Showing papers in "Journal of Hydrometeorology in 2001"

Global Precipitation at One-Degree Daily Resolution from Multisatellite Observations

Abstract: The One-Degree Daily (1DD) technique is described for producing globally complete daily estimates of precipitation on a 1 deg x 1 deg lat/long grid from currently available observational data. Where possible (40 deg N-40 deg S), the Threshold-Matched Precipitation Index (TMPI) provides precipitation estimates in which the 3-hourly infrared brightness temperatures (IR T(sub b)) are thresholded and all "cold" pixels are given a single precipitation rate. This approach is an adaptation of the Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI), but for the TMPI the IR Tb threshold and conditional rain rate are set locally by month from Special Sensor Microwave/Imager (SSM/I)-based precipitation frequency and the Global Precipitation Climatology Project (GPCP) satellite-gauge (SG) combined monthly precipitation estimate, respectively. At higher latitudes the 1DD features a rescaled daily Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) precipitation. The frequency of rain days in the TOVS is scaled down to match that in the TMPI at the data boundaries, and the resulting non-zero TOVS values are scaled locally to sum to the SG (which is a globally complete monthly product). The time series of the daily 1DD global images shows good continuity in time and across the data boundaries. Various examples are shown to illustrate uses. Validation for individual grid -box values shows a very high root-mean-square error but, it improves quickly when users perform time/space averaging according to their own requirements.

Soil Moisture Memory in Climate Models

Abstract: Water balance considerations at the soil surface lead to an equation that relates the autocorrelation of soil moisture in climate models to (1) seasonality in the statistics of the atmospheric forcing, (2) the variation of evaporation with soil moisture, (3) the variation of runoff with soil moisture, and (4) persistence in the atmospheric forcing, as perhaps induced by land atmosphere feedback. Geographical variations in the relative strengths of these factors, which can be established through analysis of model diagnostics and which can be validated to a certain extent against observations, lead to geographical variations in simulated soil moisture memory and thus, in effect, to geographical variations in seasonal precipitation predictability associated with soil moisture. The use of the equation to characterize controls on soil moisture memory is demonstrated with data from the modeling system of the NASA Seasonal-to-Interannual Prediction Project.

The Representation of Snow in Land Surface Schemes: Results from PILPS 2(d)

Abstract: Twenty-one land surface schemes (LSSs) performed simulations forced by 18 yr of observed meteorological data from a grassland catchment at Valdai, Russia, as part of the Project for the Intercomparison of Land-Surface Parameterization Schemes (PILPS) Phase 2(d). In this paper the authors examine the simulation of snow. In comparison with observations, the models are able to capture the broad features of the snow regime on both an intra- and interannual basis. However, weaknesses in the simulations exist, and early season ablation events are a significant source of model scatter. Over the 18-yr simulation, systematic differences between the models’ snow simulations are evident and reveal specific aspects of snow model parameterization and design as being responsible. Vapor exchange at the snow surface varies widely among the models, ranging from a large net loss to a small net source for the snow season. Snow albedo, fractional snow cover, and their interplay have a large effect on energy available for ablation, with differences among models most evident at low snow depths. The incorporation of the snowpack within an LSS structure affects the method by which snow accesses, as well as utilizes, available energy for ablation. The sensitivity of some models to longwave radiation, the dominant winter radiative flux, is partly due to a stability-induced feedback and the differing abilities of models to exchange turbulent energy with the atmosphere. Results presented in this paper suggest where weaknesses in macroscale snow modeling lie and where both theoretical and observational work should be focused to address these weaknesses.

