are modulated by the passage of the waves with the rainfall maxima about 50% greater than the corresponding minima. Upper air calculations show that the vertical structure of the waves varies with the mean zonal wind in such a way that the meridional wind patterns tilt in the opposite direction to the mean zonal shear.
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The Hurricane Research Division collected radar reflectivity data with a portable recorder attached to National Weather Service (NWS) WSR-57 radar as Hurricanes Alicia of 1983 and Elena of 1985 approached the coastline of the United States. The reflectivity data were used to estimate rain rates for the eyewall region, including the rain-free eye, and the rainbands in the annular area outside the eyewall, but within 75 km of the center of the eye. The rain rates include reflectivity corrections that were based upon the variation of average returned power with range in four hurricanes
This study examines the temporal and spatial variations of rain rates in the cores of Hurricanes Alicia and Elena. In Alicia, variations of area-averaged rain rate (R) in the eyewall region were caused by the growth and decay of mesoscale convective areas. In Elena, the life cycles of individual convective cells also accounted for large changes in the eyewall R. In both hurricanes, the time series of R in the rainband region was less variable than the eyewall R, because the rainband region was larger than the eyewall and contained a smaller percentage of convection.
The distribution of precipitation in the eyewall and rainband regions was asymmetric. For several hours early in the observing period, the maximum rain rates in the eyewall and rainband regions of Alicia occurred in the left-front quadrant relative to the storm motion. Then, the heaviest rain in the eyewall region shifted to the right-front quadrant and that in the rainband region moved to the right of the storm track. In Elena, the maximum rain rates in the eyewall and rainband regions remained in the right-front quadrant throughout the computational period. About 55% of the precipitation in Elena's eyewall region occurred in the right-front quadrant.
Abstract
The Hurricane Research Division collected radar reflectivity data with a portable recorder attached to National Weather Service (NWS) WSR-57 radar as Hurricanes Alicia of 1983 and Elena of 1985 approached the coastline of the United States. The reflectivity data were used to estimate rain rates for the eyewall region, including the rain-free eye, and the rainbands in the annular area outside the eyewall, but within 75 km of the center of the eye. The rain rates include reflectivity corrections that were based upon the variation of average returned power with range in four hurricanes
This study examines the temporal and spatial variations of rain rates in the cores of Hurricanes Alicia and Elena. In Alicia, variations of area-averaged rain rate (R) in the eyewall region were caused by the growth and decay of mesoscale convective areas. In Elena, the life cycles of individual convective cells also accounted for large changes in the eyewall R. In both hurricanes, the time series of R in the rainband region was less variable than the eyewall R, because the rainband region was larger than the eyewall and contained a smaller percentage of convection.
The distribution of precipitation in the eyewall and rainband regions was asymmetric. For several hours early in the observing period, the maximum rain rates in the eyewall and rainband regions of Alicia occurred in the left-front quadrant relative to the storm motion. Then, the heaviest rain in the eyewall region shifted to the right-front quadrant and that in the rainband region moved to the right of the storm track. In Elena, the maximum rain rates in the eyewall and rainband regions remained in the right-front quadrant throughout the computational period. About 55% of the precipitation in Elena's eyewall region occurred in the right-front quadrant.
