Variations In Convective Activity Research Articles

The creation and climatology of a large independent rainfall event database for Great Britain

AbstractExtreme rainfall studies generally focus on hourly or daily rainfall accumulations. Additionally, studies focus on annual maxima (AM) or ‘peak intensities’. Although this approach is useful, the use of fixed‐duration accumulations simplifies inherently continuous rainfall processes which, at a rain gauge, are experienced as rainstorms of varying duration. A gap also exists in the study of sub‐hourly rainfall extremes which are an important source of flash floods. Here, we present the first large‐scale study of rainstorms in the United Kingdom. We leverage a recently created sub‐hourly resolution rain gauge dataset for Great Britain to identify over 70,000 rainstorms that generated AM rainfall intensities across a range of sub‐hourly to daily durations at 1279 rain gauges up to 2018. Our findings are consistent with previous studies of UK rainfall extremes and their climatology, where strong east–west differences are found in the magnitude of autumn and winter long‐duration (over 12‐h) rainstorms. We observe broad similarities in the behaviour of short (under 4‐h) rainstorms across GB, which are dominated by seasonal variations in convective activity rather than by geographical location. Our study reveals substantial overlaps between AM of different durations, with single rainstorms containing AM across a wide range of durations. We also demonstrate the substantial correlation between rainstorm duration and volume. These results challenge common independence assumptions used in the statistical modelling of rainfall extremes using extreme value theory. Our results represent an opportunity to refine rainfall design methods towards improved, robust representations of rainstorms.

Cirrus observations in the tropical tropopause layer over the western Pacific

A polarization lidar was continuously operated aboard the research vessel Mirai in the tropical western Pacific over three northern winters: at 2.0°N, 138.0°E during November and December 2001; at 2.0°N, 138.5°E during November and December 2002; and at 7.5°N, 134.0°E during December 2004 and January 2005. Intensive radiosonde soundings were made from the vessel at 3‐h intervals during all three campaigns. The mechanisms that underlie the observed variations in cirrus in the tropical tropopause layer (TTL) are discussed from the viewpoint of large‐scale dynamics and transport. During the 2001 campaign, the tropopause region was cold, but the TTL was often clear, with only some subvisual cirrus. Potential vorticity data and trajectories show that the TTL during this period was strongly affected by dry air transport from the northern midlatitude lower stratosphere. During the 2002 campaign, a packet of large‐amplitude equatorial Kelvin waves was the primary control on the generation and disappearance of cirrus in the TTL. During the 2004–2005 campaign, a cold phase of large‐scale waves resulted in cirrus generation in the TTL in late December of 2004, similar to that observed during the 2002 campaign. Outflow from the South Pacific Convergence Zone (SPCZ) caused optically thick cirrus in the TTL, particularly during early January 2005, when we also observed regular diurnal variations in cirrus development within the TTL, that is, apparent sedimentation during the nighttime. We investigated two possible controlling processes, namely, horizontal advection together with diurnal variations in convective activity within the SPCZ and diurnal variations in local temperature due to tides and gravity waves. In the equatorial western Pacific, equatorial Kelvin waves are the important dynamical process that controls cirrus variations in the TTL. Dry‐air horizontal transport from the midlatitude lower stratosphere and wet‐air vertical transport near the tropical convergence regions should also be considered in fully explaining the cirrus observations in the TTL.

Variability of convective activity over the Bay of Bengal and the Arabian sea

The present study is an attempt to examine seasonal and interannual variability of convective activity over the north Indian Ocean. The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study for a period of 26 year (1979 to 2004). The period has been divided into two groups of 13 years each with period (i) from 1979 to 1991 and period (ii) from 1992 to 2004. The monthly and seasonal variation of convective activity along with its linear trends analysis during four seasons [winter, Jan-Feb (JF); pre-monsoon, Mar-May (MAM); monsoon, Jun-Sep (JJAS); & post-monsoon, Oct-Dec (OND)] are performed. 
 