An Intercomparison of Three Snow Schemes of Varying Complexity Coupled to the Same Land Surface Model: Local-Scale Evaluation at an Alpine Site

Abstract: The Interactions between Soil, Biosphere, and Atmosphere land surface scheme is currently used coupled both to atmospheric models and to a distributed hydrological model. There are two snow-scheme options available for hydrological modeling: the baseline force-restore approach, which uses a composite snow-soil-vegetation energy budget and a single snow layer; and a multilayer detailed internal-process snow model. Only the force-restore method is routinely used in atmospheric modeling applications. Recent studies have shown that hydrological simulations for mountainous catchments within the Rhone basin, France, are significantly improved using the detailed snow scheme. The main drawback is that the scheme is computationally expensive, and it is not currently feasible for routine application in atmospheric models. For these reasons, a third new intermediate-complexity scheme has been developed that includes certain key physical processes from the complex model for improved snowpack realism and hydrological depiction while attemping to keep computational requirements similar to those of the simple default scheme. In the current study, the new scheme is described, evaluated, and compared with the results from the two other schemes at a local scale at an alpine site located within the Rhone basin for two contrasting (weather) years. All schemes are able to model the basic features of the snow cover with similar errors averaged over the 2-yr period; however, there are important differences on shorter timescales. When compared with the more complex scheme, it was found that differing surface energy budget parameter-izations (turbulent transfer, albedo) were the cause for the largest differences in total snowpack snow water equivalent (SWE) simulated by the models. When compared with the simple scheme, the ability for the intermediate model to simulate snow ripening resulted in the largest differences in simulated SWE and snow temperature during melt and runoff.

Global Vegetation Root Distribution for Land Modeling

Abstract: Vegetation root distribution is one of the factors that determine the overall water holding capacity of the land surface and the relative rates of water extraction from different soil layers for vegetation transpiration. Despite its importance, significantly different root distributions are used by different land surface models. Using a comprehensive global field survey dataset, vegetation root distribution (including rooting depth) has been developed here for three of the most widely used land cover classifications [i.e., the Biosphere–Atmosphere Transfer Scheme (BATS), International Geosphere–Biosphere Program (IGBP), and version 2 of the Simple Biosphere Model (SiB2)] for direct use by any land model with any number of soil layers.

Impact of Deforestation on Regional Precipitation over the Indochina Peninsula

Abstract: It is now widely recognized that tropical deforestation can change the regional climate significantly. The increasing population and the spreading deforestation in the Indochina Peninsula, especially in Thailand, make it urgent to assess the effects of deforestation on the regional climate. Most of the previous numerical experiments generally have shown that decreases in precipitation occur as a result of deforestation. However, in most cases, these hydrometeorological changes have not been detected in observations. In this study, the nonparametric Mann‐Kendall rank test and linear regression analysis were applied to analyze precipitation data obtained over a period of more than 40 yr, for each month, at each meteorological station in Thailand. Significant decreases in precipitation over Thailand were detected only in the time series of monthly precipitation in September. Amounts of precipitation recorded at many meteorological stations in September have decreased by approximately 100 mm month21 (approximately 30% relative change) over the past three or four decades. Numerical experiments with a regional climate model based on the Regional Atmospheric Modeling System with a simple land surface scheme were carried out for the Indochina Peninsula. In these experiments, the type of vegetation in the northeastern part of Thailand was specified as either short vegetation (the current vegetation type) or forest (the former vegetation type). The experiments were carried out using the initial and boundary meteorological conditions of August and September in 1992‐94. The initial and boundary conditions were interpolated from the data of the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis. In these numerical experiments, a decrease in precipitation over the deforested area was obtained for September, but not for August. The magnitude of the mean decrease in precipitation over the whole deforested area in these experiments was 26 mm month21 (7% relative change), and the local maximum decrease was 88 mm month21 (29% relative change). Precipitation in the wet season over the Indochina Peninsula basically occurs under the influence of the Southeast Asian summer monsoon system. The strong summer monsoon westerlies bring abundant moisture to the Indochina Peninsula as a source of precipitation. The monsoon westerlies are the predominant external force influencing the regional climate. However, the strong westerlies over the Indochina Peninsula disappear in September, although that is typically the month of maximum precipitation. Accordingly, it is inferred that the effect of local deforestation appears significantly only in September because of the absence of this strong external force.

