Summer monsoon low pressure systems over the Indian region and their relationship with the sub-divisional rainfall

In the present paper performance of the monthly sub-divisional summer monsoon rainfall is studied in association with the position of the Low Pressure System (LPS) over the Indian region. Existence of the LPS over a particular location increases the rainfall activities in certain parts of the country while decreases in some other parts. For this study, the Indian region (5°-35° N and 60° -100° E) is divided into 5° Lat. ´ 5° Long. grids. The duration of LPS is taken in terms of LPS days with respect to the location of LPS in a particular grid. Monthly total number of LPS days in each of the grids are computed during the summer monsoon season, June to September for the period 1891 – 1990. Maximum number of LPS days (more than half of the total) are observed in the latitude belt between 20°-25°N. The percentages of total LPS days in this area are higher in July and August which are peak monsoon months as compared to June and September. When there is a LPS are in the area 20°-25° N and 80°-90° E, there is significant increase in the rainfall activities in the sub-divisions along mean monsoon trough while northeast India and southeast peninsular India experience significant decrease in rainfall in the months of July and August. Owing to the movement of LPS from east to west through central India, most parts of the country, excluding northeast India and south peninsular India get good rainfall activity. Correlation coefficients between monthly LPS days over the different grids and monthly sub-divisional rainfall are computed to study the relationships. The performance of sub-divisional rainfall mostly related with the occurrence of LPS in certain grid- locations. The correlation field maps may give some useful information about rainfall performance due to LPS in a particular grid locations.

Signatures of midlatitude heat waves in Rossby wave variability spectra

<p>This work aims at identifying extreme circulation conditions such as heat waves in modal space which is defined by eigensolutions of the linearized primitive equations. Here, the Rossby waves are represented in terms of Hough harmonics that are an orthogonal and complete expansion set allowing Rossby wave diagnostics in terms of their total (kinetic and available potential) energies. We expect that this diagnostic provides a more clear picture of the Rossby wave variability spectra compared to the common Fourier decomposition along a latitude belt. </p> <p>The probability distributions of Rossby wave energies are analysed separately for the zonal mean flow, for the planetary and synoptic zonal wavenumbers. The robustness is ensured by considering the four reanalyses ERA5, ERA-Interim, JRA-55 and MERRA. A single wave is characterized by Gaussianity in the complex Hough amplitudes and by a chi-square distribution for the energies. We find that the distributions of the energy anomalies in the wavenumber space are non-Gaussian with almost the same positive skewness in the four reanalyses.  The skewness increases during persistent heat waves for all energy anomaly distributions, in agreement with the recent trend of increased subseasonal variance in large-scale Rossby waves and decreased variance at synoptic scales. The new approach offers a selective filtering to physical space. The reconstructed circulation during heat waves is dominated by large-scale anticyclonic systems in northeastern Europe with zonal wavenumbers 2 and 3, in agreement with previous studies, thereby demonstrating physical meaningfulness of the skewness. </p> <p> </p>

Representation of tropical convection in a near-global convection permitting seasonal simulation with WRF

<p>Precipitation observations between March to May 2015 show several coherent propagating systems in an area between 10°N and 10°S with a lifetime of 3-4 weeks demonstrating the importance of simulations beyond a month. The eastward propagation speed is typically 1100 km day<sup>-1</sup>. The main origins of significant amounts of precipitation along this belt are the tropical warm pools in the Western Pacific around 158-174°E and the eastern Indian Ocean around 90°E as well as the tropical rainforest over South America around 69°W.</p><p>We investigated the lifetime and propagation of tropical precipitating systems based on observations and a near-global convection permitting seasonal simulation with the Weather Research and Forecasting (WRF). The latitude-belt simulation covers an area between 57°S to 65°N with a grid increment of 0.03° over a period of 5 months forced by sea surface temperature (SST) observations.</p><p>Results of this simulation with respect to tropical convection were investigated by means of comparison with satellite-based cloud and precipitation observations and ECMWF operational analysis. Wavenumber-frequency spectra of the tropical convection and the detection of various wave pattern were derived from the 3-h outgoing longwave radiation at the top of the atmosphere (TOA OLR) fields and revealed by Wheeler-Kiladis diagrams. The simulation shows the observed spectral signatures of eastward propagating EIGs and Kelvin waves.</p><p>The EOF decomposition of the monthly averaged sea level pressure fields demonstrates that 65 % of the sea surface pressure fluctuations in the ECMWF analyses can be explained by the correlation pattern shown in the 1<sup>st</sup> EOF. The agreement with the 1<sup>st</sup> EOF of the WRF simulation is excellent despite a slight underestimation of the strength of the correlations. The spatial structure is very similar and 61 % of the variance are contained in first EOF. The EOF analyses provided strong evidence that the seasonal simulation with a convection permitting horizontal resolution captures the representation of the teleconnection pattern.</p>

