Eleanor J. Burke

Modeling the Recent Evolution of Global Drought and Projections for the Twenty-First Century with the Hadley Centre Climate Model

Meteorological drought in the Hadley Centre global climate model is assessed using the Palmer Drought Severity Index (PDSI), a commonly used drought index. At interannual time scales, for the majority of the land surface, the model captures the observed relationship between the El Nino–Southern Oscillation and regions of relative wetness and dryness represented by high and low values of the PDSI respectively. At decadal time scales, on a global basis, the model reproduces the observed drying trend (decreasing PDSI) since 1952. An optimal detection analysis shows that there is a significant influence of anthropogenic emissions of greenhouse gasses and sulphate aerosols in the production of this drying trend. On a regional basis, the specific regions of wetting and drying are not always accurately simulated. In this paper, presentday drought events are defined as continuous time periods where the PDSI is less than the 20th percentile of the PDSI distribution between 1952 and 1998 (i.e., on average 20% of the land surface is in drought at any one time). Overall, the model predicts slightly less frequent but longer events than are observed. Future projections of drought in the twenty-first century made using the Special Report on Emissions Scenarios (SRES) A2 emission scenario show regions of strong wetting and drying with a net overall global drying trend. For example, the proportion of the land surface in extreme drought is predicted to increase from 1% for the present day to 30% by the end of the twenty-first century.

Evaluating Uncertainties in the Projection of Future Drought

Abstract The uncertainty in the projection of future drought occurrence was explored for four different drought indices using two model ensembles. The first ensemble expresses uncertainty in the parameter space of the third Hadley Centre climate model, and the second is a multimodel ensemble that additionally expresses structural uncertainty in the climate modeling process. The standardized precipitation index (SPI), the precipitation and potential evaporation anomaly (PPEA), the Palmer drought severity index (PDSI), and the soil moisture anomaly (SMA) were derived for both a single CO2 (1×CO2) and a double CO2 (2×CO2) climate. The change in moderate drought, defined by the 20th percentile of the relevant 1×CO2 distribution, was calculated. SPI, based solely on precipitation, shows little change in the proportion of the land surface in drought. All the other indices, which include a measure of the atmospheric demand for moisture, show a significant increase with an additional 5%–45% of the land surface in...

Understanding the Sensitivity of Different Drought Metrics to the Drivers of Drought under Increased Atmospheric CO2

AbstractA perturbed physics Hadley Centre climate model ensemble was used to study changes in drought on doubling atmospheric CO2. The drought metrics analyzed were based on 1) precipitation anomalies, 2) soil moisture anomalies, and 3) the Palmer drought severity index (PDSI). Drought was assumed to occur 17% of the time under single CO2. On doubling CO2, in general, PDSI drought occurs more often than soil moisture drought, which occurs more often than precipitation drought. This paper explores the relative sensitivity of each drought metric to changes in the main drivers of drought, namely precipitation and available energy. Drought tends to increase when the mean precipitation decreases, the mean available energy increases, the standard deviation of precipitation increases, and the standard deviation of available energy decreases. Simple linear approximations show that the sensitivity of drought to changes in mean precipitation is similar for the three different metrics. However, the sensitivity of dr...

The impact of the parameterization of heterogeneous vegetation on the modeled large‐scale circulation in CCM3‐BATS

This letter reports evidence of an unexpected large change in the planetary-scale circulation as a consequence of modifying the land cover within the Community Climate Model (CCM3). Three 10-year simulations were analyzed using (i) default land cover, i.e., a single dominant vegetation in each land grid cell; (ii) aggregate land cover that includes a representation of subpixel heterogeneity; and (iii) aggregate land cover for aerodynamic properties, but default land cover otherwise. Simulations (ii) and (iii) were similar, indicating that aerodynamic properties are influential at the planetary scale. Comparing these two runs with (i), there are significant differences in the boreal summer. These include a northward shift of the Northern Hemisphere jet that relates to a decrease in the zonally averaged aerodynamic roughness around 60°N, and a perturbation of the Southern Hemisphere jet. Field significance tests suggest these changes are likely a remote influence of the Northern Hemisphere perturbation, not a sampling fluctuation.

Measuring water content in saline sands using impulse time domain transmission techniques

This paper discusses two time domain transmission (TDT) electromagnetic methods for measuring soil water content in a sand and examines the impact of pore water salinity on the resulting measurements. The first technique calculates the time taken by an impulse traveling one way through the medium relative to its time taken in air. The second method converts this impulse to the frequency domain via a Fast Fourier Transform (FFT) and calculates the travel time from the difference in phase measured in air and that measured in the medium at each frequency, resulting in a measurement of the frequency-dependent travel time. The relationship between travel time and water content was determined for pore water electrical conductivities (EC) ranging from 0.5 to 40 dS m −1 . At 0.5 dS m −1 the relationship was similar to that found by previous researchers using time domain reflectometry (TDR) measurements. At pore water EC ≥5 dS m −1 the travel time was faster than that found for 0.5 dS m −1 at the same water content, contradicting traditional thinking based on transmission line theory and differing from results of TDR methods. In addition, for pore water EC ≥5 dS m −1 , the relationship determined between travel time and water content was shown to be independent of pore water EC, to the precision of the TDT measurement technique. As a result, the impulse TDT method and this calibration relationship may improve our ability to measure soil water content under natural field conditions and may encourage further investigation of the impact of salinity on the spectral dielectric response of porous media.