One-Dimensional Soil Moisture Profile Retrieval by Assimilation of Near-Surface Measurements: A Simplified Soil Moisture Model and Field Application

Abstract: The Kalman filter assimilation technique is applied to a simplified soil moisture model for retrieval of the soil moisture profile from near-surface soil moisture measurements. First, the simplified soil moisture model is developed, based on an approximation to the Buckingham‐Darcy equation. This model is then used in a 12month one-dimensional field application, with updating at 1-, 5-, 10-, and 20-day intervals. The data used are for the Nerrigundah field site, New South Wales, Australia. This study has identified (i) the importance of knowing the depth over which the near-surface soil moisture measurements are representative (i.e., observation depth), (ii) soil porosity and residual soil moisture content as the most important soil parameters for correct retrieval of the soil moisture profile, (iii) the importance of a soil moisture model that represents the dominant soil physical processes correctly, and (iv) an appropriate forecasting model as far more important than the temporal resolution of near-surface soil moisture measurements. Although the soil moisture model developed here is a good approximation to the Richards equation, it requires a root water uptake term or calibration to an extreme drying event to model extremely dry periods at the field site correctly.

The Impact of Detailed Snow Physics on the Simulation of Snow Cover and Subsurface Thermodynamics at Continental Scales

Abstract: The three-layer snow model is coupled to the global catchment-based Land Surface Model (LSM) of the NASA Seasonal to Interannual Prediction Project (NSIPP) project, and the combined models are used to simulate the growth and ablation of snow cover over the North American continent for the period 1987-1988. The various snow processes included in the three-layer model, such as snow melting and re-freezing, dynamic changes in snow density, and snow insulating properties, are shown (through a comparison with the corresponding simulation using a much simpler snow model) to lead to an improved simulation of ground thermodynamics on the continental scale.

Multiscale Statistical Properties of a High-Resolution Precipitation Forecast

Abstract: Small-scale (less than ∼15 km) precipitation variability significantly affects the hydrologic response of a basin and the accurate estimation of water and energy fluxes through coupled land–atmosphere modeling schemes. It also affects the radiative transfer through precipitating clouds and thus rainfall estimation from microwave sensors. Because both land–atmosphere and cloud–radiation interactions are nonlinear and occur over a broad range of scales (from a few centimeters to several kilometers), it is important that, over these scales, cloud-resolving numerical models realistically reproduce the observed precipitation variability. This issue is examined herein by using a suite of multiscale statistical methods to compare the scale dependence of precipitation variability of a numerically simulated convective storm with that observed by radar. In particular, Fourier spectrum, structure function, and moment-scale analyses are used to show that, although the variability of modeled precipitation agr...

Estimation of Rainfall Interstation Correlation

Abstract: This study discusses questions of estimating correlation coefficient of point rainfall as observed at two measuring stations. The focus is on issues such as sensitivity to sample size, extreme rain events, and distribution of rainfall. The authors perform extensive analysis based on a two-point data-driven rainfall model that simulates the intermittence and extreme variability of rainfall using a bivariate mixed-lognormal distribution. The study examines the commonly used product–moment estimator along with an alternative transformation-based estimator. The results show a high level of bias and variance of the traditional correlation estimator, which are caused mostly by significant skewness levels that characterize rainfall observations. Application using data from a high-density cluster indicated the advantages of using the alternative estimator. The overall aim of the study is to draw the attention of researchers working with rainfall to some commonly disregarded issues when they seek accurate...

Comparison of Snow Deposition, the Snow Cover Energy Balance, and Snowmelt at Two Sites in a Semiarid Mountain Basin

Abstract: Significant differences in snow deposition, development of the seasonal snow cover, and the timing of melt can occur over small spatial distances because of differences in topographically controlled wind exposure and canopy cover. To capture important intrabasin hydrological processes related to heterogeneous snow cover and energy inputs, models must explicitly account for these differences. The “SNOBAL” point snow cover energy and mass balance model is used to evaluate differences in snow cover energy and mass balance at two sites in a small headwater drainage of the Reynolds Creek Experimental Watershed (RCEW) in the Owyhee Mountains of southwestern Idaho. Though these sites are separated by only 350 m, they are located in distinctly different snow cover regimes. The “ridge” site (elevation 2097 m) is located on a broad shelf on the southern ridge of RCEW, and the “grove” site (elevation 2061 m) is sheltered by topography and forest canopy in a grove of aspen and fir trees just in the lee of th...

