Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations

Space-borne observations reveal that 20–40% of marine convective clouds below the freezing level produce rain. In this paper we speculate what the prevalence of warm rain might imply for convection and large-scale circulations over tropical oceans. We present results using a two-column radiative–convective model of hydrostatic, nonlinear flow on a non-rotating sphere, with parameterized convection and radiation, and review ongoing efforts in high-resolution modeling and observations of warm rain. The model experiments investigate the response of convection and circulation to sea surface temperature (SST) gradients between the columns and to changes in a parameter that controls the conversion of cloud condensate to rain. Convection over the cold ocean collapses to a shallow mode with tops near 850 hPa, but a congestus mode with tops near 600 hPa can develop at small SST differences when warm rain formation is more efficient. Here, interactive radiation and the response of the circulation are crucial: along with congestus a deeper moist layer develops, which leads to less low-level radiative cooling, a smaller buoyancy gradient between the columns, and therefore a weaker circulation and less subsidence over the cold ocean. The congestus mode is accompanied with more surface precipitation in the subsiding column and less surface precipitation in the deep convecting column. For the shallow mode over colder oceans, circulations also weaken with more efficient warm rain formation, but only marginally. Here, more warm rain reduces convective tops and the boundary layer depth—similar to Large-Eddy Simulation (LES) studies—which reduces the integrated buoyancy gradient. Elucidating the impact of warm rain can benefit from large-domain high-resolution simulations and observations. Parameterizations of warm rain may be constrained through collocated cloud and rain profiling from ground, and concurrent changes in convection and rain in subsiding and convecting branches of circulations may be revealed from a collocation of space-borne sensors, including the Global Precipitation Measurement (GPM) and upcoming Aeolus missions.     

Detecting shadows in multi-temporal aerial imagery to support near-real-time change detection

Multi-temporal aerial imagery captured via an approach called repeat station imaging (RSI) facilitates post-hazard assessment of damage to infrastructure. Spectral-radiometric (SR) variations caused by differences in shadowing may inhibit successful change detection based on image differencing. This study evaluates a novel approach to shadow classification based on bi-temporal imagery, which exploits SR change signatures associated with transient shadows. Changes in intensity (brightness from red–green–blue images) and intensity-normalized blue waveband values provide a basis for classifying transient shadows across a range of material types with unique reflectance properties, using thresholds that proved versatile for very different scenes. We derive classification thresholds for persistent shadows based on hue to intensity ratio (H/I) images, by exploiting statistics obtained from transient shadow areas. We assess shadow classification accuracy based on this procedure, and compare it to the more conventional approach of thresholding individual H/I images based on frequency distributions. Our efficient and semi-automated shadow classification procedure shows improved mean accuracy (93.3%) and versatility with different image sets over the conventional approach (84.7%). For proof-of-concept, we demonstrate that overlaying bi-temporal imagery also facilitates normalization of intensity values in transient shadow areas, as part of an integrated procedure to support near-real-time change detection.     

Geochemical studies on the Feshcha Springs,Dead Sea basin

The Feshcha springs issue in a 4 km long strip on the Dead Sea shores. They constitute two separate groups: a) T-N waters, similar in their salt composition, temperature and radon content to the many other members of the Rift Valley “Tiberias-Noit water association”. The hydrologic, radon, tritium and carbon-14 indicate they are mixtures of recent meteoric waters with ancient (trapped) T-N waters of an age of at least 18000 years. b) Z-Y waters which, like other members of the Dead Sea basin “Zohar-Yesha water group”, originate by a mixing of T-N waters with Dead Sea waters. This is seen in the chemical compositions and is confirmed by the oxygen-18 and deuterium data.     

Nucleation and growth of strike-slip faults in limestones from Somerset, U.K.

