Abstract: Precipitation phase forecasting is particularly important during the transformation period from autumn to winter and winter to spring because the same magnitude of different precipitation phases can lead to marked differences. As the altitude differed obviously from east to west in Gansu Province, northwest China, precipitation phase forecasting became more difficult to judge with a unified index. Then it is very necessary to determine the forecasting indexes of precipitation phase according to altitude. By using the surface observation data, sounding data and 1°×1° NCEP reanalysis data from 2003 to 2013, the temporal and spatial distribution characteristics of precipitation phase transformation were analyzed for nearly 10 years in Gansu Province and temperature forecast indexes of the phase transformation were got under the condition of complex topography. Results reveal as follows:(1) in Gansu, there was three high-incidence areas for the precipitation phase transformation that mainly occurred in the transformation seasons, spring and autumn, accounting for 83.5% of the total.(2) The phase transformation usually occurred when the circulation of plateau trough moved eastward and produced precipitation. Generally, obvious cold advection on 500 hPa along the northwest or west or north paths influenced Gansu. When moving along the northwest path, the plateau trough was deep and the cold advection the strongest, the rain and snow conversion occurred in most Hedong region of Gansu Province;when moving along the western path, plateau trough was shallow and more southerly, phase conversion occurred only in central and southern Longdong area of Gansu.(3) The arrival time of cold advection near surface was a good forecasting index for the conversion occurrence time. When cold advection intensity was less than -5×10^-5℃·s^-1 on 700 hPa and -10×10^-5℃·s^-1 on 850 hPa, cold advection control zone will occur phase transformation. It is very helpful for forecasters to predict accurately whether it is the direct solid precipitation or conversion of liquid to solid in the actual business.(4) Except for cold advection, the combination of temperature at different levels was a more important factor that affecting the phase transformation. Ground temperature higher than 3.0℃, lower than -1.0℃, between -1.0℃ and 3.0℃ were the prerequisite for rain, snow and sleet phase transformation, respectively. Under this precondition:for a region of altitude less than 1500 m, it tended to snow when temperature was less than or equal to -3.0℃ at 850 hPa and less than or equal to -6.0℃ at 700 hPa, to sleet when temperature was between -3.0℃ and 2.5℃ at 850 hPa and between -6.0℃ and -3.0℃ at 700 hPa;for a region of altitude between 1500 m and 2000 m, it tended to snow when temperature was less than or equal to -7.0℃ at 700 hPa, to sleet when temperature was between -7.0℃ and -4.0℃ at 700 hPa;for a region of attitude more than 2000 m, it tended to snow when temperature was less than or equal to -1.0℃ at 700 hPa, to sleet when temperature was between -1.0℃ and 2.0℃ at 700 hPa. Upon examination, the indexes had good forecast results.

Key words: precipitation phase forecasting, temperature indexes, plateau trough, cold advection, altitude