Maximum In Early Morning Research Articles

The differential conjugate hemisphere ionospheric response during solstice storms and the winter side maxima

The hemispheric differences in the impact of the geomagnetic storms of June 2015 and December 2015 are investigated. A meridional chain of ground observatories along 95°E ± 10°E (conjugate point-GNSS receiver/Ionosonde), satellite in-situ measurements (SWARM/COSMIC/C-NOFS), and Total Electron Content maps are utilized. Symmetric negative (positive) effects were noted during the main phase of the June (December) storm but hemispheric asymmetry was manifested during the recovery phase. The quiet time hemispheric asymmetry was reversed during both storms with positive TEC effects on the winter side. Differential VTEC and NmF2 responses exhibited seasonal variation. On 23 June 2015, the VTEC enhancements in the southern low latitude were large and consistent in contrast to the weak response in NmF2. In contrast, during the December storm, the NmF2 depletion in southern low latitude was more severe than the corresponding depletion in VTEC. The topside density/TEC enhancement in the southern low latitude recorded by SWARM was much higher than 300 % during the morning of 23 June. The SWARM Ne/TEC profile as well as the ground GPS TEC map showed a third latitudinal maximum around −45° dip angle of field lines in southern low latitudes on 23 June, in addition to the conventional EIA crests. Similarly, an early morning maximum was recorded at + 45° dip (northern hemisphere) on 21 December. The hmF2 measured by the southern hemisphere ionosonde and COSMIC satellite showed an anomalously higher altitude of the F2-layer on 23 June while C/NOFS recorded equator-ward meridional flow velocity. The reversal of the hemispheric asymmetry and the additional storm time transient maximum on the winter side is attributed to the equator-ward winds surge in the winter hemisphere due to storm time heating of the polar region.

Photosynthetic electron transport in a tropical moss Hyophila involuta

Despite their global distribution moss physiology is not well understood, particularly in tropical environments. Photosynthetic Electron Transport Rate (ETR) of Hyophila involuta was measured using PAM technology. The plants were growing in a heavily shaded habitat with irregular sunflecks of full sunlight. Two models were used for estimating photosynthetic electron transport (ETR), the Waiting-in-Line and Eilers-Peeters models. Both take photoinhibition into account and gave similar results. The population studied was growing on bricks and stonework and so periodically dried out. The same species is sometimes found in waterfall environments where it is semi-aquatic. ETRmax was low at dawn, increased to an early morning maxima and then decreased during the course of the day, partially reversed by rainstorms. Mid-morning (10:30 solar time): Eopt ≈ 521 ± 77 µmol photon m–2 s–1 which is higher than might be expected in a shaded sun-fleck environment, ETRmax ≈ 28.3 ± 2.5 µmol e- m–2 s–1 on a projected surface area basis or ≈ 57.7 ± 5.1 (µmol e- g–1 Chl a s–1), photosynthetic efficiency (Alpha, α0) ≈ 0.301 (e- photon–1 g–1 Chl a). Chl a content of Hyophila was ≈ 453 mg Chl a m–2 with Chl b/a ≈ 0.371. The O2 electrode-based respiration rate was R = 6.78 ± 0.698 µmol O2 g–1 Chl a s–1, the ETRmax is roughly equivalent to Gross photosynthesis (Pg) ≈ 14.4 ± 1.28 µmol O2 g–1 Chl a s–1 (4 e-/ O2), so the optimum Pg/R ratio is about 2.12 ± 0.289 and net photosynthesis (Pn) is ≈ 7.62 ± 1.46 µmol O2 g–1 Chl a s–1. Thus even under optimum irradiance net photosynthesis was actually rather low. pH experiments showed that Hyophila used both CO2 & HCO3- inorganic carbon sources, suggesting that a concentrating mechanism (CCM) is present. Hyophila is partially homiochlorophyllous, recovering well from desiccation after 2h of lighted rehydration and recovers more after 24h but lost ≈30% of its Chl a. The moss retained its chlorophyll content if desiccated in the dark. The moss lost ≈ 2/3 of its Chl a during the monsoonal dry season.

