Initial Moisture Profile Research Articles

Toward Parametrization of Precipitating Shallow Cumulus Cloud Organization via Moisture Variance

AbstractThe influence of the initial vertical moisture profile on precipitating shallow cumulus cloud organization in terms of the column‐averaged moisture variance is investigated using large‐eddy simulations. Five idealized simulations based on the Rain in Cumulus over the Ocean field experiment with different initial moisture profiles are investigated. All cases simulate precipitating shallow cumulus convection in a marine sub‐tropical region under large‐scale subsidence. The results show that the moisture variance is mainly generated through the interaction of the moisture flux and the moisture gradient in the gradient production term at the top of the boundary layer. The development is characterized by three regimes: initial, transition, and quasi‐steady regime. During the initial regime, the moisture gradient is built up by moisture accumulation until precipitating convection starts. Within the transition regime, precipitation enables mesoscale cloud organization with enhanced convective activity and moisture fluxes. The moisture variance increases from the moist to the dry initial moisture profiles. In a following quasi‐steady regime, the moisture variance is approximately preserved. Thereby, the initial moisture gradient between the average sub‐cloud layer and the free atmosphere is found to be an important factor for the generation of the quasi‐steady column‐averaged moisture variance. The result suggests that a resolved‐scale variable like the moisture gradient can be used to estimate the quasi‐steady state conditions resulting from cloud organization. This finding may serve as a starting point for the parametrization of the subgrid scale cloud organization caused by precipitating shallow convection.

Sensitivity of Axisymmetric Tropical Cyclone Spinup Time to Dry Air Aloft

AbstractThe sensitivity of tropical cyclone spinup time to the initial entropy deficit of the troposphere is examined in an axisymmetric hurricane model. Larger initial entropy deficits correspond to less moisture above the initial lifting condensation level of a subcloud-layer parcel. The spinup time is quantified in terms of thresholds of integrated horizontal kinetic energy within a radius of 300 km and below a height of 1.5 km. The spinup time increases sublinearly with increasing entropy deficit, indicating the greatest sensitivity lies with initial moisture profiles closer to saturation. As the moisture profile approaches saturation, there is a large increase in the low-level, area-averaged, vertical mass flux over the spinup period because of the predominance of deep convection. Higher entropy deficit experiments have a greater amount of cumulus congestus and reduced vertical mass flux over a longer duration. Consequently, the secondary circulation takes longer to build upward, and the radial influx of angular momentum is reduced. There is also a reduction in the conversion of potential available enthalpy to horizontal kinetic energy, as a result of reduced flow down the radial pressure gradient early in the spinup period. Later in the spinup period, the low-level vortex spins up relatively quickly near the nascent radius of maximum wind in the high-entropy deficit experiments, whereas the low-level vortex spins up over a wider area in the low-entropy deficit experiments.

The role of radiation in organizing convection in weak temperature gradient simulations

AbstractUsing a cloud system resolving model with the large scale parameterized by the weak temperature gradient approximation, we investigated the influence of interactive versus noninteractive radiation on the characteristics of convection and convective organization. The characteristics of convecting environments are insensitive to whether radiation is interactive compared to when it is not. This is not the case for nonconvecting environments; interactive radiative cooling profiles show strong cooling at the top of the boundary layer which induces a boundary layer circulation that ultimately exports moist entropy (or analogously moist static energy) from dry domains. This upgradient transport is associated with a negative gross moist stability, and it is analogous to boundary layer circulations in radiative convective equilibrium simulations of convective self‐aggregation. This only occurs when radiation cools interactively. Whether radiation is static or interactive also affects the existence of multiple equilibria‐steady states which either support precipitating convection or which remain completely dry depending on the initial moisture profile. Interactive radiation drastically increases the range of parameters which permit multiple equilibria compared to static radiation; this is consistent with the observation that self‐aggregation in radiative‐convective equilibrium simulations is more readily attained with interactive radiation. However, the existence of multiple equilibria in absence of interactive radiation suggests that other mechanisms may result in organization.