Extreme Rainfall and Flooding from Supercell Thunderstorms

Abstract: Supercell thunderstorms, the storm systems responsible for most tornadoes, have often been dismissed as flood hazards. The role of supercell thunderstorms as flood agents is examined through analyses of storm systems that occurred in Texas (5‐6 May 1995), Florida (26 March 1992), Nebraska (20‐21 June 1996), and Pennsylvania (18‐19 July 1996). Particular attention is given to the ‘‘Dallas Supercell,’’ which resulted in 16 deaths from flash flooding and more than $1 billion in property damage during the evening of 5 May 1995. Rainfall analyses using Weather Surveillance Radar-1988 Doppler (WSR-88D) reflectivity observations and special mesonet rain gauge observations from Dallas, Texas, show that catastrophic flash flooding resulted from exceptional rainfall rates at 5‐60-min timescales. The spatial structure of extreme rainfall was linked to supercell structure and motion. The ‘‘Orlando Supercell’’ produced extreme rainfall rates (greater than 300 mm h 21) at 1‐5-min timescales over a dense rain gauge network. The Nebraska and Pennsylvania storm systems produced record flooding over larger spatial scales than the Texas and Florida storms, by virtue of organization and motion of multiple storms over the same region. For both the Nebraska and Pennsylvania storms, extreme rainfall and tornadoes occurred in tandem. Severe rainfall measurement problems arise for supercell thunderstorms, both from conventional gauge networks and weather radar. It is hypothesized that supercell storms play a significant role in the ‘‘climatology’’ of extreme rainfall rates (100-yr return interval and greater) at short time intervals (1‐60 min) in much of the central and eastern United States.

A 36-yr Climatological Description of the Evaporative Sources of Warm-Season Precipitation in the Mississippi River Basin

Abstract: The terrestrial and oceanic sources of moisture that supply warm-season rainfall to the Mississippi River basin and its subbasins are examined over a 36-yr period (1963–98). Using hourly observed precipitation, National Centers for Environmental Prediction (NCEP) reanalyses at 6-h intervals, and a back-trajectory algorithm, the water falling during observed precipitation events is probabilistically traced to its most recent surface evaporative source, terrestrial or oceanic. Maps of these sources generally show dual maxima, one terrestrial and one oceanic, in spring and a dominance of terrestrial sources in summer. Pentad time series averaged over the 36 years show a late-summer maximum of precipitation recycling in all but the Missouri subbasin. During the 36 years analyzed, 32% of warm-season precipitation in the entire Mississippi basin originated as evaporation within the basin (recycled). About 20% of warm-season precipitation was contributed directly by evaporation from the Gulf of Mexico a...

Large-Sample Evaluation of Two Methods to Correct Range-Dependent Error for WSR-88D Rainfall Estimates

Abstract: The vertical variability of reflectivity is an important source of error that affects estimations of rainfall quantity by radar. This error can be reduced if the vertical profile of reflectivity (VPR) is known. Different methods are available to determine VPR based on volume-scan radar data. Two such methods were tested. The first, used in the Swiss Meteorological Service, estimates a mean VPR directly from volumetric radar data collected close to the radar. The second method takes into account the spatial variability of reflectivity and relies on solving an inverse problem in determination of the local profile. To test these methods, two years of archived level-II radar data from the Weather Surveillance Radar-1988 Doppler (WSR-88D) located in Tulsa, Oklahoma, and the corresponding rain gauge observations from the Oklahoma Mesonet were used. The results, obtained by comparing rain estimates from radar data corrected for the VPR influence with rain gauge observations, show the benefits of the met...

