Search Results

You are looking at 1 - 10 of 18 items for

  • Author or Editor: M. Ek x
  • Refine by Access: All Content x
Clear All Modify Search
M. Ek
and
L. Mahrt

Abstract

Data from the Hydrological and Atmospheric Pilot Experiment-Modélisation du Bilan Hydrigue (HAPEX-MOBILHY) field program and results from a one-dimensional model of the soil and atmospheric boundary layer are analyzed to study the daytime evolution of the relative humidity at the boundary layer top. This evolution is thought to control the development of boundary layer clouds. This study examines the dependence of boundary layer relative humidity on soil moisture, large-scale vertical motion, and the moisture content and temperature stratification above the boundary layer. The response of the boundary layer relative humidity to external forcing involves competing mechanisms and the net effect on relative humidity is difficult to predict without complete analysis of the relative humidity tendency equation.

As one example, drier soil leads to smaller boundary layer specific humidity but also leads to cooler temperatures at the boundary layer top due to greater boundary layer growth. When the latter effect dominates, the relative humidity at the boundary layer top is greater over drier soil. In contrast, drier soil leads to lower relative humidity at the boundary layer top when the air above the boundary layer is strongly stratified or quite dry. These and other nonlinear interactions are posed in terms of a detailed analysis of the budget equation for boundary layer top relative humidity.

Full access
A. A. M. Holtslag
and
M. Ek

Abstract

The interaction of the atmospheric boundary layer with the heterogeneous pine forest in HAPEX-MOBILHY on a scale of order 10 km is studied. A state-of-the-art, coupled atmosphere-soil-vegetation model is used and is run for 16 June 1986 in a stand-alone mode using prescribed dynamics. Published values for the effective roughness lengths of heat and momentum from different origins are used to show the impact on the surface fluxes and the boundary layer development. The modal simulations indicate that the coupled atmosphere-vegetation system is rather sensitive to the value for the roughness length of heat. This affects in particular the sensible heat flux and, as a consequence, the boundary layer height and profiles of mean quantities in the boundary layer. The model results are compared with observations made at a forest town, with radiosonde profiles, and with aircraft data. The best overall agreement for the boundary layer quantities is obtained by using a roughness length of heat that is three orders of magnitude smaller than the roughness length of momentum.

Full access
M. B. Ek
and
A. A. M. Holtslag

Abstract

The daytime interaction of the land surface with the atmospheric boundary layer (ABL) is studied using a coupled one-dimensional (column) land surface–ABL model. This is an extension of earlier work that focused on modeling the ABL for 31 May 1978 at Cabauw, Netherlands; previously, it was found that coupled land–atmosphere tests using a simple land surface scheme did not accurately represent surface fluxes and coupled ABL development. Here, findings from that earlier study on ABL parameterization are utilized, and include a more sophisticated land surface scheme. This land surface scheme allows the land–atmosphere system to respond interactively with the ABL. Results indicate that in coupled land–atmosphere model runs, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds (shallow cumulus). Subsequently, the role of soil moisture in the development of ABL clouds is explored in terms of a new relative humidity tendency equation at the ABL top where a number of processes and interactions are involved. Among other issues, it is shown that decreasing soil moisture may actually lead to an increase in ABL clouds in some cases.

Full access
S. Chang
,
D. Hahn
,
C-H. Yang
,
D. Norquist
, and
M. Ek

Abstract

An updated complete and comprehensive description of the land surface parameterization scheme in the Coupled Atmosphere–Plant–Soil (CAPS) model is presented. The CAPS model has been in development at Oregon State University and Phillips Laboratory since 1981. The CAPS model was originally designed for a global atmospheric model, but it has also been used as a stand-alone model for a variety of applications. The land surface scheme in the CAPS model is one of the two dozen schemes that participated in the Project for Intercomparison of Land Surface Parameterization Schemes (PILPS). Some unique features of the CAPS scheme are given in detail. A comprehensive dataset of one year (1987), including atmospheric forcing data and validation data from Cabauw, has been provided for PILPS by the Royal Netherlands Meteorological Institute. Using the Cabauw data, a validation study for the CAPS scheme has been carried out. The scheme’s self-consistencies in terms of surface energy balance and water budget are discussed. Finally, the results of this validation study with emphasis on the performance of surface momentum and heat fluxes are presented.

