Tropospheric Instability Research Articles

Modelling hydrometeorological extremes associated to the moisture transport driven by the Great Plains low-level jet

The Great Plains Low-Level Jet (GPLLJ) system consists of very strong winds in the lower troposphere that transport a huge amount of moisture from the Gulf of Mexico to the American Great Plains. This paper aims to study the extremes of the Transported Moisture (TM) from the GPLLJ source region to the jet domain; and, for low and high TM, to analyze the extremal dependence between the upper tail of the precipitation in the GPLLJ sink region and the lower tail of the tropospheric stability in that region, which is known as tropospheric instability. The declustered extremes of TM were analyzed using Peaks Over Threshold (POT). A non-stationary Exponential model was fitted to the cluster maxima. Estimated return levels show that the extremes of TM are expected to decrease in the future. This is meteorologically congruent with the known displacement of the western edge of the North Atlantic Subtropical High, which controls atmospheric circulation in the North Atlantic, and to a higher scale with the change of phase from negative to positive of the Atlantic Multidecadal Oscillation. Bilogistic and Logistic models were fitted to the extremes of (tropospheric instability, precipitation) for low and high TM, respectively. The extremal dependence between tropospheric instability and precipitation proves to be stronger in the case of high TM. This confirms that dynamical instability is the most important parameter for achieving high values of precipitation once there is a mechanism that allows the continuous supply of large amounts of moisture, such as the derived from a low-level jet system.

Diagnostic study of diabatic heating and potential vorticity during a case of cyclogenesis

On 16-17 November, 2015, north and middle regions of Saudi Arabia were hit by a case of cyclogenesisassociated with heavy rainfall. This work presents a diagnostic study of this heavy rainfallcase based on the analysis of diabatic heating and potential vorticity. The synoptic analysis investigate that the important dynamical factors that causes this case are the northward extension of Red Sea Trough, anticyclone over the Arabian Peninsula, a travailing midlatitude upper trough, moisture transport pathways and strong upward motion arising from tropospheric instability. The calculation of diabatic heating by the thermodynamic equation illustrate that the contribution of vertical temperature advection and the adiabatic term are opposite to each other during the period of study. The largest contribution of the horizontal cold advection occurs during the first two days while the largest contribution of the horizontal warm advection occurs during the maximum development days. The dynamics of the studied case are also investigated in terms of isobaric Potential Vorticity. It is found that the location of the low-level Potential Vorticity anomaly and the Potential Vorticity generation estimates coincides with the heating region, which implies that condensation supports a large enough source to explain the existence of the low-level Potential Vorticity anomaly.

Diurnal variation of deep convective clouds over Indian monsoon region and its association with rainfall

Diurnal cycle of deep convective clouds (DCC) over the Indian Summer Monsoon Region is studied. A harmonic analysis has been applied on the 3 hourly deep convective cloud fraction estimated from satellite observations to derive diurnal and semi-diurnal amplitudes of deep convective clouds. The diurnal cycle of deep convective clouds shows seasonal variations and amplitudes which are stronger during the northern hemispheric summer than winter and exhibits prominence over land as compared to ocean. Irrespective of seasons, deep convection peaks in the late evening hours over land whereas the peak is in the afternoon hours over majority of the oceanic regions considered in our study. The reason for the difference is attributed to the semi-diurnal oscillation which determines the oceanic diurnal cycle of deep convection. Various parameters which represent the tropospheric instability are equally affected by the semi-diurnal oscillation as in deep convective clouds over ocean. These results suggest that diurnal tides could play a key role in governing the daily cycle of deep convective clouds over oceanic regions. The relation between diurnal amplitudes of DCC and rainfall is also explored and it is shown that the strength of the daily cycle of DCC has a strong bearing on the amount of rainfall observed over the Indian summer monsoon region.