A Bias-Corrected Siberian Regional Precipitation Climatology

Abstract: A methodology for correcting the Tretyakov gauge-measured daily precipitation for wind-induced undercatch and trace amounts of precipitation is presented and applied at 61 climate stations in Siberian regions for 1986 to 1992. It is found that wind-induced gauge undercatch is the greatest error, and a trace amount of precipitation is also a significant bias, particularly in the low-precipitation regions. Monthly correction factors (corrected divided by measured precipitation) differ by location and by type of precipitation. Considerable interannual variation of the corrections exists in Siberian regions because of the fluctuation of wind speed, air temperature, and frequency of snowfall. More important, annual precipitation has been increased by 30–330 mm because of the bias corrections for the seven years (about 10%–65% of the gauge-measured yearly total). This result suggests that annual precipitation in Siberia is much higher than previously reported, particularly in the northwest sectors of h...

Vegetative and Atmospheric Corrections for the Soil Moisture Retrieval from Passive Microwave Remote Sensing Data: Results from the Southern Great Plains Hydrology Experiment 1997

Abstract: A radiative transfer model and data from the Southern Great Plains 1997 Hydrology Experiment were used to analyze the dependency of surface emissivity retrieval at 19 GHz on atmospheric and vegetative effects. Volumetric soil moisture obtained from ground measurements in the Central Facility area that show a dynamic range of 25% was highly correlated with the corresponding L-band electronically steered thinned array radiometer (ESTAR) 1.4-GHz and Special Sensor Microwave Imager 19-GHz brightness temperatures. For the Little Washita area, only the ESTAR measurements were well correlated with volumetric soil moisture. Atmospheric corrections, which were calculated from collocated radiosonde measurements, did not improve the soil moisture retrieval significantly. However, a sensitivity study at 19 GHz using a larger dataset of 241 radiosonde ascents indicates that the variability in integrated atmospheric water vapor introduces variations of 0.023 in surface emissivity. This value is ∼36% of the var...

The Most Extreme Precipitation Events over the Eastern United States from 1950 to 1996: Considerations of Scale

Abstract: The intensity or magnitude of a given heavy precipitation event is typically associated with the greatest point precipitation total. The scale or size of the heavy precipitation region, however, is important because it affects the scale of the flooding potential (e.g., local- vs regional-scale basins). In this study, a heavy-rain climatological description is constructed that identifies all precipitation events for the period of 1950–96 and estimates the heaviest mean 2-day precipitation totals over a range of spatial scales (i.e., circular regions from 2500 to 500 000 km2). Ranks of the most extreme precipitation events are provided for four regions of the study area for each of the 10 spatial scales. To develop the dataset, daily precipitation totals from the cooperative observer network are spatially interpolated onto a finescale (10 km by 10 km) grid over the eastern two-thirds of the United States. An automated algorithm is developed 1) to identify regions displaying the greatest mean 2-day ...

Using the Special Sensor Microwave Imager to Monitor Surface Wetness

Abstract: The frequencies flown on the Special Sensor Microwave Imager (SSM/I) are sensitive to liquid water near the earth’s surface. These frequencies are primarily atmospheric window channels, which receive the majority of their radiation from the surface. Liquid water near the surface depresses the emissivity as a function of wavelength. The relationship between brightness temperatures at different frequencies is used to dynamically derive the amount of liquid water in each SSM/I observation at 1/38 resolution. These data are averaged at 18 resolution throughout the globe for each month during the period of 1992‐97, and the 6-yr monthly means and the monthly anomalies of the wetness index are computed from this base period. To quantify the relationship between precipitation and surface wetness, these anomalies are compared with precipitation anomalies derived from the Global Precipitation Climate Program. The analysis was performed for six agricultural regions across six continents. There is generally a good correspondence between the two variables. The correlation generally increases when the wetness index is compared with precipitation anomalies accumulated over a 2-month period. These results indicate that the wetness index has a strong correspondence to the upper layer of the soil moisture in many cultivated areas of the world. The region in southeastern Australia had the best relationship, with a correlation coefficient of 0.76. The Sahel, France, and Argentina showed that the wetness index had memory of precipitation anomalies from the previous months. The memory is shorter for southeastern Australia and central China. The weakest correlations occurred over the southeastern United States, where the surface is covered by dense vegetation. The unique signal, strengths, and weaknesses of the wetness index in each of the six study regions are discussed.

