Search Results
Abstract
Sub-Saharan West Africa (10–20°N) receives moisture from the tropical Atlantic via low-level south-westerly flow across the southwestern coast of West Africa. This paper utilizes a 1arge data set to identify the tropical Atlantic (30°N–30°S) surface atmospheric and oceanic patterns for two years when sub-Saharan West Africa experienced anomalous weather. Comparison is made with 60-year (1911–70) average fields.
The following tropical Atlantic surface features were located/centered 300–500 km further south in the deficient sub-Saharan rainy season (July-September) of 1968 than the more abundant 1967 rainy season— the kinematic axis between the Northern and Southern Hemisphere trades, the near-equational convergence tune, the near-equatorial pressure trough, the zone of maximum sea surface temperature (SST), the mid-Atlantic maxima of precipitation frequency and total cloudiness, and the center of the North Atlantic subtropical high. Sixty-year mean positions of these features were generally intermediate between the 1967 and 1968 locations. Rainfall was more frequent immediately south of the Gulf of Guinea coast and more abundant along this coast, during the 1968 sub-Saharan drought than in 1967. During the dry July-September 1968, positive SST departures occurred south of 10°N and east of 35°W, with a southwest-northwest oriented negative SST anomaly immediately to the northwest. The opposite SST departure pattern characterized July-September 1967.
The July-September 1968 departures from 60-year average patterns were largely characteristic of April-June 1968. In contrast, the July-September 1967 anomalies showed little evidence of evolving during preceding
Abstract
Sub-Saharan West Africa (10–20°N) receives moisture from the tropical Atlantic via low-level south-westerly flow across the southwestern coast of West Africa. This paper utilizes a 1arge data set to identify the tropical Atlantic (30°N–30°S) surface atmospheric and oceanic patterns for two years when sub-Saharan West Africa experienced anomalous weather. Comparison is made with 60-year (1911–70) average fields.
The following tropical Atlantic surface features were located/centered 300–500 km further south in the deficient sub-Saharan rainy season (July-September) of 1968 than the more abundant 1967 rainy season— the kinematic axis between the Northern and Southern Hemisphere trades, the near-equational convergence tune, the near-equatorial pressure trough, the zone of maximum sea surface temperature (SST), the mid-Atlantic maxima of precipitation frequency and total cloudiness, and the center of the North Atlantic subtropical high. Sixty-year mean positions of these features were generally intermediate between the 1967 and 1968 locations. Rainfall was more frequent immediately south of the Gulf of Guinea coast and more abundant along this coast, during the 1968 sub-Saharan drought than in 1967. During the dry July-September 1968, positive SST departures occurred south of 10°N and east of 35°W, with a southwest-northwest oriented negative SST anomaly immediately to the northwest. The opposite SST departure pattern characterized July-September 1967.
The July-September 1968 departures from 60-year average patterns were largely characteristic of April-June 1968. In contrast, the July-September 1967 anomalies showed little evidence of evolving during preceding
Abstract
This study investigates the relation between tropospheric static stability and central North American growing season (May–August) rainfall for the highly contrasting years of 1975. 1976, and 1979. It uses two extensive sets of meteorological data (individual rawinsonde soundings for 38 stations; hourly rainfall totals for 854–944 locations) for the region extending from the Rocky to the Appalachian Mountains and from the Gulf Coast to approximately 55°N in Canada. The major objectives are to: (i) ascertain which of the many available methods of parameterizing static stability are most strongly related to the above (predominantly convective) rainfall; and (ii) quantify the rainfall variance fraction explained by static stability alone, as opposed to other atmospheric processes/conditions. Forty static stability indices and related thermodynamic parameters (SSITPs) are treated.
The results pertaining to objective (i) are definitive and those concerning (ii) are encouraging. The SSITPs that correlate most strongly with rainfall amount consistently include the lifting condensation level (LCL) (near-regionwide) and the convective condensation level (CCL) (western U.S. Great Plains) for the afternoon half-day, and K-type and SWEAT indices (eastern United States) and the CCL and convective temperature (U.S. Great Plains) for the morning half-day. In contrast, the SSITPs developed for forecasting severe thunderstorms and tornadoes correlate poorly with rainfall amount. Except on the U.S. Great Plains, the maximum SSITP-rainfall amount correlation magnitudes tend to be larger for the afternoon half-day (average of 0.47–0.49) than the morning half-day (0.37–0.39). Particularly high maximum afternoon SSITP-rainfall amount correlation magnitudes were obtained for the eastern United States (0.50–0.70); earlier work of this type seldom yielded correlation magnitudes above 0.32. For the SSITPs that correlate most strongly with rainfall amount on a regionwide basis (ICL variant for afternoon; modified-K index for morning), we also document the considerable spatial and intraseasonal variability of the thresholds beyond which the probability of rainfall exceeds that of no rainfall.