 
 The difference of seasonal OLR anomalies in winter season between period (ii) & period (i) indicate increase in convective activity during recent period [period (ii)] compared to that of period (i) over the south Arabian Sea & south Bay of Bengal and just the opposite occurred over the rest regions associated with a significant decreasing trend of convective activity over the north and central Arabian Sea (at 99.9% level and 99% level respectively). During MAM the Bay of Bengal and the eastern part of Arabian Sea are relatively more convective during the recent period compared to the former period with significant increasing trend of convective activity over central Bay of Bengal (99% level) and northern (95% level) Bay of Bengal. The convective activity increases mainly over the southern parts of the Arabian Sea (dominated in the month of July) and some parts of southern and eastern Bay of Bengal in recent period compared to that of former period during the monsoon season and is associated with significant increasing trend (at 95 % level) of convective activity over the north Bay of Bengal. The post monsoon season also shows significant (at 95% level) increasing trend in convective activity over south Arabian Sea.

Diurnal Variation of Convective Activity and Precipitable Water around Ulaanbaator, Mongolia, and the Impact of Soil Moisture on Convective Activity during Nighttime

Abstract The diurnal variations of convective activity and precipitable water were investigated using a C-band airport radar and GPS receivers around Ulaanbaator (UB), Mongolia; this location was considered as an example of an arid region. The convective activity exhibited a pronounced diurnal cycle; it increased rapidly at 1100 local solar time (LST; 0300 UTC), reached the maximum at 1400 LST, and almost disappeared after 1900 LST. On the other hand, no diurnal variation of precipitable water could be observed, which implied that there was no considerable evapotranspiration, and the diurnal variation of the convective activity was irrelevant to the variation of water vapor. The reason why the deep convection could not develop at night is discussed using numerical modeling from the viewpoint of soil moisture. In the moist soil conditions assumed for humid simulations, an increase in the water vapor in the boundary layer due to evapotranspiration led to a potentially unstable condition that was sustained until night. Deep convection was formed at the southern foot of mountains where topographical convergence was expected. On the other hand, in the dry soil conditions assumed for the arid simulations, deep convection did not occur during nighttime even though topographical convergence was expected over the southern foot of the mountains. These features of dry soil conditions were consistent with the results from radar observations around UB. In other words, since the soil around UB is too dry in practice to sustain an unstable condition until night, the deep convection had to decay by night and could not be initiated at night.

Rates and Mechanisms of Water Mass Transformation in the Labrador Sea as Inferred from Tracer Observations*

Time series of hydrographic and transient tracer (3H and 3He) observations from the central Labrador Sea collected between 1991 and 1996 are presented to document the complex changes in the tracer fields as a result of variations in convective activity during the 1990s. Between 1991 and 1993, as atmospheric forcing intensified, convection penetrated to progressively increasing depths, reaching 2300 m in the winter of 1993. Over that period the potential temperature (θ)/salinity (S) properties of Labrador Sea Water stayed nearly constant as surface cooling and downward mixing of freshwater was balanced by excavating and upward mixing of the warmer and saltier Northeast Atlantic Deep Water. It is shown that the net change in heat content of the water column (150–2500 m) between 1991 and 1993 was negligible compared to the estimated mean heat loss over that period (110 W m−2), implying that the lateral convergence of heat into the central Labrador Sea nearly balances the atmospheric cooling on a surprisingly short timescale. Interestingly, the 3H–3He age of Labrador Sea Water increased during this period of intensifying convection. Starting in 1995, winters were milder and convection was restricted to the upper 800 m. Between 1994 and 1996, the evolution of 3H–3He age is similar to that of a stagnant water body. In contrast, the increase in θ and S over that period implies exchange of tracers with the boundaries via both an eddy-induced overturning circulation and along-isopycnal stirring by eddies [with an exchange coefficient of O(500 m2 s−1)]. The authors construct a freshwater budget for the Labrador Sea and quantitatively demonstrate that sea ice meltwater is the dominant cause of the large annual cycle of salinity in the Labrador Sea, both on the shelf and the interior. It is shown that the transport of freshwater by eddies into the central Labrador Sea (140 cm between March and September) can readily account for the observed seasonal freshening. Finally, the authors discuss the role of the eddy-induced overturning circulation with regard to transport and dispersal of the newly ventilated Labrador Sea Water to the boundary current system and compare its strength (2–3 Sv) to the diagnosed buoyancy-forced formation rate of Labrador Sea Water.