Full access
Pierre Gentine
,
Albert A. M. Holtslag
,
Fabio D'Andrea
, and
Michael Ek

Abstract

The onset of moist convection over land is investigated using a conceptual approach with a slab boundary layer model. The authors determine the essential factors for the onset of boundary layer clouds over land and study their relative importance. They are 1) the ratio of the temperature to the moisture lapse rates of the free troposphere, that is, the inversion Bowen ratio; 2) the mean daily surface temperature; 3) the relative humidity of the free troposphere; and 4) the surface evaporative fraction. A clear transition is observed between two regimes of moistening of the boundary layer as assessed by the relative humidity at the boundary layer top. In the first so-called wet soil advantage regime, the moistening results from the increase of the mixed-layer specific humidity, which linearly depends on the surface evaporative fraction and inversion Bowen ratio through a dynamic boundary layer factor. In the second so-called dry soil advantage regime, the relative humidity tendency at the boundary layer top is controlled by the thermodynamics and changes in the moist adiabatic induced by the decreased temperature at the boundary layer top and consequent reduction in saturation water vapor pressure. This regime pertains to very deep boundary layers under weakly stratified free troposphere over hot surface conditions. In the context of the conceptual model, a rise in free-tropospheric temperature (global warming) increases the occurrence of deep convection and reduces the cloud cover over moist surfaces. This study provides new intuition and predictive capacity on the mechanism controlling the occurrence of moist convection over land.

Full access
Youlong Xia
,
Trent W. Ford
,
Yihua Wu
,
Steven M. Quiring
, and
Michael B. Ek

Abstract

The North American Soil Moisture Database (NASMD) was initiated in 2011 to provide support for developing climate forecasting tools, calibrating land surface models, and validating satellite-derived soil moisture algorithms. The NASMD has collected data from over 30 soil moisture observation networks providing millions of in situ soil moisture observations in all 50 states, as well as Canada and Mexico. It is recognized that the quality of measured soil moisture in NASMD is highly variable because of the diversity of climatological conditions, land cover, soil texture, and topographies of the stations, and differences in measurement devices (e.g., sensors) and installation. It is also recognized that error, inaccuracy, and imprecision in the data can have significant impacts on practical operations and scientific studies. Therefore, developing an appropriate quality control procedure is essential to ensure that the data are of the best quality. In this study, an automated quality control approach is developed using the North American Land Data Assimilation System, phase 2 (NLDAS-2), Noah soil porosity, soil temperature, and fraction of liquid and total soil moisture to flag erroneous and/or spurious measurements. Overall results show that this approach is able to flag unreasonable values when the soil is partially frozen. A validation example using NLDAS-2 multiple model soil moisture products at the 20-cm soil layer showed that the quality control procedure had a significant positive impact in Alabama, North Carolina, and west Texas. It had a greater impact in colder regions, particularly during spring and autumn. Over 433 NASMD stations have been quality controlled using the methodology proposed in this study, and the algorithm will be implemented to control data quality from the other ~1200 NASMD stations in the near future.

Full access
Robert J. Zamora
,
Edward P. Clark
,
Eric Rogers
,
Michael B. Ek
, and
Timothy M. Lahmers

Abstract

The NOAA Hydrometeorology Testbed (HMT) program has deployed a soil moisture observing network in the Babocomari River basin located in southeastern Arizona. The Babocomari River is a major tributary of the San Pedro River. At 0000 UTC 23 July 2008, the second-highest flow during the period of record was measured just upstream of the location where the Babocomari River joins the main channel of the San Pedro River.

Upper-air and surface meteorological observations and Special Sensor Microwave Imager (SSM/I) satellite images of integrated water vapor were used to establish the synoptic and mesoscale conditions that existed before the flood occurred. The analysis indicates that a weak Gulf of California surge initiated by Hurricane Fausto transported a warm moist tropical air mass into the lower troposphere over southern Arizona, setting the stage for the intense, deep convection that initiated the flooding on the Babocomari River. Observations of soil moisture and precipitation at five locations in the basin and streamflow measured at two river gauging stations enabled the documentation of the hydrometeorological conditions that existed before the flooding occurred. The observations suggest that soil moisture conditions as a function of depth, the location of semi-impermeable layers of sedimentary rock known as caliche, and the spatial distribution of convective precipitation in the basin confined the flooding to the lower part of the basin. Finally, the HMT soil moisture observations are compared with soil moisture products from the NOAA/NWS/NCEP Noah land surface model.