An Evaluation of a High-Resolution Hydrometeorological Modeling System for Prediction of a Cool-Season Flood Event in a Coastal Mountainous Watershed

Abstract: This study used the atmospheric Fifth-Generation Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model (MM5) and the University of Washington Distributed Hydrology‐Soil‐ Vegetation Model (DHSVM) for the simulation of a complex rain-on-snow flood event that occurred from 28 December 1996 to 3 January 1997 on the 1560-km2 Snoqualmie River watershed in western Washington. Three control simulations were created with MM5 applied at 36-, 12-, and 4-km horizontal spacing and DHSVM at a horizontal spacing of 100 m. Results showed that the accuracy of the atmospheric fields increased with higher horizontal resolution, although underforecasting of precipitation was evident for all three resolutions. Simulated river flows captured 67% (36 km), 75% (12 km), and 72% (4 km) of the total flow and 52% (36 km), 58% (12 km), and 62% (4 km) of the event peak flow. Several sensitivity simulations of the modeling system (4-km spacing only) were conducted to improve on the control simulations. Adjusting the MM5 precipitation using observations led to a streamflow forecast that captured 90% of the total flow. Reduction of the model high‐wind speed bias improved the simulated snowmelt, although the resulting effects on streamflow were relatively small. A sensitivity experiment that included the precipitation from an intense rainband that was not captured by MM5 revealed the importance of this highintensity, short-lived feature; simulated streamflow from this experiment captured 93% of the total flow and over 82% of the peak flow, with a 4-h timing error. A final set of sensitivity simulations, using both a higher- and lower-elevation observation as the sole forcing of DHSVM (no MM5), revealed strong sensitivity to the observation location; using a slightly displaced (;8 km) lower-elevation observation produced river flows that differed by over 18%. Both of the resulting simulated river flows forced by the two-station method were significantly lower than both the observed flows (35% and 53% of total observed flow) and the flows simulated with the MM5 input fields. A major cause of this low flow was that the temperatures at the observation locations were located in gap regions of the Cascade Mountains, were not representative of the basin-average temperature, and therefore caused too much precipitation to fall as snow.

Precipitation Recycling over the Central United States Diagnosed from the GEOS-1 Data Assimilation System

Abstract: Precipitation recycling has been computed for 15 yr of reanalysis data from the National Aeronautics and Space Administration Goddard Earth Observing System (GEOS-1) Data Assimilation System using monthly mean hydrological data and a bulk diagnostic recycling model. This study focuses on the central United States and the extreme summers of 1988 (drought) and 1993 (flood). It is found that the 1988 summer recycling ratio is larger than that of 1993, and that the 1988 recycling ratio is much larger than average. The 1993 recycling ratio was less than average during the summer, but it was larger than average during the springtime, when the soil was being primed for flooding. In addition, the magnitude of summertime recycled precipitation was smaller than average in both 1988 and 1993. During the summer of 1993, the extremely large moisture transport dominates evaporation as the source of water for the extreme summer precipitation. The diagnosed recycling data show that the recycled precipitation is ...

Value of Incorporating Satellite-Derived Land Cover Data in MM5/PLACE for Simulating Surface Temperatures

Abstract: The Parameterization for Land–Atmosphere–Cloud Exchange (PLACE) module is used within the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) to determine the importance of individual land surface parameters in simulating surface temperatures. Sensitivity tests indicate that soil moisture and the coverage and thickness of green vegetation [as manifested by the values of fractional green vegetation coverage (fVEG) and leaf area index (LAI)] have a large effect on the magnitudes of surface sensible heat fluxes. The combined influence of LAI and fVEG is larger than the influence of soil moisture on the partitioning of the surface energy budget. Values for fVEG, albedo, and LAI, derived from 1-km-resolution Advanced Very High Resolution Radiometer data, are inserted into PLACE, and changes in model-simulated 1.5-m air temperatures in Oklahoma during July of 1997 are documented. Use of the land cover data provides a clear improvement in afterno...

