METHOD FOR CALIBRATING MONTHLY PRECIPITATION FORECAST BY USING GAMMA-GAUSSIAN DISTRIBUTION

Abstract

The present invention provides a method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution, including the following steps: acquiring forecast data of monthly average precipitation in a watershed area and corresponding observed values of the average precipitation in the watershed area as input data; performing fitting on the input data by means of a Gamma distribution function; calculating a cumulative distribution function value of each input data in a corresponding Gamma distribution; transforming the cumulative distribution function values into variables obeying a standard normal distribution; constructing a joint normal distribution according to the variables obeying the standard normal distribution to characterize a correlation between the forecast data and the observed values in the input data; and randomly sampling the observed values according to the correlation, and inversely transforming acquired samples to obtain a calibrated forecast result.

Description

TECHNICAL FIELD

The present invention relates to the field of hydrologic forecast, and more particularly to a method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution.

BACKGROUND

An existing global climate model can provide abundant precipitation forecast information. A monthly precipitation forecast of a basin extracted from a global precipitation forecast product can provide important reference for reservoir scheduling, agricultural irrigation, flood control, and drought fight in the basin. Monthly raw forecast data and observed data have a good correlation therebetween. However, the system is complex and has a random error, which brings difficulties to the practical application of the precipitation forecast, and affects the forecast skill to a certain extent.

The patent publication No. CN108830419A (publication date: Nov. 16, 2018) provides a joint forecast method for inflow flows of cascaded reservoir group based on ECC post-processing, relates to the technical field of hydrologic forecast, and discloses: a systematic error of ensemble numerical weather forecast data is calibrated on the basis of measured data and a Gamma distribution function. However, the method of the patent still has a random error, and still has a certain impact on the forecast skill.

SUMMARY

In order to overcome the defect in the prior art that the system complexity and the random error affect the precipitation forecast skill, the present invention provides a method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution.

In order to solve the above technical problem, the technical solution of the present invention is as follows:

A method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution, including the following steps:

S1, acquiring forecast data of monthly average precipitation in a watershed area and corresponding observed values of the average precipitation in the watershed area as input data;

S2, performing fitting on the input data by means of a Gamma distribution function;

S3, calculating a cumulative distribution function value of each input data in a corresponding Gamma distribution;

S4, transforming the cumulative distribution function values into variables obeying a standard normal distribution;

S5, constructing a joint normal distribution according to the variables obeying the standard normal distribution to characterize a correlation between the forecast data and the observed values in the input data; and

S6, randomly sampling the observed values according to the correlation, and inversely transforming acquired samples to obtain a calibrated forecast result.

Preferably, in the step S2, the Gamma distribution function is used to perform fitting on the forecast data and the observed values respectively to obtain marginal distributions of the raw forecast data and the observed values; the expression formula thereof is as follows:

$\{\begin{array}{c}F~G\left({\alpha }_f,{\beta }_f\right)\\ O~G\left({\alpha }_o,{\beta }_o\right)\end{array}$

Wherein F denotes a set of K acquired forecast data [f₁, f₂, . . . , f_K]; O denotes a set of K acquired observed values [o₁, o₂, . . . , o_K]; G(*) denotes the Gamma distribution function; α_f and β_f denote Gamma distribution parameters of the forecast data obtained by means of fitting; and α_o and β_o denote Gamma distribution parameters of the observed values obtained by means of fitting.

Preferably, the Gamma distribution parameters α_f, β_f, α_o, and β_o are respectively calculated with a maximum likelihood estimation method.

Preferably, in the step S3, a cumulative distribution function in the corresponding Gamma distribution is used to calculate the cumulative distribution function values of each forecast data f_i and of each observed value o_i in the corresponding Gamma distribution; the expression formula thereof is as follows:

$\{\begin{array}{c}{P}_{f_i}={\mathrm{CDF}}_{\left({\alpha }_F,{\beta }_F\right)}\left(f_i\right)\\ P_{o_i}={\mathrm{CDF}}_{\left({\alpha }_O,{\beta }_O\right)}\left(o_i\right)\end{array}$

Wherein P_fi and P_oi respectively denote the cumulative distribution function values corresponding to the forecast data f_i and the observed values o_i in an i-th year; and CDF_(αF,βF)(f_i) and CDF_(αo,βo)(o_i) respectively denote the cumulative distribution functions in the Gamma distribution obtained by performing fitting on the forecast data f_i and the observed values o_i.