On the Relationship Between the Marine Cold Air Outbreak M Parameter and Low‐Level Cloud Heights in the Midlatitudes

AbstractFocusing on conditions of subsidence when low clouds are present, ground‐based observations in both the North Atlantic and the Southern Ocean reveal strong relationships between cloud boundary (base and top heights) and different measures of lower tropospheric instability. The difference in potential temperature between the surface and 800 hPa (a metric called M) provides a stronger relationship than measures of inversion strength such as the lower tropospheric stability and estimated inversion strength. This is because (1) inversion strength itself does not correlate well with cloud boundaries, and (2) M contains information that appears important for cloud boundaries. These include the surface forcing through the use of sea surface rather than near‐surface air temperature and an upper level close to the real cloud top. These results expand upon previous work on the importance of M as a predictor of cloud morphology. However, important differences are found in low‐cloud conditions for the North Atlantic as compared to the Southern Ocean (for a given value of M): stronger inversions, deeper boundary layers, and much larger sea level pressures. Therefore, the relationship between cloud boundaries and M differs between the two regions. A general circulation model provides similar relationships as observed between M and both cloud top height and temperature but tends to place clouds higher and at colder temperatures than observed for a given M. This might cause issues with the representation of precipitation, cloud cover and radiation in the Southern Ocean.

Impact of extreme cyclone events on coastal agriculture in Bangladesh

Extreme cyclone events are now occurring more frequently in Bangladesh. Bangladesh experiences severe 52 cyclones from 1960 to 2010 where, the approximate percentage of storm surge impact is 40%, the largest in the world. A severe cyclone in 1970 and 1991 caused loss of 300,000 and 200,000 lives. It is reported that 210000, 36000, and 3500 tonnes of boro rice, aus rice, and other food crops (e.g. potatoes and vegetables) were totally destroyed by 1991 cyclone. The storm surge killed huge livestock and caused loss of 100% of freshwater fish. Recently, the super cyclonic storm SIDR (2007) and AILA (2009) affected 10,000 and 300,000 people, respectively. Apart from these, cyclones NARGIS (2008) and MOHASEN (2013) are also mentionable. The crop production in the coastal regions of Bangladesh is most vulnerable by cyclones while, sea level rise by 2050 will inundate 17.7% of southern coastal areas. Tropical cyclones could become more frequent with more strength under recent climate change conditions. In this research, a new dimension of extreme weather assessment is done combining GCM and GIS technology and using tropospheric instability indices. The thermodynamic environment, vertical instability characteristics of severe cyclones are indispensable to cope with climate change conditions, and for planning, disaster management, and to reduce the risk of food insufficiency.
 Progressive Agriculture, Vol. 30, Suppl. 1: 33-41, 2019

Crucial role of Black Sea warming in amplifying the 2012 Krymsk precipitation extreme

Over the past 60 years, both average daily precipitation intensity and extreme precipitation have increased in many regions1, 2, 3. Part of these changes, or even individual events4, 5, have been attributed to anthropogenic warming6, 7. Over the Black Sea and Mediterranean region, the potential for extreme summertime convective precipitation has grown8 alongside substantial sea surface temperature increase. A particularly devastating convective event experienced in that region was the July 2012 precipitation extreme near the Black Sea town of Krymsk9. Here we study the effect of sea surface temperature (SST) increase on convective extremes within the region, taking the Krymsk event as a showcase example. We carry out ensemble sensitivity simulations with a convection-permitting atmospheric model and show the crucial role of SST increase in the extremeness of the event. The enhancement of lower tropospheric instability due to the current warmer Black Sea allows deep convection to be triggered, increasing simulated precipitation by more than 300% relative to simulations with SSTs characteristic of the early 1980s. A highly nonlinear precipitation response to incremental SST increase suggests that the Black Sea has exceeded a regional threshold for the intensification of convective extremes. The physical mechanism we identify indicates that Black Sea and Mediterranean coastal regions may face abrupt amplifications of convective precipitation under continued SST increase, and illustrates the limitations of thermodynamical bounds for estimating the temperature scaling of convective extremes.

Extreme precipitation events in the Middle East: Dynamics of the Active Red Sea Trough