Does Grazing Affect Regional Climate

Abstract: Before European settlement, the Great Plains of the United States contained vast herds of bison. These bison altered the landscape through their grazing. Measurement data of the disturbance that such grazing could produce, when scaled for the large population of bison, were used with a coupled atmospheric‐ecosystem model to evaluate the likely effect that this grazing had on the growing season weather in the Great Plains. A dynamically coupled meteorological and plant growth model was used to investigate the regional atmospheric conditions over a single growing season. A 50-km horizontal mesh was implemented, covering the central plains of the United States. The modeling system was then integrated, with a time step of 90 s, for a period covering 1 April 1989 through 31 August 1989 using boundary conditions obtained from an objective analysis of gridded archive data. This integration was performed with and without grazing to assess the effects on regional atmospheric and biological processes. The grazing algorithm was employed to represent presettlement North American bison and was switched on and off for different simulations. The results indicated a cooling response in daily maximum temperatures to removal of grazing. The opposite trends were found for the minimum daily temperature. It was also found that grazing produced significant perturbations in the hydrological cycle.

On the Use of the Force–Restore SVAT Model Formulation for Stratified Soils

Abstract: An approach to simulate soil moisture content with the force–restore soil–vegetation–atmosphere transfer (SVAT) model in the case of stratified soils is proposed. Typical soil profiles possess marked variation in soil hydraulic properties from the soil surface to the base of the root zone. The force–restore method is extensively used for land surface modeling in atmospheric models but without any specific consideration for dealing with stratified soils. Drainage from and recharge to the near-surface soil layer is classically estimated on the basis of the volumetric soil moisture differences between the near surface and lower root zone, with an adjustment for gravitational effects. However, moisture differences do not relate uniquely to differences in hydraulic potentials when the soil properties are vertically inhomogeneous. As a consequence, the classical force–restore formulation does not correctly represent vertical fluxes, and it results in biased time series of predicted near-surface soil mo...

An Evaluation of the Strength of Land–Atmosphere Coupling

Abstract: Two ensembles of 1-month integrations of a coupled land–atmosphere climate model that differ only in their treatment of land surface boundary conditions have been generated from initial conditions chosen from the July states taken from each year of a 17-yr integration from the second Atmospheric Model Intercomparison Project (AMIP2). Both ensembles have specified sea surface temperature from one randomly chosen year, but one ensemble has the land surface state variables specified in each member at each time step to be identical to those from a single member of the other ensemble. Comparisons with the 17-yr AMIP2 integration provide an estimate of the role of interannually varying SST in affecting climate variability. Comparison between the two ensembles helps to quantify the role of land surface variability on the variance of surface fluxes and the climate. In this model system, the impacts of suppressed ocean variability on intra-ensemble spread are generally stronger than for suppressed land su...

A Coupled Land Surface–Boundary Layer Model and Its Adjoint

Abstract: In this paper, a simple coupled land surface–boundary layer model and its adjoint are presented. The primary goal is to demonstrate the capabilities of the adjoint model as a general tool for sensitivity analysis and data assimilation. The adjoint method was chosen primarily for two reasons: 1) the adjoint model can be used not only to obtain parameter sensitivities with greater efficiency but, more important, to provide added insight into the sensitivities as compared with that obtained with traditional simulation techniques (e.g., pathways, time variations in sensitivity) and 2) the adjoint model can be used in a variational data assimilation framework to combine measurements and the model of the physical system optimally in order to estimate state variables and fluxes. Two simple examples are presented to illustrate how the framework can be used for performing both diagnostic sensitivity experiments and hydrologic data assimilation. In the sensitivity experiment, temporal patterns and total in...

Feedback between the Land Surface Energy Balance and Atmospheric Boundary Layer Diagnosed through a Model and Its Adjoint

Abstract: In this paper, the differences in (daytime) sensitivity of surface turbulent fluxes to states and parameters for a coupled land–atmospheric boundary layer model in coupled and uncoupled modes are investigated using an adjoint framework. The adjoint approach yields diagnostic insight into sensitivity pathways that is not readily obtained when using a traditional forward sensitivity framework. It is shown that the corresponding uncoupled adjoint model is a special case of the coupled adjoint model in which the atmospheric feedback pathways are “closed.” These closed sensitivity pathways are directly responsible for the differences in sensitivity between the uncoupled and coupled models. Integrating the adjoint models about a consistent and common trajectory provided by the forward model yields the sensitivities of the surface turbulent fluxes to all of the model states and parameters, which are shown to be significantly different because of the boundary layer feedbacks. Furthermore, the adjoint sen...