Abstract
This study investigates the relation between tropospheric static stability and central North American growing season (May–August) rainfall for the highly contrasting years of 1975. 1976, and 1979. It uses two extensive sets of meteorological data (individual rawinsonde soundings for 38 stations; hourly rainfall totals for 854–944 locations) for the region extending from the Rocky to the Appalachian Mountains and from the Gulf Coast to approximately 55°N in Canada. The major objectives are to: (i) ascertain which of the many available methods of parameterizing static stability are most strongly related to the above (predominantly convective) rainfall; and (ii) quantify the rainfall variance fraction explained by static stability alone, as opposed to other atmospheric processes/conditions. Forty static stability indices and related thermodynamic parameters (SSITPs) are treated.
The results pertaining to objective (i) are definitive and those concerning (ii) are encouraging. The SSITPs that correlate most strongly with rainfall amount consistently include the lifting condensation level (LCL) (near-regionwide) and the convective condensation level (CCL) (western U.S. Great Plains) for the afternoon half-day, and K-type and SWEAT indices (eastern United States) and the CCL and convective temperature (U.S. Great Plains) for the morning half-day. In contrast, the SSITPs developed for forecasting severe thunderstorms and tornadoes correlate poorly with rainfall amount. Except on the U.S. Great Plains, the maximum SSITP-rainfall amount correlation magnitudes tend to be larger for the afternoon half-day (average of 0.47–0.49) than the morning half-day (0.37–0.39). Particularly high maximum afternoon SSITP-rainfall amount correlation magnitudes were obtained for the eastern United States (0.50–0.70); earlier work of this type seldom yielded correlation magnitudes above 0.32. For the SSITPs that correlate most strongly with rainfall amount on a regionwide basis (ICL variant for afternoon; modified-K index for morning), we also document the considerable spatial and intraseasonal variability of the thresholds beyond which the probability of rainfall exceeds that of no rainfall.
Abstract
We investigate the method-dependence of large-scale vertical motion (LSVM) estimates given by four variants of the kinematic approach (Endlich-Clark triangle; Chien-Smith pentagon; Objective analysis; Kung optimization) and the Limited-area Fine-Mesh II (LFM-II) model of the U.S. National Weather Service. The treatment spans 54 rawinsonde sounding times from three contrasting periods during the 1979 summer in the central United States that each included both widespread and abundant rainfall and intervening dry spells. Quantitative LSVM intercomparisons and evaluations for 500 and 700 mb are are with respect to cloud cover and rainfall data for 138 locations.
The Kung (especially) and LFM-II estimates have the narrowest frequency distributions. A particularly broad distribution is evident for the Endlich-Clark method. The Kung and LFM-II approaches yield significantly more frequent estimates of upward LSVM for overcast and rainy conditions (64–74 percent, depending on level and weather category) than the other methods (57–66 percent). However, the LFM-II also gives significantly more frequent estimates of upward LSVM for the fair weather condition of “0–5/10 cloud cover” (44–47 percent, depending on level) than all of the other techniques (36–41 percent). The Kung method gives very low such frequencies (38–39 percent). The above fair weather result, together with more detailed frequency distribution information, suggest that the LFM-II may be biased toward giving upward LSVM at 500 and 700 mb.
The foregoing findings are supported by additional analyses that intercompare the LSVM methods with respect to (i) their full frequency distributions when the more extreme cloud/rainfall conditions prevail and (ii) the frequency of occurrence of various cloud/rainfall categories when strong LSVM (defined separately for each method) is estimated. The results collectively suggest the Kung method to be superior to the other kinemtic approaches, in some cases substantially so, and also to the LFM-II. They offer guidance for the treatment of LSVM in meso- and synoptic-scale studies and climate dynamics.