Relationship between the Indian monsoon rainfall and the tropospheric temperature over the Eurasian continent

Based on climatological data such as the all-India rainfall (AIR) for 1949–98, air temperatures in the troposphere and at the land surface for 1949–98 as well as precipitation and outgoing long-wave radiation (OLR) for 1979–98, the statistical relationship between the Indian monsoon rainfall and tropospheric temperature (TT) over the Eurasian continent is examined by using the correlation analysis and singular value decomposition (SVD) analysis methods. The main results are summarized as follows. (1) The June-September (JJAS) AIR has significant positive correlations with the 200–700 hPa TT averaged for Eurasia in JJAS. The area with the highest simultaneous correlation of upper TT with AIR for JJAS is located in western Eurasia. There is also a region with positive simultaneous correlations in the southern subtropics covering southern Africa. The JJAS AIR also shows significant positive correlations with TT over the Eurasian continent in March-May (MAM) and with the TT averaged over the Tibetan Plateau in September and October. Although some precursory springtime temperature signals for the anomalous summer monsoon are found in the upper TT over Eurasia, the variation in the upper TT appears mostly independent from that of the land surface temperature. (2) The anomalies of upper TT in JJAS are closely coupled with the variations in OLR related to the monsoon rainfall over tropical Asia and Africa. The spatial structure of the first SVD mode for OLR anomalies shows the coherent variations in convective activity from northern tropical Africa to India, while the pattern of the first SVD mode for 300 hPa temperature anomalies shows a distribution which is nearly symmetric with respect to the equator, with variations of the same sign over subtropical areas in both hemispheres and variations of the opposite sign around equatorial Africa. This pair of coupled patterns suggests that when the monsoon rains from tropical Africa to India become more intense, the upper troposphere from northern Africa to western Asia and over southern Africa becomes warmer. (3) The observed temperature variation in the upper troposphere over the Eurasian continent (especially western Eurasia) during the summer monsoon season is considered to be a result of the anomalous subsidence and thermal advection, dynamically induced by anomalous tropical heating associated with the enhanced monsoon rainfall. This study suggests that the tropical convective activity linked to the Indian monsoon rainfall has an impact on the atmospheric variability on a global scale.

Relationship between the Indian monsoon rainfall and the tropospheric temperature over the Eurasian continent

AbstractBased on climatological data such as the all‐India rainfall (AIR) for 1949–98, air temperatures in the troposphere and at the land surface for 1949–98 as well as precipitation and outgoing long‐wave radiation (OLR) for 1979–98, the statistical relationship between the Indian monsoon rainfall and tropospheric temperature (TT) over the Eurasian continent is examined by using the correlation analysis and singular value decomposition (SVD) analysis methods. The main results are summarized as follows. (1) The June‐September (JJAS) AIR has significant positive correlations with the 200–700 hPa TT averaged for Eurasia in JJAS. The area with the highest simultaneous correlation of upper TT with AIR for JJAS is located in western Eurasia. There is also a region with positive simultaneous correlations in the southern subtropics covering southern Africa. The JJAS AIR also shows significant positive correlations with TT over the Eurasian continent in March‐May (MAM) and with the TT averaged over the Tibetan Plateau in September and October. Although some precursory springtime temperature signals for the anomalous summer monsoon are found in the upper TT over Eurasia, the variation in the upper TT appears mostly independent from that of the land surface temperature. (2) The anomalies of upper TT in JJAS are closely coupled with the variations in OLR related to the monsoon rainfall over tropical Asia and Africa. The spatial structure of the first SVD mode for OLR anomalies shows the coherent variations in convective activity from northern tropical Africa to India, while the pattern of the first SVD mode for 300 hPa temperature anomalies shows a distribution which is nearly symmetric with respect to the equator, with variations of the same sign over subtropical areas in both hemispheres and variations of the opposite sign around equatorial Africa. This pair of coupled patterns suggests that when the monsoon rains from tropical Africa to India become more intense, the upper troposphere from northern Africa to western Asia and over southern Africa becomes warmer. (3) The observed temperature variation in the upper troposphere over the Eurasian continent (especially western Eurasia) during the summer monsoon season is considered to be a result of the anomalous subsidence and thermal advection, dynamically induced by anomalous tropical heating associated with the enhanced monsoon rainfall. This study suggests that the tropical convective activity linked to the Indian monsoon rainfall has an impact on the atmospheric variability on a global scale.