Low-field NMR logging sensor for measuring hydraulic parameters of model soils

Summary Knowing the exact hydraulic parameters of soils is very important for improving water management in agriculture and for the refinement of climate models. Up to now, however, the investigation of such parameters has required applying two techniques simultaneously which is time-consuming and invasive. Thus, the objective of this current study is to present only one technique, i.e., a new non-invasive method to measure hydraulic parameters of model soils by using low-field nuclear magnetic resonance (NMR). Hereby, two model clay or sandy soils were respectively filled in a 2 m-long acetate column having an integrated PVC tube. After the soils were completely saturated with water, a low-field NMR sensor was moved up and down in the PVC tube to quantitatively measure along the whole column the initial water content of each soil sample. Thereafter, both columns were allowed to drain. Meanwhile, the NMR sensor was set at a certain depth to measure the water content of that soil slice. Once the hydraulic equilibrium was reached in each of the two columns, a final moisture profile was taken along the whole column. Three curves were subsequently generated accordingly: (1) the initial moisture profile, (2) the evolution curve of the moisture depletion at that particular depth, and (3) the final moisture profile. All three curves were then inverse analyzed using a MATLAB code over numerical data produced with the van Genuchten–Mualem model. Hereby, a set of values (α, n, θr and θs) was found for the hydraulic parameters for the soils under research. Additionally, the complete decaying NMR signal could be analyzed through Inverse Laplace Transformation and averaged on the 1/T2 space. Through measurement of the decay in pure water, the effect on the relaxation caused by the sample could be estimated from the obtained spectra. The migration of the sample-related average 〈1/T2,Sample〉 with decreasing saturation speaks for a enhancement of the surface relaxation as the soil dries, in concordance with results found by other authors. In conclusion, this low-field mobile NMR technique has proven itself to be a fast and a non-invasive mean to investigate the hydraulic behavior of soils and to explore microscopical aspect of the water retained in them. In the future, the sensor should allow easy soil moisture measurements on-field.

New linearized two-parameter infiltration equation for direct determination of conductivity and sorptivity

A new two-parameter (the sorptivity, S [ L / T 0.5 ], and saturated soil hydraulic conductivity, K s [ L / T ]) vertical infiltration equation is proposed. A three-parameter accurate approximate equation is derived. The proposed two-parameter equation is a specific solution that is approximately located at the middle of the domain of real soils defined by two “limiting” behavior soils. According to the proposed equation, plotting the square of cumulative infiltration, i [ L ], divided by time, t [T], as a function of i , a linear line is obtained and it is easy to determine S squared as the intercept and K s as the slope of the line and to test the adequacy of the proposed equation. The equation approaches Philip semi-analytical infinite-series solution at small and moderate times, whereas the infiltration rate approaches the constant K s at large times. It is expressed in a form explicit in i as function of t and vice versa. The equation is validated through comparison with other common nonlinear models (with two- and three-parameters) on a number of reference soils from the literature (both real and analytical) The parameter estimates of the equation are practically insensitive to the number of data points used in fitting, as well as to random measurement errors. Two illustrative infiltration tests for more realistic conditions (non-uniform soil and initial moisture profiles) were also performed. The tests demonstrate the advantage of the linearized equation over other common nonlinear models to be able visually detect and eventually eliminate abnormalities of infiltration process.

Multiple equilibria in a single‐column model of the tropical atmosphere

A single‐column model run under the weak temperature gradient approximation, a parameterization of large‐scale dynamics appropriate for the tropical atmosphere, is shown to have multiple stable equilibria. Under conditions permitting persistent deep convection, the model has a statistically steady state in which such convection occurs, as well as an extremely dry state in which convection does not occur. Which state is reached depends on the initial moisture profile.

Burgers' equation: A general nonlinear solution of infiltration and redistribution

The general one‐dimensional solution of Burgers equation is developed using a series of transformations and the Green's function method. The solution gives the distribution of the reduced water content in a semi‐infinite and in a finite domain for arbitrary initial moisture distributions. The boundary condition at the soil surface can be either a time‐dependent flux or constant water content while the bottom boundary condition for the finite case is a constant water content value. Explicit results for a uniform, discrete, hydrostatic and steady state initial moisture profiles are derived. An analytical solution that spans the preponding to the postponding period of rainfall is also obtained. Expressions for the time to ponding and infiltration equations are presented for the various cases of initial moisture profiles and a shallow water table. These results allowed the quantification of the error in the various approximations to the time to ponding and the cumulative infiltration in the postponding period.

Flux‐Concentration Relation‐Based Solution of Constant‐Flux Infiltration Equation: I. Infiltration into Nonuniform Initial Moisture Profiles

AbstractThe quasi‐analytical solution of the infiltration equation based on the flux‐concentration relation for constant flux condition and initially uniform water content profile is extended here to nonuniform moisture profiles for fluxes either smaller or greater than the saturated hydraulic conductivity. In the latter case, the solution is developed for the postponding stage. It is shown that the measured flux‐concentration relation is well‐approximated by F(θ*) = θ* and that no significant time dependence can be observed. The quasi‐analytical solution is then successfully compared with both laboratory experiments performed on a sandy soil column and numerical solution of the Richards equation for the nonponding, preponding, and postponding stages of infiltration.