The behavior of cloud base height over a hilly remote station of North-East India using ground-based remote sensing technique

Impact of clouds and their vertical structures on the radiative budget, hydrological cycle, and climate is strong but still uncertain. There is a need to monitor and investigate cloud characteristics continuously to improve our knowledge and understanding. The behavior of the Cloud Base Height (CBH) is studied over Umiam, situated in a complex terrain of the Shillong Plateau. CBH measurements are obtained from a LIDAR Ceilometer and a comprehensive analysis of its characteristics covering from February 2019 to January 2022 has been presented here. Cloud occurrences showed distinct seasonality with maxima in wet months and minima in dry months. Multilayer cloud structures are found mostly in the monsoon season, although single-layer clouds dominated throughout the year. During the whole study period, the average frequency of occurrences of single, double, and triple layers of clouds are found at 38.5%, 11%, and 0.01% respectively against a total observation point of 78%. Near-ground cloud occurrences are ubiquitous with a maximum in July 2020 (18.08%) and a minimum in April 2021 (0.03%). CBH is found to vary with seasons too with higher bases during pre-monsoon months and lower during monsoon and post-monsoon months. Nighttime and early morning maxima in cloud occurrences are noted in some seasons. Cloud formation and occurrences are found to follow a pattern of existing circulation, dominated by local circulation during post-monsoon and winter and synoptic circulation during pre-monsoon and monsoon seasons. A comparison study of CBH with corresponding reanalysis datasets showed their inability to capture cloud representation over sloped terrain.

Extent of diurnal cycle of rainfall and its intra seasonal variation over coastal Tamil Nadu during north east monsoon season

The diurnal variation of north east monsoon rainfall of coastal Tamil Nadu represented by four coastal stations Chennai Nungambakkam (Nbk), Chennai Meenambakkam (Mbk), Nagapattinam (Npt) and Pamban (Pbn) was studied in detail based on hourly rainfall data of rainy days only, for the period 1 Oct-31 Dec for the 47/48 year period 1969-2016/2017. Mean Octet rainfall and its anomaly were computed for the 8 octets 00-03,…., 21-24 hrs of the day and the anomaly was tested for statistical significance. Various analysis for the individual months of Oct, Nov, Dec and the entire period Oct-Dec were separately conducted. The basic technique of evolutionary histogram analysis supplemented by harmonic analysis of octet mean rainfall anomaly was used to detect the diurnal cycle signal. Two indices named as diurnal variation of rainfall index and coefficient of mean absolute octet rainfall anomaly representing the intensity of diurnal variation in dimensionless numbers were defined, computed and interpreted.
 The analysis based on the above techniques revealed that the diurnal signal which shows an early morning maximum and late afternoon minimum of octet rainfall is well defined in Oct, decreases in Nov and further decreases in Dec for all the 4 stations. Though the diurnal variation manifests a well defined pattern in Dec the signal is not statistically significant in most cases. For Nbk and Mbk there is a weak secondary peak of octet rainfall anomaly occurring in the forenoon and afternoon respectively in Oct and Dec suggesting the presence of semi-diurnal variation of rainfall. Stationwise, the diurnal signal is most well defined for each month/season in Pbn followed by Npt, Nbk and then Mbk.
 
 The physical causes behind the diurnal signal and its decrease as the north east monsoon season advances from Oct to Dec have been deliberated. The well known feature of nocturnal maximum of oceanic convection influencing a coastal station with maritime climate and the higher saturation at the lower levels of the upper atmosphere in the early morning hours have been advanced as some of the causes. For the much more complex feature of decrease of diurnal signal with the advancement of the season, the decrease of minimum surface temperature over coastal Tamil Nadu from Oct to Dec causing an early morning conceptual land breeze has been shown as one of the plausible causes based on analysis of temperature and wind. Scope for further work based on data from automatic weather stations, weather satellites and Doppler Weather Radars has been discussed.

Impact of a Mixed Ocean Layer and the Diurnal Cycle on Convective Aggregation.

