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Abstract
Precursory signals in sea surface temperature anomalies (SSTAs) associated with summer precipitation over the Southwest (SW) United States are examined using data from 1900 to 1996. Two patterns of SSTAs are found to influence summer and winter precipitation evolution. One pattern shows positive anomalies extending from the central Pacific to the west coast of the Americas during winter. Warm SSTAs over the equatorial central Pacific in winter are associated with wet conditions in the SW and dry conditions over the Pacific Northwest. The following summer, SSTAs in the central Pacific diminish, but positive SSTAs remain over the tropical eastern Pacific and in the vicinity of Central America. Warm SSTs are conducive to enhanced rising motion and convection. A compensating downward branch is found over the SW favoring dry conditions over that area. Negative SSTAs have the opposite effect. This mode is similar to the SSTA pattern discussed by Higgins et al. When this mode is active, a wet (dry) SW winter is followed by a dry (wet) summer.
In addition to SSTAs over the central and eastern Pacific, another controlling factor is the SSTA over the North Pacific. Negative (positive) SSTAs over the North Pacific persist from winter to summer and favor positive (negative) rainfall anomalies over the SW. When this mode is active, a wet (dry) SW persists from winter to summer.
The authors conclude that SW rainfall evolution from winter to the following summer is modulated by both SSTAs over the equatorial Pacific and over the North Pacific. SSTAs in the equatorial Pacific alone are not sufficient to explain the observed summer rainfall variability over the southwest United States.
Abstract
Precursory signals in sea surface temperature anomalies (SSTAs) associated with summer precipitation over the Southwest (SW) United States are examined using data from 1900 to 1996. Two patterns of SSTAs are found to influence summer and winter precipitation evolution. One pattern shows positive anomalies extending from the central Pacific to the west coast of the Americas during winter. Warm SSTAs over the equatorial central Pacific in winter are associated with wet conditions in the SW and dry conditions over the Pacific Northwest. The following summer, SSTAs in the central Pacific diminish, but positive SSTAs remain over the tropical eastern Pacific and in the vicinity of Central America. Warm SSTs are conducive to enhanced rising motion and convection. A compensating downward branch is found over the SW favoring dry conditions over that area. Negative SSTAs have the opposite effect. This mode is similar to the SSTA pattern discussed by Higgins et al. When this mode is active, a wet (dry) SW winter is followed by a dry (wet) summer.
In addition to SSTAs over the central and eastern Pacific, another controlling factor is the SSTA over the North Pacific. Negative (positive) SSTAs over the North Pacific persist from winter to summer and favor positive (negative) rainfall anomalies over the SW. When this mode is active, a wet (dry) SW persists from winter to summer.
The authors conclude that SW rainfall evolution from winter to the following summer is modulated by both SSTAs over the equatorial Pacific and over the North Pacific. SSTAs in the equatorial Pacific alone are not sufficient to explain the observed summer rainfall variability over the southwest United States.
Abstract
The ensemble rainfall forecasts over the Sahel for July–September (JAS) from the NCEP Coupled Forecast System (CFS) were evaluated for the period 1981–2002. The comparison with the gauge-based precipitation analysis indicates that the predicted Sahel rainfall is light and exhibits little interannual variability. The rain belt is shifted about 4° southward.
One major source of rainfall errors comes from the erroneous sea surface temperature (SST) forecasts. The systematic SST error pattern has positive errors in the North Pacific and the North Atlantic and negative errors in the tropical Pacific and the southern oceans. It resembles the decadal SST mode, which has a significant influence on rainfall over the Sahel. Because the systematic SST errors were not corrected during the forecasts, persistent errors serve as an additional forcing to the atmosphere.
The second source of error is from the soil moisture feedback, which contributes to the southward shift of rainfall and dryness over West Africa. This was demonstrated by the comparison between simulations (SIMs) and the Atmospheric Model Intercomparison Project (AMIP) run. Both are forced with observed SSTs. The SIMs initialized at the end of June have realistic soil moisture and do not show the southward shift of rainfall. The AMIP, which predicts soil moisture, maintains the dryness through the summer over the Sahel. For AMIP, the decreased rainfall is contributed by the decreased evaporation (E) due to the dry soil and the shift of the large temperature gradients southward. In response, the African easterly jet (AEJ) shifts southward. Since this jet is the primary source of energy for the African waves and their associated mesoscale convective systems, these too shift southward. This negative feedback contributes to increased dryness over the Sahel.
