Agro-climatic onset of cropping season: A tool for determining optimum date of sowing in dry zones of southern Karnataka

The study was conducted to analyse the rainfall pattern of dry farming zones of Southern Karnataka to arrive at proper date of sowing by considering parameters like threshold rainfall (20 mm), threshold dry day (2.5 mm) and threshold dry spell period (10 days) as a main defining parameters for decision making in sowing of major crops (finger millet, pigeonpea, groundnut, etc.). In all the three zones, the agro-climatic onset of cropping season was earlier as compared to meteorological onset (June 1st week) due to bimodal distribution of rainfall having its peaks in May and September month. In Central Dry Zone, Southern Dry Zone and Eastern Dry Zone, fourteenth June, thirteenth June and twentythird May were the agro-climatic onset dates (average of all stations in each zone), respectively. Station wise analysis of the rainfall revealed different agro-climatic onset dates. Ninth May in central dry zone, eighth May in eastern dry zone and fifth May in southern dry zone were the earliest onset dates. These variations in between zonal and station specific onset dates were due to spatio-temporal variations in rainfall. Therefore, advancements in sowing of crops based on the agro-climatic onset should be taken into account for betterment of crop production.

Remote Rainfall of Typhoon Khanun (2017): Monsoon Mode and Topographic Mode

Remote rainfall related to tropical cyclones (TCs) can be attributed to interaction between the northeasterly monsoon and TC circulation (hereafter monsoon mode), and topographic blocking and lifting effects (hereafter topographic mode). Typhoon Khanun (2017) is a case in point affected by both modes. The objective of this study is to understand the key factors leading to uncertainty in the TC-induced remote rainfall. Ensemble simulations are conducted, with the ensemble members related to the monsoon mode classified into subtypes based on the geographic location of the precipitation maxima. The results demonstrate that frontogenesis and terrain-induced uplifting are the main mechanisms leading to the heavy precipitation in northeastern Taiwan, while the orographic lifting and the interaction between the TC circulation and the topographically-blocked northeasterlies result in the heavy rainfall in southeastern Taiwan. For the topographic mode, at larger rainfall threshold, strong relation is found between the inflow angle of the TC circulation and the cumulative frequency of the rainfall, while at smaller rainfall threshold, rainfall cumulative frequency is related to the ensemble track directions. Sensitivity experiments with TC-related moisture reduced (MR) and the terrain of Taiwan removed (TR) show that the average of the 3-day accumulated rainfall is reduced by 40% and more than 90% over the mountainous area in MR and TR respectively. Overall, this study highlights the fact that multiple mechanisms contribute to remote rainfall processes in Khanun, particularly the orographic forcing, thus providing better insights into the predictability of TC remote rainfall.

Predicting the Onset of the North Australian Wet Season with the POAMA Dynamical Prediction System

A forecast product focusing on the onset of the north Australian wet season using a dynamical ocean–atmosphere model is developed and verified. Onset is defined to occur when a threshold rainfall accumulation of 50 mm is reached from 1 September. This amount has been shown to be useful for agricultural applications, as it is about what is required to generate new plant growth after the usually dry period of June–August. The normal (median) onset date occurs first around Darwin in the north and Cairns in the east in late October, and is progressively later for locations farther inland away from these locations. However, there is significant interannual variability in the onset, and skillful predictions of this can be valuable. The potential of the Predictive Ocean–Atmosphere Model for Australia (POAMA), version 2, for making probabilistic predictions of onset, derived from its multimember ensemble, is shown. Using 50 yr of hindcasts, POAMA is found to skillfully predict the variability of onset, despite a generally dry bias, with the “percent correct” exceeding 70% over about a third of the Northern Territory. In comparison to a previously developed statistical method based solely on El Niño–Southern Oscillation, the POAMA system shows improved skill scores, suggesting that it gains from additional sources of predictability. However, the POAMA hindcasts do not reproduce the observed long-term trend in onset dates over inland regions to an earlier date despite being initialized with the observed warming ocean temperatures. Understanding and modeling this trend should lead to further enhancements in skill.

