CGD's Dr. Aiguo Dai

Abstract

Global precipitation data sets with high spatial and temporal resolution are needed for many applications, but they were unavailable before the recent creation of several such satellite products. Here we evaluate four different satellite data sets of hourly or 3-hourly precipitation (namely CMORPH, PERSIANN, TRMM 3B42 and a microwave-only product referred to as MI) by comparing the spatial patterns in seasonal mean precipitation amount, daily precipitation frequency and intensity, and the diurnal and semidiurnal cycles among them and with surface synoptic weather reports.

We found that these high-resolution precipitation products show spatial patterns in seasonal mean precipitation amount comparable to other monthly products for the low- and mid-latitudes, and the mean daily precipitation frequency and intensity maps are similar among these pure satellite-based precipitation data sets and consistent with the frequency derived using weather reports over land. The satellite data show that spatial variations in mean precipitation amount come largely from precipitation frequency rather than intensity, and that the use of satellite infrared (IR) observations to improve sampling does not change the mean frequency, intensity and the diurnal cycle significantly.

Consistent with previous studies, the satellite data show that sub-daily variations in precipitation are dominated by the 24-h cycle, which has an afternoon-evening maximum and mean-to-peak amplitude of 30-100% of the daily mean in precipitation amount over most land areas during summer. Over most oceans, the 24-h harmonic has a peak from midnight to early morning with an amplitude of 10-30% during both winter and summer. These diurnal results are broadly consistent with those based on the weather reports, although the time of maximum in the satellite precipitation is a few hours later (especially for TRMM and PERSIANN) than that in the surface observations over most land and ocean, and it is closer to the phase of showery precipitation from the weather reports. The TRMM and PERSIANN precipitation shows a spatially coherent time of maximum around 0300-0600 LST for a weak (amplitude <20%) semi-diurnal (12-h) cycle over most mid- to high-latitudes, comparable to 0400-0600 LST in the surface data. The satellite data also confirm the notion that the diurnal cycle of precipitation amount comes mostly from its frequency rather than its intensity over most low and mid-latitudes, with the intensity has only about half of the strength of the diurnal cycle in the frequency and amount. The results suggest that these relatively new precipitation products can be useful for many applications.

Figure caption: The phase (local solar time in hrs of the maximum, left column) and amplitude (in % of daily mean, right column) of the 24-hr harmonic estimated from the mean diurnal anomalies of JJA precipitation frequency for non-drizzle and showery precipitation from surface weather reports (top two rows), and of JJA precipitation amount from MI (3rd row), TRMM 3B42 (4th row), PRESIANN (5th row), and CMORPH (bottom row). Note the normalized amplitude is not shown (i.e., white color) over the subtropical areas where the mean precipitation is less than 0.1 mm/day.

Support: NASA Grant No. NNX07AD77G and NCAR TIIMES Water Cycle Program.

---

Sun, Y., S. Solomon, A. Dai, and R. Portmann, 2007: How often will it rain? J. Climate, in press.

Abstract

Daily precipitation data from climate change simulations using the latest generation of coupled climate system models are analyzed for potential future changes in precipitation characteristics. For the emission scenarios SRES B1 (a low projection), A1B (a medium projection), and A2 (a high projection) during the 21^st century, all the models consistently show a shift towards more intense and extreme precipitation for the globe as a whole and over various regions. For both SRES B1 and A2, most models show decreased daily precipitation frequency and all the models show increased daily precipitation intensity. The multi-model averaged percentage increase in the precipitation intensity (2.0% K^-1) is larger than the magnitude of the precipitation frequency decrease (-0.7% K^-1). However, the shift in precipitation frequency distribution towards extremes results in large increases in very heavy precipitation events (>50 mm day^-1), so that for very heavy precipitation, the percentage increase in frequency is much larger than the increase in intensity (31.2% vs. 2.4%). The climate model-projected increases in daily precipitation intensity are, however, smaller than that based on simple thermodynamics (~7% K^-1). Multi-model ensemble means show that precipitation amount increases during the 21^st century over high latitudes, as well as over currently wet regions in low- and mid-latitudes more than other regions. This increase mostly results from a combination of increased frequency and intensity. Over the dry regions in the subtropics, precipitation amount generally declines because of decreases in both frequency and intensity. This indicates that wet regions may get wetter and dry regions may become drier mostly because of simultaneous increase (decrease) of precipitation frequency and intensity.

