Ensemble prediction for nowcasting with a convection-permitting model - II: forecast error statistics

A 24-member ensemble of 1-h high-resolution forecasts over the Southern United Kingdom is used to study short-range forecast error statistics. The initial conditions are found from perturbations from an ensemble transform Kalman filter. Forecasts from this system are assumed to lie within the bounds of forecast error of an operational forecast system. Although noisy, this system is capable of producing physically reasonable statistics which are analysed and compared to statistics implied from a variational assimilation system. The variances for temperature errors for instance show structures that reflect convective activity. Some variables, notably potential temperature and specific humidity perturbations, have autocorrelation functions that deviate from 3-D isotropy at the convective-scale (horizontal scales less than 10 km). Other variables, notably the velocity potential for horizontal divergence perturbations, maintain 3-D isotropy at all scales. Geostrophic and hydrostatic balances are studied by examining correlations between terms in the divergence and vertical momentum equations respectively. Both balances are found to decay as the horizontal scale decreases. It is estimated that geostrophic balance becomes less important at scales smaller than 75 km, and hydrostatic balance becomes less important at scales smaller than 35 km, although more work is required to validate these findings. The implications of these results for high-resolution data assimilation are discussed

Reticular synthesis and the design of new materials

The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal-oxygen-carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62718/1/nature01650.pd

Tunable Porous Organic Crystals: Structural Scope and Adsorption Properties of Nanoporous Steroidal Ureas

Previous work has shown that certain steroidal bis-(N-phenyl)ureas, derived from cholic acid, form crystals in the P61 space group with unusually wide unidimensional pores. A key feature of the nanoporous steroidal urea (NPSU) structure is that groups at either end of the steroid are directed into the channels and may in principle be altered without disturbing the crystal packing. Herein we report an expanded study of this system, which increases the structural variety of NPSUs and also examines their inclusion properties. Nineteen new NPSU crystal structures are described, to add to the six which were previously reported. The materials show wide variations in channel size, shape, and chemical nature. Minimum pore diameters vary from ∼0 up to 13.1 Å, while some of the interior surfaces are markedly corrugated. Several variants possess functional groups positioned in the channels with potential to interact with guest molecules. Inclusion studies were performed using a relatively accessible tris-(N-phenyl)urea. Solvent removal was possible without crystal degradation, and gas adsorption could be demonstrated. Organic molecules ranging from simple aromatics (e.g., aniline and chlorobenzene) to the much larger squalene (Mw = 411) could be adsorbed from the liquid state, while several dyes were taken up from solutions in ether. Some dyes gave dichroic complexes, implying alignment of the chromophores in the NPSU channels. Notably, these complexes were formed by direct adsorption rather than cocrystallization, emphasizing the unusually robust nature of these organic molecular hosts

Structure and Magnetic Properties of the Spin Crossover Linear Trinuclear Complex [Fe3(furtrz)6(ptol)2(MeOH)4]·4(ptol)·4(MeOH) (furtrz: furanylidene-4H-1,2,4-triazol-4-amine ptol: p-tolylsulfonate)

The furan-functionalised 1,2,4-triazole ligand furanylidene-4H-1,2,4-triazol-4-amine (furtrz) has been incorporated into the trinuclear complex Fe3(furtrz)6(ptol)2(MeOH)4]·4(ptol)·4(MeOH) (ptol = p-tolylsulfonate) composed of μ1,2-triazole bridges between iron(II) sites, as per one-dimensional chain materials, and terminally coordinated ptol anions and methanol molecules. Magnetic susceptibility measurements reveal a gradual single-step spin crossover (SCO) behavior of one third of the iron(II) sites per trinuclear unit. Single-crystal X-ray diffraction below the transition (90 K) shows the central iron(II) sites undergo a HS to LS transition and the peripheral ones remain HS (HS = high spin; LS = low spin). This is a rare example of a cationic trinuclear SCO material where the discrete unit includes bound anions

Nanoporosity and Exceptional Negative Thermal Expansion in Single-Network Cadmium Cyanide

Accentuate the negative: Single-network cadmium cyanide displays isotropic negative thermal expansion behavior of unprecedented magnitude over a large temperature range (see graph of unit cell parameter a versus temperature). Guest molecules in the pores of this framework block the transverse vibrational modes responsible for this behavior, causing the value of the linear coefficient of thermal expansion to increase with guest occupancy

Negative thermal expansion and low-frequency modes in cyanide-bridged framework materials

We analyze the intrinsic geometric flexibility of framework structures incorporating linear metal–cyanide– metal sM–CN–M8d linkages using a reciprocal-space dynamical matrix approach. We find that this structural motif is capable of imparting a significant negative thermal expansion sNTEd effect upon such materials. In particular, we show that the topologies of a number of simple cyanide-containing framework materials support a very large number of low-energy rigid-unit phonon modes, all of which give rise to NTE behavior. We support our analysis by presenting experimental verification of this behavior in the family of compounds ZnxCd1−xsCNd2, which we show to exhibit a NTE effect over the temperature range 25–375 K more than double that of materials such as ZrW2O8

A versatile family of interconvertible microporous chiral molecular frameworks: The first example of ligand control of network chirality

Two families of molecular frameworks which grow as homochiral single crystals are described. Both consist of multiple interpenetration of the three-connected chiral (10,3)-a (Y*) network and result from the tridentate coordination of the 1,3,5-benzenetricarboxylate (btc) ligand to octahedral metal centers which act as linear connectors. The nature of the interpenetration is controlled by the auxiliary ligands bound in the equatorial plane of the metal center. Ethylene glycol (eg) binds in a unidentate fashion to form phase A which has 28% accessible solvent volume and contains four interpenetrating (10,3)-a networks. 1,2-Propanediol (1,2- pd) coordinates as a bidentate ligand to yield a phase B with a greatly enhanced 51% of solvent accessible volume, because only two (distorted) (10,3)-a' networks interpenetrate. Ligands in the void space and bound to the metal center can both be liberated thermally: the kinetics of this process allow isolation of microporous desolvated crystalline A and B. The porous phases lose crystallinity reversibly upon further loss of ligands bound to the equatorial metal: crystallinity is restored upon exposure to the vapors of simple alcohols, which can also effect conversion of B to A. Both phases present interpenetrating network topologies that are unique to chemistry and adopt space groups that are new for molecular solids: A crystallizes in P4232 and B adopts I4132. B can be grown homochirally from enantiomerically pure diol template. The stereochemistry of the alcohol bound to the metal controls the helicity of the chiral framework. The structure determination of the 1,2-propanediol phase represents the first demonstration that chiral molecules can specifically template helix handedness in a chiral porous framework solid