Abstract
The ensemble rainfall forecasts over the Sahel for July–September (JAS) from the NCEP Coupled Forecast System (CFS) were evaluated for the period 1981–2002. The comparison with the gauge-based precipitation analysis indicates that the predicted Sahel rainfall is light and exhibits little interannual variability. The rain belt is shifted about 4° southward.
One major source of rainfall errors comes from the erroneous sea surface temperature (SST) forecasts. The systematic SST error pattern has positive errors in the North Pacific and the North Atlantic and negative errors in the tropical Pacific and the southern oceans. It resembles the decadal SST mode, which has a significant influence on rainfall over the Sahel. Because the systematic SST errors were not corrected during the forecasts, persistent errors serve as an additional forcing to the atmosphere.
The second source of error is from the soil moisture feedback, which contributes to the southward shift of rainfall and dryness over West Africa. This was demonstrated by the comparison between simulations (SIMs) and the Atmospheric Model Intercomparison Project (AMIP) run. Both are forced with observed SSTs. The SIMs initialized at the end of June have realistic soil moisture and do not show the southward shift of rainfall. The AMIP, which predicts soil moisture, maintains the dryness through the summer over the Sahel. For AMIP, the decreased rainfall is contributed by the decreased evaporation (E) due to the dry soil and the shift of the large temperature gradients southward. In response, the African easterly jet (AEJ) shifts southward. Since this jet is the primary source of energy for the African waves and their associated mesoscale convective systems, these too shift southward. This negative feedback contributes to increased dryness over the Sahel.
Abstract
A reconstructed rainfall dataset, and satellite estimates are used to analyze interannual to decadal variability of austral summer precipitation over South America. Rotated empirical orthogonal function (REOF) analysis is applied to isolate dominant patterns of rainfall. Links of these patterns to sea surface temperature anomalies (SSTAs) are examined.
The leading mode is related to El Niño–Southern Oscillation (ENSO), which explains 12% of the total variance. During warm ENSO events, the positive phase of this mode shows dry conditions over northern South America and wet conditions over the subtropical plains between 25° and 35°S. The situation reverses during cold events. The second REOF 2, which explains about 10.8% of the total variance, consists of positive loadings over northeast Brazil centered at 50°W near the equator and negative loadings over Colombia and the subtropical plains. For December–January–February (DJF), REOF 2 is influenced by tropical South Atlantic SSTAs through displacements of the intertropical convergence zone. Northeast Brazil receives most rainfall in March–April–May (MAM) and it is modulated by both the Atlantic SSTAs and ENSO. In the interannual frequency band, the North Atlantic Oscillation (NAO) has very limited influence on rainfall. On the decadal timescales, the NAO leads REOF 2 by three years.
Latitudinal variations of tropical convection are through the joint contribution of REOF 2 and REOF 4. REOF 4 is similar to REOF 2, but centers are displaced about 10° south. When these two EOFs are both positive, central South America is wet. The amplitudes of REOF 2 and REOF 4 are small during the mid-1950s to the mid-1960s and they are out of phase from 1968 to 1970, periods with persistent dry conditions over the upper La Plata River basin.
Abstract
A reconstructed rainfall dataset, and satellite estimates are used to analyze interannual to decadal variability of austral summer precipitation over South America. Rotated empirical orthogonal function (REOF) analysis is applied to isolate dominant patterns of rainfall. Links of these patterns to sea surface temperature anomalies (SSTAs) are examined.
The leading mode is related to El Niño–Southern Oscillation (ENSO), which explains 12% of the total variance. During warm ENSO events, the positive phase of this mode shows dry conditions over northern South America and wet conditions over the subtropical plains between 25° and 35°S. The situation reverses during cold events. The second REOF 2, which explains about 10.8% of the total variance, consists of positive loadings over northeast Brazil centered at 50°W near the equator and negative loadings over Colombia and the subtropical plains. For December–January–February (DJF), REOF 2 is influenced by tropical South Atlantic SSTAs through displacements of the intertropical convergence zone. Northeast Brazil receives most rainfall in March–April–May (MAM) and it is modulated by both the Atlantic SSTAs and ENSO. In the interannual frequency band, the North Atlantic Oscillation (NAO) has very limited influence on rainfall. On the decadal timescales, the NAO leads REOF 2 by three years.