Preferably, in the step S4, the cumulative distribution function values are regarded as quantiles of the standard normal distribution; the cumulative distribution function values are transformed into variables obeying the standard normal distribution by means of an inverse function of the cumulative distribution function in the standard normal distribution; and the expression formula thereof is as follows:

$\{\begin{array}{c}{\hat{f}}_i={\mathrm{PPF}}_{N\left(0,1^2\right)}\left(P_{f_i}\right)\\ {\hat{o}}_i={\mathrm{PPF}}_{N\left(0,1^2\right)}\left(P_{o_i}\right)\end{array}$

Wherein PPF_N(0,12)(⋅) denotes the inverse function of the cumulative distribution function in the standard normal distribution; and {circumflex over (f)}_i and ô_i are respectively the forecast data and the observed values obtained by means of normal quantile transform. The transformed forecast data {circumflex over (F)}=[{circumflex over (f)}₁, {circumflex over (f)}₂, . . . , {circumflex over (f)}f_K] and the transformed observed values Ô=[ô₁, ô₂, . . . , ô_K] all obey normal distributions, and the expression formula thereof is as follows:

$\{\begin{array}{c}\hat{F}~N\left(0,1^2\right)\\ \hat{O}~N\left(0,1^2\right)\end{array}$

Wherein N(0,1²) denotes a standard normal distribution.

Preferably, in the step S5, a joint normal distribution is constructed according to the variables {circumflex over (F)} and Ô obeying the standard normal distribution to characterize the correlation between the forecast data and the observed values in the input data; and the expression formula thereof is as follows:

$\left[\begin{array}{c}\hat{F}\\ \hat{O}\end{array}\right]~N\left(\left[\begin{array}{c}0\\ 0\end{array}\right],\left[\begin{array}{cc}1& \rho \\ \rho & 1\end{array}\right]\right)$

Wherein ρ denotes the correlation between the variables {circumflex over (F)} and Ô.

Preferably, in the step S6, the specific steps are as follows:

S6.1, taking the forecast data {circumflex over (f)} as a predictor, taking the observed value Ô corresponding to each forecast data as a predictand, and calculating a conditional probability distribution of the predictand, wherein the calculation formula is as follows:

ô|{circumflex over (f)}˜N(ρ{circumflex over (f)},1−ρ²)

S6.2, randomly sampling the conditional probability distribution result of the observed value Ô, and inversely transforming sampled samples according to the cumulative distribution function in the standard normal distribution and the inverse function of the cumulative distribution function in the Gamma distribution obtained by performing fitting on the observed value, so as to obtain a calibrated forecast result.

Preferably, the method further includes the following step: calculating a bias value and forecast skill according to the calibrated forecast result as forecast verification metrics.

Preferably, the method further includes the following step: drawing a forecast diagnostic diagram according to the calibrated forecast result, the bias value, and the forecast skill.

Preferably, in the forecast diagnostic diagram, a calibrated forecast median is used as the x axis; a precipitation forecast distribution interval and the observed values are used as the y axis; and the calculation results of the bias value and the forecast skill are inserted in the forecast diagnostic diagram for display.

Compared with the prior art, the beneficial effects of the technical solution of the present invention are: the present invention transforms the precipitation forecast and the observed data into normal distributions by means of the Gamma distribution, thereby avoiding the complex data normalization method; furthermore, the present invention constructs a joint normal distribution according to the variables obeying the standard normal distribution to characterize the correlation between the forecast data and the observed values in the input data, and further randomly samples the observed values according to the correlation, thereby effectively quantifying the random error, solving the problem that the system complexity and the random error affect the precipitation forecast skill, and effectively improving the forecast skill.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of the method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution according to an embodiment 1;

FIG. 2 shows input data in tables according to an embodiment 2;

FIG. 3 is a quantile-quantile plot of observed precipitation values before transformation according to the embodiment 2;

FIG. 4 is a quantile-quantile plot of observed precipitation values after transformation according to the embodiment 2;

FIG. 5 is a time series diagram of raw forecast data according to the embodiment 2;

FIG. 6 is a time series diagram of calibrated forecast according to the embodiment 2;

FIG. 7 is a diagnostic diagram of the raw forecast according to the embodiment 2; and

FIG. 8 is a diagnostic diagram of the calibrated forecast according to the embodiment 2.