Measuring Spectral Dielectric Properties Using Gated Time Domain Transmission Measurements

A method to measure the frequency-dependent dielectric permittivity of simple materials based on a time domain transmission technique is described. A vector network analyzer (VNA) was connected to a twin-rod transmission line via a coaxial cable. The complex dielectric permittivity was found from the difference in phase and magnitude between a reference line surrounded by air and the same line surrounded by the substance of interest. The spectral response showed periodic variations in the dielectric permittivity as a result of multiple reflections in the experimental setup. These multiple reflections can be removed by using a time domain gate that selects only the primary transmission and filters out any subsequent reflections. It is essential that the apparatus be designed so that the first reflection is well separated from the primary transmission. This requires a long transmission line and a long coaxial cable. However, if the transmission line is too long, excessive conductive or dielectric losses make it hard to detect the primary transmission. The application of the gated time domain transmission technique to measure the frequency-dependent dielectric permittivity of water, ethanol, sand and saturated sand is demonstrated. This method does not have the typical limitations on sample volume. In addition, it does not require the assumptions necessary in previous time domain spectroscopy methods applied to open transmission lines where a probe model is used in conjunction with simple Debye relaxation and/or inverse methods.

Exploring the potential for multipatch soil-moisture retrievals using multiparameter optimization techniques

This paper explores the potential to retrieve surface soil moisture and optical depth simultaneously for several different patches of land cover in a single pixel from dual polarization, multiangle microwave brightness temperature observations such as will be provided by, for instance, the Soil Moisture and Ocean Salinity (SMOS) mission. MICRO-SWEAT, a coupled land-surface and microwave emission model, was used in a. year-long simulation to define the patch-specific soil moisture, optical depth, and synthetic, pixel-average microwave brightness temperatures similar to those that will be provided by SMOS. The microwave emission component of MICRO-SWEAT also forms the basis of an exploratory retrieval algorithm in which the difference between (synthetic) observations of microwave brightness temperatures and modeled, pixel-average microwave brightness temperatures for different input values of soil moisture and optical depth is minimized using the shuffled complex evolution (SCE) optimization procedure. Results are presented for two synthetic pixels, one with eight patches, where only the soil moisture is retrieved, and one with five patches, where both the soil moisture and the optical depth are retrieved.

Measuring the dielectric permittivity of a plant canopy and its response to changes in plant water status: An application of Impulse Time Domain Transmission

Impulse Time Domain Transmission (ITDT) has been used to measure the complex dielectric permittivity of media such as ethanol, water and variably saturated sand. This paper applies ITDT to measurements of the complex dielectric permittivity of a vegetation canopy. The dielectric permittivity of a vegetation canopy is very close to that of air and only very small changes in its value will occur with changes in plant water status. This paper presents preliminary results demonstrating that ITDT can make repeatable measurements of the complex components of the dielectric permittivity of a plant canopy. Furthermore, ITDT is shown to be highly sensitive to the very small changes in dielectric that occur as a result of changes in plant water status. Based on these preliminary results, there are potential applications foreseen for ITDT in microwave remote sensing, irrigation scheduling, plant physiological ecology, and fire susceptibility.

Impact of horizontal and vertical heterogeneities on retrievals using multiangle microwave brightness temperature data

This paper investigates the impact of heterogeneity at the land surface on geophysical parameters retrieved from multiangle microwave brightness temperature data, such as would be obtained from the Soil Moisture and Ocean Salinity (SMOS) mission. Synthetic brightness temperature data were created using the Common Land (land surface) Model, coupled with a microwave emission model and set within the framework of the North American Land Data Assimilation System (NLDAS). Soil moisture, vegetation optical depth, and effective physical temperature were retrieved using a multiobjective calibration routine similar to the proposed SMOS retrieval algorithm for a typical on-axis range of look angles. The impact of heterogeneity both in the near-surface profiles of soil moisture and temperature and in the land cover on the accuracy of the retrievals was examined. There are significant errors in the retrieved parameters over regions with steep gradients in the near-surface soil moisture profile. These errors are approximately proportional to the difference in the soil water content between the top (at 0.7 cm) and second layer (at 2.7 cm) of the land surface model. The errors resulting from heterogeneity in the land cover are smaller and increase nonlinearly with increasing land-surface heterogeneity (represented by the standard deviation of the optical depth within the pixel). The most likely use of retrieved soil moisture is through assimilation into an LDAS for improved initiation of weather and climate models. Given that information on the soil moisture profile is already available within the LDAS, the error in the retrieved soil moisture as a result of the near-surface profile can be corrected for. The potential errors as a result of land-surface heterogeneity can also be assessed for use in the assimilation process.

APPLICATION OF IMPROVED ECOSYSTEM AERODYNAMICS IN REGIONAL WEATHER FORECASTS

This paper reports the impact of applying improved representation of the area-average aerodynamic properties of South American ecosystems on 7-d weather fore- casts made with the Brazilian regional forecast model (Eta-SSiB). Two pairs of 7-d forecasts were made, each pair starting from the same initial conditions on 1 January 1999 and on 24 June 1999, respectively. In one run, the aerodynamic properties of the ecosystem used for each modeled grid square were those of the most common vegetation present. In the second, the grid-average aerodynamic properties of the vegetation were calculated using aggregation rules in combination with a high-resolution land-cover data set. There are marked differences in the aerodynamic properties of the underlying ecosystems in these two runs in some regions, notably around the Amazon River basin, in regions where there has been land-cover change, and in portions of southern Brazil. The 7-d average, predicted ecosystem-surface exchanges, near-surface temperature, and precipitation for the two pairs of runs were compared. There were differences between the forecasts that were most no- ticeable in regions where the ecosystems aerodynamic properties were most dissimilar, but also elsewhere. Given the limited nature of the experiments reported, these preliminary results should be treated with care. Nonetheless, they suggest the need for more systematic study of the significance of ecosystems on regional weather forecasts in South America.