Latitudinal variations of tropical convection are through the joint contribution of REOF 2 and REOF 4. REOF 4 is similar to REOF 2, but centers are displaced about 10° south. When these two EOFs are both positive, central South America is wet. The amplitudes of REOF 2 and REOF 4 are small during the mid-1950s to the mid-1960s and they are out of phase from 1968 to 1970, periods with persistent dry conditions over the upper La Plata River basin.
Abstract
Upper-ocean temperature variability in the tropical Atlantic is examined from the Comprehensive Ocean–Atmosphere Data Set (COADS) as well as from an ocean model simulation forced by COADS anomalies appended to a monthly climatology. The findings are as follows: Only the sea surface temperatures (SST) in the northern Tropics are driven by heat fluxes, while the southern tropical variability arises from wind-driven ocean circulation changes. The subsurface temperatures in the northern and southern Tropics are found to have a strong linkage to buoyancy forcing changes in the northern North Atlantic. Evidence for Kelvin-like boundary wave propagation from the high latitudes is presented from the model simulation. This extratropical influence is associated with wintertime North Atlantic oscillation (NAO) forcing and manifests itself in the northern and southern tropical temperature anomalies of the same sign at depths of 100–200 m as result of a Rossby wave propagation away from the eastern boundary in the wake of the boundary wave passage. The most apparent association of the southern tropical sea surface temperature anomalies (SSTA) arises with the anomalous cross-equatorial winds that can be related to both NAO and the remote influence from the Pacific equatorial region. These teleconnections are seasonal, so that the NAO impact on the tropical SST is the largest at midwinter but in spring and early summer the Pacific remote influence competes with NAO. However, NAO appears to have a more substantial role than the Pacific influence at low frequencies during the last 50 years. The dynamic origin of SSTA is indirectly confirmed from the SST–heat flux relationship using ocean model experiments that remove either anomalous wind stress forcing or atmospheric forcing anomalies contributing to heat exchange.
Abstract
Upper-ocean temperature variability in the tropical Atlantic is examined from the Comprehensive Ocean–Atmosphere Data Set (COADS) as well as from an ocean model simulation forced by COADS anomalies appended to a monthly climatology. The findings are as follows: Only the sea surface temperatures (SST) in the northern Tropics are driven by heat fluxes, while the southern tropical variability arises from wind-driven ocean circulation changes. The subsurface temperatures in the northern and southern Tropics are found to have a strong linkage to buoyancy forcing changes in the northern North Atlantic. Evidence for Kelvin-like boundary wave propagation from the high latitudes is presented from the model simulation. This extratropical influence is associated with wintertime North Atlantic oscillation (NAO) forcing and manifests itself in the northern and southern tropical temperature anomalies of the same sign at depths of 100–200 m as result of a Rossby wave propagation away from the eastern boundary in the wake of the boundary wave passage. The most apparent association of the southern tropical sea surface temperature anomalies (SSTA) arises with the anomalous cross-equatorial winds that can be related to both NAO and the remote influence from the Pacific equatorial region. These teleconnections are seasonal, so that the NAO impact on the tropical SST is the largest at midwinter but in spring and early summer the Pacific remote influence competes with NAO. However, NAO appears to have a more substantial role than the Pacific influence at low frequencies during the last 50 years. The dynamic origin of SSTA is indirectly confirmed from the SST–heat flux relationship using ocean model experiments that remove either anomalous wind stress forcing or atmospheric forcing anomalies contributing to heat exchange.
Abstract
This study employs observations and the model simulations from the U.S. Climate Variability and Predictability (CLIVAR) Drought Working Group to examine extreme precipitation events like drought and wet spells that persist more than one season over South America. These events tend to persist over northeastern Brazil, the Guianas, and the west coast of Colombia, Ecuador, and Peru. They are least likely to persist over southeastern South America, which includes Uruguay, southern Brazil, and northeastern Argentina.
The U.S. CLIVAR simulations, particularly those of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3.5 (CAM3.5), capture satisfactorily the impact of the El Niño–Southern Oscillation (ENSO) and the north tropical Atlantic (NTA) sea surface temperature anomaly (SSTA) signals on persistent extreme events and reproduce the mechanisms inducing the teleconnection patterns. The cold (warm) ENSO favors wetness (dryness) over Venezuela, Colombia, and northeastern Brazil and dryness (wet spells) over southeastern South America and southern Argentina. The NTA SSTAs alone tend to have a more local impact affecting mostly over northern South America in March–May.