The Influence of Precipitation and Consecutive Dry Days on Burned Areas in Yunnan Province, Southwestern China

Precipitation is among the more limiting meteorological factors affecting the occurrence and extent of forest fire. We examined the correlation between burned area of individual wildfires and the rainfall amounts occurring on the day of the burn and the number of consecutive dry days for a range of limiting daily rainfall amounts (0–6mm) used to define a “dry” day. Daily threshold rainfall levels that most significantly affected area burned were determined for each ecoregion in Yunnan province, a major fire-prone area, in southwestern China. Results showed that the burned area of a wildfire decreased exponentially with increasing rainfall amounts on the day of burning. Burned area was also positively correlated to the number of consecutive dry days prior to burning. The threshold rainfall value providing the highest correlation between burned area and the number of consecutive dry days prior to a burn varied between ecoregions. Consecutive dry days with rainfall less than the specified threshold predominantly affected large fires (>100 ha) rather than more frequently occurring small fires. These results will help forest managers evaluate regionalfire danger indices for forest fire prevention, particularly for catastrophic forest wildfires causing significant economic losses and threats to human life and environment.

Rain-induced attenuation of deep-water waves

A laboratory investigation has been undertaken to quantify water wave attenuation rates as a function of rainfall rate. Vertical artificial rainfall is shown to generate weak near-surface velocity fluctuations that decline systematically away from the free surface and are independent of rainfall rate across the range of rainfall rates investigated (40–$170~\mathrm{mm} ~{\mathrm{h} }^{- 1} $). In the absence of rain, the observed attenuation of gravity waves is at levels consistent with classical viscous theory, but with a systematic finite-amplitude effect observed above a mean steepness of 0.10. Wave attenuation rates were found to be independent of the mean wave steepness and identical when artificial rainfall rates of 108 and$141~\mathrm{mm} ~{\mathrm{h} }^{- 1} $were applied. Reassessment of complementary theoretical and experimental studies of individual droplets impacting on undisturbed water surfaces indicates that above a weak threshold rainfall rate of$30~\mathrm{mm} ~{\mathrm{h} }^{- 1} $, the surface irradiation becomes so frequent that droplet-generated violent surface motions directly interact with the incoming droplets. Present evidence is that a matching of time scales develops between the incoming surface irradiation and surface water motions generated by antecedent droplets as the rainfall rate increases. Consequently, at high rainfall rates, a highly dissipative surface regime is created that transmits little of the incident rainfall kinetic energy to the aqueous layers below. Rainfall-induced wave attenuation rates are compared with measurements of other wave attenuation processes to obtain a hierarchy of strength in both the laboratory and the field. Comparison is also made with wave attenuation theories that incorporate momentum and energy flux considerations. Rain-induced wave attenuation rates are weak or very strong depending on whether they are expressed in terms of energy scaling obtained from above or below the surface respectively, due to the high dissipation rate that occurs in the vicinity of the interface.

The development and termination of the 1968 plague of the desert locust, Schistocerca gregaria (Forskål) (Orthoptera, Acrididae)

The population dynamics of the desert locust, Schistocerca gregaria* (Forsk.), in Africa, the Middle East and Asia from 1966 to 1969 have been analysed to determine the causes of the beginning and the abrupt end of the plague of 1968. Evidence indicates that in 1966 there were no persistent sequences of breeding by gregarious populations, but in 1967 and early 1968 there were four sequences of breeding by initially solitary behaving locusts, each resulting in a substantial increase in numbers and density. These events led to a plague that ended by mid-1969.The threshold rainfall requirement for successful breeding, defined as an increase in numbers from mature adults of the parent generation to filial fledglings, has been related to the rainfall required for the germination of ephemerals and their growth to sizes that can provide food and shelter for hoppers throughout their development. It is suggested that the minimum quantity of rainfall required initially over a large area is 25 mm. This requirement is usually met in the summer breeding areas. Rainfall in the winter-spring breeding areas is more variable, and falls exceeding the threshold are most commonly associated with deep troughs in the mid- and upper-tropospheric westerly winds in winter and spring. A season in which many such troughs occur may be essential for plague development. It is also suggested that the timing of successive periods of rainfall allowing successful breeding is critical in determining the occurrence of upsurges.Control against non-gregarious infestations failed to contain the upsurge, but later control measures against hopper bands and swarms contributed to the plague decline. The most economical strategy for plague prevention might be one of upsurge elimination, that is to say control in selected areas against gregarious locusts only. The implications of adopting such a strategy are discussed.