Relationships between Stability and Monsoon Convection

Abstract Rawinsonde data from the Australian Monsoon Experiment are analyzed to determine the manner in which the atmospheric stratification of density and moisture respond to large amounts of convective latent heat release. The study focuses on time series of data from a ship located at the northern end of the Gulf of Carpentaria during active and break periods of the monsoon. Variations in lapse rate or vertical stratification through the depth of the troposphere are found to occur mainly between active and break periods, rather than on a day-to-day basis. This is interpreted as being due to midtropospheric temperature being adjusted by dynamical processes over large scales rather than in situ response to localized convection. Between active and break periods large changes occurred in midtropospheric moisture. Variations in convective activity are well related to variations in lower and middle tropospheric moisture content. The break coincided with a drying due to large-scale horizontal advection. Conve...

The Influence of the Madden–Julian Oscillation on Ocean Surface Heat Fluxes and Sea Surface Temperature

The Madden‐Julian oscillation (MJO) involves pronounced variations in convection and large-scale circulation throughout the tropical troposphere. In addition, the MJO is also related to dynamic and thermodynamic variability near the surface and the upper ocean. This study uses observational data to characterize the changes in surface heat fluxes and sea surface temperature (SST) during the life cycle of the MJO. Variations in convective activity are described with outgoing longwave radiation (OLR) during the period January 1985 through September 1994. International Satellite Cloud Climatology Project data (January 1985‐ April 1991) and European Centre for Medium-Range Weather Forecasts surface analyses (January 1985‐December 1994) are used to derive surface fluxes of net shortwave radiation (SW), latent heat ( E), their difference (Q 5 SW 2 E), and SST. The spatial patterns of OLR, SW, E, Q, and SST anomalies reveal that the region of positive OLR anomalies that precede the occurrence of enhanced convection is associated with positive SW and negative E anomalies, which result in positive Q anomalies. The prevailing conditions in the region of positive Q anomalies favor the development of positive SST anomalies, which lead to variations of enhanced convection. In contrast the region of negative OLR anomalies is associated with negative SW and positive E anomalies. These conditions induce negative Q anomalies, which favor the formation of negative SST anomalies. The above results suggest a possible feedback between the oscillation and intraseasonal variations in SST and this may be an important mechanism for numerical simulations of the life cycle of the MJO.

ＧＭＳ‐ＩＲデータを用いたインドシナ半島域における対流活動の日周変化の解析

ＧＭＳ‐ＩＲデータを用いたインドシナ半島域における対流活動の日周変化の解析

The thermal response of the tropical atmosphere to variations in equatorial Pacific sea surface temperature

For the period 1966–1982, monthly mean temperature anomalies were determined for 30 pressure levels from 1000 mbar (0 km altitude) to 15 mbar (28 km) at five tropical radiosonde stations, four in the western Pacific Ocean (Yap, Majuro, Wake Island, and Pago Pago) and one in the Caribbean (Curacao). Long‐term trends in the data were removed, and the detrended time series were cross correlated with an index of nonseasonal Pacific sea surface temperature (SST) anomalies with lags ranging up to ±30 months. At each station the correlation diagrams show a clear division of the atmosphere into four distinct regions, each having a different response to SST variations. In the main troposphere (850–150 mbar; 2–14 km) the correlation is positive, with a maximum at a lag of 1–4 months, in general agreement with earlier studies. In the lower stratosphere (100–40 mbar; 16–22 km) the correlation is negative, with a similar delay, while the surface layer (1000–850 mbar; 0–2 km) and the upper troposphere (150–100 mbar; 14–16 km) show responses that vary from station to station. The correlation drops essentially to zero above 40 mbar (22 km). Examination of the lags at the different stations indicate that the tropospheric inphase and stratospheric out‐of‐phase responses appear to propagate zonally from an apparent point of origin in the east central equatorial Pacific with a speed of the order of 1 m s−1. The possible mechanisms underlying this propagation are discussed, and it is concluded that simple advection of air from centers of convective activity is unlikely to explain both the tropospheric and the stratospheric responses. An alternative explanation involving variations in the rate of tropospheric subsidence and stratospheric upwelling accompanying variations in convective activity is suggested. The long‐term trends in atmospheric temperature are shown to have a variation with height that parallels the variation in the SST correlation. This may indicate that the atmosphere is responding to a corresponding long‐term trend in Pacific sea surface temperatures.