DETAILED DESCRIPTION OF EMBODIMENTS

The drawings are used for illustrative purpose only, but should not be considered as a limitation to the present patent.

For a person skilled in the art, it is understandable that certain commonly known structures in the figures and the descriptions thereof can be omitted.

The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

Embodiment 1

The present embodiment provides a method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution. FIG. 1 shows a flow chart of the method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution according to the present embodiment.

The method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution provided by the present embodiment includes the following steps:

S1, acquiring forecast data of monthly average precipitation in a watershed area and corresponding observed values of the average precipitation in the watershed area as input data.

S2, performing fitting on the input data by means of a Gamma distribution function.

Specifically, the Gamma distribution function is used to perform fitting on the forecast data and the observed values respectively to obtain marginal distributions of the raw forecast data and the observed values; the expression formula thereof is as follows:

$\{\begin{array}{c}F~G\left({\alpha }_f,{\beta }_f\right)\\ O~G\left({\alpha }_o,{\beta }_o\right)\end{array}$

Wherein F denotes a set of K acquired forecast data [f₁, f₂, . . . , f_K]; O denotes a set of K acquired observed values [o₁, o₂, . . . , o_K]; G(⋅) denotes the Gamma distribution function; α_f and β_f denote Gamma distribution parameters of the forecast data obtained by means of fitting; and α_o and β_o denote Gamma distribution parameters of the observed values obtained by means of fitting.

Wherein the Gamma distribution parameters α_f, β_f, α_o, and β_o are respectively calculated with a maximum likelihood estimation method.

S3, calculating a cumulative distribution function value of each input data in a corresponding Gamma distribution.

Specifically, a cumulative distribution function in the corresponding Gamma distribution is used to calculate the cumulative distribution function value of each forecast data f_i and of each observed value o_i in the corresponding Gamma distribution; the expression formula thereof is as follows:

$\{\begin{array}{c}{P}_{f_i}={\mathrm{CDF}}_{\left({\alpha }_F,{\beta }_F\right)}\left(f_i\right)\\ P_{o_i}={\mathrm{CDF}}_{\left({\alpha }_O,{\beta }_O\right)}\left(o_i\right)\end{array}$

Wherein P_fi and P_oi respectively denote the cumulative distribution function values corresponding to the forecast data f_i and the observed values o_i in an i-th year; and CDF_(αF,βF)(f_i) and CDF_(αo,βo)(o_i) respectively denote the cumulative distribution functions in the Gamma distribution obtained by performing fitting on the forecast data f_i and the observed values o_i.

S4, transforming the cumulative distribution function values into variables obeying a standard normal distribution.

Specifically, the cumulative distribution function values are regarded as quantiles of the standard normal distribution; the cumulative distribution function values are transformed into variables obeying the standard normal distribution by means of an inverse function of the cumulative distribution function in the standard normal distribution; and the expression formula thereof is as follows:

$\{\begin{array}{c}{\hat{f}}_i={\mathrm{PPF}}_{N\left(0,1^2\right)}\left(P_{f_i}\right)\\ {\hat{o}}_i={\mathrm{PPF}}_{N\left(0,1^2\right)}\left(P_{o_i}\right)\end{array}$

Wherein PPF_N(0,12)(⋅) denotes the inverse function of the cumulative distribution function in the standard normal distribution; and {circumflex over (f)}_i and ô_i are respectively the forecast data and the observed values obtained by means of normal quantile transform. The transformed forecast data {circumflex over (F)}=[{circumflex over (f)}₁, {circumflex over (f)}₂, . . . , {circumflex over (f)}_K] and the transformed observed values Ô=[ô₁,ô₂, . . . , ô_K] all obey normal distributions, and the expression formula thereof is as follows:

$\{\begin{array}{c}\hat{F}~N\left(0,1^2\right)\\ \hat{O}~N\left(0,1^2\right)\end{array}$

Wherein N(0,1²) denotes a standard normal distribution.

S5, constructing a joint normal distribution according to the variables obeying the standard normal distribution to characterize a correlation between the forecast data and the observed values in the input data.