We investigate how ocean feedbacks and the diurnal cycle impact convective aggregation using a slab ocean coupled to a cloud resolving model. With a 20 m mixed layer ocean, aggregation occurs after 25 days. Thinner ocean layers slow the onset of clustering, with a 1 m ocean layer needing around 43 days. The delay is due to anomalous solar radiation in clear sky regions, causing a relative surface warming balanced by a matching cooling in cloudy areas. The resulting gradient in surface heat fluxes opposes low level convergence into convecting regions. On aggregation, the convective regions are surrounded by moist, clear sky regions with the hottest SSTs, toward which convection migrates, while a cold SST patch forms under dry suppressed regions due to enhanced latent heat fluxes and longwave radiation. The next experiment allows a diurnal cycle of 2.5°C in the domain mean SST. This causes convective rainfall to shift from a weak morning maximum to a sharper evening peak, reminiscent of undisturbed tropical observations. However, convection reverts to a weak early morning maximum once aggregation starts due to spatially heterogeneous radiative forcing. This implies thin mixed ocean layers are a necessary but non‐sufficient condition for an afternoon maximum of convection; limited spatial water vapor variability is also necessary. The imposition of the mean diurnal cycle has no statistically significant impact on the mean timing of clustering onset.

A 40‐year climatology of summer heavy hourly rainfall over mountainous Shanxi in China

AbstractIn this study, a statistical analysis of the spatiotemporal characteristics of summer heavy hourly rainfall (HHR) events during 1979–2018 over mountainous Shanxi Province is performed in order to examine regional climate change over such a transition region between North China and Northwest China. Results show that the regional 40‐year annual mean summer rainfall is about 272 mm, about 37% of which is associated with HHR events. High‐frequency and high‐intensity HHR events tend to take place in eastern mountainous subregions, whereas most basin subregions experience HHR events with low frequency and low intensity, showing the important influences of local topography and its interaction with prevailing large‐scale flows. The important topographical influences are also reflected by the observations that most mountainous subregions exhibit a major late‐afternoon peak with a secondary early morning maximum, and that the associated diurnal variations are much greater in amplitude than those in basin subregions. It is found that the regional total annual summer rainfall has a decreasing trend during the first 20 years, mostly by non‐HHR contributions, but a rapid increasing trend during the recent 20 years, with dominant HHR contributions. Most subregions with the increasing trends of HHR frequency are those dominated by rapidly increasing surface temperatures. In particular, more frequent HHR events occur over major urbanization areas, that are in close proximity to complex topography, during the recent two decades. The decadal trends in HHR are found to be correlated to the distribution and intensity changes of the East Asian summer monsoon. The above results have important implications to the operational prediction of HHR events, and a better understanding of regional climate change over mountainous Shanxi Province.

Spatiotemporal Variations of Precipitation in Bangladesh Revealed by Nationwide Rain Gauge Data

Bangladesh is under the strong influence of the South Asian monsoon. Extensive damages caused by frequent severe flood events in this country call for a study of nationwide precipitation climatology. Here, we examine spatiotemporal variations of precipitation in Bangladesh using rain gauge data at 35 stations for the period 2003–2016. The annual precipitation amount in Bangladesh is 2263 mm, and it shows large spatial variations. The annual precipitation amount is large in the southeastern coastal region of the country and the region close to the Meghalaya Plateau, and it is small in the west central region. 17% and 73% of the annual precipitation amount are observed in the pre-monsoon season (March to May) and monsoon season (June to September), respectively. The pattern of the diurnal variation of precipitation differs depending on regions of Bangladesh. In the northern region of the country, the precipitation maximum occurs in the late night to early morning in both the pre-monsoon and monsoon seasons. The late night to early morning maximum of precipitation is associated with the large horizontal convergence of low-level water vapor flux. The Himalaya Mountains and the Meghalaya Plateau together with more nighttime moisture transport due to the strengthened nighttime low-level wind seem to play roles in enhancing nighttime precipitation. In the southwestern region of the country, the precipitation maximum is observed in the early evening in the pre-monsoon season, which is associated with the large convective instability in the afternoon, and it is observed in the early afternoon in the monsoon season. In the southeastern region of the country, the diurnal variability of precipitation is smallest in the pre-monsoon season compared with other regions of the country in the pre-monsoon and monsoon seasons and the precipitation maximum occurs in the late night to early morning in the monsoon season.