The 30–50 Day Variation of Convective Activity over the Western Pacific Ocean with Emphasis on the Northwestern Region

Coherent 30–50 day oscillations between regions north and south of 20°N were observed using the latitude–time (y–t) diagram of a convective index (Ic) along 140°E over the entire 1979 summer (May–September). The oscillation in the northern region exhibits a relatively regular north–south movement of deep convective clouds, while the oscillation in the southern region shows enhancement or suppression fluctuations of broad and intensive deep convective clouds with a period of 30–50 days. The southward (northward) movement of the midlatitude oscillation coincides with the suppression (enhancement) of the tropical deep convective clouds. The midlatitude deep convective clouds are usually maintained by the divergent circulation and are associated with fronts. The empirical orthogonal function (EOF) analysis of velocity potential at 200 mb shows that the north–south, 30–50 day oscillation of deep convective clouds over the northwestern Pacific is a response of the regional weather system to the eastward propagation of this low-frequency mode. The surface charts in the vicinity of Japan show that 1ocations of fronts passing through 140°E north of 20°N undergo a similar oscillation as indicated by the y–t diagram of Ic. With both the EOF analysis of velocity potential, and the comparison between the time series of the 850-mb height field averaged over the center of Pacific anticyclone and those over the equatorial trough of the western Pacific, it is shown that the coherent 30–50 day oscillations in the tropics and midlatitudes revealed in this study are induced by the eastward propagation of the 30–50 day mode.

Diurnal Variations in Convective Activity and Precipitation During Phases II and III of GATE

Abstract Harmonic analysis is used to determine the phase and normalized amplitude of the diurnal variations in convective activity and precipitation during phases II and III of GATE. Satellite results are based on the percent coverage of squares, 3° of latitude and longitude in dimensions, by convective clouds, as estimated subjectively from SMS-1 infrared images. Rainfall results are based on hourly precipitation records for 33 land stations and hourly or 3-hourly rainfall amounts from 13 ships in the A/B-scale networks. The rainfall data for the land stations were combined into four geographically distinct groups and the ship data into a single group before making the harmonic analyses. The long-term representativeness of the results for the land areas is judged by comparing them with the results obtained from analysis of data presented by Burpee (1976) on the frequencies of occurrence of thunder, moderate to heavy precipitation and light precipitation during the period June-September 1966–69. The main...

Interannual Variations in Convective Activity Over the GATE Area

Based on five years of satellite brightness data the year-to-year variations in convective activity are investigated over the GATE area. The monthly-mean brightness data suggest that in the B-scale region, centered at 8.5N, 23.5W, the probability of intense convective activity during the summer of 1974 (the GATE period) is quite large. During the majority of the summer months investigated the intertropical convergence zone was found close to the latitude of the B-scale network. The daily brightness records show that one may expect a high day-to-day variability with a predominant period of about 4 to 5 days. This may limit the range of extrapolative predictability of cloud clusters in the B-scale area to only a few days.

Response of the Joint Ocean-Atmosphere Model to the Seasonal Variation of the Solar Radiation

Abstract The effect of the seasonal variation of solar radiation is incorporated into the joint ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration, and the resulting system is integrated for the 11/2-yr model time. The purpose of this study is to analyze the response of the joint air-sea model to seasonal changes in the solar zenith angle rather than to obtain a true equilibrium state. Comparisons are also made with results previously presented for the case of annual mean conditions. The most important feature that emerges as a direct result of this seasonal variation is a significant warming of the lower troposphere in high latitudes. This warming is found to be caused by (1) the removal of the snowpack during the summer season, which decreases the earth's albedo there during this time, and (2) a net rise in the temperature of the ocean surface in high latitudes as a result of the seasonal variation of convective activity i...

A STUDY OF CONVECTIVE PRECIPITATION AS REVEALED BY RADAR OBSERVATION, TEXAS 1958–59

Data collected during the summer of 1958 and the late spring and summer of 1959, by use of an AN/CPS-9 radar located in the Department of Oceanography and Meteorology on the campus of the A. & M. College of Texas, were analyzed to study convective precipitation processes. The formation and growth of convective echoes were studied in relation to the mechanisms of precipitation formation. Consideration also was given to day-to-day variations in convective activity. Pronounced variations were noted in echo growth and subsidence, level of first-echo formation, and echo movements. It is shown that maximum echo tops are largely dependent on the depth of the moist layer. It is also revealed that precipitation in the Central Texas region is frequently initiated by a process involving only liquid water droplets.