Abstract
We investigate the method-dependence of large-scale vertical motion (LSVM) estimates given by four variants of the kinematic approach (Endlich-Clark triangle; Chien-Smith pentagon; Objective analysis; Kung optimization) and the Limited-area Fine-Mesh II (LFM-II) model of the U.S. National Weather Service. The treatment spans 54 rawinsonde sounding times from three contrasting periods during the 1979 summer in the central United States that each included both widespread and abundant rainfall and intervening dry spells. Quantitative LSVM intercomparisons and evaluations for 500 and 700 mb are are with respect to cloud cover and rainfall data for 138 locations.
The Kung (especially) and LFM-II estimates have the narrowest frequency distributions. A particularly broad distribution is evident for the Endlich-Clark method. The Kung and LFM-II approaches yield significantly more frequent estimates of upward LSVM for overcast and rainy conditions (64–74 percent, depending on level and weather category) than the other methods (57–66 percent). However, the LFM-II also gives significantly more frequent estimates of upward LSVM for the fair weather condition of “0–5/10 cloud cover” (44–47 percent, depending on level) than all of the other techniques (36–41 percent). The Kung method gives very low such frequencies (38–39 percent). The above fair weather result, together with more detailed frequency distribution information, suggest that the LFM-II may be biased toward giving upward LSVM at 500 and 700 mb.
The foregoing findings are supported by additional analyses that intercompare the LSVM methods with respect to (i) their full frequency distributions when the more extreme cloud/rainfall conditions prevail and (ii) the frequency of occurrence of various cloud/rainfall categories when strong LSVM (defined separately for each method) is estimated. The results collectively suggest the Kung method to be superior to the other kinemtic approaches, in some cases substantially so, and also to the LFM-II. They offer guidance for the treatment of LSVM in meso- and synoptic-scale studies and climate dynamics.
Abstract
This paper describes aspects of a strong moisture surge over the Gulf of California that was observed during the 2004 North American Monsoon Experiment. Although a variety of special observation platforms aid the analyses, the authors focus on observations collected during two NOAA research aircraft flights made on 12 and 13 July. These flights sampled the initial and mature phases of a strong surge associated with Tropical Storm Blas. The first flight is identified by both a convective outflow and another feature, both deeper and with larger spatial scale, ahead of the outflow in association with the surge’s leading edge. The surge speed, ~18 m s−1, was identified from anomaly analysis of surface station pressure data. Observations show interesting multiscale features associated with the surge during its initial stages but do not allow for unambiguous identification of the surge’s forcing mechanism or dynamical properties. Data from the second flight were used to describe the along- and cross-gulf structure of the enhanced low-level flow associated with the surge event. The strongest winds were over the northernmost gulf, with weaker winds over the surrounding coastal areas. The kinematic moisture flux increased toward the northern gulf; wind speed is the main control on the flux as the moist layer shows only small horizontal gradients. Over the northern gulf, the combination of a very shallow moist layer and a shallow low-level jet yield maximum moisture fluxes near 950 hPa that are almost an order of magnitude larger than those at 850 hPa.
Abstract
This paper describes aspects of a strong moisture surge over the Gulf of California that was observed during the 2004 North American Monsoon Experiment. Although a variety of special observation platforms aid the analyses, the authors focus on observations collected during two NOAA research aircraft flights made on 12 and 13 July. These flights sampled the initial and mature phases of a strong surge associated with Tropical Storm Blas. The first flight is identified by both a convective outflow and another feature, both deeper and with larger spatial scale, ahead of the outflow in association with the surge’s leading edge. The surge speed, ~18 m s−1, was identified from anomaly analysis of surface station pressure data. Observations show interesting multiscale features associated with the surge during its initial stages but do not allow for unambiguous identification of the surge’s forcing mechanism or dynamical properties. Data from the second flight were used to describe the along- and cross-gulf structure of the enhanced low-level flow associated with the surge event. The strongest winds were over the northernmost gulf, with weaker winds over the surrounding coastal areas. The kinematic moisture flux increased toward the northern gulf; wind speed is the main control on the flux as the moist layer shows only small horizontal gradients. Over the northern gulf, the combination of a very shallow moist layer and a shallow low-level jet yield maximum moisture fluxes near 950 hPa that are almost an order of magnitude larger than those at 850 hPa.