Near-global-scale high-resolution seasonal simulations with WRF-Noah-MP v.3.8.1

The added value of global simulations on the convection-permitting (CP) scale is a subject of extensive research in the earth system science community. An increase in predictive skill can be expected due to advanced representations of feedbacks and teleconnections in the ocean–land–atmosphere system. However, the proof of this hypothesis by corresponding simulations is computationally and scientifically extremely demanding. We present a novel latitude-belt simulation from 57∘ S to 65∘ N using the Weather Research and Forecasting (WRF)-Noah-MP model system with a grid increment of 0.03∘ over a period of 5 months forced by sea surface temperature observations. In comparison to a latitude-belt simulation with 45 km resolution, at CP resolution the representation of the spatial-temporal scales and the organization of tropical convection are improved considerably. The teleconnection pattern is very close to that of the operational European Centre for Medium Range Weather Forecasting (ECMWF) analyses. The CP simulation is associated with an improvement of the precipitation forecast over South America, Africa, and the Indian Ocean and considerably improves the representation of cloud coverage along the tropics. Our results demonstrate a significant added value of future simulations on the CP scale up to the seasonal forecast range.

On PSI Interactions in Internal Gravity Wave Fields and the Decay of Baroclinic Tides

Barotropic tidal oscillations over seafloor topography generate baroclinic tides that may be damped in turn via nonlinear triad interactions with internal gravity waves, fueling the ambient wave field. We derive the kinetic equations for this tidal damping and the energy transfer to the ambient wave field and compute damping times and energy transfer rates for the M2 tide and a Garrett–Munk-like ambient wave field. We show that parametric subharmonic instability (PSI) interactions are important, where the tide interacts resonantly with two background waves, each of half the tidal frequency. PSI is restricted to the latitude belt 28.8°N/S and yields under typical conditions damping times of about 20 days for tides with low vertical wavenumber. Damping times decrease with equivalent mode number j roughly as 1/j2. Outside the critical latitudes PSI is not possible, and damping times are from one to two orders of magnitude larger. The energy transfer to the ambient wave field is concentrated at half the tidal frequency ω at all latitudes within the critical latitude belt. Outside, the transfer is much smaller and peaks at ω + f and N. An estimation of the tidal spectral transfer on the global scale is hampered by insufficient knowledge of the baroclinic energy distribution over the vertical modes. Using results from a numerical circulation model with tidal forcing, we find an energy transfer from the tide to the ambient wave field of typically 0.3 TW, about half of what is currently proposed for the conversion of barotropic to baroclinic energy.

Mapping the Photochemistry of European Mid-Latitudes Rivers: An Assessment of Their Ability to Photodegrade Contaminants

The abiotic photochemical reactions that take place naturally in sunlit surface waters can degrade many contaminants that pose concern to water bodies for their potentially toxic and long-term effects. This works aims at assessing the ability of European rivers to photoproduce reactive transient intermediates, such as HO• radicals and the excited triplet states of chromophoric dissolved organic matter (3CDOM*), involved in pollutant degradation. A photochemical mapping of the steady-state concentrations of these transients was carried out by means of a suitable modeling tool, in the latitude belt between 40 and 50°N. Such a map allowed for the prediction of the photochemical lifetimes of the phenylurea herbicide isoproturon (mostly undergoing photodegradation upon reaction with HO• and especially 3CDOM*) across different European countries. For some rivers, a more extensive dataset was available spanning the years 1990–2002, which allowed for the computation of the steady-state concentration of the carbonate radicals (CO3•−). With these data, it was possible to assess the time trends of the photochemical half-lives of further contaminants (atrazine, ibuprofen, carbamazepine, and clofibric acid). The calculated lifetimes were in the range of days to weeks, which might or might not allow for efficient depollution depending on the river-water flow velocity.

Near global scale high-resolution seasonal simulations with WRF-NOAHMP v.3.8.1

The added value of global simulations on the convection-permitting (CP) scale is a subject of extensive research in the earth system science community. An increase in predictive skill can be expected due to advanced representations of feedbacks and teleconnections in the ocean-land-atmosphere system. However, the proof of this hypothesis by corresponding simulations is computationally and scientifically extremely demanding. We present a novel latitude-belt simulation from 57° S to 65° N using the WRF-NOAHMP model system with a grid increment of 0.03° km over a period of 5 months forced by sea surface temperature observations. In comparison to a latitude-belt simulation with 45 km resolution, at CP resolution the representation of the spatial-temporal scales as well as the organization of tropical convection are improved considerably. The teleconnection pattern are very close to that of the operational ECMWF analyses. The CP simulation is associated with an improvement of the precipitation forecast over South America, Africa, and the Indian Ocean and considerably improves the representation of cloud coverage along the tropics. Our results demonstrate a significant added value of future simulations on the CP scale up to the seasonal forecast range.