Specifically, a joint normal distribution is constructed according to the variables {circumflex over (F)} and Ô obeying the standard normal distribution to characterize the correlation between the forecast data and the observed values in the input data; and the expression formula thereof is as follows:

$\left[\begin{array}{c}\hat{F}\\ \hat{O}\end{array}\right]~N\left(\left[\begin{array}{c}0\\ 0\end{array}\right],\left[\begin{array}{cc}1& \rho \\ \rho & 1\end{array}\right]\right)$

Wherein ρ denotes the correlation between the variables {circumflex over (F)} and Ô.

S6, randomly sampling the observed values according to the correlation, and inversely transforming acquired samples to obtain a calibrated forecast result. The specific steps thereof are as follows:

S6.1, taking the forecast data {circumflex over (f)} as a predictor, taking the observed value Ô corresponding to each forecast data as a predictand, and calculating a conditional probability distribution of the predictand, wherein the calculation formula is as follows:

ô|{circumflex over (f)}˜N(ρ{circumflex over (f)},1−ρ²);

S6.2, randomly sampling the conditional probability distribution result of the observed value Ô, and inversely transforming sampled samples according to the cumulative distribution function in the standard normal distribution and the inverse function of the cumulative distribution function in the Gamma distribution obtained by performing fitting on the observed value, so as to obtain a calibrated forecast result.

Further, the method further includes the following steps: calculating a bias value and forecast skill according to the calibrated forecast result as forecast verification metrics; and drawing a forecast diagnostic diagram according to the calibrated forecast result, the bias value, and the forecast skill.

Wherein in the forecast diagnostic diagram, a calibrated forecast median is used as the x axis; a precipitation forecast distribution interval and the observed values are used as the y axis; and the calculation results of the bias value and the forecast skill are inserted in the forecast diagnostic diagram for display.

The method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution provided by the present embodiment can be implemented on an open source language platform Python.

In a specific implementation process, a function read_csv of an open source third party library Pandas of Python is used to read the precipitation forecast and the observed data in a pre-stored file, so as to obtain the input data to be acquired in the step S1. Then, the mathematical calculation processes in the steps S2-S6 are programmed on the language platform Python mainly by using third party libraries Numpy and Scipy. The steps are encapsulated to form class functions in the class ( ) and def ( ) forms. And the class functions can be called to calibrate the precipitation forecast.

On the basis that the calibrated forecast is obtained, the forecast verification metrics such as the bias and the forecast skill CRPSS are calculated by means of Numpy, and then a forecast diagnostic diagram is drawn by means of a third party library Matplotlib of the Python, so as to compare and analyze the improvement effect of the calibrated forecast result in the present embodiment.

In the present embodiment, the precipitation forecast and the observed data are transformed into normal distributions by means of the Gamma distribution, thereby avoiding the complex data normalization method; furthermore, a joint normal distribution is constructed according to the variables obeying the standard normal distribution to characterize the correlation between the forecast data and the observed values in the input data, and the observed values are randomly sampled according to the correlation, thereby effectively quantifying the random error, solving the problem that the system complexity and the random error affect the precipitation forecast skill, and effectively improving the forecast skill.

Embodiment 2

The present embodiment provides a specific implementation process. In such process, the method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution provided by the embodiment 1 is used to calibrate, on the platform Python, the monthly precipitation forecast ECMWF-S2S of Beijiang in the Pearl River basin.

First, forecast data of monthly average precipitation in a watershed area and corresponding observed values of the average precipitation in the watershed area are acquired as input data; the input data of the present embodiment are stored in a csv file, as shown in table 1 and table 2.