The analysis of circadian rhythm of heart rate variability in patients with drug-resistant epilepsy

ObjectiveThe aim of this study was to investigate the variations and the circadian rhythm of multi-domain parameters of heart rate variability (HRV) in patients with drug-resistant epilepsy. MethodsTime domain HRV parameters of 24-hour data, frequency and non-linear domain HRV parameters were calculated using a 24-hour ambulatory electrocardiogram (ECG) from subjects in a resting and awake state (5-min data from 9:00-10:00 a.m.). Circadian fluctuation (24 one-hour segments) and the night (00:00 - 05:00)/ day (07:30 - 09:30) ratio of each HRV parameter were measured in (i) refractory epilepsy (n = 51) and (ii) healthy, age and sex matched controls (n = 50). Cosinor fit method was used to quantify the pattern of the circadian rhythm of HRV. ResultsCompared to healthy controls, patients with drug-resistant epilepsy had significantly decreased time domain (mean_RR, STDRR, RMSSD, pNN50, HRV_triangular_index), frequency domain (VLF_power, LF_power, HF_power, Total_power) and non-linear domain (Poincaré SD1, SD2, D2) HRV measurements. There were also overall reductions in amplitude and the baseline of circadian curves of HRV parameter in the patients group, especially nocturnal inhibition of cardiac autonomic modulations was stronger than that in daytime. The majority of differences in HRV values between the two groups reached a maximum in early morning (usually 5:00 or 6:00 a.m.). No significant differences were found in the night/day ratios of multi-domain HRV measurements between patients and healthy controls (p = 0.862). ConclusionsThe significant suppression of HRV parameters in patients with refractory epilepsy may reflect the inhibitions of both sympathetic and parasympathetic activities. This is notably the first time that marked circadian diversity of HRV measurments between the patient and control groups were found and quantified by cosinor fit method. The difference in amplitude increased during night with peak occurrence between 5:00 or 6:00 a.m. These findings may be linked to cardiac autonomic dysfunction and subsequent sudden unexpected death in epilepsy (SUDEP), which is reported to often occur during night or early morning. Taken together, HRV parameters may be potential bio-markers for predicting SUDEP.

Characteristics and mechanisms of the diurnal variation of winter precipitation in Taiwan

Using gridded precipitation data remapping from rain‐gauge observations of the time period of 2001–2015 winter months (December, January and February), this study examines the spatio‐temporal characteristics of the climatological diurnal winter precipitation in Taiwan. Our results show that the timings of the maximum values of the diurnal precipitation in Taiwan exhibit clear east–west regional differences, with night‐time maxima over eastern Taiwan but early morning maxima over western Taiwan. Analyses also show that the climatological characteristics of diurnal winter precipitation documented in this study are not controlled by some unusual, heavy precipitation events and can be seen as the common features. By examining the changes in meteorological variables (including winds, relative humidity and temperature) extracted from the in situ observations and the MERRA (Modern‐Era Retrospective Analysis for Research and Applications) reanalysis, this study further suggests that (a) the radiative cooling effect and (b) the surface wind convergence induced by the interaction between the local orography, the local land–sea breeze and the large‐scale diurnal circulation changes over the East Asian–western North Pacific (EAWNP) region are two of the possible formation mechanisms responsible for the spatio‐temporal differences of the climatological diurnal winter precipitation in Taiwan. This finding sheds light on the importance of understanding the role of the large‐scale diurnal circulation changes over the EAWNP region in modulating local diurnal precipitation properties during the winter months.

Mesoscale precipitation characteristics near the Western Ghats during the Indian Summer Monsoon as simulated by a high‐resolution regional model