AbstractThe Active Red Sea Trough (ARST) is an infrequent weather phenomenon that is associated with extreme precipitation, flash floods, and severe societal impacts in the Middle East (ME). Using reanalysis (ERA‐Interim) and observational precipitation (Aphrodite and stations) data, we investigate its underlying dynamics, geographical extent, and seasonality. Twelve ARST events affecting the Levant have the same dynamical characteristics as those associated with a major flood in Jeddah (Saudi Arabia) on 25 November 2009. Hence, the Jeddah flooding was caused by an ARST, which implies that ARSTs can affect a much larger part of the ME than previously assumed. We present an ARST concept involving six dynamical factors: (1) a low‐level trough; the Red Sea Trough (RST), (2) an anticyclone over the Arabian Peninsula; the Arabian Anticyclone (AA), (3) a transient midlatitude upper trough, (4) an intensified subtropical jet stream, (5) moisture transport pathways, and (6) strong ascent resulting from tropospheric instability and the synoptic‐scale dynamical forcing. We explain the ARST as the interaction of a persistent stationary wave in the tropical easterlies (i.e., the RST) with a superimposed amplifying Rossby wave, resulting in northward propagating moist air masses over the Red Sea. Our findings emphasize the relevance of the AA, causing moisture transport from the Arabian and Red Seas. The particular topography in the Red Sea region and associated low‐level circulation makes the ARST unique among tropical‐extratropical interactions. The ARST seasonality is explained by the large‐scale circulation and in particular the seasonal cycle of the semipermanent quasi‐stationary RST and AA.

Information content of MPEF DIVergence product in diagnosing the environment of deep convection

The use of divergence fields retrieved by tracking cloud and humidity features in satellite water vapour imagery to characterise preconvective environments in cases of strong upper-level dynamics is explored. An hourly upper-troposphere DIVergence (DIV) product is inferred from Atmospheric Motion Vectors (AMVs) tracked in the 6.2μm channel of the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) onboard Meteosat Second Generation (MSG). This is an advanced satellite product operationally available since 2008.A specific colour palette is introduced to highlight the information content of the DIV product in mid latitudes. To make this product suitable for using in support to deep convection forecasting, an interpretation scheme of DIV product overlaid by the cloud top brightness temperature lower than −40°C as derived from the corresponding 10.8μm infrared (IR) image is proposed.The potential for operational applications is considered, taking into account that the divergent/convergent upper-level flow at the onset of deep moist convection is a sign of the presence of positive/negative factors, respectively, for the further development of the storm. In this study, the deep convective cells considered are limited to those, reaching the middle troposphere or above. For that purpose, 1166 convective cells with cloud top temperature lower than −50°C that developed over Europe and the Mediterranean coast of Africa from 16 June to 22 July 2010 were investigated. The results show that 76% of all studied deep convective cells developed in areas of divergence (as seen by MPEF DIV product). In 92% of the cases of deep convection appearance (at either divergent or convergent upper-level flow) the convective cells further developed in areas of divergence (as derived by the MPEF DIV product).This relation can be useful for the purposes of convection nowcasting by considering three types of observational information derived by MSG satellite data: composite images of the divergence fields, IR channel 10.8μm images and instability indices, as well as NWP parameters of tropospheric instability and upper-level dynamics.

Tall clouds from small eruptions: the sensitivity of eruption height and fine ash content to tropospheric instability

A critical factor in successfully monitoring and forecasting volcanic ash dispersion for aviation safety is the height reached by eruption clouds, which is affected by environmental factors, such as wind shear and atmospheric instability. Following earlier work using the Active Tracer High Resolution Atmospheric Model for strong Plinian eruptions, this study considered a range of eruption strengths in different atmospheres. The results suggest that relatively weak volcanic eruptions in the moist tropics can trigger deep convection that transports volcanic material to 15–20 km. For the same volcanic strength there can be ~9 km difference between eruption heights in moist tropical and dry subpolar environments (a larger height difference than previously suggested), which appears consistent with observations. These results suggest that eruption intensity should not be estimated from eruption height alone for tropospheric eruptions and also that the average height of volcanic eruptions may increase if the tropical atmospheric belt widens in a changing climate. Ash aggregation is promoted by hydrometeors (particularly liquid water), so the smaller modelled eruptions in moist atmospheres, which have a relatively small ash content for their height and water content, result in a relatively small proportion of fine ash in the dispersing cloud when compared to a dry atmosphere. This in turn makes the ash clouds much more difficult to detect using remote sensing than those in dry atmospheres. Overall, a weak eruption in the tropics is more likely to produce a plume above cruising levels for civil aviation, harder to detect and track, but with a lower concentration of fine ash than a mid-latitude or polar equivalent. There is currently no defined ‘acceptable’ concentration of ash for aircraft, but as these results suggest low-grade encounters in the tropics from undetected clouds are likely, it would be desirable to explore that issue.