Figure caption: (a) globally (dashed line) and land (solid line) averaged distribution of daily precipitation frequency as a function of precipitation intensity (bin size is 1 mm day ^-1) from observations (GTS_3D, 1980-1999) and 11 model ensemble of simulations for present (20C3M, 1980-1999), and future (2088-2099) climates under SRES B1, A1B, and A2 scenarios; (b) Same as (a) but for percentage changes; (c) Same as (a), but for precipitation amount (mm) in each bin; (d) Same as (c) but for percentage changes.

Support: NRC, NOAA, NSF Grant #ATM-0233568, NCAR's Water Cycle Program, Chinese NSF Grant #40605020.

---

Qian, T., A. Dai, K. E. Trenberth, 2007: Hydroclimatic trends in the Mississippi River Basin from 1948-2004. J. Climate, 20, 4599-4614.

Abstract

The trends of the surface water and energy budget components in the Mississippi River basin from 1948 to 2004 are investigated using a combination of hydrometeorological observations and observation-constrained simulations of the land surface conditions using the latest version of the Community Land Model version 3 (CLM3). The atmospheric forcing data for the CLM3 were constructed by adding the intra-monthly variations from the 6-hourly National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis to observation-based analyses of monthly precipitation, surface air temperature and cloud cover. Our model-based analysis suggests that for the surface water budget, the observed increase in basin-averaged precipitation is compensated by increases in both runoff and evapotranspiration. For the surface energy budget, the decrease of net shortwave radiation associated with observed increases in cloudiness is compensated by decreases in both net longwave radiation and sensible heat flux, while the latent heat flux increases in association with wetter soil conditions. Both the simulated surface water and energy budgets support the view that evapotranspiration has increased in the Mississippi River basin from 1948-2004. Sensitivity experiments show that the precipitation change dominates the evapotranspiration trend, while the temperature and solar radiation changes have only small effects. Large spatial variations within the Mississippi River basin and the contiguous United States are also found. However, the increased evapotranspiration is ubiquitous despite spatial variations in hydrometeorology.

Figure caption: Time series of annual (water-year) surface water budget components averaged over the Mississippi River basin and associated linear trends (straight lines, b is the slope). Except for the observed precipitation (P, solid line) and runoff at Vicksburg, Mississippi (R_obs, solid line with triangles) and the water budget-derived evapotranspiration (P-R_obs-dW/dt, solid line with dots), other components are from CLM3 simulations and include runoff (R, dashed line with triangles), evapotranspiration (E, dashed line with dots), change of land water storage (dW/dt, dashed line with quadrangles, note the slope b for this quantity is d(dW/dt)/dt) and land water storage (W, dashed line, b=0.20 mm yr^-1 is the trend in W) (bottom panel).

Support: NSF Grant ATM-0233568 and NCAR TIIMES Water Cycle Program.

---

Qian, T., A. Dai, K. E. Trenberth, K. W. Oleson, 2006: Simulation of global land surface conditions from 1948 to 2004: Part I: Forcing Data and Evaluations. J. Hydrometeorol., 7, 953-975.

Abstract

Because of a lack of observations, historical simulations of land surface conditions using land surface models are needed for studying variability and changes in the continental water cycle and for providing initial conditions for seasonal climate predictions. Atmospheric forcing datasets are also needed for land surface model development. The quality of atmospheric forcing data greatly affects the ability of land surface models to realistically simulate land surface conditions. Here a carefully constructed global forcing dataset for 1948-2004 with 3-hourly and T62 (_1.875°) resolution is described, and historical simulations using the latest version of the Community Land Model version 3.0 (CLM3) are evaluated using available observations of streamflow, continental freshwater discharge, surface runoff, and soil moisture. The forcing dataset was derived by combining observation-based analyses of monthly precipitation and surface air temperature with intramonthly variations from the National Centers for Environmental Prediction - National Center for Atmospheric Research (NCEP-NCAR) reanalysis, which is shown to have spurious trends and biases in surface temperature and precipitation. Surface downward solar radiation from the reanalysis was first adjusted for variations and trends using monthly station records of cloud cover anomaly and then for mean biases using satellite observations during recent decades. Surface specific humidity from the reanalysis was adjusted using the adjusted surface air temperature and reanalysis relative humidity. Surface wind speed and air pressure were interpolated directly from the 6-hourly reanalysis data. Sensitivity experiments show that the precipitation adjustment (to the reanalysis data) leads to the largest improvement, while the temperature and radiation adjustments have only small effects.