The simulations show that when the two modes (ENSO and NTA) act in concert, the effects may become noticeable in different and remote areas of the continent, as they shift the probability of drought and persistent wet spells over different regions of South America. The impact is strong when the ENSO and the NTA are in opposite phases. For the cold (warm) Pacific and warm (cold) Atlantic, droughts (persistent wet spells) are intensified over southeastern South America, while persistent wet spells (droughts) are favored over the northern part of the continent. The changes in the patterns are regional and not as intense when both oceans are warm (or cold).
Abstract
This study employs observations and the model simulations from the U.S. Climate Variability and Predictability (CLIVAR) Drought Working Group to examine extreme precipitation events like drought and wet spells that persist more than one season over South America. These events tend to persist over northeastern Brazil, the Guianas, and the west coast of Colombia, Ecuador, and Peru. They are least likely to persist over southeastern South America, which includes Uruguay, southern Brazil, and northeastern Argentina.
The U.S. CLIVAR simulations, particularly those of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3.5 (CAM3.5), capture satisfactorily the impact of the El Niño–Southern Oscillation (ENSO) and the north tropical Atlantic (NTA) sea surface temperature anomaly (SSTA) signals on persistent extreme events and reproduce the mechanisms inducing the teleconnection patterns. The cold (warm) ENSO favors wetness (dryness) over Venezuela, Colombia, and northeastern Brazil and dryness (wet spells) over southeastern South America and southern Argentina. The NTA SSTAs alone tend to have a more local impact affecting mostly over northern South America in March–May.
The simulations show that when the two modes (ENSO and NTA) act in concert, the effects may become noticeable in different and remote areas of the continent, as they shift the probability of drought and persistent wet spells over different regions of South America. The impact is strong when the ENSO and the NTA are in opposite phases. For the cold (warm) Pacific and warm (cold) Atlantic, droughts (persistent wet spells) are intensified over southeastern South America, while persistent wet spells (droughts) are favored over the northern part of the continent. The changes in the patterns are regional and not as intense when both oceans are warm (or cold).
Abstract
The effect of tropical latent heat release in accelerations of the Southern Hemisphere subtropical jet is discussed based on a case study for 6–8 August 1979 and general circulation model simulations. This jet is a main feature of the winter time circulation extending over Australia and the western Pacific Ocean. An intensification of this jet was observed for 8 August, with peak values of 93 m s−1. Satellite pictures and daily precipitation amounts revealed that the 6–8 August period was characterized by extensive rainfall in the tropics, with precipitation diminishing from that day on. Level III-b data for the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory for these dates was projected into the normal modes of a primitive equation model linearized about a basic state at rest. The analysis shows that the jet accelerations were due both to external and internal Rossby modes, with somewhat stronger contributions from the external mode. An enhancement of the local meridional circulation is due to these extensive precipitation areas which project mostly in internal gravity modes with maximum values upstream of the jet accelerations.
Conjecture that the jet stream is reacting in this short time scale to tropical latent heat release is tested with the Goddard Laboratory for the Atmospheres fourth order general circulation model. A 15 day integration starting on 1 August 1979 is used as the control case. This run produced peak values of 82 m s−1 for the zonal wind at 200 mb during August 8 centered over Australia at about 26°S in conjunction with increased divergent circulations at about 6°N about 50° longitude upstream from the jet maximum. A simulation was started on 4 August and run for ten days suppressing tropical Pacific heating from 90°E through 120°W. Normal mode contributions were also obtained for the control and “no tropical heating” experiment. The case study and GCM simulations suggest the following time scale of response of subtropical latitudes to tropical latent heat release: divergent circulations which project mostly into inertia-gravity waves react to changes in latent heat release in 1–2 days, its impact in subtropical latitudes is felt in 2–4 days; and becomes fully established after about 6 days.