Performance and application of a multi-wavelength, ground-based microwave radiometer in intense convective weather

Thermodynamic and dynamic quantities, such as the K-index and GUSTEX (a wind gust estimate), are commonly used in the nowcasting of intense convective weather. In the past, they were derived from conventional upper-air ascent measurements, which were normally only available twice a day. In the tropics, the thermodynamic property of the troposphere could change rapidly in rain episodes and the conventional upper-air ascent data alone are not sufficient for nowcasting purposes. This paper discusses the use of a multi-wavelength, ground-based microwave radiometer to provide frequently updated (once every 5 minutes) thermodynamic profiles of the troposphere up to 10 km for the nowcasting of severe convective weather during a field experiment in Hong Kong in 2004. The accuracy of the radiometer's measurements is first established by comparing with the temperature and humidity profiles of upper-air ascents and the integrated water vapour of Global Positioning System (GPS) receivers. The humidity profile and K-index from the radiometer in a number of rainstorm cases are then studied. They are found to give useful indications of the accumulation of water vapour and the increasing degree of instability of the troposphere before the occurrence of the heavy rain. The continuous availability of the thermodynamic profiles from the radiometer also makes it possible to study the correlation between K-index and the degree of instability of the troposphere. In this study, the tropospheric instability is expressed in terms of the total number and the rate of lightning strikes within 20 km or so from the radiometer. It is found to have good correlation with the time-averaged K-index from the radiometer during the heavy rain episodes in the field experiment period. Furthermore, the feasibility of combining the thermodynamic profiles from the radiometer and wind profiles given by radar wind profilers in the continuous estimation of wind gusts is studied. The estimated and the actual gusts are reasonably well correlated. The wind gusts so estimated are found to provide better indications of the strength of squalls, with a lead time of about one hour, compared with those based on radiosonde measurements. Overall speaking, the microwave radiometer is able to provide very useful data in the nowcasting of intense convective weather.

Analysis of cyclogenesis parameter for developing and nondeveloping low-pressure systems over the Indian Sea

A cyclone genesis parameter, termed the genesis potential parameter (GPP), for the Indian Sea is proposed. The parameter is defined as the product of four variables, namely vorticity at 850 hPa, middle tropospheric relative humidity, middle tropospheric instability, and the inverse of vertical wind shear. The variables are calculated using the National Centers for Environmental Prediction (NCEP), USA, reanalysis data, averaged within a circle of 2.5° radius around the centre of cyclonic system. The parameter is tested with a sample dataset of 35 nondeveloping and developing low-pressure systems that formed over the Indian Sea during the period 1995–2005. The result shows that there is a distinction between GPP values for nondeveloping and developing systems in more than 85% cases. The composite GPP value is found to be around three to five times greater for developing systems than for nondeveloping systems. The analysis of the parameter at early development stage of a cyclonic storm appears to provide a useful predictive signal for intensification of the system.

Surface and upper air meteorological features during onset phase of 2003 monsoon

The second campaign of the Arabian Sea Monsoon Experiment (ARMEX-II) was conducted in two phases viz., March–April and May–June 2003. In the present work, the buoy and ocean research vessel data collected during the second phase of ARMEX-II have been analysed to bring out the characteristic features of monsoon onset. The results have shown that the thermodynamical features such as build up of lower tropospheric instability and increased height of zero degree isotherm occurred about a week before the monsoon onset over Kerala and adjoining southeast Arabian Sea. There was a sharp fall in the temperature difference between 850 and 500 hPa, and the height of zero degree isotherm about 2–3 days before the monsoon onset. The flux of sensible heat was positive (sea to air) over south Arabian Sea during the onset phase. Over the Bay of Bengal higher negative (air to sea) values of sensible flux prevailed before the monsoon onset which became less negative with the advance of monsoon over that region.The pre-onset period was characterized by large sea surface temperature (SST) gradient over the Arabian Sea with rapid decrease towards north of the warm pool region. The buoy observations have shown that SST remained close to 30.5°C in the warm pool region during the pre-onset period in 2003 but only 2–3 degrees away (north of this region) SSTs were as low as 28.5–29°C. An interesting aspect of sea level pressure (SLP) variability over the Indian seas during the onset phase of summer monsoon 2003 was undoubtedly, the highest SLP in the warm pool region inspite of very high SSTs.