When forced by this dataset, the CLM3 reproduces many aspects of the long-term mean, annual cycle, interannual and decadal variations, and trends of streamflow for many large rivers (e.g., the Orinoco, Changjiang, Mississippi, etc.), although substantial biases exist. The simulated long-term-mean freshwater discharge into the global and individual oceans is comparable to 921 river-based observational estimates. Observed soil moisture variations over Illinois and parts of Eurasia are generally simulated well, with the dominant influence coming from precipitation. The results suggest that the CLM3 simulations are useful for climate change analysis. It is also shown that unrealistically low intensity and high frequency of precipitation, as in most model-simulated precipitation or observed time-averaged fields, result in too much evaporation and too little runoff, which leads to lower than observed river flows. This problem can be reduced by adjusting the precipitation rates using observed-precipitation frequency maps.

Figure caption: Observed soil moisture content within the top 0.9 m depth (solid) compared with the CLM simulated values (dashed) over Illinois for the same period: (a) monthly anomaly time series; (b) mean annual cycle (with the +/- one standard deviation error bars); (c) mean soil moisture tendency for each month. Also shown on top of each panel is the correlation coefficient (r) between the two curves, in (a) the second value is the correlation coefficient including the annual cycle.

Support: NSF Grant ATM-0233568 and NCAR's Water Cycle Program.

---

Rasmussen, R., A. Dai, K. E. Trenberth, 2007: Impact of climate change on precipitation. Chapter 16. Large-scale Disasters: Prediction, Control and Mitigation, Gad-el-Hak, Ed., Cambridge University Press, 453-472.

Summary

The above discussion highlights some of the key issues needed to be addressed in order to properly simulate the water cycle in climate models. These include:

1. The proper simulation of the frequency, intensity, duration, timing, and phase of precipitation.
2. The proper treatment of local evapo-transpiration vs. large scale moisture advection.
3. The proper treatment of water runoff and soil infiltration in order to properly model soil moisture and its impact on latent and sensible heat fluxes.

It is critical that scientists address these issues in order to provide natural disaster managers and other users of climate information the proper guidance to make the difficult decisions facing our global society in the near future.

Figure caption: Example of two hypothetical surface weather stations receiving the same total amount of precipitation, but with different frequency and intensity.

Support: NCAR TIIMES Water Cycle Program.

---

Tian, X., A. Dai, D. Yang, and Z. Xie, 2007: Effects of precipitation-bias corrections on surface hydrology over northern latitudes. J. Geophys. Res.-Atmospheres, 112, D14101, doi:10.029/2007JD008420.

Abstract

Under-catch errors in precipitation gauge records can be as large as 50-100% during the cold season at high latitudes. To quantify the impacts of these errors on hydrometeorological fields, a comprehensive land surface model, namely the Community Land Model version 3 (CLM3), is run forced with (COF) and without (CON) precipitation-bias corrections and other identical atmospheric forcing from 1973 to 2004. It is found that the enhanced snowfall induced by the bias corrections increases snow accumulation on the ground (by 6-18 cm for December to February), which in turn increases May to July runoff by 0.4-0.6 mm day_1 and streamflow by 5-25% for most major rivers in the northern latitudes (north of 45_N). The precipitation-bias corrections also improve the model-simulated mean annual cycle and temporal variations of streamflow for the major northern rivers during 1973-2004. As a result, the simulation of the freshwater discharge into the Arctic Ocean is also improved. Only small and statistically insignificant changes are found in soil moisture content, surface evaporation, and sensible heat flux between the CON and COF runs. Nevertheless, the results still suggest that it is important to use bias-corrected precipitation in terrestrial water balance analyses and land surface modeling.

Figure caption: Long-term mean annual cycle of freshwater discharge (in Sv, 1 Sv = 10⁶ m⁶ s^-1) into the Arctic Ocean from observations (solid line, from Dai and Trenberth 2002) and the CLM3 COF (short-dashed line, with precipitation bias corrections) and CON (long-dashed line, without the corrections) runs.

Support: NSF, NCAR's Advanced Study Program (ASP), NCAR TIIMES Water Cycle Program. This study was also partly supported by CAS International Partnership Creative Group through the project entitled "The Climate System Model Development and Application Studies", the Knowledge Innovation Key Project of Chinese Academy of Sciences under grant no. KZCX2-YW-217, and the National Natural Science Foundation of China under grant no. 90411007.

---

Trenberth, K. E., and A. Dai, 2007: Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering. Geophys. Res. Lett., 34, L15702, doi:10.1029/2007GL030524.