Abstract
The effect of tropical latent heat release in accelerations of the Southern Hemisphere subtropical jet is discussed based on a case study for 6–8 August 1979 and general circulation model simulations. This jet is a main feature of the winter time circulation extending over Australia and the western Pacific Ocean. An intensification of this jet was observed for 8 August, with peak values of 93 m s−1. Satellite pictures and daily precipitation amounts revealed that the 6–8 August period was characterized by extensive rainfall in the tropics, with precipitation diminishing from that day on. Level III-b data for the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory for these dates was projected into the normal modes of a primitive equation model linearized about a basic state at rest. The analysis shows that the jet accelerations were due both to external and internal Rossby modes, with somewhat stronger contributions from the external mode. An enhancement of the local meridional circulation is due to these extensive precipitation areas which project mostly in internal gravity modes with maximum values upstream of the jet accelerations.
Conjecture that the jet stream is reacting in this short time scale to tropical latent heat release is tested with the Goddard Laboratory for the Atmospheres fourth order general circulation model. A 15 day integration starting on 1 August 1979 is used as the control case. This run produced peak values of 82 m s−1 for the zonal wind at 200 mb during August 8 centered over Australia at about 26°S in conjunction with increased divergent circulations at about 6°N about 50° longitude upstream from the jet maximum. A simulation was started on 4 August and run for ten days suppressing tropical Pacific heating from 90°E through 120°W. Normal mode contributions were also obtained for the control and “no tropical heating” experiment. The case study and GCM simulations suggest the following time scale of response of subtropical latitudes to tropical latent heat release: divergent circulations which project mostly into inertia-gravity waves react to changes in latent heat release in 1–2 days, its impact in subtropical latitudes is felt in 2–4 days; and becomes fully established after about 6 days.
Abstract
Persistent anomalies with recurrent spatial patterns play an important role in the atmosphere's low-frequency variability. We establish a connection between statistical and dynamical methods of description and prediction of persistent anomalies. This is done by computing and analyzing the empirical orthogonal functions (EOFs) in a simple deterministic model, on the one hand, and in Southern Hemisphere geopotential heights, on the other.
The dynamical model is governed by the fully nonlinear, equivalent-barotropic vorticity equation on the sphere, with simplified forcing, dissipation and topography. Model solutions exhibit persistent anomalies identifiable with blocked, zonal and wave-train anomalies in Northern Hemisphere atmospheric data. Flow structures similar to the patterns above occur as high-variance EOFs of this nonlinear model.
The Southern Hemisphere data we analyze consist in gridded daily maps of 500 mb heights from June 1972 to July 1983. Two types of persistent anomalies appear in this time series, both having a strong wavenumber-three component; they differ by the value of the constant phase of this wave and by the strength of the wavenumber-one component. The first two EOFs bear a striking resemblance to these two patterns.
We conclude that the dynamical interpretation of EOFs is their pointing from the time mean to the most populated regions of the system's phase space. Pursuing this interpretation, we introduce a Markov-chain formulation of transitions from one persistent anomaly regime to another, and discuss the implications for long-range forecasting.
Abstract
Persistent anomalies with recurrent spatial patterns play an important role in the atmosphere's low-frequency variability. We establish a connection between statistical and dynamical methods of description and prediction of persistent anomalies. This is done by computing and analyzing the empirical orthogonal functions (EOFs) in a simple deterministic model, on the one hand, and in Southern Hemisphere geopotential heights, on the other.
The dynamical model is governed by the fully nonlinear, equivalent-barotropic vorticity equation on the sphere, with simplified forcing, dissipation and topography. Model solutions exhibit persistent anomalies identifiable with blocked, zonal and wave-train anomalies in Northern Hemisphere atmospheric data. Flow structures similar to the patterns above occur as high-variance EOFs of this nonlinear model.
The Southern Hemisphere data we analyze consist in gridded daily maps of 500 mb heights from June 1972 to July 1983. Two types of persistent anomalies appear in this time series, both having a strong wavenumber-three component; they differ by the value of the constant phase of this wave and by the strength of the wavenumber-one component. The first two EOFs bear a striking resemblance to these two patterns.
We conclude that the dynamical interpretation of EOFs is their pointing from the time mean to the most populated regions of the system's phase space. Pursuing this interpretation, we introduce a Markov-chain formulation of transitions from one persistent anomaly regime to another, and discuss the implications for long-range forecasting.
Abstract
We have studied a barotropic flow over sinusoidal topography in a beta channel in the presence of forcing and dissipation. Our work can be divided into two main parts: In the first part, we have computed numerically the stationary solutions of the model, using Newton's method, together with a finite difference approximation, and we compare our solutions with the solutions of the linear theory; in the second part, we successfully develop a nonlinear theory that reproduces our numerical solutions very closely.