Small-scale structures along strike-slip fault zones in limestones exposed around the Bristol Channel, U.K., suggest that pressure solution plays a key role during fault nucleation and growth. Incipient shear zones consist of enéchelon veins. The first generation of solution seams form due to bending of the intact rock (bridge) between overlapping veins. As the bridge rotates, slip occurs along the seams, linking the veins, causing cm-scale calcite-filled pull-apart structures to form and allowing fault displacement to increase. A second generation of solution seams forms at the tip of the sliding seams. As displacement increases further, causing larger rotation, slip also can occur along these second-generation solution seams, producing the third generation of solution seams as well as tail cracks (pinnate veins) at their tips. These three generations of solution seams all contribute to the formation of individual fault segments. Fourth and fifth generations of solution seams occur within larger-scale contractional oversteps between side-stepping fault segments. The oversteps are breached by slip along these localized solution seams, eventually leading to the formation of a distinct through-going fault with several metres of displacement.The initial enéchelon veins, solution seams of various generations and tail cracks progressively fragment the fault-zone material as fault slip accumulates. Slip planes nucleate on these pre-existing discontinuities, principally along the clay-enriched, weaker solution seams. This can be observed at a variety of scales and suggests that Mode II shear fracturing does not occur as a primary fracture mechanism, but only as a macroscopic phenomenon following Mode I (veins and tail cracks) and anti-mode I (solution seams) deformation. It appears that solution seams can play a similar role to microcracks in localizing a through-going slip plane. This micromechanical model of faulting may be applicable to some other faults and shear zones in host rocks which are prone to pressure solution.     

The Late Cenozoic uplift – climate change paradox

The geologic evidence for worldwide uplift of mountain ranges in the Neogene is ambiguous. Estimates of paleoelevation vary, according to whether they are based on the characteristics of fossil floras, on the masses and grain sizes of eroded sediments, or on calculations of increased thickness of the lithosphere as a result of faulting. Detrital erosion rates can be increased both by increased relief in the drainage basin and by a change to more seasonal rainfall patterns. The geologic record provides no clear answer to the question whether uplift caused the climatic deterioration of the Neogene or whether the changing climate affected the erosion system in such a way as to create an illusion of uplift. We suggest that the spread of C₄ plants in the Late Miocene may have altered both the erosion and climate systems. These changes are responsible for the apparent contradictions between data supporting uplift and those supporting high elevations in the past.     

Aspartic acid racemization in teeth from Ketef Hinnom,a late Holocene burial cave in Jerusalem: Evidence for reuse

The Ketef Hinnom tombs, lying just outside the walls of the old city of Jerusalem, were carved out of bedrock during the 7th–5th century B.C. Several artefacts dating to the Late Hellenistic Period (ca. 100 B.C.) have also been found within the tombs. Aspartic acid (Asp) racemization in 31 samples of human tooth dentine from these tombs was analyzed in order to evaluate the possible reuse of the tombs during this later period. A range of D/L Asp values was found: a group with values averaging 0.062 and another group averaging 0.043. These D/L values are close to those expected for the two archeological age groups as calculated from (1) the kinetics of Asp racemization as established from experimental studies at higher temperatures and in vivo racemization rates, and (2) the effective annual temperature in the tombs, as measured using the Pallman method. These results thus support the later reuse of the tombs for burials. Several factors contribute to the accuracy of Asp racemization dating, including the age of the individual at death, uncertainties concerning Asp racemization kinetics, and effects of temperature history. © 1999 John Wiley & Sons, Inc.     

Normalizing shadows in multi-temporal aerial frame imagery using relative radiometric adjustments to support near-real-time change detection

This study addresses the problem of shadows in multi-temporal imagery, which is a key issue with change detection approaches based on image comparison. We apply image-to-image radiometric normalizations including histogram matching (HM), mean-variance (MV) equalization, linear regression based on pseudo-invariant features (PIF-LR), and radiometric control sets (RCS) representing high- and low-reflectance extrema, for the novel purpose of normalizing brightness of transient shadows in high spatial resolution, bi-temporal, aerial frame image sets. Efficient shadow normalization is integral to remote sensing procedures that support disaster response efforts in a near-real-time fashion, including repeat station image (RSI) capture, wireless data transfer, shadow detection (as precursor to shadow normalization), and change detection based on image differencing and visual interpretation. We apply the normalization techniques to imagery of suburban scenes containing shadowed materials of varied spectral reflectance characteristics, whereby intensity (average of red, green, and blue spectral band values) under fully illuminated conditions is known from counterpart reference images (time-1 versus time-2). We evaluate the normalization results using stratified random pixel samples within transient shadows, considering central tendency and variance of differences in intensity relative to the unnormalized images. Overall, MV equalization yielded superior results in our tests, reducing the radiometric effects of shadowing by more than 85 percent. The HM and PIF-LR approaches showed slightly lower performance than MV, while the RCS approach proved unreliable among scenes and among stratified intensity levels. We qualitatively evaluate a shadow normalization based on MV equalization, describing its utility and limitations when applied in change detection. Application of image-to-image radiometric normalization for brightening shadowed areas in multi-temporal imagery in this study proved efficient and effective to support change detection.