Abstract

The problem of global warming arises from the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine by emulating a volcanic eruption. In between the incoming solar radiation and the OLR is the entire weather and climate system and the hydrological cycle. The precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from El Chichón in March 1982 and Pinatubo in June 1991, taking into account changes from El Niño-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991-September 1992. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions.

Figure caption: Top: Adapted time series of 20°N to 20°S ERBS non-scanner wide-field-of-view broadband shortwave, longwave and net radiation anomalies from 1985 to 1999 [Wielicki et al., 2002a, 2002b] where the anomalies are defined with respect to the 1985 to1989 period with Edition 3_Rev 1 data [Wong et al., 2006]. Bottom: Time series of the annual water year (Oct. to Sep.); note slight offset of points plotted vs tick marks indicating January) continental freshwater discharge and land precipitation (from Fig. 1) for the 1985 to 1999 period. In both panels, the period clearly influenced by the Mount Pinatubo eruption is indicated by grey shading.

---

Trenberth, K. E., L. Smith, T. Qian, A. Dai and J. Fasullo, 2007: Estimates of the global water budget and its annual cycle using observational and model data. J. Hydrometeor., 8, 758-769.

Abstract

A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. A new estimate is provided of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979 to 2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental land masses, zonal means over land, hemispheric land means and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high frequency information taken from atmospheric reanalyses. Results for P-E are contrasted with those from atmospheric moisture budgets based on ERA-40 reanalyses. The latter show physically unrealistic results, because evaporation often exceeds precipitation over land especially in the tropics and subtropics.

Figure caption: The hydrological cycle. Estimates of the main water reservoirs, given in plain font in 10³ km³, and the flow of moisture through the system, given in slant font in 10³ km³/yr, equivalent to Exagrams (10¹⁸g) per year.

---

Wang, J., L. Zhang, A. Dai, T. Van Hove, and J. Van Baelen, 2007: A near-global, 2-hourly data set of atmospheric precipitable water from ground-based GPS measurements. J. Geophys. Res., 112, D11107, doi:10.1029/2006JD007529.

Abstract

A 2-hourly data set of atmospheric precipitable water (PW) has been produced from the zenith path delay (ZPD) derived from ground-based Global Positioning System (GPS) measurements. The PW data are available every 2 hours from 80 to 268 International GNSS Service (IGS, formally International GPS Service) ground stations from 1997 to 2004. The accuracy of the IGS ZPD product is roughly 4 mm. An analysis technique is developed to convert ZPD to PW on a global scale. Special efforts are made on deriving surface pressure (P_s) and water-vapor-weighted atmospheric mean temperature (T_m), which are two key parameters for converting ZPD to PW. P_s is derived from global, 3-hourly surface synoptic observations with temporal, vertical and horizontal adjustments. T_m is calculated from NCEP/NCAR reanalysis with temporal, vertical and horizontal interpolations. The derived P_s and T_m at the GPS location and height have root-mean-square (rms) errors of 1.65 hPa and 1.3 K, respectively. A theoretical error analysis concludes that typical PW error associated with the errors in ZPD, T_m and P_s is on the order of 1.5 mm. The PW data set is compared with radiosonde, microwave radiometer (MWR) and satellite data. The GPS and radiosonde PW comparisons at 98 stations around the globe show a mean difference of 1.08 mm (drier for radiosonde data) with a standard deviation of differences of 2.68 mm, which corresponds to mean percentage difference and standard deviation of 5.5% and 10.6%, respectively. The bias is primarily due to known dry biases in the Vaisala radiosonde data. The RMS difference between GPS and radiosonde/MWR data ranges from 1.2 mm to 2.83 mm. The latitudinal and seasonal variations of PW derived from the GPS data agree well with that from International Satellite Cloud Climatology Project (ISCCP) data if the ISCCP data are sampled only at grid boxes containing GPS stations. The large difference between GPS and ISCCP data in the subtropics is interesting, but is not easily explained. The comparisons did not reveal any systematic bias in GPS PW data and show that a RMS difference of less than 3 mm between GPS-derived PW and other data sets is achieved. The comparison study also illustrates the value of GPS-estimated PW for examining the quality of other data sets, such as those from radiosondes and MWR. Preliminary analysis of this data set shows interesting and significant diurnal variations in PW in four different regions.

Figure caption: Geographic distribution of all IGS stations as of Feb. 10, 2006 (circle) and the stations with ZPD data (triangle), and with PW data (dot) in 2004.

Support: NSF, the NCAR Director Office's Opportunity Fund, NCAR TIIMES Water Cycle Program and NOAA grant No. NA06OAR4310117.