In the first part of the paper we show that multiple equilibria do exist in our problem; i.e., multiple equilibria are not artificially introduced by the mathematical technique of severe truncation previously used, but are a consequence of the resonance present in our physical system. Then we show that even if we are dealing with a small topography (we have considered Fourier components of the real topography that have amplitudes of a few hundred meters, small values compared with the depth of the atmosphere) the linear theory fails when the system comes close enough to resonance to produce forced waves with an amplitude of any relevance.
In the second part, we study the nonlinear interactions, using an appropriate spectral decomposition, and we have pointed out how the linear solution must be modified when the system approaches resonance. We show how to split the nonlinear interaction term into two pieces, one always negligible (for low topography) and another one crucial when the system is close to the resonance. Based on these considerations, we have developed a nonlinear theory that successfully explains the qualitative and quantitative features of our numerical calculations.
Abstract
We have studied a barotropic flow over sinusoidal topography in a beta channel in the presence of forcing and dissipation. Our work can be divided into two main parts: In the first part, we have computed numerically the stationary solutions of the model, using Newton's method, together with a finite difference approximation, and we compare our solutions with the solutions of the linear theory; in the second part, we successfully develop a nonlinear theory that reproduces our numerical solutions very closely.
In the first part of the paper we show that multiple equilibria do exist in our problem; i.e., multiple equilibria are not artificially introduced by the mathematical technique of severe truncation previously used, but are a consequence of the resonance present in our physical system. Then we show that even if we are dealing with a small topography (we have considered Fourier components of the real topography that have amplitudes of a few hundred meters, small values compared with the depth of the atmosphere) the linear theory fails when the system comes close enough to resonance to produce forced waves with an amplitude of any relevance.
In the second part, we study the nonlinear interactions, using an appropriate spectral decomposition, and we have pointed out how the linear solution must be modified when the system approaches resonance. We show how to split the nonlinear interaction term into two pieces, one always negligible (for low topography) and another one crucial when the system is close to the resonance. Based on these considerations, we have developed a nonlinear theory that successfully explains the qualitative and quantitative features of our numerical calculations.
Abstract
A 32-km high-resolution modified Palmer drought severity index (MPDSI) based on the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (RR) from 1979 to 2004 is presented. The assumptions of Palmer, such as the water balance equation, the difference between observed precipitation and the climatologically expected precipitation over the maximum conditions, and the changes of the index as a function of the current index, are preserved. Many deficiencies of the original PDSI are eliminated by taking fields directly from the RR or by making better estimates. For example, fields such as potential evapotranspiration, evaporation, runoff, total soil moisture, and soil moisture change in a given month are obtained directly from the RR. The potential recharge is defined as the total soil moisture needed to reach the maximum total soil moisture at each grid point for each calendar month. The potential precipitation is defined as the maximum precipitation at each grid point for a given calendar month. The underground volumetric soil moisture includes both frozen and liquid form. Therefore, the contribution of snowmelt is taken into account inexplicitly. The questionable assumptions of two-layer soil model and the available soil moisture capacity are no longer needed. Overall, the MPDSI, when averaged over a large area and long time, often resembles the traditional PDSI based on the Palmer formula and the climate-division data. The MPDSI obeys Gaussian distribution, and so it can also be used to assess the potential for floods. Together with a consistent suite of soil moisture, surface energy, and atmospheric terms from the RR, the MPDSI can be used to monitor and diagnose drought and floods.
Abstract
A 32-km high-resolution modified Palmer drought severity index (MPDSI) based on the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (RR) from 1979 to 2004 is presented. The assumptions of Palmer, such as the water balance equation, the difference between observed precipitation and the climatologically expected precipitation over the maximum conditions, and the changes of the index as a function of the current index, are preserved. Many deficiencies of the original PDSI are eliminated by taking fields directly from the RR or by making better estimates. For example, fields such as potential evapotranspiration, evaporation, runoff, total soil moisture, and soil moisture change in a given month are obtained directly from the RR. The potential recharge is defined as the total soil moisture needed to reach the maximum total soil moisture at each grid point for each calendar month. The potential precipitation is defined as the maximum precipitation at each grid point for a given calendar month. The underground volumetric soil moisture includes both frozen and liquid form. Therefore, the contribution of snowmelt is taken into account inexplicitly. The questionable assumptions of two-layer soil model and the available soil moisture capacity are no longer needed. Overall, the MPDSI, when averaged over a large area and long time, often resembles the traditional PDSI based on the Palmer formula and the climate-division data. The MPDSI obeys Gaussian distribution, and so it can also be used to assess the potential for floods. Together with a consistent suite of soil moisture, surface energy, and atmospheric terms from the RR, the MPDSI can be used to monitor and diagnose drought and floods.