The seismicity of central and north-east Himalayan region

The Himalayan range extends upto 2400 km arc from Indus river valley in the west to Brahmaputra river valley in the east of India. Due to distinct geological structures of Himalayan seismic belt, seismicity in Himalaya is inhomogeneous. The inhomogeneity in seismicity is responsible for a number of seismic gaps in the Himalayan seismic belt. Thus Iin the present study, we proposed the study of spatial and temporal evolution of seismicity in entire central and north-east Himalayan region by using Gutenberg-Richter relationship. A detailed study on the behavior of natural seismicity in and around the seismic gap regions is carried out. The study region is segmented in four meridional regions (A) 80°E to 83.5°E, (B) 83.5°E to 87.5°E, (C) 87.5°E to 90°E and (D) 90°E to 98°E along with a fixed latitude belt. The homogeneous catalogue with 3 ≤ Mb ≤ 6.5 is used for the spatial and temporal analysis of seismicity in terms of b-value. It is find out that pockets of lower b-values are coinciding over and around stress accumulated regions. The observed low b-value before occurrence of the Nepal earthquake of 25th April, 2015 supports the argument of impending occurrence of moderate to large magnitude earthquake in Sikkim and north-east Himalayan region in future.

Spatial and Temporal Characteristics of Cirrus Clouds over the Tibetan Plateau Based on CALIPSO and AIRS Observations

The spatial and temporal distribution characteristics of high clouds over the Tibetan Plateau (TP) were studied using the Atmospheric Infrared Sounder (AIRS) and the GCM-Oriented CALIPSO Cloud Product (CALIPSO-GOCCP) monthly mean cloud products from 2007 to 2017. The high clouds over the TP are dominated by cirrus and show seasonal variation characteristics. There were two distinct areas with the high occurrence of cirrus clouds in the area (0°–60°N, 75°–103°E). One was located in the regions from the equator to 25°N, and the other was within the latitude belt from 30° to 40°N. From January to May, cirrus clouds mainly occurred in the central and northern parts of the TP (30° to 40°N), and the cirrus cloud fraction increased from January and reached its maximum (∼0.51) in April. From June to August, cirrus clouds mainly occurred in the southern part of the TP during summer. The cirrus clouds over the southern TP were relatively high (located in 10–17 km) and manifested northward and southward movements. The ice clouds in the southern TP are associated with plateau deep convection activities and abundant vapor transmitted by the Asian monsoon. Cirrus clouds over the northern and central TP may be relevant to the atmospheric lift by an approaching cold front and topographic lifting. Moreover, the high clouds below 11 km are dominated by opaque clouds, while the nonopaque (or thin) and opaque (or thick) clouds above 11 km are comparable.

Characteristics of ionospheric irregularities near the northern equatorial anomaly crest

This paper detects the ionospheric irregularities with rate of total electron content (TEC) change index, ROTI from GPS observation at Taoyuan (24.95° N, 121.16° E) for the solar medium and minimum years of 2003 and 2008 in the declining phase of cycle 23, the solar maximum of 2014 in solar cycle 24. Local occurrence rate (LOR) is proposed to clarify the characteristics of the irregularities together with monthly occurrence rate (MOR) and ROTI maximum for 3 latitude belts, 20–23° N, 23–26° N, 26–29° N, around the equatorial anomaly crest. MOR in May/June is larger than those in equinoxes in 2008 and 2003, which is different from that of equatorial plasma bubbles. In 2014 although MOR maximum is observed in equinoxes, the MOR in May and June is much larger than that in September. Moreover, MORs in May to August at higher latitude belt 26–29° N are larger than those in lower latitude belts and smaller in the equinoxes. The latitudinal dependence of the LORs tends to be similar to that of MORs. Seasonal variations of LORs have a similar trend for different solar activities. Maximum LORs are observed in Feb/Mar and Sep/Oct, and moderate around June, which resemble those of plasma bubbles in seasonal variations, except for latitude belt 26–29° N where maximum LORs are seen in May–Jul. The seasonal variation of ROTI maximum conforms to that of the LOR. The results suggest that irregularities near the crest in May to August are mainly originated from nonequatorial process, which is more frequently happened but weaker than plasma bubble in both spatiotemporal scale and strength.