This study investigates (i) the role of orography in precipitation along and upstream of the Western Ghats (WG) and (ii) a diurnal cycle of precipitation in western India during the summer monsoon, using a high‐resolution meteorological model and a network of surface rain‐gauges over the WG. The Weather Research and Forecasting model (WRF‐ARW) was used to simulate the 2008, 2009 and 2010 summer monsoons at 5 km horizontal grid spacing and allows resolved convection, with initial and boundary conditions provided by ERAInterim. The highest daily mean precipitation is found immediate to the WG escarpment and coastal plain between 11.5° and 18°N, but areas receiving the most rainfall do not necessarily receive it most frequently. The greatest percentage of rainy days occurs over the escarpment of the WG and slightly inland, corresponding to the topography, and high percentages (over 75%) of rainy days occur along the coast, along the coastal plain and the WG. These findings are in agreement with several recent studies using high spatial resolution Tropical Rainfall Measuring Mission (TRMM) precipitation data. Analysis of WRF output at time increments of 30 min reveals a clear diurnal pattern of rainfall, with an early morning maximum offshore and afternoon maxima over inland regions that occur later in the day with distance inland. A weak land breeze circulation is observed, as nocturnal cooling of the land surface results in deceleration of westerly flow upstream of the WG. Offshore moisture convergence and destabilization of low‐level air results in the offshore morning maximum. Rainfall maxima over inland regions indicate that, while orography is the primary impetus for lift, rainfall is also convectively driven. Analysis of convective parameters and landsurface variables such as soil moisture and latent and sensible heat fluxes supports this weak land–sea breeze circulation embedded in prevailing westerly monsoonal flow.

High Levels of Daytime Molecular Chlorine and Nitryl Chloride at a Rural Site on the North China Plain.

Molecular chlorine (Cl2) and nitryl chloride (ClNO2) concentrations were measured using chemical ionization mass spectrometry at a rural site over the North China Plain during June 2014. High levels of daytime Cl2 up to ∼450 pptv were observed. The average diurnal Cl2 mixing ratios showed a maximum around noon at ∼100 pptv. ClNO2 exhibited a strong diurnal variation with early morning maxima reaching ppbv levels and afternoon minima sustained above 60 pptv. A moderate correlation (R2 = 0.31) between Cl2 and sulfur dioxide was observed, perhaps indicating a role for power plant emissions in the generation of the observed chlorine. We also observed a strong correlation (R2 = 0.83) between daytime (10:00-20:00) Cl2 and ClNO2, which implies that both of them were formed from a similar mechanism. In addition, Cl2 production is likely associated with a photochemical mechanism as Cl2 concentrations varied with ozone (O3) levels. The impact of Cl2 and ClNO2 as Cl atom sources is investigated using a photochemical box model. We estimated that the produced Cl atoms oxidized slightly more alkanes than OH radicals and enhanced the daily concentrations of peroxy radicals by 15% and the O3 production rate by 19%.

Quantitative and qualitative assessment of diurnal variability in tropospheric humidity using SAPHIR on-board Megha-Tropiques

The global diurnal variability of relative humidity (RH) from August 2012 to May 2014 is discussed for the first time using ‘Sounder for Atmospheric Profiling of Humidity in the Inter-tropical Regions (SAPHIR)’, a microwave humidity sounder onboard Megha-Tropiques (MT). It is superior to other microwave satellite humidity sounders in terms of its higher repetitive cycle in the tropics owing to its low-inclination orbit and the availability of six dedicated humidity sounding channels. The six layers obtained are 1000–850, 850–700, 700–550, 550–400, 400–250 and 250–100hPa. Three hourly data over a month has been combined using equivalent day analysis to attain a composite profile of complete diurnal cycle in each grid (2.5°×2.5°). A distinct diurnal variation is obtained over the continental and the oceanic regions at all the layers. The magnitude in the lower tropospheric humidity (LTH), middle tropospheric humidity (MTH) and the upper tropospheric humidity (UTH) show a large variability over the continental regions compared to that over oceans. The monthly variability of the diurnal variation over the years has also been discussed by segregating into five different continental and four different oceanic regions. Afternoon peaks dominate in the LTH over the land and the desert regions. The MTH is found to vary between the evening and the early morning hours over different geographical regions and not as consistent as that of the LTH. The UTH maximum magnitude is generally observed during the early morning hours, over the continents. Interestingly, the Oceanic regions are found to have a dominant magnitude in the afternoon hours similar to that of the continents in the LTH, evening maximum in the MTH and the early morning maximum in the UTH. The underlying mechanisms involved in the variability of humidity over different regions are also discussed. The study reveals the complexity involved in the understanding the diurnal variability over the continents and open oceans.