TABLE 1
Raw forecast
	ensemble	ensemble	ensemble	ensemble	ensemble	ensemble	ensemble	ensemble	ensemble	ensemble
1998	84.89508	158.2451	63.64825	90.194	136.3437	83.64299	121.8655	195.1327	159.4989	199.9484
1999	210.7963	282.4414	299.9787	153.9599	275.1155	254.3405	285.8323	232.0072	258.1213	343.6084
2000	127.4814	244.6343	185.5596	123.2649	204.8612	117.7302	218.9883	225.839	186.2492	175.8637
2001	155.2512	127.8865	151.159	185.1081	206.232	168.6413	184.9953	192.1127	253.0905	187.5727
2002	424.8881	358.8516	417.2825	327.7083	281.1337	247.4314	418.7246	314.0324	452.9201	256.9427
2003	220.9197	255.5282	244.9292	352.9966	229.0499	321.9108	226.9213	206.379	337.4745	242.678
2004	169.4463	171.3873	191.8005	274.4523	152.4409	205.8134	187.518	182.6235	376.7231	124.2183
2005	210.6669	174.537	217.237	96.37896	222.444	140.273	221.1545	148.9361	151.6495	186.2353
2006	314.1927	229.059	158.3293	277.9718	257.0493	270.7997	341.6748	292.7564	301.6001	434.7465
2007	157.244	174.5628	408.7821	205.1021	311.279	285.2153	180.4027	369.587	234.6741	371.9234
2008	177.0635	195.2237	150.3481	179.5046	199.7789	212.0726	123.3357	314.6701	207.4794	165.2393
2009	292.9606	183.2678	180.7569	215.0623	189.6606	202.9507	120.1503	164.0583	222.1882	179.579
2010	253.562	184.6896	157.7112	134.7514	134.4464	208.7838	166.5391	276.7147	114.3669	355.7373
2011	85.35877	131.6285	220.3555	161.4225	323.5723	153.066	200.9787	173.9212	145.6553	166.1913
2012	372.181	199.1791	254.5669	217.1036	200.0598	251.6132	232.2245	206.4078	193.8979	88.02964
2013	173.7736	241.7922	110.7662	262.0269	280.3135	220.5056	164.9861	178.9081	178.2092	338.9
2014	183.4094	133.0556	193.7379	98.55543	96.57263	253.7574	111.3084	197.1161	128.5301	167.8946
2015	170.1865	66.03219	230.7332	188.6372	297.0111	136.1666	259.9242	226.3555	154.2498	146.2803
2016	295.4615	258.1794	140.8144	442.074	219.1649	133.1191	329.9726	154.9931	233.2877	246.0335
2017	252.0542	168.3717	105.1056	208.1071	155.3844	199.3643	177.8557	129.2297	151.5439	221.1763

TABLE 2
Observed value
1998 133.8236
1999 376.794
2000 223.1297
2001 295.3824
2002 259.8187
2003 198.187
2004 174.8477
2005 167.2891
2006 160.7498
2007 309.4858
2008 119.806
2009 133.8806
2010 128.273
2011 96.40596
2012 171.6868
2013 407.3324
2014 297.7172
2015 195.5005
2016 278.2253
2017 145.0358
2018 276.8951

Wherein the precipitation forecast data is cumulative precipitation amount in a thirty-day forecast period forecast at the beginning of January to December.

During implementation, the raw forecast data to be calibrated and the observed data are read by means of the function read_csv, and are respectively stored in the variables temp_x and temp_y.

A monthly precipitation forecast calibration model based on a Gamma-Gaussian distribution is constructed. The mathematical calculation processes in the steps S2-S6 in the embodiment 1 are executed mainly by means of the third party library Scipy, mainly including Gamma distribution fitting, joint normal distribution construction, and conditional probability distribution.

Specifically, a function stats.gamma.fit is used to respectively perform Gamma distribution fitting on mean value of the raw forecast data and the observed values; and the Gamma distribution parameters are obtained with the maximum likelihood estimation method, and are stored in the variables para_x and para_y.

A function stats.gamma.cdf is used to calculate the cumulative distribution function values of the raw forecast data and the observed data according to the fitted Gamma distribution parameters.

A function stats.norm.ppf is used to transform the cumulative distribution function values into variables obeying a normal distribution according to the calculated cumulative distribution function values, so as to normalize the raw forecast data and the observed values; the normalized data are respectively stored in variables trans_x and trans_y, facilitating subsequent modeling. Furthermore, a function pyplot in the Matplotlib and a function stats.proplot in the Scipy are used to draw the quantile-quantile plot of the observed precipitation values before and after the transformation to verify the normality thereof, as shown in FIGS. 3 and 4 which are respectively quantile-quantile plot of the observed precipitation values before and after the transform.

A joint normal distribution model is constructed; a correlation coefficient between the transformed forecast data variable trans_x and the observed value variable trans_y is calculated by means of a function stats.pearson, so as to characterize the correlation therebetween.

Conditional probability distribution parameters of the observed values, namely the mean and the standard deviation sigma, are calculated; the mean and the sigma are inputted into a function stats.norm.rvs as parameters to perform random sampling, so as to obtain 1000 samples.