Abstract
The broadscale aspects of the annual cycle of monthly mean global water vapor flux, flux divergence, evaporation, and precipitation derived from the National Meteorological Center (NMC) global analysis and forecast products were examined with two objectives in mind: 1) a critical evaluation of the usefulness of recent NMC products for descriptive and diagnostic studies of the global hydrological cycle and in the process and 2) to provide additional information on the behavior of the annual cycle of selected hydrological parameters over the globe in general and over the United States in particular. The 2-year period August 1991–July 1993, was chosen for study.
The global-scale characteristics of the NMC vertically integrated vapor flux fields are described in terms of the rotational and divergent components of the stationary and transient parts of the vapor flux field. Values of the zonally averaged meridional vapor flux derived from the NMC analyses are broadly similar to those presented by Peixoto and Oort, but differences are substantial in the Southern Hemisphere. The magnitude of the NMC meridional flux is generally larger in the middle latitudes, partly because of a larger transient flux.
The seasonally varying spinup characteristics of the NMC evaporation (E) and precipitation (P) forecasts are examined in terms of differences between the 0–6 h and 12–36 h forecasts. Spinup in P is much larger than spinup in E and is most pronounced (spindown) in the latitudes of the equatorial convergence zones. Comparisons of annual precipitation derived from the NMC 0–6 h forecast with values from the Legates and Willmott and the Jaeger climatologies, and with values from the GPI satellite IR algorithm for the Tropics, are summarized and discussed. Questionable zonally averaged and regional features are identified, but differences are often hard to reconcile due to uncertainties in all the methods of estimation.
The value of the NMC vapor flux analyses for studies of continental-scale hydrology is examined by comparing annual P-E derived from the vapor flux divergence field with annual surface runoff derived from mean annual river discharge. Ale comparison reveals an unrealistic flux divergence field over the continent that appears to be terrain related. A similar bias is not apparent in the forecast P-E field. suggesting that it arises from problems of model resolution and data assimilation.
Abstract
The broadscale aspects of the annual cycle of monthly mean global water vapor flux, flux divergence, evaporation, and precipitation derived from the National Meteorological Center (NMC) global analysis and forecast products were examined with two objectives in mind: 1) a critical evaluation of the usefulness of recent NMC products for descriptive and diagnostic studies of the global hydrological cycle and in the process and 2) to provide additional information on the behavior of the annual cycle of selected hydrological parameters over the globe in general and over the United States in particular. The 2-year period August 1991–July 1993, was chosen for study.
The global-scale characteristics of the NMC vertically integrated vapor flux fields are described in terms of the rotational and divergent components of the stationary and transient parts of the vapor flux field. Values of the zonally averaged meridional vapor flux derived from the NMC analyses are broadly similar to those presented by Peixoto and Oort, but differences are substantial in the Southern Hemisphere. The magnitude of the NMC meridional flux is generally larger in the middle latitudes, partly because of a larger transient flux.
The seasonally varying spinup characteristics of the NMC evaporation (E) and precipitation (P) forecasts are examined in terms of differences between the 0–6 h and 12–36 h forecasts. Spinup in P is much larger than spinup in E and is most pronounced (spindown) in the latitudes of the equatorial convergence zones. Comparisons of annual precipitation derived from the NMC 0–6 h forecast with values from the Legates and Willmott and the Jaeger climatologies, and with values from the GPI satellite IR algorithm for the Tropics, are summarized and discussed. Questionable zonally averaged and regional features are identified, but differences are often hard to reconcile due to uncertainties in all the methods of estimation.
The value of the NMC vapor flux analyses for studies of continental-scale hydrology is examined by comparing annual P-E derived from the vapor flux divergence field with annual surface runoff derived from mean annual river discharge. Ale comparison reveals an unrealistic flux divergence field over the continent that appears to be terrain related. A similar bias is not apparent in the forecast P-E field. suggesting that it arises from problems of model resolution and data assimilation.