Spatial Variations in the Diurnal Pattern of Precipitation over Nepal Himalayas

The Central Himalayan Region (Nepal Himalayas), comprised of two clear sub-parallel mountain ranges, is atypical location for studying the impact of rugged topography on spatio temporal variations of precipitation. The relationship between topography and diurnal cycles of rainfall have been investigated utilizing 13-year (1998–2010) high resolution (0.05° × 0.05°) Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data. An investigation of diurnal cycle of precipitation revealed an afternoon maximum during the pre-monsoon season (March–May) and midnight–early morning maximum during the summer monsoon season (June–August)over the southern slopes of the Himalayas. The summer monsoon exhibited a robust spatial variation of diurnal cycle of precipitation, during afternoon-evening time, primary rainfall peak appeared along the Lesser Himalayas (~2,000–2,200 m above mean sea level), while early-morning rain in contrast showed maximum concentration along the southern margin of the Himalayas (~500–700 m above MSL). An afternoon-evening rainfall peak was attributed to higher rain frequency, whereas early-morning rainfall peak was attributed to fewer but rather intense rainfall. It is suggested that, confluence between down slope and moist south easterly monsoon flow triggers convection near the foothills of the Himalayas during early morning period. The results further suggested the morning precipitation moves southward in the mature monsoon season.DOI: http://dx.doi.org/njst.v15i2.12116Nepal Journal of Science and Technology Vol. 15, No.2 (2014), 57-64

Progress in studies of the precipitation diurnal variation over contiguous China

This paper summarizes the recent progress in studies of the diurnal variation of precipitation over contiguous China. The main results are as follows. (1) The rainfall diurnal variation over contiguous China presents distinct regional features. In summer, precipitation peaks in the late afternoon over the southern inland China and northeastern China, while it peaks around midnight over southwestern China. In the upper and middle reaches of Yangtze River valley, precipitation occurs mostly in the early morning. Summer precipitation over the central eastern China (most regions of the Tibetan Plateau) has two diurnal peaks, i.e., one in the early morning (midnight) and the other in the late afternoon. (2) The rainfall diurnal variation experiences obvious seasonal and sub-seasonal evolutions. In cold seasons, the regional contrast of rainfall diurnal peaks decreases, with an early morning maximum over most of the southern China. Over the central eastern China, diurnal monsoon rainfall shows sub-seasonal variations with the movement of summer monsoon systems. The rainfall peak mainly occurs in the early morning (late afternoon) during the active (break) monsoon period. (3) Cloud properties and occurrence time of rainfall diurnal peaks are different for long- and short-duration rainfall events. Long-duration rainfall events are dominated by stratiform precipitation, with the maximum surface rain rate and the highest profile occurring in the late night to early morning, while short-duration rainfall events are more related to convective precipitation, with the maximum surface rain rate and the highest profile occurring between the late afternoon and early night. (4) The rainfall diurnal variation is influenced by multi-scale mountain-valley and land-sea breezes as well as large-scale atmospheric circulation, and involves complicated formation and evolution of cloud and rainfall systems. The diurnal cycle of winds in the lower troposphere also contributes to the regional differences in the rainfall diurnal variation. (5) Evaluation of the model performance shows that the present numerical models are weak in simulating the rainfall diurnal variation over contiguous China. The simulations are not significantly improved by increasing the model horizontal resolution alone. The key is to reduce the uncertainty in physical parameterizations related to the rainfall processes.

Atmospheric CO2 Variations in Two Contrasting Environmental Sites Over India

We analyzed the influence of environmental parameters on the temporal variation of atmospheric carbon dioxide (CO2) mixing ratios in two environmentally contrasting Indian sites, Dehradun (30.1°N, 77.4°E, humid subtropical station) and Gadanki (13.5°N, 79.18°E, dry tropical station), from October 2010 to September 2011. The annual range of mixing ratios is low in Gadanki as compared to those of Dehradun because of relatively less monthly variation in temperature and relative humidity (RH) at Gadanki. At both the stations, the minimum mixing ratios are present during the high ecosystem productivity seasons in the afternoon hours. The maximum values are in the early morning hours. However, low wind speed conditions control the unexpected afternoon high mixing ratios in Gadanki during the pre-monsoon season. The early morning maximum is high during monsoon and post-monsoon seasons in Dehradun and Gadanki, respectively, whereas morning inflexion occurred earlier in Gadanki compared with Dehradun. The effect of cloudiness on the CO2 uptake depends on the canopy cover.