The cumulative distribution function values of the 1000 randomly sampled samples in the standard normal distribution are calculated; and finally the samples are inversely transformed according to the Gamma distribution parameter para_y to obtain a calibrated forecast result.

For the forecast in each month, the raw forecast data is calibrated according to the above steps, and finally a group of calibrated forecast results of the month are obtained. The forecast data of 12 months are sequentially calibrated to obtain 12 groups of calibrated forecast results. And the 12 groups of raw forecast data and the calibrated forecast results are put together to perform forecast verification.

Specifically, a percentile function in Numpy is used to respectively calculate the quantiles 10, 25, 50, 75, and 90 of the raw forecast and the calibrated forecast; and a function pyplot.plot in Matplotlib is used to draw precipitation forecast time series diagrams by taking year as the x-axis and the precipitation amount as the y-axis, as shown in FIG. 5 and FIG. 6 which are respectively the time series diagrams of the raw forecast data and the calibrated forecast according to the present embodiment. Then, functions mean and sum in the Numpy are used to calculate the biases and the forecast skill CRPSS of the raw forecast and the calibrated forecast; furthermore, the function pyplot.plot in the Matplotlib is used to draw a forecast diagnostic diagram by taking ensemble forecast medians as the x axis and taking the precipitation forecast distribution interval and the observed values as the y axis; a function pyplot.text is used to interpolate the bias and the forecast skill calculation results in the diagram as shown in FIGS. 6 and 7 which are respectively a diagnostic diagram of the raw forecast and a diagnostic diagram of the calibrated forecast according to the present embodiment.

In FIG. 7, the biases of the raw forecast from January to December are respectively 27.74%, 37.04%, 22.19%, 36.29%, −3.00%, 4.00%, 17.69%, −0.34%, 51.56%, −9.37%, 28.97%, and 54.11%; and the forecast skills CRPSS of the raw forecast from January to December are respectively −18.28%, −24.7%, −48.46%, −32.94%, 17.17%, 6.14%, 18.25%, 4.52%, −88.80%, 31.13%, −8.08%, and −3.37%.

In FIG. 8, the biases of the calibrated forecast from January to December are respectively −1.34%, −1.00%, 0.20%, −0.42%, −0.86%, −0.50%, 0.80%, 0.56%, −0.88%, 1.04%, −1.46%, and −0.26%; and

The forecast skills CRPSS of the calibrated forecast from January to December are respectively −2.71%, 1.68%, −4.77%, 27.84%, 15.00%, 7.79%, 19.83%, 3.68%, −8.17%, 31.34%, −0.68%, and 33.41%.

By comparing the above biases and the forecast skills CRPSS, it can be seen that after the method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution is used, it is obvious that the biases of the calibrated forecast result are effectively reduced than that of the raw forecast data; the biases in the calibrated forecast are basically within 1.5%, and the forecast skill CRPSS thereof are more stable.

In addition, the present embodiment can use the sentences class ( ) and def ( ) in the Python to encapsulate each step of the precipitation forecast calibration to form class functions which are respectively gamma_fit, trans_norm, back_trans, conditional_distribution, and gamma_gaussian; the class functions are stored in a .py file; when in use, the precipitation forecast in a basin can be calibrated only by calling the class functions by means of an import sentence.

The same or similar reference signs correspond to the same or similar components.

The words for describing position relationships in the drawings are used for illustrative purpose only, but should not be considered as a limitation to the present patent.

It is obvious that the above embodiments of the present invention are only examples for clearly illustrating the present invention, but not limitations to the embodiments of the present invention. A person skilled in the art may make various modifications or variations in other forms on the basis of the above description. It is unnecessary and impossible to exhaust all the embodiments herein. Any modifications, equivalent substitutions, improvements and the like made within the spirit and principles of the present invention are all intended to be concluded in the scope of protection of the claims of the present invention.

Claims

1. A method for calibrating monthly precipitation forecast by using a Gamma-Gaussian distribution, comprising the following steps: S1, acquiring forecast data of a monthly average precipitation in a watershed area and corresponding observed values of the average precipitation in the watershed area as input data;S2, performing fitting on the input data by means of a Gamma distribution function;S3, calculating a cumulative distribution function value of each input data in a corresponding Gamma distribution;S4, transforming the cumulative distribution function values into variables obeying a standard normal distribution;S5, constructing a joint normal distribution according to the variables obeying the standard normal distribution to characterize a correlation between the forecast data and the observed values in the input data; andS6, randomly sampling the observed values according to the correlation, and inversely transforming acquired samples to obtain a calibrated forecast result.

2. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 1, wherein in the step S2, the Gamma distribution function is used to perform fitting on the forecast data and the observed values respectively to obtain marginal distributions of raw forecast data and the observed values; the expression formula thereof is as follows:

3. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 2, wherein the Gamma distribution parameters αf, βf, αo, and βo are respectively calculated with a maximum likelihood estimation method.

4. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 2, wherein in the step S3, a cumulative distribution function in the corresponding Gamma distribution is used to calculate the cumulative distribution function values of each forecast data fi and of each observed value oi in the corresponding Gamma distribution; the expression formula thereof is as follows:

5. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 4, wherein in the step S4, the cumulative distribution function values are regarded as quantiles of the standard normal distribution; the cumulative distribution function values are transformed into variables obeying the standard normal distribution by means of an inverse function of the cumulative distribution function in the standard normal distribution; and the expression formula thereof is as follows:

6. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 5, wherein in the step S5, a joint normal distribution is constructed according to the variables {circumflex over (F)} and Ô obeying the standard normal distribution to characterize the correlation between the forecast data and the observed values in the input data; and the expression formula thereof is as follows:

7. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 6, wherein in the step S6, the specific steps are as follows: S6.1, taking the forecast data {circumflex over (f)} as a predictor, taking the observed value ô corresponding to each forecast data as a predictand, and calculating a conditional probability distribution of the predictand, wherein a calculation formula is as follows: ô|{circumflex over (f)}˜N(ρ{circumflex over (f)},1−ρ2);S6.2, randomly sampling a conditional probability distribution result of the observed value ô, and inversely transforming sampled samples according to the cumulative distribution function in the standard normal distribution and the inverse function of the cumulative distribution function in the Gamma distribution obtained by performing fitting on the observed value, so as to obtain a calibrated forecast result.

8. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 1, further comprises the following step: calculating a bias value and a forecast skill according to the calibrated forecast result as forecast verification metrics.

9. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 8, further comprises the following step: drawing a forecast diagnostic diagram according to the calibrated forecast result, the bias value, and the forecast skill.

10. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 9, wherein in the forecast diagnostic diagram, a calibrated forecast median is used as an x axis; a precipitation forecast distribution interval and the observed values are used as a y axis; and calculation results of the bias value and the forecast skill are interpolated in the forecast diagnostic diagram for display.

11. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 2, further comprises the following step: calculating a bias value and a forecast skill according to the calibrated forecast result as forecast verification metrics.

12. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 3, further comprises the following step: calculating a bias value and a forecast skill according to the calibrated forecast result as forecast verification metrics.

13. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 4, further comprises the following step: calculating a bias value and a forecast skill according to the calibrated forecast result as forecast verification metrics.

14. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 5, further comprises the following step: calculating a bias value and a forecast skill according to the calibrated forecast result as forecast verification metrics.

15. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 6, further comprises the following step: calculating a bias value and a forecast skill according to the calibrated forecast result as forecast verification metrics.

16. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 7, further comprises the following step: calculating a bias value and a forecast skill according to the calibrated forecast result as forecast verification metrics.

17. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 11, further comprises the following step: drawing a forecast diagnostic diagram according to the calibrated forecast result, the bias value, and the forecast skill.

18. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 17, wherein in the forecast diagnostic diagram, a calibrated forecast median is used as an x axis; a precipitation forecast distribution interval and the observed values are used as a y axis; and calculation results of the bias value and the forecast skill are interpolated in the forecast diagnostic diagram for display.

19. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 12, further comprises the following step: drawing a forecast diagnostic diagram according to the calibrated forecast result, the bias value, and the forecast skill.

20. The method for calibrating monthly precipitation forecast by using the Gamma-Gaussian distribution according to claim 19, wherein in the forecast diagnostic diagram, a calibrated forecast median is used as an x axis; a precipitation forecast distribution interval and the observed values are used as a y axis; and calculation results of the bias value and the forecast skill are interpolated in the forecast diagnostic diagram for display.