Short-Term and Seasonal Variability of the Atmospheric Water Vapor Transport through the Mackenzie River Basin

Abstract: The transport of water vapor through the Mackenzie River basin, a typical high-latitude river basin, is examined for the years 1979–93, with the European Centre for Medium-Range Weather Forecasts reanalysis dataset (ERA). It is shown that the transport of water vapor through the Mackenzie basin is highly variable in space and time. This transport has two distinct modes. During the autumn, winter, and spring, moisture is transported into the basin from the southwest by extratropical cyclones. The source of this moisture is argued to be the subtropical and midlatitude central Pacific Ocean. During the summer, moisture enters the basin from the northwest, with the source region being the Arctic Ocean. The values of monthly water vapor budgets obtained with the objectively analyzed fields are compared with those obtained from interpolated radiosonde data and with other known quantitative information about the water budget of the basin. It is found that the ERA data seriously overestimate the values o...

Climate Drift in a Coupled Land–Atmosphere Model

Abstract: A coupled land–atmosphere climate model is examined for evidence of climate drift in the land surface state variable of soil moisture. The drift is characterized as pathological error growth in two different ways. First is the systematic error that is evident over seasonal timescales, dominated by the error modes with the largest saturated amplitude: systematic drift. Second is the fast-growing modes that are present in the first few days after either initialization or a data assimilation increment: incremental drift. When the drifts are robust across many ensemble members and from year to year, they suggest a source of drift internal to the coupled system. This source may be due to problems in either component model or in the coupling between them. Evidence is presented for both systematic and incremental drift. The relationship between the two types of drift at any given point is shown to be an indication of the type and strength of feedbacks within the coupled system. Methods for elucidating p...

Impact of Small-Scale Rainfall Variability on Larger-Scale Spatial Organization of Land–Atmosphere Fluxes

Abstract: A coupled modeling framework is used in this study to investigate the effect of subgrid-scale rainfall variability on the spatial structure of the evolving storm and on other surface variables and water and energy fluxes. The Fifth-Generation Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model coupled with the Biosphere‐Atmosphere Transfer Scheme is combined with a dynamical/statistical scheme for statistically downscaling rainfall. Model simulations with and without including subgrid-scale rainfall variability are compared at the grid scale to quantify the propagation of small-scale rainfall heterogeneities through the nonlinear land‐atmosphere system. It was found that including subgrid-scale rainfall variability (here on the order of 3 km) affects the spatial organization of the storm system itself, surface temperature, soil moisture, and sensible and latent heat fluxes. These effects were found to occur at spatial scales much larger than the scale at which rainfall variability was prescribed, illustrating the pronounced nonlinear spatial dynamics of the land‐atmosphere system and its important role on hydrometeorological predictions.

Verification of Patch- and Regional-Scale Energy Balance Estimates Derived from Microwave and Optical Remote Sensing during SGP97

Abstract: The 1997 Southern Great Plains Hydrology Experiment (SGP97) was designed and conducted to extend surface soil moisture retrieval algorithms based on passive microwave observations to coarser resolutions, larger regions with more diverse conditions, and longer time periods. The L-band Electronically Scanned Thinned Array Radiometer (ESTAR) on an airborne platform was used for daily mapping of surface soil moisture over an area of approximately 40 km × 260 km for a 1-month period. Results showed that the soil moisture retrieval algorithm performed the same as in previous investigations, demonstrating consistency of both the retrieval and the instrument. This soil moisture product at 800-m pixel resolution together with land use and fractional vegetation cover information is used in a remote sensing model for computing spatially distributed fluxes over the SGP97 domain. Validation of the model output is performed at the patch scale using tower-based measurements and at regional scale using aircraft ...