Mechanism of Orographic Precipitation around the Meghalaya Plateau Associated with Intraseasonal Oscillation and the Diurnal Cycle

Abstract Mesoscale processes that cause tremendous amounts of precipitation around the Meghalaya Plateau, northeast India, were investigated using a regional atmospheric model, with a particular focus on the possible role of an intraseasonal oscillation (ISO) and the diurnal cycle. A numerical experiment was conducted using the Weather Research and Forecasting Model (WRF) to simulate a prominent submonthly-scale ISO event observed during June–July 2004. A WRF experiment successfully simulated the timing and magnitude of precipitation during the first active period and subsequent inactive period of the ISO, despite large biases in the second active period. The WRF experiment revealed that the magnitude of prevailing southwesterly winds in the lower troposphere differs significantly between active and inactive periods; strong southwesterly flow transports abundant moisture to the southern slope of the Meghalaya Plateau during the active period, which triggers forced lifting and brings orographic precipitation. However, low-level wind during the inactive period is too weak to overcome the vertical stratification barrier; hence, it is unable to reach the condensation level. The WRF experiment suggested that the diurnal cycle of precipitation is enhanced during the active period, exhibiting an evening to early morning maximum, as observed by the Tropical Rainfall Measuring Mission (TRMM). The experiment also indicated a prominent nocturnal low-level jet (LLJ) appearing at 900 hPa. The direction of the LLJ varies clockwise with time, which accelerates the preexisting southwesterly flow during 1800–0600 LT. The diurnal variation of the LLJ is responsible for the evening to early morning maximum of precipitation, thus contributing to the precipitation variability in the ISO.

Diurnal Cycle of Summer Precipitation over Subtropical East Asia in CAM5

Abstract The simulations of summertime diurnal cycle of precipitation and low-level winds by the Community Atmosphere Model, version 5, are evaluated over subtropical East Asia. The evaluation reveals the physical cause of the observed diurnal rainfall variation in East Asia and points to the source of model strengths and weaknesses. Two model versions with horizontal resolutions of 2.8° and 0.5° are used. The models can reproduce the diurnal phase of large-scale winds over East Asia, with an enhanced low-level southwesterly in early morning. Correspondingly, models successfully simulated the diurnal variation of stratiform rainfall with a maximum in early morning. However, the simulated convective rainfall occurs at local noontime, earlier than observations and with larger amplitude (normalized by the daily mean). As a result, models simulated a weaker diurnal cycle in total rainfall over the western plain of China due to an out-of-phase cancellation between convective and stratiform rainfalls and a noontime maximum of total rainfall over the eastern plain of China. Over the East China Sea, models simulated the early-morning maximum of convective precipitation and, together with the correct phase of the stratiform rainfall, they captured the diurnal cycle of total precipitation. The superposition of the stratiform and convective rainfalls also explains the observed diurnal cycle in total rainfall in East Asia. Relative to the coarse-resolution model, the high-resolution model simulated slight improvement in diurnal rainfall amplitudes, due to the larger amplitude of stratiform rainfall. The two models, however, suffer from the same major biases in rainfall diurnal cycles due to the convection parameterization.

Influence of summer monsoon diurnal cycle on moisture transport and precipitation over eastern China