Full access
Youlong Xia
,
Michael B. Ek
,
Yihua Wu
,
Trent Ford
, and
Steven M. Quiring

Abstract

Soil moisture observations from seven observational networks (spanning portions of seven states) with different biome and climate conditions were used in this study to evaluate multimodel simulated soil moisture products. The four land surface models, including Noah, Mosaic, Sacramento soil moisture accounting (SAC), and the Variable Infiltration Capacity model (VIC), were run within phase 2 of the North American Land Data Assimilation System (NLDAS-2), with a ⅛° spatial resolution and hourly temporal resolution. Hundreds of sites in Alabama, Colorado, Michigan, Nebraska, Oklahoma, West Texas, and Utah were used to evaluate simulated soil moisture in the 0–10-, 10–40-, and 40–100-cm soil layers. Soil moisture was spatially averaged in each state to reduce noise. In general, the four models captured broad features (e.g., seasonal variation) of soil moisture variations in all three soil layers in seven states, except for the 10–40-cm soil layer in West Texas and the 40–100-cm soil layer in Alabama, where the anomaly correlations are weak. Overall, Mosaic, SAC, and the ensemble mean have the highest simulation skill and VIC has the lowest simulation skill. The results show that Noah and VIC are wetter than the observations while Mosaic and SAC are drier than the observations, mostly likely because of systematic errors in model evapotranspiration.

Full access
Youlong Xia
,
Michael B. Ek
,
Yihua Wu
,
Trent Ford
, and
Steven M. Quiring

Abstract

In this second part of a two-part paper, the impacts of soil texture and vegetation type misclassification and their combined effect on soil moisture, evapotranspiration, and total runoff simulation are investigated using the Noah model. The results show that these impacts are significant for most regions and soil layers, although they vary depending on soil texture classification, vegetation type, and season. The use of site-observed soil texture classification and vegetation type in the model does not necessarily improve anomaly correlations and reduce mean absolute error for soil moisture simulations. Instead, results are mixed when examining all regions and soil layers. This is attributed to the compensation effects (e.g., effect of ill-calibrated model parameters), as Noah has been more or less calibrated with model-specified soil texture classification and vegetation type. The site-based analysis shows that Noah can reasonably simulate the variation of daily evapotranspiration, soil moisture, and total runoff when soil texture classification (vegetation type) is corrected from loam (forest) to clay (grasslands) or vice versa. This suggests that the performance of Noah can be further improved by tuning model parameters when site-observed soil texture and vegetation type are used.

Full access
Gary M. Lackmann
,
Kermit Keeter
,
Laurence G. Lee
, and
Michael B. Ek

Abstract

During episodes of sustained moderate or heavy precipitation in conjunction with near-freezing temperatures and weak horizontal temperature advection, the latent heat released (absorbed) by the freezing (melting) of falling precipitation may alter thermal profiles sufficiently to affect the type and amount of freezing or frozen precipitation observed at the surface. Representation of these processes by operational numerical weather prediction models is incomplete; forecaster knowledge of these model limitations can therefore be advantageous during winter weather forecasting. The Eta Model employs a sophisticated land surface model (LSM) to represent physical processes at the lower-atmospheric interface. When considering the thermodynamic effect of melting or freezing precipitation at the surface, it is shown that limitations in the current version of the Eta LSM can contribute to biases in lower-tropospheric temperature forecasts. The Eta LSM determines the precipitation type reaching the surface from the air temperature at the lowest model level; subfreezing (above freezing) temperatures are assumed to correspond to snow (rain) reaching the surface. There is currently no requirement for consistency between the LSM and the Eta grid-scale precipitation scheme. In freezing-rain situations, the lowest model air temperature is typically below freezing, and the Eta LSM will therefore determine that snow is falling. As a result, a cold bias develops that is partly caused by the neglected latent heat release accompanying the freezing of raindrops at the surface. In addition, alterations in surface characteristics caused by erroneous snowfall accumulation in the model may also contribute to temperature biases. In an analogous fashion, warm biases can develop in cases with melting snow and above-freezing air temperatures near the surface (the LSM assumes rain). An example case is presented in which model misrepresentation of freezing rain is hypothesized to have contributed to a lower-tropospheric cold bias. A simple temperature correction, based on the first law of thermodynamics, is applied to lower-tropospheric model temperature forecasts; the neglect of latent heat released by freezing rain in the model is shown to contribute substantially to a cold bias in near-surface temperature forecasts. The development of a spurious snow cover likely exacerbated the bias.

Full access