Diurnal variability of the summer monsoon over China, a key element affecting regional climate, is examined using the latest reanalysis dataset and satellite rain estimates. Diurnal variation of low‐level wind is found to be pronounced over South China during active monsoon days. Mean wind speed attains a maximum (minimum) in early morning (afternoon), with a diurnal range of 2–3 m/s, double that of inactive monsoon days. The largest amplitude typically appears at 850 hPa, consistent with radiosonde observations. Such a monsoon diurnal cycle can strengthen low‐level moisture transport at night by about 20% more than during the day. Nocturnal moisture fluxes converge toward Central China and lead to a meso‐synoptic‐scale moisture sink during the late night and morning, which plays a role in regulating the regional water budget on a diurnal time scale. Monsoon flow also helps provide substantial moisture over low‐lying areas in the morning hours. Correspondingly, morning rainfall undergoes a remarkable increase and greatly contributes to the diurnal cycle of summer rainfall. The strongest response comes from meso‐α‐scale rain events that not only become prominent during active monsoon days but also possess a dominant morning peak. These morning events occur mainly on the basins and plains of Central China, where the monsoon diurnal cycle promotes nighttime mesoscale convection. These tend to shift northward from June to August, with the progress of the monsoon diurnal cycle, thereby producing the morning‐peak summer rainband. The findings point to an efficiency of nocturnal monsoon flow influencing the warm‐season weather and climate over eastern China.

Diurnal rainfall variability over the Upper Blue Nile Basin: A remote sensing based approach

In this study we aim to assess the diurnal cycle of rainfall across the Upper Blue Nile (UBN) basin using satellite observations from Tropical Rainfall Measuring Mission (TRMM). Seven years (2002–2008) of Precipitation Radar (PR) and TRMM Microwave Imager (TMI) data are used and analyses are based on GIS operations and simple statistical techniques. Observations from PR and TMI reveal that over most parts of the basin area, the rainfall occurrence and conditional mean rain rate are highest between mid- and late-afternoon (15:00–18:00 LST). Exceptions to this are the south-west and south-eastern parts of the basin area and the Lake Tana basin where midnight and early morning maxima are observed. Along the Blue Nile River gorge the rainfall occurrence and the conditional mean rain rate are highest during the night (20:00–23:00 LST). Orographic effects by large scale variation of topography, elevation and the presence of the UBN river gorge were assessed taking two transects across the basin. Along transects from north to south and from east to west results indicate increased rainfall with increase of elevation whereas areas on the windward side of the high mountain ranges receive higher amount of rainfall than areas on the leeward side. As such, mountain ranges and elevation affect the rainfall distribution resulting in rain shadow effect in the north-eastern parts of Choke-mountain and the ridges in the north-east of the basin. Moreover, a direct relation between rainfall occurrence and elevation is observed specifically for 17:00–18:00 LST. Further, results indicate that the rainfall distribution in the deeply incised and wide river gorge is affected with relatively low rainfall occurrence and low mean rainfall rates in the gorge areas. Seasonal mean rainfall depth is highest in the south-west area and central highlands of the basin while areas in the north, north-east and along the Blue Nile gorge receive the least amount of rainfall. Statistical results of this work show that the diurnal cycle of rainfall occurrence from TRMM estimates show significant correlation with the ground observations at 95% confidence level. In the UBN basin, the PR conditional mean rain rate estimates are closer to the ground observations than the TMI. Analysis on mean wet season rainfall amount indicates that PR generally underestimates and TMI overestimates the ground observed rainfall.

The diurnal cycle of rainfall in South Africa in the austral summer

AbstractThe diurnal cycle of precipitation over South Africa during summer is analysed for the first time using hourly precipitation data from 103 stations for the period 1998–2007. Using harmonic analysis, we found the presence of a distinct diurnal cycle over most of South Africa regarding both the frequency and amount of precipitation events. The standardized amplitudes, indicative of the strength of the diurnal cycle across a region, are strongest over the interior and along the east coast of South Africa with up to 70% explained variance associated with the diurnal cycle. The time of maximum precipitation is late afternoon to early evening in the interior, and midnight to early morning along the Agulhas Current as well as inland in the northeast of the county. The proximity of the warm Agulhas Current plays a role in the diurnal cycle of rainfall at coastal stations. There is an early morning maximum in precipitation in the South West of the country with small amplitude in the diurnal cycle there. On average, the peak in precipitation amount leads the peak in frequencies of precipitation by 30 min to 1 h. Together with a high resolution climatological summer rain rate presented here and a detailed table for all stations, this study is a benchmark upon which model output or satellite estimate of the diurnal cycle can be compared within the region. Copyright © 2012 Royal Meteorological Society